GB1567553A

GB1567553A - Digital ground proximity warning systems

Info

Publication number: GB1567553A
Application number: GB24862/77A
Authority: GB
Original assignee: Litton Industries Inc
Current assignee: Litton Industries Inc
Priority date: 1976-06-14
Filing date: 1977-06-14
Publication date: 1980-05-14

Description

(54) DIGITAL GROUND PROXIMITY WARNING SYSTEMS (71) We, LITTON INDUSTRIES, INC., a corporation organised and existing under the laws of the State of Delaware, United States of America, having an office at 360 North Crescent Drive, Beverly Hills, California 90210, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to digital ground proximity warning systems.

The FAA has promulgated regulations and specifications for ground proximity warning systems for installation on commercial aircraft. These specifications are referenced at 14 Code of Federal Regulations, Parts 37 and 121, as published in the Federal Register (F.R. 30839) on July 23,1975, and are disclosed in graphic form in the accompanying drawings.

According to one aspect of the present invention, there is provided a ground proximity warning system comprising: digital data processing circuit means for checking ground proximity warning conditions in either of a first and a second mode, the first mode being for detecting a predetermined condition of loss of altitude of an aircraft on take-off or missed approach and the second mode being for detecting a predetermined condition of proximity to ground with landing gear or flaps not in a landing configuration; means for providing a warning signal if in either mode the predetermined condition is met; a mode continuity circuit connected to the processing circuit means and including a storage device having a first state when one of the two modes is operative and a second state when the other of the two modes is operative; and means for checking the state of said device upon resumption of power following a power interruption to place said processing circuit means in the mode corresponding to the device state.

According to another aspect of the invention, there is provided a digital ground proximity warning system, comprising: low-level semiconductor digital data processing circuit means for iteratively checking ground proximity warning conditions of an aircraft; first means for operating said digital data processing circuitry in a first mode for detecting loss of altitude of an aircraft on take off or missed approach; means included in said digital data processing circuitry for providing a warning signal if said first detecting means indicates loss of altitude beyond predetermined limits for a predetermined length of time; second means for operating said digital data processing circuitry in a second mode for detecting proximity to ground with landing gear or flaps not in the landing configuration;; means included in said digital data processing circuitry for providing a warning signal if either of said detecting means indicates ground proximity warning conditions for a predetermined number of iterations; a mode continuity circuit including a two state storage means compatible with said low-level semiconductor circuitry and having a relatively long time constant for state retentions; means for setting said storage means to one of its two states when the ground proximity system is in the second of said two modes; and means for promptly checking the discrete input associated with said mode continuity circuit upon resumption of power following a power interruption, and for placing said digital data processing circuitry into said second mode if said storage means is in said one state.

A preferred embodiment has in its continuity circuit a storage element having a long time constant for signal retention, and which is consistent with low-level semiconductor circuitry. It serves to bridge brief power outages, and indicate to the ground proximity warning system on resumption of power whether it is in a take off (Mode 3) ground warning situation or in a landing (Mode 4) warning situation. In the absence of such a mode continuity circuit, the ground proximity system could pick up in the wrong mode and give a false alarm. In this embodiment, the mode continuity circuit includes a capacitor having a relatively long discharge time, and means for continuously charging this capacitor while it is in one of the two modes.

Following a power interruption, an initialization sequence determines whether the capacitor is still charged or not, and places the ground proximity system in the appropriate mode reflecting the state of the capacitor. Because of the critical nature of the landing warning mode, the capacitor is charged whenever the aircraft enters Mode 4 warning conditions. Then, immediately after a brief power interruption, the ground proximity warning system will pick up in Mode 4 without delay. Mode 3 conditions are indicated by a discharge capacitor, and the full initialization routine, which is moderately time-consuming, is accomplished when the mode continuity circuit includes a discharged capacitor.

In the foregoing a relatively long time constant and a prompt response by the checking means are mentioned. Preferably the time constant and the promptness are such that a power interruption of 1/5 of a second, or as much as 3/10 of a second, can be bridged.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figures 1 A and 1 B together make up an overall block diagram of a ground proximity warning system illustrating the principles of the present invention; Figure 2 is a diagram showing an aircraft in a flight altitude which might bring the present ground proximity warning system into play; Figures 3 through 8 show various modes of flight of an aircraft which would trigger warnings from the present system under certain specified conditions; Figure 9 is a generalized ground proximity warning mode envelope.

Figures 11 a through 11 e constitute a second diagram giving sequence steps for determining whether the aircraft is within one of the ground proximity warning envelopes of Figures 3 through 8; Figure 12 is a third diagram indicating initialization steps for the present ground proximity warning system; Figure 13 is a block diagram of one possible implementation of warning and clear counter circuitry, indicating how and when the warning circuits are energized; Figure 14 is a circuit diagram of the mode continuity circuit; and Figure 15 is a circuit diagram of the analog to digital converter employed in the ground proximity warning system.

Referring more particularly to the drawings, Figure 1, including sheet IA and sheet 1B, is a block diagram of the ground proximity warning system, together with associated sources of input signals. The ground proximity warning system receives input signals from other aircraft systems, including radar altimeter 12, barometric altimeter 14 or 14', and glide slope receiver 20. The ground proximity warning system 10 may be adapted to receive warning system 10 may also be adapted to receive inputs from Scott T transformers 24 which receives synchro signals from the barometric altimeter.In the block circuit diagram of Figures 1A and IB, the principal circuits include an analog input signal conditioner 36; an analog multiplexer 38; and analog to digital converter 40; a discrete signal input buffer 44; a digital multiplexer 72; a computer 74 which includes microprocessor 76, a read only memory or "ROM" 78, a random access memory or "RAM" 82, and clock circuit 84; and warning circuit 86. Warning circuit 86 includes discrete buffer 88, the voice synthesizer 89, two audio amplifiers 92 and 94 and the additional discrete buffer circuit 96. The circuit components utilized in fabricating ground proximity warning system 10 may include TTL, Schottky Bipolar and MOS monolithic devices or other equivalent devices, or equivalent circuit.

Analog input signal conditioner 36 receives signals which include a DC radio or radar height signal 13, a DC altitude rate signal 15, an AC altitude rate signal 17, an air data reference signal 19, a high glide slope deviation signal 21, and a low glide slope deviation signal 23. In most applications either DC rate signal 15 or AC rate signal 17 would be received from a barometric altimeter 14 but not both.

Scott T transformer 24 transforms altitude synchro signal 25 into sine signal 27 and cosine signal 29, with a reference signal being supplied on lead 31. The signals received by signal conditioner 38 are normally scaled to the requirements of some specification controlling the characteristics of equipment used on aircraft. For example, the equipment utilized on commercial aircraft flying within the United States produces output signals which meet the requirement of ARINC characteristics.

The different types of signals received by the ground proximity warning system from the various equipment such as the radar altimeter 12, barometric altimeter 14 or 14', air data computer 18 and glide slope receiver 20 are monitored by computer 74.

In order to monitor these different types of electrical signals they must first be converted to a uniform format. Signal conditioner 36 transforms each analog signal into a DC signal having a range from -5 to +5 volts. Analog multiplexer 38 selects one of the transformed incoming signals at any one time for monitoring by a computer 74. Computer 74 controls the selection of a particular incoming signal via address control bus 77. The selected signal is transmitted on bus 41 to analog-to-digital converter 41 which transforms it into a 12bit digital word which represents the analog magnitude and the polarity of that signal.

Output signals from analog-to-digital converter 41 are transmitted on bus 43 to digital multiplexer 72.

Discrete input signal buffer 44 transforms discrete input signals received by ground proximity warning system 10 to TTI compatible voltage levels. In the block diagram of Figure 1, there are shown selected sources of discrete input signals which may be required by the present airborne ground proximity warning system.

