GB2085250A - A method and apparatus for self- calibration of a Loran-C receiver - Google Patents

Abstract

A method and apparatus for self- calibration of a LORAN-C navigation receiver, the time difference of signal arrival of master station pulse trains from a LORAN-C chain selected by group repetition interval (GRI) information input to the receiver becoming a frequency standard to which the output of an oscillator and counter internal to the receiver is compared to determine frequency error. The error is interpolated over each GRI and a correction factor is added or subtracted to each count output of the counter used to make time difference of signal arrival measurements. <IMAGE>

Description

SPECIFICATION A method and apparatus for self-calibration of a LORAN-C receiver This invention relates to navigational equipment and more particularly to hyperbolic navigational equipment utilizing the time difference in the propagation of radio frequency pulses from synchronized ground transmitting stations.

Throughout maritime history navigators have sought an accurate, reliable method of determining their position on the surface of the earth and many instruments such as the sextant were devised. During the second world war, a long range radio navigation system, LORAN-A, was developed and was implemented under the auspices of the United States Coast Guard to fulfill wartime operational needs. At the end of the war there were seventy LORAN-A transmitting stations in existence and all commercial ships, having been equipped with LORAN-A receivers for wartime service, continued to use this navigational system. This navigational system served its purpose but shortcomings therein were overcome by a new navigational system called LORAN-C.

Presently, there are eight LORAN-C multi-station transmitting chains in operation. This new navigational system will result in an eventual phase-out of the earlier LORAN-A navigational system.

LORAN-C is a pulsed low-frequency (100 kilohertz), hyperbolic radio navigation system. LORAN-C radio navigation systems employ three or more synchronized ground stations that each transmit radio pulse chains having, at their respective start of transmission, a fixed time relation to each other. The first station to transmit is referred to as the master station while the other stations are referred to as the secondary stations.

The pulse chains are radiated to receiving equipment that is generally located on aircraft or ships whose positions is to be accurately determined. The pulse chains transmitted by each of the master and secondary stations is a series of pulses wherein each pulse has an exact envelope shape, each pulse chain is transmitted at a constant precise repetition rate, and each pulse is separated in time from a subsequent pulse by a precise fixed time interval. In addition, the secondary station pulse chain transmission are delayed a sufficient amount of time after the master station pulse train transmissions to assure that their time of arrival at receiving equipment anywhere within the operational area of the particular LORAN-C system will follow receipt of the pulse chain from the master station.

Since the series of pulses transmitted by the master and secondary stations is in the form of pulses of elecromagnetic energy which are propagated at a constant velocity, the difference in time of arrival of pulses from a master and a secondary station represents the difference in the length of the transmission paths from these stations to the LORAN-C receiving equipment.

The focus of all points on a LORAN-C chart representing a constant difference in distant from a master and a secondary station, and indicated by a fixed time difference of arrival of their 100 kilohertz carrier pulse chains, describes a hyperbola. The LORAN-C navigation system makes it possible for a navigator to exploit this hyperbolic relationship and precisely determine his position using a LORAN-C chart. By using a moderately low frequency such as 100 kilohertz, which is characterized by low attenuation, and by measuring the time difference between the reception of the signals from master and secondary stations, the modern day LORAN-C system provides equipment position location accurate within two hundred feet and with a repeatability of within fifty feet.

The theory and operation of the LORAN-C radio navigation system is described in greater detail in an article by W.P. Frantz, W. Dean, and R. L. Frank entitled "A Precision Multi-Purpose Radio Navigation System", 1957 I.R.E. Convention Record, Part 8, page 79. The theory and operation of the LORAN-C radio navigation system is also described in a pamphlet put out by the Department of Transportation, United States Coast Guard, Number CG-462, dated August, 1974, and entitled "LORAN-C User Handbook".

The LORAN-C system of the type described in the aforementioned article and pamphlet and employed at the present time is a pulse type system the energy of which is radiated by the master station and by each secondary station in the form of pulse trains which include a number of precisely shaped and timed bursts of radio frequency energy as previously mention. All secondary stations radiate pulse chains of eight discrete time-spaced pulses, and all master stations transmit the same eight discrete time-spaced pulses but also transmit an identifying ninth pulse which is accurately spaced from the first eight pulses. Each pulse of the pulse chains transmitted by the master and secondary stations has a 100 kilohertz carrier frequency so that it may be distinguished from the much higher frequency carrier used in the predecessor LORAN-A system.

