ERROR CORRECTION METHOD AND APPARATUS FOR PULSE AMPLITUDE COMPARISON SYSTEMS

Abstract: A method and apparatus for correcting errors introduced into a pulse amplitude comparison system particularly suitable for correcting errors introduced by base line shift (26', 28') and pulse amplitude droop (30', 32') alters the sequence of the pulses to be compared in such a manner that the errors tend to cancel (40, 42, 44, 46, 66, 72), and averages the results of the comparisons to reduce the comparison error (96).

Description: ERROR CORRECTION METHOD AND APPARATUS FOR PULSE AMPLITUDE COMPARISON SYSTEMS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to a method of apparatus for reducing the errors introduced into a pulse amplitude comparison system as a result of base line shift or variations in the amplitudes of the received pulses, and is particularly useful in approach guidance systems for aircraft. 2. Background of the Invention Approach guidance systems of the prior art typically utilize a radio transmitter that alternately transmits pulses in overlapping beams. Such systems typically have two horizontally overlapping beams to provide azimuth information and two vertically over¬ lapping beams to provide elevation information. The systems generally use separate transmitters to provide the pulses, with one transmitter alternately applying pulses to the upper and lower beams and the other transmitter applying pulses to the left and right beams. The pulses are received by a receiver in an approaching aircraft and the amplitudes of the pulses received from the various beams are compared to provide position information. While the above-described systems do provide a way to obtain elevation and azimuth information, the pulse amplitude comparison circuitry employed in such systems can suffer from various errors, including base line shift which particularly affects capacitivelv coupled processors, and pulse droop which results from variations in the power output of the transmitter generating the pulses being compared. Among the reasons that these errors are introduced is the fact that the pulses ' to be compared are generally transmitted as pairs of pulses in rapid 10 succession at predetermined time intervals. Because the pulses forming each pair are transmitted in rapid succession, the first pulse in each pair can introduce a base line shift in a capacitively coupled system that shifts the relative amplitude of the second pulse , ς in the pair. Also, the power required to transmit the first pulse often causes the voltage of the power supply in the transmitter to droop, thus causing the second pulse in each pair to have a lower amplitude than the first pulse. Although base line variations can be mini¬ 20 mized by utilizing direct coupling in the pulse com¬ parison unit and pulse droop can be minimized by using larger components in the transmitter, such solutions are costly, and the use of larger, heavier components may be impractical in a portable tactical system where 25 the transmitting units must be manually handled. In addition, the use of direct coupled circuitry raises the complexity of the processing circuitry and intro¬ duces alignment problems. SUMMARY OF THE INVENTION 30 Thus, it is an object of the present inven¬ tion to provide a method and apparatus for comparing the amplitudes of pulses that overcomes many of the disadvantages of the prior art systems. It is another object of the present invention 35 to provide a pulse amplitude comparison system that is particularly usable for approach guidance systems for aircraft. It is another object of the present invention to provide an approach guidance system for aircraft that minimizes the problems caused by transmitted pulse droop without increasing the size and weight of the transmitters. It is yet another object of the present invention to provide a method and apparatus for reduc¬ ing the effects of base line shift in an amplitude comparison circuit without the use of direct coupled circuitry. Briefly, in accordance with the present invention, rather than transmitting the pulses consti¬ tuting the pairs of pulses to be compared in the same sequence as is done in the prior art systems, the pairs of pulses containing the azimuth or elevation information are alternately transmitted in reverse order. For example, in an azimuth guidance system, in the first pair of pulses transmitted, the left pulse may precede the right pulse, and in the next pair, the right pulse may precede the left pulse, with the sequence of the two pulses being reversed for each successive pair of pulses. A similar tech¬ nique is employed with the up down pulses in an eleva¬ tion guidance system. At the receiving unit, after each comparison is made, the results of each comparison are fed into an accumulator such as an averaging filter in order to aver the results of the comparisons. Because the pulses are transmitted in reverse order in succes¬ sive transmissions, the effects of base line shift and pulse droop will tend to cancel, thus improving system accuracy. DESCRIPTION OF THE DRAWING These and other objects and advantages of the present invention will become readily apparent upon consideration of the following detailed descrip¬ tion and attached drawing, wherein: FIG. 1 is a plan view of an aircraft runway showing the overlapping azimuth guidance beams of a typical system; FIG. 2 is an elevational view of an aircraft runway showing the overlapping elevation guidance beams of a typical system; FIG. 3 is a diagram illustrating the pulse sequence