CA1114480A

CA1114480A - Method and apparatus for automatically calibrating a radio altimeter

Info

Publication number: CA1114480A
Application number: CA304,243A
Authority: CA
Inventors: Robert J. Weber; Roy E. Robertson
Original assignee: Rockwell International Corp
Current assignee: Boeing North American Inc
Priority date: 1977-05-26
Filing date: 1978-05-25
Publication date: 1981-12-15
Also published as: GB1589047A; FR2392396A1; FR2392396B1

Abstract

METHOD AND APPARATUS FOR AUTOMATICALLY
CALIBRATING A RADIO ALTIMETER

Abstract of the Disclosure A method and apparatus for automatically calibrating and correcting for nonlinearities in a CWFM radio altimeter transmitter frequency sweep includes a coupling element to sample the transmitted signal. A calibration delay line receives and delays the sampled signal, and a mixer recombines the delayed signal with the currently sampled transmitted signal to provide a calibration signal for use as a time base against which the ground reflected transmitted signal also mixed with the currently transmitted signal is compared. The calibration signal, derived from the actual transmitted signal, varies in accordance with nonlinearities of the transmitted frequency sweep and thereby cancels the effects of such nonlinearities in the ground returned mixed signal. A phase locked loop to which the calibration signal is applied generates pulses at a higher frequency to increase the resolution available for determining the period of the ground returned signal. A period converter which receives the ground returned mixed signal as well as the calibration signal for compari-son therebetween provides outputs that can be accumulated and processed by a microprocessor and associated memory for output to derive appropriate altitude indicators.

Description

Background of the Invention 1. Field of the Invention .
This invention relates to improvements in distance ~easuring apparatus, and9 more particularly, to improvements in CWFM radio altimeter circuits, and still more particularly to a circuit for providing an autocalibration timing signal against which a returned signal mixed with a currently transmitted signal can be compared to minimize the effects of nonlinearities -in the frequency of the trans-mitted signal.

2. Description of the Prior Art Typical radio altimeters transmit a signal to be reflected from the underlying terrain. The signal carries a known modulation signal, usually a frequency sweep varying in accordance with a tri-angular or saw-toothed waveform configuration. The frequency sweep in the prior art devices must be critically controlled to be linearly time varying or have a known average ~f/~t.
The reflections from the terrain are detected by a receiver portion of the altimeter, and the frequency offset between the modula-tion of the received signal and the currently transmitted signal is determined. The altitude or distance between the aircraft and the terrain is then calculated from the frequency offset, which is directly proportional to the two-way time of travel of the transmitted signal.
The larger the difference in frequency between the received and cur-rently transmitted signals, the higher the altitude.
As mentioned, the slope and linearity of the frequency sweep transmitted in the prior art radio altimeter devices have to be pre cisely controlled for an accurate altitude measurement. If the average slope of the transmitted saw toothed ramp were not constant, for example, then the ~f/Qt measurement would not accurately reveal the proper fre-quency difference, resulting in erroneous measurements.

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lypically, the saw-toothed linearity and slope control is achieved by using a special (and expensive) filter in the modulator stage of the transmitter, and by using extremely high quality trans-mitter circuits and components. This results in a larger weight and volume in the altimeter, requiring valuable space on board the particular aircraft with which it is associated.
Another provisîon typically incorporated into prior art altimeter devices is a calibration feedback circuit which controls the overall slope or rate of change of the ~Frequency sweep modulated onto the transmitted signal. Thus, in addition to the weight and circuitry problems necessitated by the use of the linearity maintainin~ circuits mentioned, the circuitry used to control the slope of the modulation frequency must be taken into consideration, as well. Furthermore, because of the numerous variations in the transmitter components, which may result in frequency drift and the like, the prior art radio altimeters had to be necessarily frequently calibrated to ensure their continued accuracy.
Summary of the Invention In light of the above, it is, therefore, an object of the invention to provide a radio altimeter which automatically corrects for transmitter and transmission line nonlinearities to enable less critical transmi'ter control.
It is another object of the invention to provide an altitude indicating apparatus which relaxes the transmitter carrier and modula-tion fre~uency control requirements.
It is another object oF the invention to provide a CWFM radioaltimeter which has improved accuracy from radio altimeters heretofore used.
It is another object of the invention to provide a radio altimeter utili~ing digital data processing techniques incorporating :. .
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a clock frequency ~Ihich varies with nonlinearities in transmitted signal ~requencies and frequency rate of cnange.
It is another object of the invention to provide a ~ethod ~or automatically calibratiny a CWFM radio altimeter to correct for or cancel the effect of the nonlinearities in the frequency sweep of the transmitted signal.
It is another object of the invention to provide a radio altimeter which can be used to analyze segments of the transmitted and received data curves, rather than the total curve as was hereto-fore necessary.
These and other objects, features, and advantages will becomeapparent to those skilled in the art from the following detailed description when read in conjunction with the accompanying drawing and appended claims.
In its broad aspect, the invention provides an autocalibration circuit for use in a CWFM radio altimeter. The circuit includes means for transmitting a signal which has been frequency modulated with a frequency sweep signal. Reflection detecting means receives the delayed transmitted signal, and a first mixing means produces an altitude determining signal of frequency equal to the difference of the trans-mitted signal and the detected reflections. Means for sampling the transmitted signal produces a sample signal, and means for delaying the sample signal a predetermined time produces a delayed signal.
Second mixing means produces a calibration signal of frequency equal to the difference of the transmitted and delayed signals, and means for comparing the ratio of the altitude determining signal and ~he calibration signal determines the period of the altitude determining signal.
In another aspect of the invention, a method of automatically calibrating a CWFM radio altimeter inc!udes the steps of developing a '- :