The discrete pulses radiated by each master and each secondary LORAN-C transmitter are characterized by an extemely precise spacing of 1,000 microseconds between adjacent pulses. Any given point on the precisely shaped envelope of each pulse is also separated by exactly 1,000 microseconds from the corresponding point on the envelope of a preceding or subsequent pulse within the eight pulse chains. To insure such precise time accuracy, each master and secondary station transmitter is controlled by a cesium frequency standard clock and the clocks of master and secondary stations are synchronized with each other.

As mentioned previously, LORAN-C receiving equipment is utilized to measure the time difference of arrival of the series of pulses from a master station and the series of pulses from a selected secondary station, both stations being within a given LORAN-C chain. It is clear that any inaccuracies in measuring time difference of arrival of signals from master and secondary transmitting stations results in position determination errors. This requires that oscillators internal to the LORAN-C receiver be calibrated frequently in order to avoid measurement errors caused by oscillator inaccuracy.

The signals presently received by LORAN-C navigation receivers have very low signals to noise ratios and it is difficult to locate the third cycle positive zero crossing conventionally used in making the time difference measurements between signals received from the master and secondary stations. This problem is exacerbated by noise generated within the circuitry of LORAN-C navigation receivers and particularly in the receiver front and circuitry in the signal path immediately following the receiver antenna.

Thus, there is a need in the art for improved circuitry and techniques to minimize the noise generated internal to LORAN-C receivers. It is a feature of this invention to minimize the effect of noise generated internally to a receiver by averaging out the noise.

There is also a need in the art for inexpensive oscillators within LORAN-C receivers that never require calibration yet the operation of the receivers is as if the oscillators are as accurate as a laboratory standard oscillator. Such oscillators increase the accuracy and reliability of navigation information output from the receiver.

The present invention eliminates much of the complex and costly automatic acquisition and tracking circuitry in prior art LORAN-C navigation receivers and provide a small, iight weight, inexpensive receiver using relatively little electrical power, minimizing the effects of noise generated to the receiver and requiring no calibration of the receiver oscillator/clock.

Fourthumbwheel switches on my LORAN-C equipment are used by the operator to enter the group repetition interval information for a selected LORAN-C chain covering the area within which the LORAN-C equipment is being operated. This information entered via the thumbwheel switches is used in the process of locating the signals from the master and secondary stations of the chosen LORAN-C chain and providing an output.

The receiver of the equipment receives all signals that appear within a small bandwidth centered upon the 100 KHz operating frequency of the LORAN-C network. A shift register clocked at 100 KHz is coupled with logic circuitry continuously check all received signals to search for the unique pulse trains transmitted by LORAN-C master and secondary stations. The microprocessor and other circuits internal to my novel LORAN-C equipment analyze outputs from the register and associated logic circuitry indicating that signals from master or secondary stations have been received to first determine which received signals match the group repetition interval rate for the selected LORAN-C chain. Once the receiver has identified the pulse trains from the selected master station and can predict future receipt of same, the microprocessor causes other circuitry to go into a fine search mode.

In the fine search mode the microprocessor enables a phase-lock-loop made up of a computer program and other circuitry, including a cycle detector, to analyze and locate the third cycle positive zero crossing point of each receiver master station pulse. In the event the third cycle positive zero crossing of each master station pulse is not located at the time calculated by the microprocessor, the cycle detector provides outputs used by the microprocessor to determine whether multiples of 10 microseconds should be added to or subtracted from the calculated time. The microprocessor then repeats the fine search mode analyzation process. This analyzation process and revision of the calculated time is repeated using feedback from the cycle detector untii the third cycle positive zero crossing of each pulse of the master station pulse train is located.

Once the third cycle positive zero crossing of each pulse from the master transmitting station of the selected LORAN-C chain is located, the receiver operates to locate the associated secondary stations. The microprocessor creates a small number of time bins between the arrival of each pulse train from the master station and creates a coarse histogram by putting a count in an appropriate bin when a secondary station signal is detected. Once particular bins are found to contain counts representing receipt of signals from secondary stations, the microprocessor breaks those particular bins down into a large number of time bins creating a fine histogram to more closely determine the time of signal arrival of secondary station signals.