generated by a typical prior art azimuth guidance system; FIG. 4 illustrates the base line shift and pulse droop encountered in a system utilizing the pulse sequence illustrated in FIG. 3; FIG. 5 illustrates the pulse sequence uti¬ lized by an azimuth guidance system according to the present invention; FIG. 6 is an illustration of the pulse se¬ quence utilized by an elevation guidance system accord¬ ing to the present invention; FIG. 7 is a schematic block diagram of a pulse transmitter usable in an azimuth or elevational guidance system according to the present invention; and FIG. 8 is a schematic block diagram of a pulse receiving and comparison system usable in an elevation or azimuth guidance system. DETAILED DESCRIPTION Referring to FIG. 1, there is shown a runway 10 equipped with an azimuth guidance system. As shown, a typical azimuth guidance system utilizes a pair of antennas that transmit pulses in two overlapping beams illustrated by lobes 12 and 14 that overlap in an area 16. On approach, the aircraft compares the ampli¬ tude of the pulses received from the lobes 12 and 14. As long as the aircraft is on course, i.e., within the area 16, the amplitudes of the pulses received fro the lobes 12 and 14 will be approximately equal. However, should the aircraft stray off course, the pulses received from one of the lobes 12 and 14 will be larger than those received from the other. FIG. 2 illustrates an elevation guidance unit for the runway 12. The elevation unit operates similarly to the azimuth unit except that the two beams are displaced vertically rather than horizontal¬ ly. For example, the elevation unit in FIG. 2 has an upper lobe 20 and a lower lobe 22 that overlap in an area 24. As in the case of the azimuth unit, as long as the aircraft is on course, i.e., on the proper glide slope, the size of the pulses received from the lobes 20 and 22 will be approximately equal. However, should the aircraft fly above or below the glide slope, the amplitude of the pulses received from the two lobes will no longer be equal. In a typical system, pulses transmitted on the two lobes of an elevation or azimuth guidance system are transmitted as pairs of pulses in rapid succession. For example, as is illustrated in FIG. 3, for an ideal azimuth guidance system, a left lobe pulse 26 is transmitted followed very shortly by a right lobe pulse 28. After a predetermined time inter¬ val, another left lobe pulse 30 is transmitted followed by another right lobe pulse 32. Succeeding pairs of pulses are then transmitted in the same sequence, which may be a left right sequence as illustrated in FIG. 3, or if desired in a continuous right left se¬ quence. Only the pulses that provide left and right (or up and down) information are shown in FIGS. 3 through 6 for purposes of clarity, but in a practical system each left and right (or up and down) pulse would be preceded by a preamble consisting of a plur¬ ality of pulses (e.g., four pulses) that identify whether the following pulse is an up, down, left or right pulse. When the pulses illustrated in FIG. 3 are received by a receiver aboard an aircraft, the pulses may cause a shift in the base line of the pulse pro¬ cessing circuitry within the receiver, as is illus- ^ trated in FIG. 4. For example, the base line shift caused by the pulses, particularly in a capacitively coupled processing system wherein the pulses tend to charge the coupling capacitors, the received pulse 26' (corresponding to the transmitted pulse 26) may Q be offset from the received pulse 28' (corresponding to the transmitted pulse 28) . The magnitude of the error is illustrated by the distance between the hori¬ zontal lines level with the peaks of the pulses 26' and 28' . - The transmitted pulses illustrated in FIG. 3 are idealized pulses showing no amplitude deteriora¬ tion or pulse droop. However, in a practical system, the pulses tend to droop and are not necessarily equal in amplitude. An example of a pair of typical pulses 0 is illustrated by the pulses 30' and 32' which corres¬ pond to, for example, the idealized pulses 30 and 32. As is apparent, the amplitude of the pulses decreases with time, and in addition, the peak amplitude of the second pulse is generally smaller than the peak aπipli- 5 tude of the first pulse as is illustrated by the dif¬ ference in the peak amplitudes of the pulses 30' and 32*. The errors introduced by each of the two phenomena illustrated in FIG. 4 can be significant, particularly when the effects of the two phenomena are combined. 0 In order to reduce the problems caused by the base line shift and pulse droop illustrated in FIG. 4, and also to reduce the effects of multi-path reception, in accordance with an important aspect of the present invention, the sequence in which the pairs 5 of pulses are transmitted is alternated each time a pair of pulses is transmitted. For example, as illus¬ trated in FIG. 5, a left lobe pulse 40 precedes a right lobe pulse 42 during the first transmission, but during the next transmission, a right lobe pulse 44 precedes a left lobe pulse 46. A similar scheme may be employed for an elevation guidance system as illustrated in FIG. 6, wherein an upper lobe pulse 50 precedes a down lobe pulse 52 on the first transmis¬ sion and a down lobe pulse 54 precedes an upper lobe pulse 56. The reversal of the sequence permits the effects of pulse droop to be minimized, because the 0 pulse that was larger during the first transmission (e.g., the left lobe pulse) will be the smaller of the two pulses in the second transmission, and by averaging the pulses received on successive transmis¬ sions, or by averaging the results of the comparisons _ taken during successive transmissions, the effects of pulse droop are substantially reduced. In addition, the reversal of the pulses on successive transmissions also reduces the effects of base line shift if the comparisons taken during successive transmissions are _ averaged. A pulse transmitter usable with the invention is illustrated in block diagram form in FIG. 7. The transmitter, generally designated by the reference numeral 60, utilizes a pulse generator 62 to generate a pulse train consisting of a series of paired pulses. 