The discrete input signals which are required are determined by the performance specification covering the operation of the ground proximity warning system. Discrete input signals 45--69 are shown by way of example. The format and logic of these discrete input signals may be of several types, such as a ground and an open connection denoting a true logic state (or binary "1"), and a false logic state (or binary "0"), respectively; or +28 volts DC may mean a true logic state and zero volts DC may mean a false logic state, etc.

Various types of circuitry may be utilized in buffer 44 to transform the various types of discrete input signals to +5 volts or zero volt output signals. The discrete input signals are all continuous signals.

It may be noted in passing that there may be some apparent duplication of the analog and discrete inputs. This is a result of the applicability of the present system to any type of commercial aircraft, including aircraft having different types of analog and discrete signals. Accordingly, the system provides different inputs for different types of analog and discrete inputs; but only one barometric altitude signal or landing gear signal, for specific examples, would be used at one time.

With reference to the discrete inputs to the buffer unit 44, they include radar altimeter circuit 12, barometric altimeter circuit 14, landing gear circuitry 16, flap position switch circuitry 22, the self-test circuit 24, inhibit switch circuitry 26, glide slope receiver 20', glide slope circuit 22, the Mode 4 continuity circuit 26 together with its associated pulse source 28. As mentioned above, these discrete signals may have different forms on different aircraft, and certain blocks which appear in Figure 1A have been repeated in Figure 1B for completeness. Note in connection with the flap position switch that there are two output leads 53 and 55.These actually relate to separate input leads which require different buffering circuits to translate them to the required zero volts to +5 volts required at the output of buffer circuit 44.

More specifically, in one aircraft the flap discrete signal could be +28 volts indicating that the flaps are in one state and zero volts indicating the flaps are in the other state; while in another aircraft the two states could be represented by ground and an open circuit, respectively. The two inputs 53 and 55 are not both used for any one installation. Instead, when the ground proximity warning system is installed in an aircraft having the +28 and zero volt discrete, input lead 53 is employed: while aircraft having the open circuit and ground discrete signal for the flap position switch circuit connected to the lead 55.

Digital multiplexer 72 receives a sequence of digital words from analog-todigital converter 40 on circuits 43, with the digital words representing the magnitude and polarity of input analog signals 13-35; and multiplexer 72 also continuously receives discrete input signals from buffer 44 via bus 71. Digital multiplexer 72 includes several circuits each of which multiplexes digital signals from 8 lines to one line. That is, it selects one from up to eight eight-bit parallel data input signals to present to the input of computer 74. More particularly, eight digital words of eight bits each are received by digital multiplexer 72.

Digital multiplexer 72 transmits one eight bit word at a time computer 74 via bus 73, which includes eight leads. Selection is accomplished under the control of computer 74 via address control bus 79.

In the present specification and claims, reference will be made both to random access memories, referred to as "RAMS" or "RAM" memories, and to "read-only" memories, which will be referenced by the acronym "ROMS", or by the designation "ROM memories".

Microprocessor 76 is preferably a single integrated circuit device such as an Intel 8080 microprocessor chip. ROM memory 80 comprises an ultraviolet erasable read only memory. Four ROM chips each including storage for 1,094 words of 8 bits each are utilized for instructions and as a memory for constants. RAM memory 82, which comprises two 256x4 bit random access memory chips, is employed as a "scratch pad" memory. That is, during computations intermediate solutions may be temporarily written in RAM memory 82, recalled when needed, and then erased.

Computer 74 processes digital words representative of radar height signal 13, barometric altitude rate signal 15, and guide slope deviation signals to provide a warning when the aircraft is on a dangerous flight path. Computer 74 includes permanently stored sequencing arrangements to provide a warning of dangerous flight paths due to an excessive rate of descent, an excessive terrain closure rate, a significant descent after takeoff, insufficient clearance with respect to the terrain when not in a landing configuration (flaps and landing gear down) or an excessive deviation from the proper glide slope.Dangerous flight paths are identified in terms of aircraft flight parameters which fall within any one of several predetermined envelopes which are functions of radar height above the terrain and one of several other variable flight conditions including radar altitude rate, barometric altitude rate, barometric altitude loss, or glide slope deviation; and these envelopes may be modified by various flight parameters such as flap state or the like. Sets of constants are stored in ROM memory 80 which define lines dividing clearly safe or normal flight conditions from dangerous conditions where the pilot should be alerted by the "proximity of the ground", as measured by the above-described combinations of input signals.Such conditions where it is desirable to alert the pilot by ground proximity warnings shall be referred to in the present specification as "ground proximity warning" or GPW conditions.

Incidentally, before considering the Mode 1 characteristics in detail, attention is directed to the Mode 4 continuity circuit 26 and its associated pulse source 28. The Mode 4 continuity circuit discrete input 69 differs from all of the other discrete inputs to buffer 44 in that its state is controlled by computer 74. More specifically, circuit 26 is provided to indicate whether, in the case of a brief power interruption, the ground proximity circuit has been in the Mode 4 or the Mode 3 state. Mode 4 involves landing of the aircraft while Mode 3 involves ground proximity conditions when the plan is taking off. In the absence of a continuity circuit such as circuit 26 upon resumption of power after a brief interrupt, a false warning might be produced. When the system is in Mode 4, a capacitor in continuity circuit 26 is charged up.This is accomplished under the control of lead 60 which enables the application of pulses from source 28 to circuit 26. Following resumption of power, the computer checks discrete input 69 from continuity circuit 26 and returns the computer to Mode 4 if a Mode 4 indication is provided at the output of continuity circuit 26.

Now that an overview of the system has been presented, the specific characteristics of each of the warning mode envelopes will be considered in graphical form.

Figures 2 and 3 show, respectively, a diagram of a ground proximity warning situation, and a plot of the so-called "Mode 1" conditions under which an alarm signal is given. In Figure 2 an aircraft 212 is approaching the ground or terrain 214 under conditions which may actuate the ground proximity warning system. In Figure 2 the radar height at any instant is indicated by the vertical arrows 216.

In the present ground proximity warning system, the height above the terrain, otherwise referred to as the radar or radio height, is plotted against various parameters and various warnings are given if the radar height is below certain prescribed levels, and if other criteria are met. Thus, as shown in Figure 3, representing "Mode 1" warning conditions, a warning is given if the radar height and the barometric altitude sink rate are such as to fall to the right of characteristic 218. By way of specific example, if the plane was losing altitude at a rate of 4,000 feet per second and was at a height of 1,000 feet, the resulting point designated 220 in Figure 3 would be to the right and below the characteristic 218, and after a suitable delay a warning signal would be given. On a more general basis, let us assume that a plane is losing altitude at a rate of 4,000 feet per minute, and is thus descending along line 222. At point 224 it would cross characteristic 218, thus indicating that it is entering the region where a ground proximity warning signal is appropriate. However, a delay is introduced in the actual implementation of the warning, to avoid false warnings, as described below in connection with the sequence and circuit diagrams.

It may be noted in passing that no warning is given when the plane is on the ground or within 50 feet of the ground, as indicated by the horizontal line 225 in Figure 3.

Figures 4 through 8 represent other conditions involving different parameters and input signals, plotted against radar height, under which it is desired to give a ground proximity warning signal. More specifically, Figure 4 shows the so-called Mode 2 plots 230 and 232 of terrain closure rate against the height above the terrain, or radar height, for two different aircraft flap configurations. As compared with Figure 3, Figure 4 uses the radar altimeter sink rate instead of the barometric altitude sink rate.

In Figure 4, characteristic 230 is effective only when the flaps are not in the landing configuration, while characteristic 232 is utilized when the flaps are down, in the landing configuration. From an operational standpoint, note that for a radar height of 1,000 feet and a radar sink rate of 5,000 feet, shown at point 234 in Figure 4, that a warning would be given if the flaps were up, but not if they were down in the landing configuration. This confirms the expected relationships of the warning envelopes 230 and 232, i.e., greater tolerance without warning is appropriate when the flaps are down.