The cycle detector is then utilized in conjunction with the microprocessor in a phase-lock-loop mode to identify the third cycle positive zero crossing of each received pulse from a secondary station.

The microprocessor then makes accurate time difference of arrival measurements between the time of arrival of signals from the master station and the secondary stations. The equipment operator utilizes other thumbwheel switches to indicate secondary stations, the time difference of signal arrival information which is to be visually displayed. The operator of the LORAN-C equipment plots these visual read-outs on a LORAN-C hydrographic chart to locate the physical position of the LORAN-C receiver on the surface of the earth.

Our LORAN-C navigation receiver need never have its internal oscillator calibrated unlike prior art receivers. The microprocessor, having the GRI (Group Repetition Interval) input thereto by the receiver operator, knows how many cycles of the internal oscillator must occur within the cesium clock standard GRI between two consecutive received master station pulse trains. Any error is noted and interpolated over the GRI period and correction factors are added or subtracted to internal circuit clock counts of interest to thereby achieve highly accurate time difference of signal arrival measurement.

The Applicant's LORAN-C navigation receiver will be better understood upon a reading of the description given hereinafter with reference to the single figure of the accompanying drawing which is a general block schematic diagram of the Applicant's LORAN-C navigation receiver.

In Figure there is seen a block diagram of the LORAN-C navigation equipment utilizing the novel self-calibration method. Filter and preamplifier 1 and antenna 2 are of a conventional design of the type used in all LORAN-C receivers and is permanently tuned to a center frequency of 100 KHz, which is the operating frequency of all LORAN-C transmitting stations. Fiiter 1 has a bandpass of 20 Kilohertz. Received signals are applied via inverting amplifier 81 to cycle detector 82 and to zero crossing detector 6.

The signal input to zero crossing detector 6 is first amplitude limited so that each cycle of each pulse is represented by a binary one and each negative half cycle is represented by a binary zero. The leading or positive edge of each binary one exactly corresponds to the positive slope of each sine wave comprising each pulse. Thus, detector 6 is a positive zero-crossing detector. As will be described in detail further in this specification logic circuit 16 also provides an input to zero crossing detector 6, not shown in Figure 1, which sets a 10 microsecond window only within which the leading edge of each binary 1 may be detected. The end result is that only the positive zero-crossing of the third cycle of each pulse of the train pulse trains transmitted by each LORAN-C station is detected and an output is provided by detector 6.

It can be seen that latch 5 has its input from zero crossing detector 6. Clock/counter 7 is a crystal controlled clock which is running continuously while my novel LORAN-C receiver is in operation. The count present in counter 7 at the moment that zero crossing detector 6 indicates a third cycle positive zero crossing is stored in latch 5, the contents of which are then applied to multiplexer 8. Multiplexer 8 is a time division multiplexer used to multiplex the many leads from logic circuit 16, logic circuit 4, cycle detector 82, latch 5, clock/counter 7, and thumbwheel switches 11 and 12, through to microprocessor 9. The count in latch 5 indicates to microprocessor 9 the time at which each positive zero crossing is detected.

The signal input to smart shift register 3 from detector 6 is a pulse train of 1's and 0's which is shifted through the shift register digital delay line which is tapped at 1 millisecond intervals. Because of the logic circuits connected to each tap thereof, only the pulse trains from LORAN-C master and secondary stations will result in outputs from the logic circuits of register 3. The logic circuits within register 3 are used to analyze the contents of the shift register delay line to first determine if the signals represent a pulse train from a LORAN-C master or secondary station, and secondly, to indicate the particular phase coding of the signals being received. Logic circuit 4 stores information from register 3 indicating whether a pulse train is from a master or secondary station and further indicating the particular phase code transmitted.This information stored within logic circuit 4 is applied to microprocessor 9 via multiplexer 8 for use in processing received LORAN-C signals. At the same time that information is stored within logic circuit 4, detector 6 causes latch 5 to store the present count in clock/counter 7 which indicates the time of coccurrence. It should be noted that clock/counter 7 also has an input to multiplexer 8 so that microprocessor 9 can keep track of continuous running time as indicated by recycles of counter 7.

Thumbwheel switches 11 are used to input the GRI of a selected LORAN-C chain to the receiver. The output ofthumbwheel switches 11 are also input to multiplexer 8to apply the GRI of the selected LORAN-C chain to microprocessor 9.

