Pulsed radar

Abstract

According to a first aspect, a pulsed radar comprises a transmitter; wherein the pulsed radar is arranged to generate a string of binary values; wherein the transmitter comprises a pulse generator arranged to generate a pulse signal comprising a series of transmit pulses with polarities determined in accordance with the string of binary values; wherein a first substring comprises a first series of values; wherein a second substring comprises a second series of values; wherein the second substring is different from the first substring; and wherein each value in the second series of values is either the same as or different from the corresponding value in the first series of values according to a repeating pattern; and wherein the string of binary values comprises at least the first substring and the second substring concatenated together and each optionally being reversed before concatenation.

Claims

1. A pulsed radar comprising a transmitter; wherein the pulsed radar is arranged to generate a string of binary values; wherein the transmitter comprises a pulse generator arranged to generate a pulse signal comprising a series of transmit pulses with polarities determined in accordance with the string of binary values; wherein a first substring comprises a first series of values; wherein a second substring comprises a second series of values; wherein the second substring is different from the first substring; and wherein each value in the second series of values is either the same as or different from the corresponding value in the first series of values according to a repeating pattern; and wherein the string of binary values comprises at least the first substring and the second substring concatenated together and each optionally being reversed before concatenation.

2. The pulsed radar as claimed in claim 1, wherein the first substring is the inverse of the second substring.

3. The pulsed radar as claimed in claim 1, wherein the values of the second substring are alternately equal to the corresponding value of the first substring and equal to the inverse of the corresponding value in the first substring.

4. The pulsed radar as claimed in claim 1, wherein the string of binary values comprises a plurality of substrings concatenated together and wherein the plurality of substrings are related to each other such that when all pulses are accumulated in a receiver, pulses from at least one ambiguity range are cancelled while pulses from at least one different ambiguity range are summed.

5. The pulsed radar as claimed in claim 4, wherein the plurality of substrings are related to each other such that pulses from at least two adjacent ambiguity ranges are cancelled while pulses from at least one different ambiguity range are summed.

6. The pulsed radar as claimed in claim 1, wherein the string of binary values comprises at least four substrings concatenated together, wherein values of a second substring are the inverse of corresponding values in a first substring, wherein values of a third substring are alternately equal to the corresponding value of the first substring and equal to the inverse of the corresponding value in the first substring, and wherein values of a fourth substring are the inverse of corresponding values in the third substring.

7. The pulsed radar as claimed in claim 6, wherein the string of binary values comprises at least eight substrings concatenated together, and wherein values of a fifth substring are formed by dividing the first substring into adjacent groups of four adjacent values starting from the beginning of the first substring and forming the corresponding group of the fifth substring by copying the first two values of the group and inverting the last two values of the group, wherein values of a sixth substring are the inverse of corresponding values in the fifth substring, wherein values of a seventh substring are formed by dividing the first substring into adjacent groups of four adjacent values starting from the beginning of the first substring and forming the corresponding group of the seventh substring by copying the first and last values of the group and inverting the middle two values, and wherein values of an eighth substring are the inverse of corresponding values in the seventh substring.

8. The pulsed radar as claimed in claim 1, wherein the string of binary values is a random or pseudo-random string of binary values.

9. The pulsed radar as claimed in claim 1, further comprising: a receiver comprising a signal combining device that combines the received signal with a chopping signal that switches at the pulse repetition frequency in accordance with the binary values of the string so as to recover the signal from a desired ambiguity range.

10. The pulsed radar as claimed in claim 1, wherein the transmit pulses are equally spaced in time.

11. The pulsed radar as claimed in claim 1, wherein the receiver comprises a quantizer arranged to receive the received signal and a threshold signal and arranged to output a binary value quantized signal based on a comparison of the received signal with the threshold signal and wherein the threshold is swept through a defined range of voltages at least once for each substring.

12. The pulsed radar as claimed in claim 11, wherein the length of each substring is equal to an integer multiple of the number of discrete voltages in the sweep through the defined range of voltages.

13. A method of transmitting pulses in a pulsed radar, comprising: generating a string of binary values; and transmitting a series of transmit pulses with polarities determined in accordance with the string of binary values; wherein a first substring comprises a first series of values; wherein a second substring comprises a second series of values; wherein the second substring is different from the first substring; and wherein each value in the second series of values is either the same as or different from the corresponding value in the first series of values according to a repeating pattern; and wherein the string of binary values comprises at least the first substring and the second substring concatenated together and each optionally being reversed before concatenation.

14. The method as claimed in claim 13, wherein the first substring is the inverse of the second substring.

15. The method as claimed in claim 13, wherein the values of the second substring are alternately equal to the corresponding value of the first substring and equal to the inverse of the corresponding value in the first substring.

16. The method as claimed in claim 13, wherein the string of binary values comprises a plurality of substrings concatenated together and wherein the plurality of substrings are related to each other such that when all pulses are accumulated in a receiver, pulses from at least one ambiguity range are cancelled while pulses from at least one different ambiguity range are summed.