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correction signal from a current and a delayecl transmitte~ signal, and comparing the correction signal to a signal of frequency equal to the difference of a current and a reflected transmitted signal to produce an altitude indication.
Brief Description of the Drawin~s The invention is illustrated in the accompanying drawing, wherein:
Fig. 1 is a box diagram of a radio altimeter usiny the automatic calibration circuit and method in accorclance with the present invention.
Fig. 2 is a graph of t~e frequency versus time showing ideal transmitted and received frequency sweeps for use in altitude determi-nation.
Fig. 3 is a graph of a more realistically encountered trans-mitted and received frequency sweeps, showing the inaccuracies resultant therefrom in an altitude determination.
Fig. 4 is a graph of the calibration frequency derived in accordance with the invention versus time.
Fig. ~ is a graph of the amplitude of the mixed and limited ground return signal, with respect to time.
Fig. 6 is a graph of an ideal clock signal, with respect to time, in relative scale to the ground return and calibration frequencies of Figs. ~ and 5.
And Fig. 7 is a detailed schematic drawing of the automatic calibration circuit in accordance with the invention, used in conjunc-tion with a data processing system employing a direct memory access control for utilization of the data derived, which has been calibrated by the method and apparatus in accordance with the invention.
In the drawings of Figs. 2-6, the relative scales and propor-tions of the curves and the number of pulses shown have been exaggerated or distorted for ease of descripti~n and clarity of illustration.

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Detailed Description of the Preferred En~bod;ments The autocalibration circuit 10 of the invention is shown in Fig. 1 in operative relationship with a transmitter section 11 and receiver section 12 of a CWFM radio altimeter 13. Briefly, the trans-mitter sect;on 11 includes a transmitter 14 modulated by a modulator 15 to deliver a signal to an antenna 16 upon a transmission line 18.
Typically, the transmitter is operated at a frequency of about 4,300 MHz, and is modulated with a triangular wave of frequency of about 100 Hz sweeping over a frequency of about 100 MHz.
The signal radiated from the antenna 16 is directed to the underlying terrain 17 or other object from which distance is desired to be measured, and reflections from the underlying terrain 17 are detected upon a receiving antenna 19. A coupling element 20 adjacent ; the transmission line 18 samples the transmitted signal, and the sample is mixed in a mixer 22 with the ground reflected signal received upon the antenna 19. The mixed ground return signal is amplified by a pre-amplifier 23, and mixed with an intermediate frequency and limited in an IF/limiter stage 24. The output from the IF/limiter stage 24, which is in the form of a square wave (see Fig. 5, below described), is then applied to a period converter 26, which determines the period of the returned signal, from which determination the altitude can be directly derived.
If desired, a number of adjacent periods of the returned signal . can be produced and listed in a memory means, such as a random access ~ memory (RAM) 27. The data in the RAM 27 can be further processed by a computer means such as a microprocessor 28 or the like to produce digital outputs which, if desired, can be converted to analog form by a ; digital-to~analog (D/A~ converter 3Q. The output of the D/A converter 30 can drive a conYentional altitude display indicator 31. If desired, the digital data can be converted to a serial format in a parallel-to-serial (P/S~ converter 32 to drive an indicator 34.

