CA2024930A1 - Multiple target ranging system - Google Patents
Multiple target ranging systemInfo
- Publication number
- CA2024930A1 CA2024930A1 CA002024930A CA2024930A CA2024930A1 CA 2024930 A1 CA2024930 A1 CA 2024930A1 CA 002024930 A CA002024930 A CA 002024930A CA 2024930 A CA2024930 A CA 2024930A CA 2024930 A1 CA2024930 A1 CA 2024930A1
- Authority
- CA
- Canada
- Prior art keywords
- target
- range
- threshold
- signal
- returned
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
- 230000010363 phase shift Effects 0.000 claims abstract description 3
- 238000000034 method Methods 0.000 description 6
- 238000001914 filtration Methods 0.000 description 3
- 230000005540 biological transmission Effects 0.000 description 2
- 230000000875 corresponding effect Effects 0.000 description 2
- 230000003111 delayed effect Effects 0.000 description 2
- 238000000605 extraction Methods 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 238000001228 spectrum Methods 0.000 description 2
- 230000003321 amplification Effects 0.000 description 1
- 230000002596 correlated effect Effects 0.000 description 1
- 230000010354 integration Effects 0.000 description 1
- 238000003199 nucleic acid amplification method Methods 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
- G01S7/35—Details of non-pulse systems
- G01S7/352—Receivers
- G01S7/354—Extracting wanted echo-signals
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
- G01S13/02—Systems using reflection of radio waves, e.g. primary radar systems; Analogous systems
- G01S13/06—Systems determining position data of a target
- G01S13/08—Systems for measuring distance only
- G01S13/32—Systems for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated
- G01S13/325—Systems for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated using transmission of coded signals, e.g. P.S.K. signals
Landscapes
- Engineering & Computer Science (AREA)
- Radar, Positioning & Navigation (AREA)
- Remote Sensing (AREA)
- Computer Networks & Wireless Communication (AREA)
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Radar Systems Or Details Thereof (AREA)
Abstract
A continuous wave ranging system, comprising a modulator 2 for modulating an r.f. carrier wave in accordance with a pseudo-random code 3, a transmitting antenna 5 for radiating the modulated signal towards a target, a receiving antenna 6 and receiver 7 for detecting the signal reflected back from the target, a correlator 8 for correlating the reflected signal with a transmitted code with a selected phase shift 9 corresponding to the current range gate to be tested, and means 10, 11, 12 for processing the range/amplitude data from the correlator 8 to discriminate between reflections due to the target and those due to other objects adjacent to the target.
Description
202~930 MULTIPLE TARGET RANGING SYSTEM
This invention relates to a continuous wave ranging system and, in one aspect, to an aircraft radar altimeter system.
Such systems usually comprise a means of microwave transmission upon which some form of coding has been added, and antenna for directing the energy to the target, an antenna for receiving the returned energy and, after amplification, a means of determining the amount of delay that has occurred on the signal, and hence the range of the target. The coding on the transmission had in the past been pulse or frequency modulation, but more recently phase modulation from a pseudo-random code has been used. This form of modulation has the property of producing a noise-like transmitted spectrum which is difficult to detect and hence finds applications where covertness is of importance. Covertness can be enhanced by reducing the transmitted power such that the returned signal is just sufficient for ranging measurement.
In such phase-modulated systems, the received - 2UZ4g30 signal is correlated with a delayed version of the transmitted code, the delay being gradually increased in steps, and samples of the output of the correlator are detected and stored in an array. From this stored data, the delay, and hence the range, where the received signal return occurs can be found.
Existing direct sequence spread spectrum ranging systems use techniques such as delay locked or Tau dither loops to track target ranges. These techniques result in a narrow tracking window and tracking loops with excellent dynamic performance. However, the narrowness of the tracking window restricts the ability of such systems to see any targets at ranges other than that being tracked.