17. The method as claimed in claim 16, wherein the plurality of substrings are related to each other such that pulses from at least two adjacent ambiguity ranges are cancelled while pulses from at least one different ambiguity range are summed.

18. The method as claimed in claim 13, wherein the string of binary values comprises at least four substrings concatenated together, wherein values of a second substring are the inverse of corresponding values in a first substring, wherein values of a third substring are alternately equal to the corresponding value of the first substring and equal to the inverse of the corresponding value if in the first substring, and wherein values of a fourth substring are the inverse of corresponding values in the third substring.

19. The method as claimed in claim 18, wherein the string of binary values comprises at least eight substrings concatenated together, and wherein values of a fifth substring are formed by dividing the first substring into adjacent groups of four adjacent values starting from the beginning of the first substring and forming the corresponding group of the fifth substring by copying the first two values of the group and inverting the last two values of the group, wherein values of a sixth substring are the inverse of corresponding values in the fifth substring, wherein values of a seventh substring are formed by dividing the first substring into adjacent groups of four adjacent values starting from the beginning of the first substring and forming the corresponding group of the seventh substring by copying the first and last values of the group and inverting the middle two values, and wherein values of an eighth substring are the inverse of corresponding values in the seventh substring.

20. The method as claimed in claim 13, wherein the string of binary values is a random or pseudo-random string of binary values.

21. The method as claimed in claim 13, further comprising: receiving a received signal; and combining the received signal with a chopping signal that switches at the pulse repetition frequency in accordance with the binary values of the string so as to recover the signal from a desired ambiguity range.

Description

(1) Preferred embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

(2) FIG. 1 shows a standard chopping architecture and its operation and associated signals;

(3) FIG. 2 shows a swept-threshold receiver architecture and associated signals;

(4) FIG. 3 illustrates the problem of range ambiguity;

(5) FIG. 4 shows a swept-threshold receiver architecture employing biphase coding;

(6) FIG. 5 illustrates a problem with using a swept-threshold receiver with biphase coding;

(7) FIG. 6 shows a system architecture with chopping implemented at the system level; and

(8) FIG. 7 illustrates how ghost pulses can be cancelled by appropriate pulse sequencing.

(9) FIGS. 1 to 5 have been described above and therefore no further description is provided here.

(10) FIG. 6 shows how the system implements signal chopping as part of the bi-phase coding scheme. The transmitter 60 generates pulses with either positive or negative polarity, conceptually shown here by multiplying positive only pulses with waveform Vc at 62. Vc alternates between +1 and −1 as illustrated in the second signal graph of FIG. 6. Notably, this first injection of the chopping signal Vc occurs on the transmitter side of the system, not on the receiver side of the system as would be the case in conventional signal chopping arrangements.

(11) The receiver 64 receives these transmitted pulses and adds an unwanted offset, Voff to the signal at 66 (these being illustrated in the third signal graph of FIG. 6). This signal is then multiplied by the same waveform Vc at 68 after amplification by amplifier 70, thus generating a new signal Vout which consists of the original stream of positive only pulses, together with the Voff signal multiplied by Vc as shown in the fourth signal graph of FIG. 6.

(12) The signal Vout is then sampled at regular intervals (indicated by t0 in FIG. 6), coherent with the pulse transmission. These samples are then summed up, averaging out the unwanted offset.

(13) Thus, with this arrangement, the two injections of the chopping signal occur on opposite sides of the system, one on the transmit side and one on the receive side.

(14) While the chopping signal, Vc shown in FIG. 6 is a regular square wave, alternating between +1 and −1, it will be appreciated that this was for illustration of the concept. In practical implementations the chopping signal is in fact a randomised square waveform which swaps between +1 and −1 in accordance with a pseudo-random sequence. The exact same pseudo-random sequence must of course be used on both sides of the system, i.e. at 62 and 68 in FIG. 6 and therefore the pseudo-random sequence must be stored and re-used or regenerated (e.g. from the same random number generator and the same seed) so that the transform applied on the transmit side is exactly reversed by the receive side. With a long enough pseudo-random sequence the number of positive and negative pulses can be expected to be roughly equal (i.e. it is expected to have a mean of zero) so that the averaging step eliminates (or at least significantly reduces) the unwanted offset Voff (as well as other noise).

(15) This system-level chopping is well suited to pulsed radars where bi-phase coding is particularly useful technique for spectrum spreading. The offset cancellation may also be particularly beneficial in interleaved ADCs, where variations in offset between the individual ADCs can be a challenge.

(16) As discussed above, while bi-phase coding can reduce the impact of pulses from other ambiguity ranges, they still appear in the received signal as ghost pulses in the form of a bump in the noise floor. By applying a more specific pattern to the chopping waveform Vc, a perfect cancellation of the ghost pulses can be achieved. The previously discussed pulse distortion can also be fixed. This is achieved by sending a second sequence of pulses which is the exact inverse of the original chopping sequence. Thus the pseudo-random sequence is inverted (+1 is changed to −1 and vice versa) and sent again immediately following the original sequence. The number of positive polarity pulses in the full sequence now exactly equals the number of negative polarity pulses in the full sequence. This eliminates the pulse distortion problem that was described in relation to FIG. 5 as every threshold has been tested with both a positive and a negative polarity pulse. The result of the counters is therefore the sum of the upper left graph and the upper right graph of FIG. 5, which recovers the pulse shape accurately, rather than the distorted results shown in the lower row of FIG. 5.