5 The pulses consist of radio frequency pulses which may be of any suitable RF frequency, but which in the present embodiment consist of radio frequency bursts at approximately 9.3 GHz at a repetition rate of ap¬ proximately 100 pulse trains per second. Each of the left and right (or up and down) pulses is encoded by a multiple pulse preamble so that it may be recognized by a receiver aboard an aircraft. For example, each preamble may contain a code indicating whether it is an upper lobe pulse (up pulse) , a lower lobe pulse (down pulse) , a left lobe pulse (left pulse) or a right lobe pulse (right pulse) . Typically, two trans- itters such as the transmitter 60 are used, one for azimuth and one for elevation. The pulses produced by the generator 62 are amplified by an amplifier 64 and applied to a switching circuit 66 which is illus¬ trated as a simple, single pole double throw switch for purposes of illustration, but various types of switches including electronic switches may be used. The switch 66 couples the signal from the amplifier 64 to one of a pair of antennas 68 and 70 which may 10 be left lobe and right lobe antennas or upper lobe and down lobe antennas. Also, the antennas 68 and 70 are illustrated as separate antennas, but may be dif¬ ferent elements in a single antenna. A controller 72 controls the operation of the transmitter by synchroniz- lt . ing the pulse generator with the switching circuit 66 so that the pulses of alternating pairs of pulses are applied to the antennas 68 and 70 in opposite sequence. A receiving and processing system suitable for use aboard an aircraft to receive the pulses trans- 2 _ mitted by the transmitter 60 is illustrated in FIG. 8 and generally designated by the reference numeral 80. The receiving and processing system 80 includes a receiver 82 which may be a conventional receiver that receives the pulses from an airborne antenna 84. The _ receiver 82 applies the received pulses to a switching circuit 86 which is also illustrated for purposes of clarity as a mechanical single pole double throw switch, but which could be any type of switch including a software function. A synchronization circuit 88 synch¬ ronizes the operation of a switch 86 with the received 30 pulses which are then applied to one of a pair of sample and hold circuits 90 and 92. Only one type of pulse is applied to the sample and hold circuit 90 and the other type of pulse is applied to the sample and hold circuit 92. Thus, for example, only left 35 lobe pulses would be applied to the sample and hold circuit 90 and only right lobe pulses would be applied to the sample and hold circuit 92. The outputs of the sample and hold circuits 92 are applied to the inputs of a comparator 94 which provides an output indicative of the difference in amplitude between the last two received pulses to an accumulator 96 and, after scaling, thence to a display 98. The function of the accumulator 96 is to average or integrate the values of the amplitude comparison signals received from the amplifier 94 for more than one comparison result. The accumulator 96 may be a digital accumula¬ tor that digitally computes the average of more than one comparison result, or it may be an averaging filter such as an integrator or low pass filter (e.g., a resistor 95 and a capacitor 97) whose time constant is selected to integrate the results of the number of comparisons that is desired. For example, the time constant of the integrator may be selected so that the results of at least the last two comparisons are averaged. Preferably, an even number of comparisons should be averaged because the pulse sequence is re¬ versed every other transmission. In the system de¬ scribed herein, the accumulator 96 consists of an integrator having a time constant of approximately 0.2 second which is long enough to provide a clean signal because many pulses are average 1 , but short enough to avoid impairing the operation of any auto pilot coupled to the system. The results are then coupled to the auto pilot 100 and/or displayed on the display 98 to provide guidance to the pilot. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. For example, it has been found ad¬ vantageous in propeller driven aircraft to compare individual pulses and then to average the result as discussed above because the position of the propeller may have an affect on the amplitudes of the received pulses; however it is possible to first average the amplitudes of the left and right (or up and down) pulses and take the difference of the averaged ampli¬ tude. Also, the sequence of the transmitted pulses need not be reversed each time a pair of pulses is sent; but more than one pair may be sent in each se¬ quence before the sequence is reversed. It is also possible that the elevation and azimuth functions may be combined into a single system. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.