According to the invention, a continuous wave ranging system comprises a modulator for modulating an r.f. carrier wave in accordance with a pseudo-random code, a transmitting antenna for radiating the modulated signal towards a target, a receiving antenna and receiver for detecting the signal reflected back from the target, a correlator for correlating the reflected signal with the transmitted code with a selected phase shift corresponding to the current range gate to be tested, and means for processing the range/amplitude data from the correlator to discriminate between reflections due to the target and those due to other objects adjacent to the target.
The pseudo-random code used in the invention is preferably a maximal length code, a sequence of numbers generated by a shift register with certain feedbacks on it. For the system of the present invention, a code length of 2047 digits is preferred.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:-Figure 1 shows a schematic view of a systemaccording to one embodiment of the inventor, Figure 2 shows a diagrammatic view of the application of the system and a typical signal received from such a system, Figure 3 shows one algorithm from the extraction of feature and terrain ranges from the system, and 202493~ . P/8249/US/CA
Figure 4 shows a typical signal reading divided into range cells.
Referring now to Figure 1, the system shown therein comprises a transmitter having a signal generator 1; a modulator 2 for modulating the signal in accordance with a pseudo-random code; a transmitter amplifier 4 and a transmitter antenna 5. A receiver includes a receiver antenna 6; a receiver amplifier 7, a correlator 8 for correlating the received signal with a delayed version of the pseudo-random code 9 according to the range being determined; an amplitude detector 10; a memory array 11 and a processor 12 for analysing the signal stored in the array 11 to determine terrain and feature ranges.
A range scanning technique be used in the above system, where the receiver code is preferably dwelled at a given delay (range) for a fixed integration period enabling signal strength to be measured for each delay period. In turn a picture of signal strength versus range is constructed for the entire measurement range of the system in the array 11. This picture will thus contain signal/range data for all targets as well as environment noise information, a typical result being shown in Figure 2. From this picture, the predominant target range (terrain) R2 and less dominant shorter ranged targets (feature) Rl may be extracted by use of the processor 12. In order to formulate a robust predominant target extraction techn;que, regard must be given to target dynamics. It can be shown that a partial area algorithm applied about the predominant target range can significantly discriminate this target from shorter range returns which occur close to it.
One particular method of extracting feature and terrain ranges will now be described with reference to Figures 3 and 4.
Referring now to Figures 3 and 4, the array of amplitudes or signal strengths in the various range bands or range cells is read and the amplitudes are modified to compensate for the law of signal strength versus range, signals reducing at 9 dB/octave due to propagation factors. The cell with the largest amplitude (after compensation) is noted (C) and the amplitude value measured (P).
The method of determining the range of a feature is as follows (left hand side of Figure 3):-- 2U24931) A threshold (T1) is set at a fixed value below P.
This is typically 12 dB and a check is made that T1 is above the general noise level. A scan is made of the cells below C starting at a fixed number of cells below C. Typically the scan would start at the equivalent of 300 feet below C although a scan would not normally cover the first few cells, corresponding to ranges below say 15 feet. The cell having the shortest range which has energy above Tl is determined (Cl). Interpolation is then made based on the energy in C1 and the energy in the next cell below and from this interpolation R1 is calculated as the range where the T1 threshold is crossed and after filtering is output.
The method of determining the range of the terrain is as follows (right hand side of Figure 3):-A threshold (T2) is set at a fixed value below P.This need not be the same as T1 but is typically 12 dB
when a good signal to noise ratio is obtained. Under poorer signal to noise conditions the threshold will be closer to P. A scan is then made of pairs of cells, comprising one cell in above C and the other below, both by the same amount. When energy in either cell of the scanned pair falls below T2 the scanning is halted. The - 2024g30 range of pairs of cells (C2 - C3) that have energy above T2 is determined. (In the example of Figure 4 only the adjacent pair of cells meets this criterion). The energy in the range of cells C2 - C3 is calculated and the area that contains a fixed fraction K of the total energy in C2 - C3 is calculated, its upper boundary giving the value of R2 (see Figure 4). Typically a value of K is 0.375. After filtering R2 is output as the range to the terrain. The amount of filtering applied to the terrain output can be greater than that of the feature if required.