Modes 3A and 3B warning conditions are set forth in Figures 5 and 6. The alternate Modes 3A and 3B involve take-off conditions approaching a stall. More specifically, Figure 5 involves a negative climb rate, before acquiring 700 feet terrain clearance after takeoff or a missed approach, defined by the warning envelope 236 of Figure 5. Mode 3B of Figure 6 is an alternative to Figure 5, and the warning envelope 238 in Figure 6 is a plot of barometric altitude loss vs radar altitude. It is further noted that Mode 3 is implemented by either Mode 3A as shown in Figure 5 or by Mode 3B as shown in Figure 6, but not both.

In general, Mode 4, as represented by Figure 7, involves flight into the terrain but with less than 500 feet terrain clearance and the aircraft not in the landing configuration.

With reference to Figure 7, the horizontal line 240 defines a warning envelope and indicates that if the plane flies below 500 feet with the landing gear up, a warning will be given. Line 242 defines additional warning conditions involving the flaps. If the flaps are up, even if the landing gear is down, a warning will be given when the aircraft conditions are below or to the right of line 242.

Figure 8 shows the warning envelopes for Mode 5 involving glide slope deviation plotted against radar height. In Mode 5, the left-hand side 252 of the envelope occurs at 1.3 "dots" of deviation below the glide path and the line 254 indicates that no warning will be given if the aircraft has a radar height above 1,000 feet. The zone A of the warning envelope is defined on its lower side by the horizontal line 256 which occurs at 300 feet height above the ground. Within the region A, a low level audio signal is employed and the repeat time of the audio signal "glide slope" may be increased in accordance with the formula: t± 7205882x+ . 0070y+ .9367647 Alternatively, the repetition rate of a flashing warning light or a warning tone may be increased.

At lower levels of altitude and/or at higher deviations below the glide slope, the aircraft conditions are more hazardous and it is desirable that a more vigorous warning be given. Accordingly, in region B the repeat time is at a frequency of .875 seconds, so that audio signal "glide slope" is repeated more than once per second. In region C, if desired, a higher level audio signal may be employed.

A specific example of determining whether or not an aircraft is within the ground proximity warning envelope, when a warning signal should be given, will now be developed. In this connection, reference is made to Tables I, II, III and IV, which appear near the end of the present specification. These tables essentially define each of the characteristics shown in Figures 3 through 7, and relate them to the standardized format of Figure 9, and the associated equations noted above. Thus, in Table II, the y-intercepts for the line segments of Modes 1 through 4 are given; in Table III the slopes of the line segments are defined in terms of their reciprocals; and in Table IV the number of line segments and the key radar height values for line segment changes for Modes 1 through 4 are given.

More specifically, note in Figure 3 that the characteristic 218 includes the straight line segments A and B corresponding generally to the segments A and B shown in the generalized mode plot of Figure 9. In Figure 9, segment A is also designated by the reference numeral 260 and segment B is also designated by the reference numeral 262.

In the determination as to whether an aircraft is within the ground proximity warning envelope, when it has a predetermined radar height above the ground and a predetermined sink rate, the system utilizes the figures stored in Tables II, III and IV. These tables define the end points and the y axis intercepts, as well as the slopes of the line segments.

We will now consider a specific example in which an aircraft has a radar altitude or height above the ground of 2,000 feet and a sink rate of 4,500 feet per minute, shown by point 270 in Figure 4.

As indicated in the common mode sequence of Table I, the first step is to determine whether the radar height is above or below the positive cutoff value of 2,450 feet above the terrain, as indicated by the upper right-hand horizontal portion of characteristic 218 in Figure 3. Obviously, with a height above the terrain of 2,000 feet, the aircraft is not above the positive cutoff height of 2,450 feet: accordingly, the next step of comparison is undertaken. As indicated in Table IV, the intersection of line segments A and B in Figure 3 for Mode 1 occurs at 1.225 feet of radar height. The actual height of the aircraft, 2,000 feet, is then compared with 1,225 feet, and it is determined that the aircraft height above the terrain falls within the height range in which the ground proximity warning envelope is defined by line segment B.

The next step in the computation is to determine the barometric altitude sink rate defined by line segment B for a radar height of 2,000 feet. Each line segment A and B is defined by the equation: y=mx+b where y is the radar altitude, x is the other parameter in Modes 1--4, m is the slope, and b is the y intercept of the line segment.

Solving for the value of x, which corresponds to the sink rate in the present example, it is determined by the relationship: 1 x=y-b) m In Table II, the B intercept is shown as being equal to -45 feet. Incidentally, because most of the intercepts are negative, the negative value of the intercept is listed in the second column of Table II. In the present instance, where the intercept is at +45 feet, the table entry is -45. In Table III, the reciprocal of the slope of the line segment B is shown equal to 2.00.

Solving the formula set forth above for the value of x, the resulting value of the sink rate is 3,910 feet per minute, see dash-dot line 271. This means that, at a radar height of 2,000 feet, a sink rate of 3,910 feet per minute would fall precisely on line segment B in Figure 3 which divides the regions in which a warning is desired from aircraft flight conditions in which no warning is desired. In the present example, of course, where the sink rate is 4,500 feet per minute, this value falls at point 270, as shown in Figure 3, which is well within the ground proximity warning area. The mathematical calculation which determines this fact involves a comparison of the actual or computed sink rate of 4,500 feet per minute with the computed 3,910 figure which defines the ground proximity warning envelope for a radar height of 2,000 feet.Of course, with 4,500 representing a sink rate in excess of the allowable 3,910 feet per minute, a determination is made that the aircraft is within the ground proximity warning envelope. This determination is one step in a timing and counting cycle as described below which results in a warning signal, either audible or visible, after a predetermined number timing and counting cycles elapse with the aircraft in GPW danger conditions.

The intercepts, slope functions, number of line segments, and line segment end points for Modes 1 through 4 are set forth in Tables II, III and IV, and calculations are accomplished relative to each of the other modes in the manner set forth above for Mode 1 represented in Figure 3.

The generalized program steps as defined in Table I are stored in the "read only memory" or ROM 80 of Figure iB.

Similarly, the values set forth in Tables II, III and IV are stored in the ROM 80. By using the same subroutine set forth in Table I for the determination as to whether the aircraft flight conditions for each mode are within the appropriate ground proximity warning envelope, a great saving in equipment and expense is achieved. In each case, suitable mode identification pointers are set up and, in going through the common mode subroutine, the data processor calls up the values for Mode I when the Mode 1 calculation is undertaken, and systematically calls up the constants pertaining to Mode 2 when the Mode 2 calculation is undertaken, etc.

Incidentally, it is noted that the values set forth in Tables II, III, and IV, as well as the common mode subroutine define in Table I, are permanently stored in the ROM chip 80, and are thus part of the permanent characteristics of the system of the present invention.

Figures 10 and 11 are sequence flow charts for the present invention. The steps indicated in Figures 10 and 11 are implemented under the control of the read only memory 80 of Figure 1B. The sequence of Figure 10 is referred to as the "real time" sequence, because it is accomplished every 1/16th of a second, and involves the calculation of parameters such as radar height, barometric sink rate, and the like which are needed for the calculations to determine whether the aircraft is in one of the ground proximity warning conditions.

The sequence shown in Figure 11 is known as the On-Line Sequence", and includes the sequence steps in the actual mode calculations which determine whether the aircraft is within any one of the five GPW envelopes. Concerning the timing of the sequences of Figures 10 and 11, the real time calculations occur regularly 16 times per second, and take about 10 milliseconds.

The remainder of the time is occupied with the "On-Line Sequence" of mode calculations set forth in Figure 11 and these are repeated several times during the course of the remaining 53 or 54 milliseconds until interrupted on a regular basis by the initiation of the "real time" sequence.

The sequence of Figure 10 starts by interrupting the mode calculations which will be disclosed below in connection with Figure 11. This interruption is indicated by the "Enter" block 282 of Figure 10. Block 284 designated "Save Registers and Condition Bits", indicates that information, such as previously calculated radar height and radar sink rate, for examples, are retained for future use. The next step as indicated by block 286 is to "Read Radar Height" from the A/D converter. Of course, the A/D converter 40 of Figure 1A is that which is referenced in block 286.