With the various types of information being input to microprocessor 9 via multiplexer 8 from the circuits previously described, microprocessors 9 determines when received signals are from the master and secondary stations of the selected LORAN-C chain. Once microprocessor 9 closely locates the signal from the selected master station, as determined by a match of the GRI number input thereto via thumbwheel switches 11 with the difference in time of receiving each pulse train transmitted by the master station of the selected chain, the receiver goes into a fine search mode utilizing a phase-lock-loop implemented with a computer program in microprocessor 9 and the loop being closed by an input from cycle detector 82 to locate the desired radio frequency carrier third cycle positive zero crossing in conjunction with zero crossing detector 6.The receiver then switches to locate the secondary station signals of the selected chain. To locate the secondary stations, microprocessor 9 first creates a coarse histogram and then a fine histogram by storing the time of receiving all secondary station signals in time slot bins created by the microprocessor in its own memory between the arrival of any two consecutive master station pulse trains. When signals from the secondary stations of the selected LORAN-C chain are located by secondary station signal counts appearing in the coarse histogram time slot bins at the same rate as the GRI of the selected LORAN-C chain, the microprocessor 9 creates a fine histogram having time slot bins of shorter time duration. In this manner, microprocessor 9 closely determines the time of arrival of pulse trains from the secondary stations of the selected LORAN-C chain.

Once microprocessor 9 closely determines the time of receiving secondary station signals and can calculate the time of receipt of substantially received secondary station pulse trains, the microprocessor causes the receiver to go into a fine search mode utilizing the same phase-locked-loop arrangement generally described above to accurately locate the third cycle positive zero crossing of each pulse of the secondary station pulse trains.

Again, control circuit 76 is provided to monitor the level of the received radio frequency signal and automatically adjust the gain of inverting amplifier 81. Logic circuit 16 also controls the inverting operation of amplifier 81 to periodically switch the phase of signals applied via amplifier 81 to the remainder of the receiver circuitry to remove the effects of noise internal to the receiver.

Once microprocessor 9 functioning with the other circuits in my LORAN-C receiver has located and locked onto the pulse trains being transmitted by the master and secondary stations of the selected LORAN-C chain, it makes the desired time difference of arrival measurements that are required in LORAN-C operation.

Microprocessor 9 then causes a visual indication to be given via display 12. The output information is plotted on a LORAN-C hydrographic chart in a well-known manner to locate the physical position of the LORAN-C receiver.

There are lamps on the front panel display 12 of the receiver which initially all flash on and off when the receiver is first turned on. As the signals of the master and each secondary station of the selected LORAN-C chain are located and it is determined by microprocessor 9 that each station's signals can be utilized to make accurate time difference of signal arrival measurements, the lamp associated with that station is changed to be lit steady. This gives an indication to the receiver operator of the confidence he may have in selecting stations with switches 11 to make time difference of signal arrival measurements.

In accordance with the teaching of the invention the oscillator internal to the LORAN-C receiver never needs calibration, unlike prior art receivers. Microprocessor 9 knows exactly the time difference of signal arrival of the pulse trains from the master station of the selected chain because of the GRI input thereto via switches 11. This information is compared with the output of a master oscillator within the receiver to determine the frequecy error of the oscillator. Microprocessor 9 then interpolates the error over the time period between receipt of signals from the master station and a correction factor is added or subtracted to internal clock indications of time of receipt of all pulses from the master and secondary stations to thereafter make accurate time difference of signal arrival measurements.

A phase shifting function is accomplished within the receiver to average out internally generated noise within the front end circuitry of the receiver, which noise normally creates a bias level which seriously affects the ability to locate the third cycle positive zero crossing of each pulse. After the receipt of two master station pulse trains the phase of all signals is periodically inverted within the receiver to average out the noise.

Smart shift register 3 provides an output signal each time signals from a master or secondary station are detected, which output signal is applied via logic circuit 4 and multiplexer 8 to microprocessor 9.

Microprocessor 9 determines which master and secondary station signals are from the selected LORAN-C station chain and provides a signal to logic circuit 16 each time the selected chain master station signal is detected. Logic circuit 16 counts these signals from microprocessor 9 and applies a signal to the control input of inverting input of amplifier 81. This causes all received signals to undergo a 180 degree phase shift every other time the signal from the selected master station is detected. The effect of this periodically alternating phase shift is removed at zero crossing detector 6 where internally generated noise is no longer a problem. Each time the inverting signal applied to inverting amplifier 81 changes state another inverting signal applied to zero crossing detector 6 also changes state to remove the effects of the phase inversion introduced at amplifier 81.

