Multiple target ranging system

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.

BACKGROUND OF THE INVENTION

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 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.

SUMMARY OF THE INVENTION

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, e.g., the terrain or ground, 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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now toFIG. 1, the system shown therein comprises a transmitter having a signal generator1, a modulator2for modulating the signal from the generator in accordance with a pseudo-random code, a transmitter amplifier4, and a transmitter antenna5. A receiver includes a receiver antenna6, a receiver amplifier7, a correlator8for correlating the received signal with a delayed version of the pseudo-random code9according to the range being determined, an amplitude detector10, a memory array11and a processor12for analysing the signal stored in the array11to determine and output respective terrain and feature ranges.

A range scanning technique is 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 array11. This picture will thus contain signal/range data for all targets as well as environment noise information, a typical result being shown inFIG. 2. From this picture, the predominant target range (terrain) R2and less dominant shorter ranged targets (feature) R1may be extracted by use of the processor12. In order to formulate a robust predominant target extraction technique, 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 the predetermine target.

One particular method of extracting feature and terrain ranges will now be described with reference toFIGS. 3 and 4.

Referring now toFIGS. 3 and 4, in the processor12the array of amplitudes or signal strengths in the various range bands or range cells is read (in a first step S1), and the amplitudes are modified (S2) 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 (S3).

The method of determining the range of a feature is as follows (left hand side of FIG.3):—

A threshold (T1) is set at a fixed value below P as indicated at S4. This is typically 12 dB and a check is made that T1is above the general noise level. A scan is made (S5) 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 C1having the shortest range which has energy above T1than is determined (S6). Interpolation is then made based on the energy in cell C1and the energy in the next cell below cell C1(S7), and from this interpolation R1is calculated as the range where the T1threshold is crossed and, after filtering, is output (S8).

The method of determining the range of the terrain is as follows (right hand side of FIG.3):—

A threshold (T2) is set at a fixed value below P (S9). This need not be the same as T1but is typically 12 dB when a good signal to noise ratio is obtained. Under poorer signal to noise conditions the threshold T2will be closer to P. A scan (S10) is then made of pairs of cells, comprising one cell above cell C and the other below cell C, both by the same amount. When energy in either cell of the scanned pair falls below T2the scanning is halted. The range of pairs of cells (C2–C3) that have energy above T2is determined (S11). (In the example ofFIG. 4, only the adjacent pair of cells meets this criterion). The energy in the range of cells C2–C3is calculated (S12) and the area that contains a fixed fraction K of the total energy in cells C2–C3is calculated (S13) its upper boundary giving the value of R2(seeFIG. 4). Typically a value of K is 0.375. After filtering, R2is output (S14) 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 T1and T2are chosen so that features such as trees and buildings are accepted and measured, whilst returns from clouds and chaff are ignored.