As indicated by leads 15, 17, 27 and 29 in Figure IA, the barometric rate input may appear in several forms, depending on the aircraft in which the computer is installed.

Block 288 and the subsequent steps are required for the accommodation of these various possible sources of barometric rate input. In the present drawings, sequence steps involving alternate subsequent steps are in the form of a diamond. Block 290 represents a sequence step asking the question, "Is the Barometric Rate Given in DC form?" If the barometric rate is in DC form, step 292 involves the selection of the DC input 15 of Figure I A and the preloading of A/D converter 40 with this barometric rate input. In the event the barometric rate was not DC, block 294 determines whether the barometric rate is in AC or synchro form, and either step 296 or 298 are implemented to provide the barometric rate signal at the output of the A/D converter 40.

The next four steps indicated by blocks 302, 304, 306 and 308 involve the computation of the basic parameters which determine whether the aircraft is in a ground proximity warning state or not.

These basic parameters include the computation of the radar height in block 302, the radar sink rate in block 304, the barometric sink rate in block 306 and the deviation below the glide slope in block 308.

Block 310 asks the question, "Should the Monitor Light be Flashing?". The monitor flashing light indicates that something is amiss with the ground proximity warning system in that one of the basic inputs is not present or the like, and step 312 causes the monitor light to be flashed under such circumstances. Block 314 checks whether any warning flags requiring ground proximity warning light signals are needed and if so, step 316 causes energization of the appropriate warning light.

The next two blocks 318 and 320 involve system procedural requirements including updating of the system clock (which is periodically stepped down as the 1/16th of a second interrupt interval expires), restoring all registers and condition bits. The final block 322 and 324 indicate shifting back over to the "On-Line Sequence" in which mode calculations are performed, as set forth in Figure 1 IA through Figure 1 lE.

Figure 11 includes successive calculations for Modes 1 through 5 to determine whether the aircraft is within the warning envelopes shown in Figures 3 through 8, for example.

The first step of the Mode 1 calculation involves a determination as to whether the aircraft is or is not within Mode 1 envelope.

This is indicated by block 328 labeled "Inside Mode 1 Envelope?" This diamond 328 has a double line boundary to indicate that it involves the Common Mode Subroutine set forth in Table I which appears at the end of the present specification.

If the Mode 1 subroutine indicates a "Yes" answer, then the clear counter is reset to zero as indicated by step 330.

Further, the warning or "warn" counter is advanced and a check is made as to whether the warn counter has reached the preset level at which a warning should be given.

This step is indicated by block 332 labeled "Is Mode 1 Warn Count Up?". If the answer to this question is "Yes", the Mode 1 warning flag will be set in the RAM 82 as indicated by box 334. If not, no additional action beyond advancing the warn counter will occur.

Returning to the alternative output from block 328, if the aircraft is not within the Mode 1 envelope, the clear counter is advanced and a determination is made, as indicated by block 336 as to whether the Mode 1 clear counter has reached a preset count. If the clear count has reached a predetermined count (usually 2 or 3 counts is sufficient), the warn delay counter and timer is reset to zero, as indicated by block 338 and the Mode 1 warn flag is set to zero, as shown by block 340. If the clear count has not reached its preset level, it will be advanced or updated as indicated by block 342.

In general, the warn counter and the clear counter are advanced or other action taken every 1/16th of a second. The full count for the clear counter is normally 2 or 3 counts so that if the aircraft is not within the GPW envelope for two successive sampling cycles, the warn counter is reset to zero and any warning signal is halted.

On the other hand, the warning counter may have 40 counts applied to it before it actuates a warning signal. Upon the arrival of each warning signal, the clear count is reset to zero and the warning counter is advanced. If the aircraft stays continually within the warning envelope, and the full count for the warning counter is 40, the time required for the initiation for a warning signal would be 2-1/2 seconds resulting from 40 counts at 1/16th second intervals. On the other hand, if the aircraft has alternate positions within and without the warning envelope during successive sampling intervals, then the warning counter will receive a count only every other cycle, or every 1/8th of a second, and will then require 5 seconds before actuation.This longer period of time also permits reset of the warning counter upon the occurrence of two clear counts, which is likely to occur during the 5 second interval with the aircraft flight conditions bordering on the GPW envelope.

Continuing with Figure 11, the Mode 2 check starts below the horizontal line 326 and implements the check shown diagrammatically in Figure 4. More specifically, note Figure 4 includes Mode 2A which is applicable when the flaps are not in the landing configuration and Mode 2B which comes into play when the flaps are in the landing configuration. Accordingly block 344 asks the initial Mode 2 question "Are the Flaps Down?". If this question is answered in the negative, Mode 2A is checked to determine whether the flight parameters bring the aircraft within the Mode 2A envelope, as indicated by common mode diamond 346. On the other hand, common mode diamond 348 is utilized to determine whether the aircraft is within Mode 2B if the flaps are down.The "Yes" outputs from diamonds 346 and 348, indicating that a Mode 2 GPW envelope has been entered, serve to reset the Mode 2 clear counter as indicated by block 350 and to advance the Mode 2B warning counter, as indicated by diamond 352.

When the Mode 2 warning count reaches the maximum, the warning flag is set, as indicated by block 354. The path resulting from "No" answers from diamonds 346 or 348, indicating that the aircraft is outside the warning envelopes, leads to step 356 and successive steps 358, 360 and 362 corresponding substantially to the clear count action noted in connection with Mode 1 and blocks 336, 338, 340 and 342.

Essentially, if the clear count has reached its maximum, the warning counter is reset and the warning is stopped, and the clear count is merely advanced if it has not reached its maximum value.

Following Mode 2, the diamond 364 in Figure 11B indicates a decision as to whether Mode 3 or Mode 4 should be considered. This is made on the basis of a "flag" or indication bit set in the random access memory or RAM 82 of Figure 1B.

This flag is set to the Mode 4 state after the aircraft has exceeded 700 feet of radar height for four seconds. The flag is reset to the Mode 3 state only after the following conditions have been met to indicate a take off or a missed landing: 1. The radar altitude is below 500 feet and the flaps and/or gear have been shifted from the landing configuration to the "Not in Landing" configuration; or 2. The radar altitude must remain below 50 feet for 5 seconds; and 3. Three seconds must have elapsed for transition from Mode 4 to Mode 3.

If the RAM flag indicates that the system is in the Mode 3 state, the next step is indicated by diamond 366 which asks the question "Over 700 feet for 4 Seconds?" Upon the receipt of a "Yes" answer, block 368 indicates that the Mode 4 warn delay timer is reset and the Mode 3 warning flag is reset to zero. In addition, the RAM flag distinguishing between Mode 3 and Mode 4 is set to the "Mode 4" state. A "No" answer to the question of "Over 700 feet for 4 Seconds?" leads to step 370 which asks the question "And the Flaps Down?". A "No" answer to this question leads to the common mode diamond 372 which determines whether the aircraft is within the Mode 3 warning envelope, either Mode 3A or Mode 3B depending on the system selected by the aircraft user.A "Yes" answer to the question of whether the flaps are down from block 370 or a "No" answer to the mode termination question of block 372 both lead to the diamond 374 on the basis that there is no Mode 3 problem.

Blocks 376, 378 and 380 correspond in function to blocks 338, 340 and 342 of the Mode 1 program appearing in Figure 1 IA, and involve the resetting of the warn delay timer and resetting of the warning flag or advancing of the Mode 3 clear counter, if it has not yet reached its full count. On the other hand, a "Yes" answer to the Inside Mode 3?" question of diamond 372 causes the resetting of the Mode 3 clear count (block 382) and advancing of the Mode 3 warn counter or a setting of the Mode 3 warn flag, as indicated by blocks 384 and 386.