Claims: What is claimed is: 1. A method for comparing the amplitudes of first and second pulses within a pulse train, com¬ prising: ordering said pulses in groups each contain¬ ing one of said first and second pulses with one of the first pulses alternately preceding and following one of the second pulses in successive groups; comparing the amplitudes of the first pulses and second pulses in each group to provide a signal representative of the difference in amplitude of the compared first and second pulses; and accumulating the difference representative signal to provide an error compensated difference representative signal. 2. An aircraft approach guidance system comprising: means for sequentially transmitting pulses along first and second spatially overlapping radio beams, said transmitting means including means for alternately transmitting the pulses along the first and second beams in a first time sequence and then in a second time sequence opposite to said first time sequence; and means for receiving the transmitted pulses and comparing the amplitudes thereof, said receiving and comparing means including means for successively comparing one of the pulses received along said first beam with one of the pulses received along the second beam and accumulating the results of the comparisons. 3. An approach guidance system comprising: means for repeatedly transmitting guidance pulses in pairs of first and second guidance pulses on first and second overlapping directional beams; and control means for rendering said transmitting means operative to transmit said first guidance pulses on said first beam and said second pulses on said second beam in one of a first and second time sequence. 4. An approach guidance system as recited in claim 3 wherein said control means includes means for periodically altering the time sequence between said first time sequence and said second time sequence. 5. An approach guidance system as recited in claim 4 further including means for receiving said first and second pulses and determining the relative amplitudes thereof, and for providing a guidance signal that is determined by the accumulated relative ampli¬ tudes of a plurality of said first and second pulses. 6. An approach guidance system as recited in claim 5 wherein said first and second sequences are opposite sequences. 7. A pulse amplitude comparison system usable for comparing the amplitudes of first pulses received from a first pulse source with the amplitudes of second pulses received from a second pulse source, wherein said first and second pulses are serially received, comprising: means for alternately receiving and comparing the amplitudes of the first and second pulses in a first sequence wherein the first pulses precede the second pulses and in a second sequence wherein the second pulses precede the first pulses; and means responsive to said receiving and com¬ paring means for providing a relative amplitude signal in response to at least one comparison performed by said receiving and comparing means on pulses received in said first sequence and one comparison performed on pulses received in said second sequence. 8. An approach guidance system comprising: means for transmitting first pulses on a first directional beam and second pulses on a second directional beam that is spatially displaced from but partially overlaps said first directional beam; means for controlling the operation of said transmitting means for causing said transmitting means to transmit said first and second pulses in an alter¬ nating sequence, said control means being operative periodically to alter the sequence from a first pulse sequence wherein the first pulse precedes the second pulse to a second pulse sequence wherein the second pulse precedes the first pulse; and means for receiving said first and second pulse sequences, comparing the amplitude of the first pulses from each sequence with the amplitude of the second pulses from each sequence to provide a guidance signal. 9. An approach guidance system as recited in claim 8 wherein said receiving and comparing means includes means for comparing the amplitudes of the first and second pulses in each sequence to provide a signal representative of the relative amplitudes there¬ of for each comparison, and means for accumulating a plurality of relative amplitude representative signals to provide a guidance signal. 10. An approach guidance system as recited in claim 9 wherein said accumulating means includes a low pass filter. 11. An approach guidance system as recited in claim 10 wherein said low pass filter has a time constant approximately equal to 0.2 second. 12. An aircraft approach guidance system comprising: means for transmitting first pulses along a first directional beam and for transmitting second pulses along a second spatially displaced directional beam partially overlapping said first directional beam; control means coupled to said transmitting means for rendering said transmitting means operative repeatedly to transmit two first pulses followed by two second pulses; means for receiving said first and second pulses and comparing the amplitude of each of said first pulses with the amplitude of an immediately preceding or following second pulse and providing a guidance signal having a value that is determined by the difference in amplitude of the compared pulses. 13. An aircraft approach guidance system as recited in claim 12 wherein said receiving and comparing means includes means for providing a signal representative of the relative amplitudes of said first and second pulses obtained during each comparison and means for accumulating a plurality of relative " amplitude signals to provide said guidance signal. 14. An aircraft approach guidance system as recited in claim 13 wherein said accumulating means includes a low pass filter. 15. An aircraft approach guidance system as recited in claim 14 wherein said low pass filter has a time constant approximately equal to 0.2 second. 16. An aircraft approach guidance system as recited in claim 12 wherein said receiving and comparing means includes means for providing a signal representative of the relative amplitudes of said first and second pulses obtained during each comparison and means for integrating said relative amplitude signal to provide said guidance signal. 17. An aircraft approach guidance system as recited in claim 16 wherein said integrating means includes a low pass filter. 18. An approach guidance system as recited in claim 17 wherein said low pass filter has a time constant approximately equal to 0.2 second.

Inventors: DAVIDSON CHARLES W, DAVIDSON, CHARLES, W.
Applicants: SUNDSTRAND DATA CONTROL, SUNDSTRAND DATA CONTROL, INC.
Application year: 1990
Main classification: G01S1
IPC codes: G01S1, H03K5