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With the system thus constituted, the operation to produce an altitude indication is as follows, with reference particularly to Figs. 2-6. The transmitter modulation frequency, indicated by the saw-tooth waveform f~ (Fig. 2) is transmitted from the antenna l6. The received signal, indicated by the sawtooth waveform fr is received on the antenna 19, but displaced in time from the transmitted signal an amount corresponding to the two-~!ay travel time of the signal. At any ~iven time, the difference in the frequencies of the transmitted and received signals can be observed, shown by the portion ~f9 which is directly relatable to the altitude or distance over which the radio waves have traversed. In the prior art, as above indicated, in order to produce an accurate altitude indication, steps have been taken to ensure that the transmitted frequency is as linear as possible in its variations with respect to time. ~lowever, even with such linearity assuring measures, the transmitted frequency assumes a waveform ~hich has nonlinear segments, such as shown in exaggerated form in Fig. 3.
; Thus, the transmitted curve ft ma~ be rounded in a concave or convex direction, as shown. The received waveform, of course, follo~s a similar path, but displaced in time. However, depending upon the time in ~lhich the change in frequency bet~een the transmitted and received waveforms is taken, different frequenc~ measurements may be obtained, as indicated by the distances Qfl and ~f2 shown. Thusg an uncertainty is introduced as to the precise altitude of the aircraft.
In order to eliminate this uncertainty, regardless of the wave~
form of the transmitted frequency, the autocalibration circuit lO of the invention is presented. With reference again to Fiy. 1, the auto-calibration circuit lO inc1udes a coupler 38 to sample the transmitted frequency. A dela~ line 39 is used to delay the sampled frequency a predetermined time~ corresponding to any convenient time (or altitude) such s lO0 feet. The sa pled nd delayed signals are mixed in a ~7-. ~ , - ,, . .

mixer ~0, then amplified and limited in an amplifier and IF/limiter stage 42. The a~plified and limited signal is then applied to a phase-locked loop 43 and applied as clock pulses to the period converter 26.
The operation of the autocalibration circuit 10 of the inven-tion can be seen from the graphs of Figs. 4-6. Speci~ically, Fig. 4 shows the change in frequency with respect to time of the calibration signal produced by the m;xed calibration delay s;gnal and the sampled transmitted signal. Any nonlinearities in the signal are reflected in a change in frequency of this calibration signal. Thus, as sho~m, the calibration signal may increase in frequency with respect to time. (It should be noted that although the QfcAL is shown having a time increase in frequency, other nonlinear variations may be seen depending upon the particular nonlinear variations of the transmitted frequency.) The returned signal difference, as shown in Fig. 5, will also exhibit a nonlinear time varying function, since it is derived from the transmitted signal having the precise nonlinearities detected by the autocalibration circuit 10. Thus, for instance, at a particular alti-tude, the frequency of the output of the phase-locked loop ~3 may be four times the frequency of the signal produced at the output of the limiter stage ?~. This can be seen in a comparison of Figs. 4 and 5.
Thus, as noted, each comparison of the period of the returned frequency and the calibration frequency has the same ratio, i.e., 4, even though the actual time of the return frequency period may be increasing or decreasin3~ If the returned frequency were to be compared, for instance, to a standard nonvarying clock frequency, illustrated in Fig. 6, the ratio would change from, for example~ a period of 4 to a period of .5 (using $he arbitrary curves of the dra~ing~. I
It can therefore ~e seen that by utilizing the sampled and delayed actual transmitted signal as a calibrating signal, the effects of the nonlinearities of the transmitted frequencies are cancelled.