As noted above, if block 364 indicates that Mode 4 is selected, the Mode 4 program is undertaken with the first step being indicated by the block 392 involving "Charge the Remember Mode 4 Capacitor". This step is included to insure that the Mode 4 capacitor is charged.

More generally, the Mode 4 capacitor is charged when the aircraft is not in Mode 3 but is in Mode 4. This initial step is undertaken to insure that Mode 4 capacitor properly represents the system state.

The remainder of the Mode 4 program may be better understood by reference to Figure 7 which shows the warning pattern involving flight into terrain with the gear or the flaps not in the landing configuration, as more fully described above. Returning to Figure 1 it, the next step indicated by diamond 394 is a determination as to whether the radar height is above 520 feet.

Before considering the next subsequent steps, it is useful to note that Modes 3 and 4 are mutually exclusive, and one of the criteria for determining transition to Mode 3 is the changing of the flaps from the landing configuration (down) to the "not in landing configuration" where the flaps are up. This involves the situation where the pilot initially starts to land but decides to "fly around" and make a new landing approach.

Returning to Mode 4, the block 394 involves a determination as to whether the radar height is above 520 feet. If the answer is "Yes", showing no Mode 4 problems, the blocks 396, 398, 400 and 402 representing the indicated method steps are implemented. First, the flap down "flag" or memory indication is set to the state indicating that the flaps are up (whether or not the flaps are down); similarly, the "flag" indicating the state of the landing gear is set to indicate the landing gear is up; and the "under 50 feet timer" is reset to 5 seconds.

This last step indicated by block 400 is regularly performed to avoid reversion into the Mode 3 state. Finally, the Mode 4 warning, if it should be actuated, is cleared, as indicated by block 402.

In the event of a "No" answer from diamond 394 indicating that the aircraft is below 520 feet, the sequence advances to diamond 404 which checks to determine whether there has been a flap transition from down to up. Of course, if there has been such a transition, this means that the pilot is not proceeding with a landing, but is planning to "go around" and attempt another landing. Accordingly, block 406 indicates change to Mode 3, block 408 indicates that the Mode 4 warning is removed, block 410 indicates that the flap down "flag" indication in the memory is shifted to the "up" state, the "gear down flag" is shifting to the "up" state as indicated by block 412, and finally the Mode 4 to Mode 3 transition timer is set to three seconds as indicated in step 414.As noted above, the passage of the three second interval is a necessary condition for transition to Mode 3 from Mode 4.

Returning to block 404, a "No" answer as to whether there has been a flap transition from down to up brings us to block 416, as we progress with the Mode 4 sequence.

Block 416 updates the state of the memory signal or "flag" relating to the condition of the flaps. In diamond 418 a check is made to determine if there has been a transition of the landing gear from down to up. An affirmative answer, of course, causes a "Change to Mode 3", at sequence block 406. A "No" answer to the diamond 418 with regard to a gear transition brings the system to step 422 involving the updating of the gear down flag. Diamond 424 asks if the aircraft is below 50 feet. If so, diamond 426 asks the question as to whether the under 50 feet timer has expired. If this question is also answered in the affirmative, the sequence reverts to "change to Mode 3" block 406. If the timer has not expired, then step 430 indicates that the Mode 4 warning should be stopped.Returning to the question posed by diamond 424, if the aircraft is not under 50 feet, the under 50 feet timer is reset as indicated by step 432 and the common mode is employed to determine whether the aircraft is in the far right-hand position of the Mode 4 envelope (shown by envelope 242 in Figure 7), as indicated by the double diamond 434. A negative answer leads to diamond 436 which determines whether the gear is down. If the answer is negative, the Mode 4 warning is set. With reference to Figure 7 showing Mode 4, the last mentioned sequence means that the aircraft is below the line 240 on the left-hand side, and the gear is not down. A "Yes" answer to question 436 indicates that the gear is down, bringing the system to step 440 by which the Mode 4 warning is removed.

Returning to the double diamond 434 which indicates the use of the common mode, a "Yes" answer indicates that the.

aircraft is to the right of line A in Figure 7 (within GPW envelope of line 242), and therefore that both the gear and flaps should be down. Diamond 442 asks this question.

Of course, if they are both down, the resulting affirmative answer brings the system to step 444 which clears the Mode 4 warning. A negative answer brings the system to step 446 which sets the Mode 4 warning.

Continuing with Figure 11D, Mode 5 involves as a first step the diamond 448 which determines whether the glide slope is enabled by the discrete input which indicates a glide slope signal is being received. A "No" answer routes the system to circle 450 and step 45 which sets the glide slope flag to "0". The system then reverts to Mode 1. A "Yes" answer to the question of diamond 448 leads the system to check the alternative glide slope inputs as indicated by diamonds 454 and 456. A "Yes" answer to the high level input sets the glide slope flag to the "I" state, as indicate by block 458, while a negative answer routes the system to diamond 456 which inquires as to the validity of the low level glide slope input.A "yes" answer to the question of the validity of the low level glide slope signal sets the glide slope flag into the "-1" state as indicated by block 459. On the other hand, a "No" answer sets the glide slope flag to the "0" state as indicated by block 460.

The next step of the program is indicated by diamond 462 which asks the question "Is the radar height more than 1,000 feet?" This is of course the highest level at which the glide slope warning may be energized, as indicated by Figure 8 relating to Mode 5. A "Yes" answer to the question that brings the system to step 464 which resets or clears the manual inhibit flag which may be energized by the pilot in the cockpit. If the plane is under 1,000 feet, the next step is indicated by diamond 466 which determines whether a manual inhibit has been established by the pilot (see discrete input 65 in Figure 1B). A "Yes" answer to the question brings the system to step 468 which involves setting of the manual inhibit flag in the RAM 82 of Figure 1B.

Mode 5 then includes a number of specific checks prior to full implementation.

As indicated by diamond 470, the Mode 1 through Mode 4 warning takes precedence and the system is not energized if any of these systems are in the warning state. The discrete glide slope validity signal is then checked as indicated by diamond 472.

Diamond 474 will negate the glide slope warning mode, on the basis that the aircraft is in Mode 4 which takes precedence. The glide slope warning mode will not go into effect unless the landing gear is down, as indicated by diamond 476. Diamond 478 checks to determine whether the aircraft is within the lower envelope of the glide slope warning mode shown in Figure 8. A "Yes" answer to the question of diamond 478 sets "glide slope" warning flags as indicated by step 480. Diamond 482 represents a check as to whether the aircraft is within the upper envelope of the glide slope warning pattern, see Figure 8. A "No" answer takes the system out of the Mode 5. A "Yes" answer to the question of diamond'482 brings the system to check the manual inhibit, indicated by diamond 484.Of course, an affirmative answer on the inhibit question again takes the system out of Mode 5. A "No" answer sets the "glide slope" warning flag, as indicated by step 482.

The sequence at the right-hand side of Figure 11 E involves completion of the on line loop or cycle and verifying that any warnings which are actually being given conform to the desired output given by the calculations and logic sequence steps as outlined above. Diamond 488 asks the question "Are Any Mode Warning Flags Set?" A "No" answer takes the system back to the beginning on the "on-line loop" at the start of Figure 11. A "Yes" answer from the question of diamond 488 brings the system to diamond 489 which inquires "Is the System Warning Flag Set?" A "No" answer to the question brings us to the series of steps designated 490 through 493 which involves first, setting the system warning flags, second, setting the appropriate fault balls, issuing audio and visual warnings and resetting the audio reduce timer.Following step 493, the system reverts to the beginning of the online loop.

Returning to diamond 489 relative to whether the warning flags are set, a "Yes" answer leads to diamond 494 inquiring "Is the Reduce Audio Discrete Set?" A "No" answer returns the system to the beginning of the on-line mode at the start of Figure 11; while a "Yes" answer brings the system to diamond 496 which asks the question "Is the Reduce Audio Timer Up?" A "No" answer again returns the system to the start of Figure 11, while a "Yes" answer shifts the system to block 497 which reduces the audio level. Following this step, the system reverts to the beginning of the on-line loop at the start of Figure 11.