The following program listing shows the complete source programs for the operation of microprocessor 9 in my LORAN-C receiver. The programs are written in the PL/M language of Intel Corporation and must be run through a compiler to obtain the machine code to be loaded into the 8080 microprocessor used in my receiver. Descriptive headings are provided throughout the program listing to identify sub routines that implement various functions of the program.

Claims (12)

CLAIMS 1. Apparatus for self-calibrating a navigation receiver-indicator that includes an internal oscillator/clock and provides navigation information by receiving and utilizing the output of said oscillator/clock to measure differences in the time of arrival of signals periodically transmitted by each of a plurality of pairs of navigation transmitters the signal transmissions from each of which are very accurately controlled on a time basis comprising: means for entering the periodic rate of transmission of the signals transmitted by each of said navigation transmitter into said receiver-indicator, and a first means performing the following functions: a) comparing the time difference between the receipt of successive signal transmissions from one of said transmitters with an output of said oscillator/clock to determine the error in time counts output from said oscillator/clock. b) interpolating said time count error over the interval between the receipt of successive signal transmissions from said one of said transmitters to get correction counts, and c) algebraically adding said correction counts to said time counts obtained from said oscillator/clock output before being used for said time difference of signal arrival measurements from said pairs of navigation transmitters to thereby achieve accurate time difference of signal arrival measurements. 2. A method for self-calibration of a navigation receiver-indicator includes an internal oscillator/clock and provides navigation information by receiving and measuring differences in the time of arrival of signals periodically transmitted by each of a plurality of pairs of transmitters the signal transmissions from each of which are very accurately controlled on a time basis, and the time difference measurements are plotted on a navigation chart to determine the position of the receiver-indicator and comprising the steps of:: entering the periodic rate of transmission of the signals from said transmitters into said receiver-indicator so that said receiver-indicator knows the exact time difference between the receipt of successive signal transmissions from one of said transmitters, comparing the time difference between the receipt of successive signal transmissions from said one of said transmitters to an output of said oscillators/clock to determine the error in time counts output from said oscillator/clock, interpolating said time count error over the interval between the receipt of successive signal transmissions from said one of said transmitters to get correction counts, and algebraically adding said correction counts to said time counts obtained from said oscillator/clock output which is used for said time difference of signal arrival measurements from said pairs of transmitters to thereby achieve accurate time difference of signal arrival measurements used to plot position on said navigation chart. 3. Apparatus as claimed in claim 1 substantially as described herein with reference to the single figure of the accompanying drawings. 4. A method as claimed in claim 2 substantially as described herein with reference to the single figure of the accompanying drawings. New claims or amendments to claims filed on 11 December 1981 Superseded claims All New or amended claims:

1. Apparatus for self-calibrating a navigation receiver that provides navigation information by measuring the interrelationship in the time of arrival of signals periodically transmitted by a plurality of navigation transmitters the signal transmissions from each of which are very accurately controlled on a time basis comprising: means for receiving the transmissions of the signals transmitted by each of said navigation transmitters; and means for utilizing the time intervals between the receipt of successive signal transmissions from one of said transmitters as a reference standard for measuring the inter-relationship between times of arrival of the signals generated by said navigation transmitters to obtain accurate navigation information.

2. The system claimed in claim 3 wherein said receiving means is a radio receiver.

3. The system claimed in claim 3 wherein said utilizing means is a microprocessor that calculates the correct different in time of arrival between said transmitters by using the successive time of arrival of one of said navigation transmitters as a time reference standard.

4. Apparatus for self-calibrating a navigation receiver that provides navigation information by measuring differences in the time of arrival of signals periodically transmitted by each of a plurality of pairs of navigation transmitters the signal transmissions from each of which are very accurately controlled on a time basis comprising: means for receiving the transmissions of the signals transmitted by each of said navigation transmitters; and means for utilizing the time intervals between the receipt of successive signal transmissions from each of a number of transmitters in the same transmission chain and using the average time intervals to derive the reference standard for measuring the difference between times of arrival of the signals generated by said navigation transmitters to obtain navigation information.