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The autocalibration circuit of the invention is shown in detail in Fig. 7, used in conjunction with a microprocessing and direct memory accesss data acquisition and controllin~ system. The mixed return signal is amplified in the amplifier 23, mixed with an intermediate frequency and limited in the IF/limiter stage 24 to be applied to clock a D-type flip-flop 50 to a set state. The output Q of the D
flip-flop 50 is connected to one input of a NAND gate 51, to enable the calibration pulses derived as below described to pass through the NAND gate 51 to the clock terminal of a counter 52.
10The output from the IF/limiter 24 is additionally applied to the ciock input terminal of a second counter 54 to produce an output after a predetermined number of mixed return signal periods has been received. Such number of periods may conveniently be one, ten, sixteen, a hundred, or any conveniently handleable number.
15- The mixed calibration signal, after being amplified and limited in the ampli~ier and IF/limiter state 42 is applied to a "signal in"
terminal of a phase-locked loop circuit 43. The particular phase-locked loop circuit illustrated is of tne RCA type CD4046, although it will be apparent that any equivalent type circuit can be equally advantageously employed. The VC0 output of the phase-locked loop 43 is connected to a clock terminal of the counter G2, and an output derived, for instance upon the Q4 output terminal,is connected to the QII input of the phase-locked loop. With the phase-locked loop 43 and counter fi2 thus configured, the output frequency upon the VC0 output terminal can be adjusted to be 16 times the frequency of the mixed calibration - signal derived from the amplifier and IF/limiter 42. The VC0 output is applied to another terminal of the NAND gate 51 to be gated thereby to the clock input of the counter 52.
Thus, in operation, when a pulse is detected of the mixed return signal, the flip-flop 50 changes to a "set" state, to enable the ' _g_ NAND gate 51 to pass the output from the phase-locked loop ~3. The mixed calibration signal, multiplied by the output frequency control provided by the counter 62 is then counted by the counter 52 until the counter 54 reaches the preselected count. At that time, the QN output terminal of the counter 54 changes state, signaling the termination o~
the desired period to a direct memory access (DMA) control 64. The DMA
control 64 then latches the count presented at the various output terminals Ql-QN of the counter 52 into a microprocessor peripheral interface circuit 65. Additionally, the DMA control 64 resets the flip-flop 50, and the counters 52 and 54 so that an additional subsequent period count can be made. It should be noted that at this point, the data produced by the counter 52 latched into the interface circuit 65 is representat~ve of a predetermined number of periods (for instance, one, ten, a hundred, etc.) which can be directly used for producing an output altitude signal to the digital-to~analog (D/A) converter 30 or the parallel to serial (P/S) converter 32 to provide an altitude indication upon indicator 31 or 34, respectively.
It may, nevertheless, be desirable to accumulate a number of such period indicating data. Con;equently, the random access memory (RAM) circuit 27 is supplied, into which the data produced at the interface circuit 65 can be successively written by the DMA control 64.
The microprocessor system 28 together with an assoclated preprogrammed read only memory (ROM) 66 can then access the RAM 27 to perform the ~ predefined processing procedures to the data therein. Such processingi~ 25 procedures may, for example, include detection and rejection of contaminated data, averaging a number of period indlcating data, and so forth. The mlcroprocessor system can then produce outputs to the digital-to-analog and parallel-to-serial converters 30 and 32 via an input output device 67.
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Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made by way of example only and that numerous changes in the arrangement and combination of parts can be resorted to without departing from the spirit and scope of the invention as hereinafter claimed.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. An automatic calibration circuit for a CWFM radio altimeter, comprising:
means for transmitting a signal frequency modulated with a frequency sweep signal, means for detecting reflections of said transmitted signal, first mixing means for producing an altitude determining signal of frequency equal to the difference of said transmitted signal and said detected reflections, means for sampling said transmitted signal to produce a sample signal, means for delaying said sampled signal a predetermined time to produce a delayed signal, second mixing means for producing a calibration signal of frequency equal to the difference of said transmitted and delayed signals, and means for directly comparing the ratio of the frequencies of a segment of said altitude determining signal and a contemporaneously occurring segment of said calibration signal for determining the period of said altitude determining signal.

2. The automatic calibration circuit of claim 1 wherein said comparing means comprises means for counting the number of cycles of said calibration signal between a predetermined number of cycles of said altitude determining signal to produce an altitude indicating signal.

3. The automatic calibration circuit of claim 2 further comprising means for accumulating a predetermined number of sequential altitude indicating signals, and for averaging said accumulated signals to produce an average altitude indicating signal.

4. Apparatus for use with A CWFM distance measuring device of the type in which a frequency swept signal is transmitted, reflected from an object, and detected, and a difference signal between the transmitted and detected signal having a period representative of the propagation delay is generated, comprising:
means for sampling the transmitted signal, means for generating a delayed sample signal, and means for mixing said sampled and said delayed signals for producing a signal having a period which varies relatably to nonlinear frequency variations in the frequency swept signals thereby providing a time base signal for comparison against the difference signal occurring concurrently therewith in determining the period of said difference signal,

5. The apparatus of claim 4 further comprising:
a limiter to which said signal having a varying period is applied to produce a square wave signal therefrom, counter means, gate means to which said signal having a varying period is applied to pass said signal to increment said counter, and means for counting a selected number of periods of said difference signal, operative to enable said gate to pass said signal having a varying period during said selected number of periods.

6. The apparatus of claim 5 further comprising a pulse generator operative to generate pulses at a rate controlled by said time base providing signal, said rate being faster than the rate of said time base providing signal to increase the resolution of said comparison.

7. An instrument for measuring the distance of an aircraft from an object comprising:
means for transmitting a radio signal modulated by a varying frequency, means for detecting reflections of said radio signal from said object, means for producing a signal of square wave pulses at a frequency equal to the difference between the frequency of the detected reflections and the frequency of the transmitted signals when the reflections are detected, means for delaying the transmitted signal a predetermined time and for mixing the delayed and transmitted signals to produce a second difference signal, means triggered by said second difference signal for generating a predetermined plurality of clock pulses after each difference pulse, means for determining a number of said clock pulses occurring between adjacent pulses of said square wave pulse signal and for averaging the determined number of said clock pulses for a preselected number of square wave pulses, and means for producing a distance indicating output from said averaged number of said clock pulses.

8. A method of automatically calibrating a CWFM radio altimeter comprising:
transmitting a frequency swept signal toward an object, detecting reflections of the signal from the object, and mixing the detected reflections with the transmitted signal to produce a mixed altitude indicating signal, developing a signal from the transmitted signal delayed a predetermined fixed time, developing a correction signal from the delayed signal, and directly comparing said correction signal to the instantaneously concurrent altitude indicating mixed signal to produce a calibrated altitude indication.