Turning now to the initialization sequence of Figure 12, the first step following "Turn On", or resumption of power as indicated by the oval 502, is the step of disabling interrupts indicated by block 504. As previously mentioned, the real time calculations are normally performed every 1/16th of a second; however, during the initialization sequence, such interrupts are suspended.

The next step in the initialization sequence is indicated by the question in diamond 506; "Is the Mode 4 Capacitor High?" This sequence step requires a sensing of the output of the Mode 4 continuity circuit 26 having output lead 69 connected to the discrete signal input buffer 44 in Figure 1B. In the present system, the Mode 4 continuity circuit distinguishes between Mode 3 conditions involving takeoffs, and Mode 4 conditions involving excessive closure rate with respect to - the terrain without the landing gear or flaps down.

Without the continuity circuit 26 and the sequence step 506, a momentary interruption of power could switch the computer from the Mode 3 to the Mode 4 state, or vice versa, with the result that a false warning or no warning, when one is needed, might occur.

A "Yes" answer to the question of diamond 506 leads to sequence step 508 involving setting the Mode 4 flag in the random access memory or RAM 82. A "No" answer to the Mode 4 question of diamond 506 results in setting the Mode 3 flag, as indicated by step 510. Following step 510, a 1.2-second delay is introduced as indicated by block 512. This permits certain transient circuit conditions to be dissipated to avoid improper operation of the circuitry.

Block 514 involves the steps of resetting the random access memory to the "0" state and sensing the discrete output signals through input buffer 44. The warning time periods are established by sequence step 516, entitled "Input Thumb Wheels, Set Up Delay Times". Step 518 enabies the interrupts, thus countermanding prior step 504 and permitting the computer to sense the analog inputs on the regular 1/16th of a second basis employed in normal system operation. Steps 520 and 522 show repetitive sensing of the discrete inputs following a 17.5 millisecond delay indicated by block 524. The two sets of discrete inputs are compared, as indicated by step 526 and, if they do not match, the local loop involving Qeps 520, 522 and 524 is repeated until the successive sensing of the discrete signals gives a full check.

The next step in the initialization sequence involves the question asked in diamond 528: "Is One Rate Excitation On?" This involves several alternative sources of barometric altitude rate signals which are available in different aircraft, as indicated in Figure 1A and as discussed above. If one source of barometric rate signals is not connected to the system and "On", the next step 530 sets an appropriate flag in the RAM 82 for flashing the monitor light, indicating, of course, that the GPW equipment is not functioning properly. A "Yes" answer to the question of diamond 528 is followed by step 532: "Stop Flashing Monitor Light".As indicated by diamond 534, the state of the Mode 4 continuity circuit 26 is again checked by the sequence step asking the question: "Is Mode 4 Capacitor High?" A "Yes" answer takes us into the regular "on-line loop" of Figures IlA through 11F, as indicated by oval 536.

A "No"answer to the question of diamond 534 introduces a nine-second delay, as indicated by block 538 prior to proceeding to the regular on-line loop of Figures 1 lA through IIF.

In Figure 11, the steps which are taken by the system of Figures 1A and 1B with regard to the actuation of the warning circuits are of particular interest. In connection with Figure 11, these warning signals have been described in connection with the sequencing steps such as indicated with regard to Mode 1 by blocks 338, 340 and 342, as well as blocks 330, 332 and 334. Of course, the program steps which are involved are set up in the form of permanent modifications in the read only memory 80 of Figure 1B. In view of the fact that the read only memory 80 is permanent and is not changed except by removing it from the circuitry and modifying the stored information through the use of a special technique involving ultraviolet light, it is considered to be a permanent part of the system structure.

Figure 13 shows an alternative implementation of the warning circuitry of the present system employing separate warning counters and clear counters in place of the microprocessor circuitry described elsewhere in the present specification. In Figure 13, the computer logic circuit 602 is operated under instructions from the sequence control circuitry 604, and makes use of the permanent memory 606 in which the ground proximity tables for the various modes are stored If the aircraft conditions indicate that the aircraft is within the ground proximity warning envelope for the particular mode under consideration, an output signal or pulse is applied to one of the "Yes" leads 611 through 615.In the event the aircraft conditions indicate that the aircraft is outside of the ground proximity warning envelope, a pulse or signal is applied to one of the "No" output leads 621 through 625 from the logic circuit 602.

Also included in the circuit of Figure 13 are the clear counters 626 and 628 for Modes 1 and 3, respectively, and the warning counters 630 and 632 for the two modes under consideration. The full count for the warning counters 630 and 632 may be varied by the thumbwheel count setting circuits 634 and 636, respectively. The warning signal devices 638 and 640 are actuated when the warning counters reach full count. As mentioned above, the full count for the warning counters might characteristically be 40 counts, while that for the clear counter might be 2 counts.

With a sampling rate for each mode of 16 samples per second, the warning signal device 638, for example, could not be actuated prior to 2-1/2 seconds, resulting from 16 counts per second for 2-1/2 seconds to produce the full count of 40.

On the other hand, in the case of alternate counts applied on lead 621 to the clear counter 626 and over lead 611 to the warn counter 630, the time required for the warn counter 630 to reach its full count would be extended, thus minimizing the likelihood of false warnings. In addition, of course, the extended time required prior to energization of the warning signal device 638 for Mode 1, increases the likelihood that two clear counts will be received by clear counter 626, so that the warning counter will be reset to its initial state, thus reducing the likelihood of unnecessary warnings. Concerning the output from clear counter 626, note that, in addition to lead 642 which resets the warn counter 630, a signal is applied on lead 644 to turn off the warn signal device 638.With regard to signals on lead 611 indicating the aircraft is within the Mode 1 envelope, note that, in addition to advancing the ~ warn counter 630 via lead 646, the signal also resets the clear counter 626 via lead 648.

The same mode of operation also occurs relative to Mode 3, and circuits 628, 632 and 640. Of course, a different full count may be established for the warn counter 632 by a different adjustment of the thumb wheel setting circuit 636.

From the foregoing description of Figure 13, it may be seen that the permanently stored sequencing arrangements provided by the RAM circuit 80 of Figure 1 B may also be implemented by separate clear and warn counters for each mode, and the accompanying energization and logic circuitry.

Figure i4 is a circuit diagram of the "Mode 4 Continuity Circuit" 26 of Figure 1 B. The Mode 4 continuity circuit of Figure 14 includes as its key element capacitor 702. This capacitor 702 has sometimes been.

referred to as the "Remember Mode 4" capacitor, and it has positive pulses applied to it to maintain it in the charged state while the aircraft is within Mode 4 conditions. It is intended to permit short interruptions of power for up to about 3/10 of a second and still have the computer revert to Mode 4, instead of Mode 3, when the computer cycle starts up, following resumption of power.

Considering the other components included in the circuit of Figure 14, they include the inverter 704, resistor 706, two diodes 708 and 710, and the bleeder resistor 712. Pulses are supplied from inverter 704 to capacitor 702 for the duration of Mode 4 conditions. This maintains the capacitor 702 charged to 3 volts DC. The buffer operational amplifier 714, which is noninverting, provides an output voltage at the input to the voltage comparator 716 of +9 volts. The capacitor 702 is coupled to the voltage follower 714 by resistor 718. The circuitry associated with amplifier 714 includes gain control resistors 720 and 722.

The voltage comparator 716 will stay in its "low" output state, if the input is greater than +3 volts. However, it will shift to its "high" output state, if its input drops below +3 volts. With amplifier 714 increasing the level at the output of the capacitor 702 three-fold, the voltage across capacitor 702 may drop to 1 volt before the output of inverting voltage comparator 716 will shift to its high output state. With the discharge path for capacitor 702 including the very high resistor 712 (22 megohms) and the somewhat lower input impedance of the unpowered operational amplifier 714, the switching action will occur after approximately 300 milliseconds to 1 second have elapsed.Accordingly, when power is restored, and computer operations resumed, the initial sensing of the discrete signal from the output of circuit 716, corresponding to lead 69 of Figure 1B (as discussed in connection with the Initialization Sequence of Figure 12) will indicate to the computer that Mode 4 is to be resumed.