Thresholds Tl and T2 are chosen so that features such as trees and buildings are accepted and measured whilst returns from clouds and chaff are ignored.
This invention relates to a continuous wave ranging system and, in one aspect, to an aircraft radar altimeter system.
Such systems usually comprise a means of microwave transmission upon which some form of coding has been added, and antenna for directing the energy to the target, an antenna for receiving the returned energy and, after amplification, a means of determining the amount of delay that has occurred on the signal, and hence the range of the target. The coding on the transmission had in the past been pulse or frequency modulation, but more recently phase modulation from a pseudo-random code has been used. This form of modulation has the property of producing a noise-like transmitted spectrum which is difficult to detect and hence finds applications where covertness is of importance. Covertness can be enhanced by reducing the transmitted power such that the returned signal is just sufficient for ranging measurement.
In such phase-modulated systems, the received - 2UZ4g30 signal is correlated with a delayed version of the transmitted code, the delay being gradually increased in steps, and samples of the output of the correlator are detected and stored in an array. From this stored data, the delay, and hence the range, where the received signal return occurs can be found.
Existing direct sequence spread spectrum ranging systems use techniques such as delay locked or Tau dither loops to track target ranges. These techniques result in a narrow tracking window and tracking loops with excellent dynamic performance. However, the narrowness of the tracking window restricts the ability of such systems to see any targets at ranges other than that being tracked.
According to the invention, a continuous wave ranging system comprises a modulator for modulating an r.f. carrier wave in accordance with a pseudo-random code, a transmitting antenna for radiating the modulated signal towards a target, a receiving antenna and receiver for detecting the signal reflected back from the target, a correlator for correlating the reflected signal with the transmitted code with a selected phase shift corresponding to the current range gate to be tested, and means for processing the range/amplitude data from the correlator to discriminate between reflections due to the target and those due to other objects adjacent to the target.
The pseudo-random code used in the invention is preferably a maximal length code, a sequence of numbers generated by a shift register with certain feedbacks on it. For the system of the present invention, a code length of 2047 digits is preferred.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:-Figure 1 shows a schematic view of a systemaccording to one embodiment of the inventor, Figure 2 shows a diagrammatic view of the application of the system and a typical signal received from such a system, Figure 3 shows one algorithm from the extraction of feature and terrain ranges from the system, and 202493~ . P/8249/US/CA
Figure 4 shows a typical signal reading divided into range cells.
Referring now to Figure 1, the system shown therein comprises a transmitter having a signal generator 1; a modulator 2 for modulating the signal in accordance with a pseudo-random code; a transmitter amplifier 4 and a transmitter antenna 5. A receiver includes a receiver antenna 6; a receiver amplifier 7, a correlator 8 for correlating the received signal with a delayed version of the pseudo-random code 9 according to the range being determined; an amplitude detector 10; a memory array 11 and a processor 12 for analysing the signal stored in the array 11 to determine terrain and feature ranges.
A range scanning technique be used in the above system, where the receiver code is preferably dwelled at a given delay (range) for a fixed integration period enabling signal strength to be measured for each delay period. In turn a picture of signal strength versus range is constructed for the entire measurement range of the system in the array 11. This picture will thus contain signal/range data for all targets as well as environment noise information, a typical result being shown in Figure 2. From this picture, the predominant target range (terrain) R2 and less dominant shorter ranged targets (feature) Rl may be extracted by use of the processor 12. In order to formulate a robust predominant target extraction techn;que, regard must be given to target dynamics. It can be shown that a partial area algorithm applied about the predominant target range can significantly discriminate this target from shorter range returns which occur close to it.
One particular method of extracting feature and terrain ranges will now be described with reference to Figures 3 and 4.