5. The system claimed in claim 6 wherein said receiving means is a radio receiver.

6. The system claimed in claim 6 wherein said utilizing means is a microprocessor that calculates the correct difference in time of arrival between said transmitters by using the successive time of arrival of one of said navigation transmitters as a time reference standard.

7. Apparatus for self-calibrating a navigation receiver-indicator that includes an internal oscillator/clock and provides navigation information by receiving and utilizing the output of said oscillator/clock to measure differences in the time of arrival of signals periodically transmitted by each of a plurality of pairs of navigation transmitters the signal transmissions from each of which are very accurately controlled on a time basis comprising, means for entering the periodic rate of transmission of the signals transmitted by said navigation transmitters into said receiver-indicator, and a first means performing the following functions:: a. comparing the time difference between the receipt of successive signal transmissions from one of said transmitters with an output of said oscillator/clock to determine the error in time counts output from said oscillator/clock, and b. modifying said time counts obtained from said oscillator/clock output in accordance with the determined error in time counts before being used for said time difference of signal arrival measurements to thereby achieve accurate time difference of signal arrival measurements.

8. A method of self-calibration of a navigation receiver-indicator includes an internal oscillator/clock and provides navigation information by receiving and measuring differences in the time of arrival of signals periodically transmitted by each of a plurality of pairs of transmitters the signal transmissions from each of which are very accurately controlled on a time basis, and the time difference measurements are plotted on a navigation chart to determine the position of the receiver-indicator and comprising the steps of: entering the periodic rate of transmission of the signals from said transmitters into said receiver-indicator so that said receiver-indicator known the exact time difference between the receipt of successive signal transmissions from one of said transmitters; comparing the time difference between the receipt of successive signal transmissions from said one of said transmitters with an output of said oscillator/clock to determine the error in time counts output from said oscillator/clock; and modifying said time counts obtained from said oscillator/clock output in accordance with the determined error in time counts before being used for said time difference of signal arrival measurements to thereby achieve accurate time difference of signal arrival measurements.

9. Apparatus for self-calibrating a navigation receiver-indicator that includes an internal oscillator/clock and provides navigation information by receiving and utilizing the output of said oscillator/clock to measure differences in the time of arrival of signals periodically transmitted by each of a plurality of pairs of navigation transmitters the signal transmissions from each of which are very accurately controlled on a time basis comprising, means for entering the periodic rate of transmission of the signals transmitted by each of said navigation transmitter into said receiver-indicator, and a first means performing the following functions:: a. comparing the time difference between the receipt of successive signal transmissions from one of said transmitters with an output of said oscillator/clock to determine the error in time counts output from said oscillator/clock, b. interpolating said time count error over the interval between the receipt of successive signal transmissions from said one of said transmitters to get correction counts, and c. algebraically adding said correction counts to said time counts obtained from said oscillator/clock output before being used for said time difference of signal arrival measurements from said pairs of navigation transmitters to thereby achieve accurate time difference of signal arrival measurements.

10. A method for self-calibration of a navigation receiver-indicator includes an internal oscillator/clock and provides navigation information by receiving and measuring differences in the time of arrival of signals periodically transmitted by each of a plurality of pairs of transmitters the signal transmissions from each of which are very accurately controlled on a time basis, and the time difference measurements are plotted on a navigation chart to determine the position of the receiver-indicator and comprising the steps of:entering the periodic rate of transmission of the signals from said transmitters into said receiver-indicator so that said receiver-indicator knows the exact time difference between the receipt of succcessive signal transmissions from one of said transmitters; comparing the time difference between the receipt of successive signal transmissions from said one of said transmitters to an output of said oscillator/clock to determine the error in time counts output from said oscillator/clock, interpolating said time count error over the interval between the receipt of successive signal transmissions from said one of said transmitters to get correction counts, and algebraically adding said correction counts to said time counts obtained from said oscillator/clock output which is used for said time difference of signal arrival measurements from said pairs of transmitters to thereby accurate time difference of signal arrival measurements used to plot position on said navigation chart.

11. Apparatus as claimed in claim 1 substantially as described herein with reference to the single figure of the accompanying drawings.

12. A method as claimed in claim 8 substantially as described herein with reference to the single figure of the accompanying drawings.