For completeness, it will be noted that the voltage comparator circuit 716 has associated with it the resistor 719 and capacitor 721 to set the switching threshold of circuit 716.

Applied to the lead 722, for an interval of approximately 125 milliseconds after power is resumed, is a signal 723 placing lead 722 at ground potential. This precludes the application of transients to capacitor 702 which could otherwise charge this capacitor and falsely indicate that the computer was in the Mode 4 condition.

For completeness, it may be noted that the values of the resistors and capacitors employed in the Mode 4 continuity circuit of Figure 14 are as follows: Resistor 706 - 1000 ohms Resistor 718 - 3.9 megohms Resistor 722 - 2000 ohms Resistor 720 - 3900 ohms Resistor719 39,000 ohms Capacitor 721 - .01 microfarads Capacitor 702 - 0.1 microfarads For completeness, several of the matters discussed above particularly in connection with Figures IA and 1B will now be considered in slightly greater depth. In connection with Figure 1A, a Scott-T transformer was disclosed. These are described in detail in a treatise entitled "Static Electromagnetic Devices" by William T. Hunt, Jr. and Robert Stein, Allyn and Bacon, Inc., Boston, 1963, pp.

237-241, especially p. 239.

With regard to the digital multiplexer 72, it may be formed using eight chips of a commercially available semiconductor switching circuit type No. DM7121D from National Semiconductor. Three address control leads are connected in parallel to each of the eight chips to select any desired one of the eight eight bit words which are connected to the 64 inputs of the eight chips. The output on the eight lead bus 73 is thus a digital word as selected by the three lead input bus 79.

Figure 15 of the drawings shows the details of the analog-to-digital converter 40 of Figure IA in considerable detail. In Figure 15, the analog multiplexer 38 appears in the upper left-hand corner of the Figure, with the analog inputs from the barometer altimer, the radar altimeter, etc. shown at 13, 15, 17... etc. The analog multiplexer 38 may be of any suitable semiconductor circuitry such as Harris chip type No. 508, for specific example. Selection of a single analog circuit is accomplished by the three input leads 77 which are applied to multiplexer circuit 38. Following selection by appropriate energization of leads 77, a single selected analog output appears on lead 752 at the input to buffer amplifier 754.

The next step in the analog-to-digital conversion process is matching of the output of the digital-to-analog converter 756 and 758 to the analog signal on lead 760 and applying the sum to operational amplifier 762 through lead 764. Incidentally, chips 756 and 758 include ladder resistance networks for developing a current proportional to the digital signal applied on leads 761 from the up/down counter chips 766, 768 and 770, as well as a summing circuit. If the analog signal on lead 760 is greater than the output from the two digitalto-analog converter chips 756 and 758, then the signal on lead 764 will have one polarity, while, if the relative magnitudes are reversed, the reverse polarity will appear on lead 764.

As noted above, the inputs to the digitalto-analog converter chips 756 and 758 are provided on leads 761, by the up/down counter including chips 766, 768 and 770. If the polarity on lead 764 is of one sense, the counter including chips 766, 768 and 770 will be stepped in one direction towards equalizing the voltages developed by the digital-to-analog converter with the analog signal on lead 760; however when the polarity on lead 764 is reversed, the up/down counter is stepped in the opposite direction. Accordingly, the digital output from the up/down counter made up of chips 766, 768 and 770 will oscillate about the digital value representing the analog input voltage on lead 760. The digital output on bus 43 includes 12 parallel digital signals.

Incidentally, the feedback loop between the operational amplifier 762 and the up/down counter includes the voltage comparator 772, the logic inverter 774 and the flip-flop 776.

The logic circuit and comparator mentioned above may be implemented by commercially available chips including: for circuits 754 and 762, Advanced Microdevices part No. 747; for voltage comparator 772, National Semiconductor part No. LM Ill; for circuit 774, use a commercially available logic inverter type No. 5404; for circuit 776, use JK flip-flop Texas Instruments part No. 54107; and for the up/down counter chips 766, 768 and 770, use Texas Instruments part No. 54191. In addition, the voltage follower circuit 777 which provides +5 volt reference potential to point 779 may be a National Semiconductor part No. LM1IOH.

In successive intervals, the digital output bus 43 must change its output from one analog input signal on lead 13, for example, to another analog input signal on lead 15 or 17. In the absence of auxiliary circuitry, this could require a counting operation by the up/down counter through its entire counting sequence. This would, of course, be time consuming and either require a very rapid counter or else delay the sampling operation. To overcome these difficulties, the chips 766 and 768 are pre-loaded via bus 778 with numerical values equal to the previous digital output which appeared on bus 43 for the selected analog input.In this way, the number of steps required by the up/down counter is held to a minimum, and, with a limited change in the analog input signals applied to leads 13, 15 and 17, for example, from one sampling interval to the next, the output from the up/down counter chips 766, 768 and 770 soon matches the sampled analog input.

Additional circuitry shown in Figure 15 includes the two NAND circuits 782 and 784, in addition to the control flip-flop 786.

The function of these circuits is to stop the counting action of the up/down counter when the count reaches a maximum level or a minimum level, and further to stop counting action when the output on bus 43 from the counter is being sampled. To consider the details of the foregoing mode of operation, note that each of the chips 766, 768 and 770 includes a "min/max" output lead 788, a "load" input lead 790 and an enable lead 792.

First, considering the stopping of counting action when the counter reaches its maximum or minimum state, note that all three of the minimum/maximum output leads 788 are connected to the input to NAND circuit 782. With a high, or a binary " I " output signal on each of leads 788 representing the minimum or maximum condition, the output from NAND gate 782 will be a binary "0" representing the low state. The output from NAND gate 784 will then be a binary "1", and will thus be high.

A high signal on the enable leads 792 to each of the chips 766, 768 and 770 will stop additional counting and prevent false indications which might otherwise occur if the counter were stepped beyond its maximum state in the upward direction or beyond its minimum state in the downward direction.

Incidentally, it may be noted that preload takes precedence over counting.

Accordingly, when the chips 776, 768 and 770 are being preloaded and a preload control signal is applied on lead 794 to all of the input leads 790 of the up/down counter chips, counting is stopped and the chips accept the preload information from input bus 778.

When the digital multiplexer is sampling the digital output on bus 43, a high or "1" signal is applied to control lead 796 to the "J" input of flip-flop 786. Under these conditions the Q output on lead 798 is "0", making the output from NAND gate 784 a "1", and thus disabling counting action.

The lead 794 carrying the preload signal is normally in the high or "1" state. It goes low, to the "0" state when preloading action occurs. Simultaneously, lead 800 to the clear input of flip-flop 786 resets flip-flog 786 to the state where it has "1" or high Q output. With the output from NAND gate 782 also normally high, the output from NAND gate 784 is low. Counting action is normally enabled by such a low signal being applied on input leads 792. However, the load signal applied directly to leads 790 from preload lead 794 takes precedence, and blocks counting for the brief duration of the loading interval. Immediately thereafter, with the "0" input on enabling leads 792, counting resumes.

The foregoing completes the detailed description of the present system, with the exception of Tables I through IV which were mentioned hereinabove and which are set forth below.

TABLE I Common Mode Sequence Step I Set pointers to table of values for particular mode.

Step 2 How many segments in the characteristic for the selected mode? Step 3 Is the actual radar height above the radar height listed in the table for the top segment of the characteristic? If "yes", jump to the start of the section of the sequence for the next mode (because the aircraft is outside the ground proximity warning or "GPW" area).

Step 4 Obtain radar height of the lower end of the second segment of the characteristic from the stored table of values for the selected mode as determined by the "pointers" (see Step 1).

Step 5 Determine if the actual radar height is above the radar height of the lower end of the line segment. If "yes", then follow the following subroutine.