Referring now to Figures 3 and 4, the array of amplitudes or signal strengths in the various range bands or range cells is read and the amplitudes are modified to compensate for the law of signal strength versus range, signals reducing at 9 dB/octave due to propagation factors. The cell with the largest amplitude (after compensation) is noted (C) and the amplitude value measured (P).
The method of determining the range of a feature is as follows (left hand side of Figure 3):-- 2U24931) A threshold (T1) is set at a fixed value below P.
This is typically 12 dB and a check is made that T1 is above the general noise level. A scan is made of the cells below C starting at a fixed number of cells below C. Typically the scan would start at the equivalent of 300 feet below C although a scan would not normally cover the first few cells, corresponding to ranges below say 15 feet. The cell having the shortest range which has energy above Tl is determined (Cl). Interpolation is then made based on the energy in C1 and the energy in the next cell below and from this interpolation R1 is calculated as the range where the T1 threshold is crossed and after filtering is output.
The method of determining the range of the terrain is as follows (right hand side of Figure 3):-A threshold (T2) is set at a fixed value below P.This need not be the same as T1 but is typically 12 dB
when a good signal to noise ratio is obtained. Under poorer signal to noise conditions the threshold will be closer to P. A scan is then made of pairs of cells, comprising one cell in above C and the other below, both by the same amount. When energy in either cell of the scanned pair falls below T2 the scanning is halted. The - 2024g30 range of pairs of cells (C2 - C3) that have energy above T2 is determined. (In the example of Figure 4 only the adjacent pair of cells meets this criterion). The energy in the range of cells C2 - C3 is calculated and the area that contains a fixed fraction K of the total energy in C2 - C3 is calculated, its upper boundary giving the value of R2 (see Figure 4). Typically a value of K is 0.375. After filtering R2 is output as the range to the terrain. The amount of filtering applied to the terrain output can be greater than that of the feature if required.
Thresholds Tl and T2 are chosen so that features such as trees and buildings are accepted and measured whilst returns from clouds and chaff are ignored.
Claims (6)
1. A continuous wave ranging system, comprising an r.f. generator for generating an r.f. carrier wave, a modulator for modulating said r.f. carrier wave in accordance with a pseudo-random code, a transmitting antenna for radiating a modulated signal from said modulator towards a target, a receiving antenna and receiver for detecting a signal reflected back from said target, a correlator for correlating said signal reflected back from said target with said pseudo random code which incorporates a selected phase shift corresponding to a current range gate to be tested, and processing means for processing range/amplitude data from said correlator to discriminate between reflections due to said target and those due to other objects adjacent to said target.
2. A system as claimed in claim 1, wherein a first threshold is determined with regard to an amplitude of received signals such that signals immediately above this threshold are signals returned from said other objects.
3. A system as claimed in claim 2, wherein a second threshold is set such that an analysis of the energy distribution of so returned signals above said second threshold allows determination of said target range.
4. A system as claimed in claim 1, wherein said target is the ground and said other objects are features on the ground.
5. A system as claimed in claim 2, wherein an increasing range scan is made of returned signals from below a range of maximum returned signal strength until a returned signal strength is above said first threshold.
6. A system as claimed in claim 3, wherein a scan is made of pairs of signals above and below a range of maximum returned signal strength until one of said pairs includes a signal below said second threshold, the total energy of said pairs above said first threshold is calculated, and the range of a fixed fraction of said total energy determined
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002024930A CA2024930A1 (en) | 1990-09-10 | 1990-09-10 | Multiple target ranging system |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002024930A CA2024930A1 (en) | 1990-09-10 | 1990-09-10 | Multiple target ranging system |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2024930A1 true CA2024930A1 (en) | 1996-10-09 |
Family
ID=4145929
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002024930A Abandoned CA2024930A1 (en) | 1990-09-10 | 1990-09-10 | Multiple target ranging system |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2024930A1 (en) |
-
1990
- 1990-09-10 CA CA002024930A patent/CA2024930A1/en not_active Abandoned
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| FZDE | Dead |