Step (a) Obtain slope of second segment of characteristic from table.

Step (b) Determine ordinate intercept point of the second segment of the characteristic from table applicable to this mode.

Step (c) Using an ordinate value equal to the actual radar height, and the line equation for the segment, compute the sink rate, or rate of descent (or other x axis parameter) of the GPW envelope corresponding to the actual radar height.

Step (d) Compare computed x-axis parameter (such as sink rate) with the actual x-axis parameter (such as the actual sink rate). If the actual parameter is greater than the computed parameter, the aircraft is within the GPW area. If less, the aircraft is outside of GPW area.

Step 6 If actual radar height is less than lower breakpoint for second segment, then program Step 5 is repeated for the lowest segment of the characteristic, using values from the table, to determine whether aircraft is within or outside of GPW envelope.

Step 7 Apply a warn count advance pulse or a clear count advance pulse to the warning circuit control circuitry, depending on whether aircraft is within GPW envelope or outside of it, respectively.

TABLE II Mode Intercepts Definition Value - (mode 1, segment A, intercept) 1391.04 -(model, segment B, intercept) -45.00 -(mode 2A, segment A, intercept) 1721.43 -(mode 2A, segment B, intercept) -1000.00 -(mode 2B, segment C, intercept) 1490.00 -(mode 3A, segment A, intercept) 7.14 -(mode 3B, segment B, intercept) 58.33 -(mode 4, segment A, intercept) 16.83 TABLE III Reciprocals of Line Segment Slopes Definition Value l/(mode 1, segment A, slope) 0.9021277 l/(mode 1, segment B, slope) 2.00 l/(mode 2A, segment A, slope) 1.129032 l/(mode 2A, segment B, slope) 6.250 l/(mode 2B, segment C, slope) 1.315789 l/(mode 3A, segment A, slope) 0.6363636 I/(mode 3B, segment B, slope) 0.0923077 I/(mode 4, segment A, slope) 2.645161 TABLE IV Tabulation of Line Segments and Radar Height of Line Segment Transitions Number of Line Radar Height of Mode Segments Transitions 1 3 2,4500 feet 1,225 feet 50 feet 2A 3 1,800 feet 1,600 feet 50 feet 2B 2 860 feet 220 feet 3A 3 700 feet 150 feet 50 feet 3B 2 700 feet 50 feet 4 3 520 feet 210 feet 50 feet For completeness, reference may also be made to descriptive material available in the literature and from manufacturers of the various semiconductor chips, logic circuits, memories, and subsystems identified herein.

One pertinent publication is entitled "Intel 8080 Microcomputer System Manual", dated January 1975, and copyrighted in 1975 by Intel Corporation, 3065 Bowers Avenue, Santa Clara, California 95051. Also of interest is the Intel material on the ROM, or read only memory, employed in the present disclosed ground proximity warning system. This material is entitled "Intel Silicon Gate MOS 8707/8704". Also of interest is the "Intel 8080 Assembly Language Programming Manual", copyright 1974. This Manual is extensive discussing the hexadecimal represenation used in working with the 8080 microprocessor and how to accomplish the desired data processing functions, such as those described in diagram form in the present specification Three granted patents which may be of interest as relating to the field of ground proximity warning systems include U.S.

Patent No. 3,715,718, granted February 6, 1973; U.S. Patent No. 3,946,358, granted March 23, 1976; and U.S. Patent No.

3,248,728, granted April 26, 1966.

In closing, the present invention involves improved arrangements for the economical realization of an effective digital ground proximity warning system, and for the prevention of false warnings in such system.

Attention is also drawn to the corresponding description and drawings, and related claims of copending application No. 24864/77 (Serial No. 1,567,554).

WHAT WE CLAIM IS: 1. A ground proximity warning system comprising: digital data processing circuit means for checking ground proximity warning conditions in either of a first and a second mode, the first mode being for detecting a predetermined condition of loss of altitude of an aircraft on take-off or missed approach and the second mode being for detecting a predetermined condition of proximity to ground with landing gear or flaps not in a landing configuration; means for providing a warning signal if in either mode the predetermined condition is met;

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. TABLE III Reciprocals of Line Segment Slopes Definition Value l/(mode 1, segment A, slope) 0.9021277 l/(mode 1, segment B, slope) 2.00 l/(mode 2A, segment A, slope) 1.129032 l/(mode 2A, segment B, slope) 6.250 l/(mode 2B, segment C, slope) 1.315789 l/(mode 3A, segment A, slope) 0.6363636 I/(mode 3B, segment B, slope) 0.0923077 I/(mode 4, segment A, slope) 2.645161 TABLE IV Tabulation of Line Segments and Radar Height of Line Segment Transitions Number of Line Radar Height of Mode Segments Transitions

1 3 2,4500 feet 1,225 feet

50 feet 2A 3 1,800 feet 1,600 feet

50 feet 2B 2 860 feet

220 feet 3A 3 700 feet

150 feet

50 feet 3B 2 700 feet

50 feet

4 3 520 feet

210 feet

50 feet For completeness, reference may also be made to descriptive material available in the literature and from manufacturers of the various semiconductor chips, logic circuits, memories, and subsystems identified herein.

3,248,728, granted April 26, 1966.

a mode continuity circuit connected to the processing circuit means and including a storage device having a first state when one of the two modes is operative and a second state when the other of the two modes is operative; and means for checking the state of said device upon resumption of power following a power interruption to place said processing circuit means in the mode corresponding to the device state.

2. A system according to claim 1, wherein the storage device requires power supply to maintain its second state for more than a given time, the time constant of this device being long enough to maintain its second state for a duration of x seconds (where x is at least equal to 1/5) plus the time required for the checking means to identify the state of the device.

3. A system according to claim 1, wherein the storage device comprises capacitance, there being means for charging the capacitance when the other of the modes is operative, and the checking means being responsive to at least a predetermined charge on the capacitance to identify the second state.

4. A system according to claim 3, wherein the time constant of the capacitance is sufficiently long and the checking means operates promptly enough to enable a power interruption of x seconds (where x is at least 1/5) to be bridged.

5. A system according to claim 2 or 4, wherein x is at least 3/10.

6. A ground proximity warning system according to any one of claims 2 to 5, wherein the second state defines said second mode.

7. A system according to any one of the preceding claims and arranged to issue the warning signal only when the predetermined condition is met for a predetermined pattern of iterations.

8. A ground proximity warning system according to any one of preceding claims, wherein the checking means includes means for performing a full initialization procedure for said ground proximity warning system if said storage device is in its first state.

9. A ground proximity warning system according to any one of the preceding claims, wherein the processing circuit means is composed of low-level semiconductor digital data processing circuitry.

10. A digital ground proximity warning system, comprising: low-level semiconductor digital data processing circuit means for iteratively checking ground proximity warning conditions of an aircraft; first means for operating said digital data processing circuitry in a first mode for detecting loss of altitude of an aircraft on take off or missed approach; means included in said digital data processing circuitry for providing a warning signal if said first detecting means indicates loss of altitude beyond predetermined limits for a predetermined length of time; second means for operating digital data processing circuitry in a second mode for detecting proximity to ground with landing gear or flaps not in the landing configuration;; means included in said digital data processing circuitry for providing a warning signal if either of said detecting means indicates ground proximity warning conditions for a predetermined number of iterations; a mode continuity circuit including a two state storage means compatible with said low-level semiconductor circuitry and having a relatively long time constant for state retentions; means for setting said storage means to one of its two states when the ground proximity system is in the second of said two modes; and means for promptly checking the discrete input associated with said mode continuity circuit upon resumption of power following a power interruption, and for placing said digital data processing circuitry into said second mode if said storage means is in said one state.

11. A ground proximity warning system substantially as hereinbefore described with reference to the accompanying drawings.

12. A ground proximity warning system according to any one of the preceding claims in operable combination with detecting devices for detecting aircraft flight conditions for the system.