(17) This process may be illustrated by sequence multipliers that show how the pseudo-random sequence is transformed to generate each new sequence. In the case described above, the two sequences are achieved with the following two multipliers:

(18) Multiplier 1: 1, 1, 1, 1, 1, . . .

(19) Multiplier 2: −1, −1, −1, −1, −1, . . .

(20) This principle can be extended to effect cancellation of pulses from other ambiguity ranges that are not of interest. This is achieved by sending several sequence of pulses, each derived from the same base sequence (e.g. pseudo-random sequence), but with different transformations applied each time so as to cause reflections from different ambiguity ranges to add up or to cancel out.

(21) For example, consider the following set of four sequences (which may be transmitted in any order):

(22) Multiplier 1: 1, 1, 1, 1, 1, . . .

(23) Multiplier 2: −1, −1, −1, −1, −1, . . .

(24) Multiplier 3: 1, −1, 1, −1, 1, . . .

(25) Multiplier 4: −1, 1, −1, 1, −1, . . .

(26) The effect of this set of sequences is shown in FIG. 7. In FIG. 7 ‘a’ is the pulse received from a reflector in the unambiguous range interval (i.e. the desired pulse), and ‘b’ is the pulse received from a reflector in the first ambiguous range interval (the undesired pulse which we want to remove). The cancellation of the ‘b’ pulse is illustrated by summing the pulse samples from the same position in each sequence. The final result is found by summing all samples, which gives the desired result of exactly zero contribution from the ‘b’ pulse.

(27) The four sequence pattern described above will cancel pulses from any odd-numbered ambiguity range interval (i.e. 1, 3, 5, etc.) More generally, pulses from any range interval can be cancelled by selecting the right set of codes. In order to cancel the reflections from the 2.sup.nd, 6.sup.th, 10.sup.th, etc. ambiguity range intervals, the above four sequences can be transmitted, followed by the same set again, but this time multiplying them all with (1, 1, −1, −1, 1, 1, . . . ), giving the following set of sequences:

(28) Multiplier 1: 1, 1, 1, 1, 1, . . .

(29) Multiplier 2: −1, −1, −1, −1, −1, . . .

(30) Multiplier 3: 1, −1, 1, −1, 1, . . .

(31) Multiplier 4: −1, 1, −1, 1, −1, . . .

(32) Multiplier 5: 1, 1, −1, −1, 1, . . .

(33) Multiplier 6: −1, −1, 1, 1, −1, . . .

(34) Multiplier 7: 1, −1, −1, 1, 1, . . .

(35) Multiplier 8: −1, 1, 1, −1, −1, . . .

(36) This principle can be further extended to cancel pulses from the 4.sup.th, 12.sup.th, 20.sup.th, etc. ambiguity intervals by transmitting the above 8 sequences, then retransmitting those 8 sequences transformed by multiplying by (1, 1, 1, 1, −1, −1, −1, −1, 1, . . . ). This process may be repeated to generate patterns that will cancel pulses from other ambiguity intervals as needed.

(37) These patterns have the particular property that the cancellation happens for each offset/position in the sequence, i.e. we don't need to look at the sequence as a whole to achieve the desired cancellation. (Except the first few positions which don't carry the necessary history.)

(38) As discussed above, the fact that there is an equal number of positive and negative pulses at each offset/position in the sequence is a useful property in systems based on the swept threshold conversion technique. Because each threshold step is guaranteed to sample both positive and negative pulses the exact same number of times, no loss of data/additional distortion will occur.

(39) The method will also cancel out unwanted pulses from “negative” ambiguity intervals; this is useful if you want to observe a range which does not start at 0 meters.

(40) One particular advantage of this technique is that it makes bi-phase coding a suitable technique so that the transmitter can be designed as a bi-phase coding transmitter. This can be simpler and more cost-effective than the alternative of time staggering techniques (e.g. randomising the PRF). However, it will be appreciated that the technique described here can be combined with time staggering techniques too if desired.

(41) The theoretically perfect cancellation of ghost pulses beyond the MUR is highly desirable. As mentioned above, with the conventional techniques, the unwanted pulse reflections create a bump in the noise floor and can potentially hide small but desired targets closer to the radar. However, these unwanted reflections can be largely eliminated or suppressed here, allowing much more reliable detection of small signal reflections.

Examples of Pseudorandom String Formation from Two Substrings

(42) Consider the following pseudo-random substring (although it will be appreciated that this is short compared with most practical implementations):

(43) First substring: 1, −1, −1, 1, −1, −1, 1, 1

(44) Repeating Pattern (length 2): [1, −1] or equivalently [copy, invert]

(45) A second substring formed from the first substring according to the repeating pattern is obtained by multiplying the values of the first substring by those of the pattern as follows:

(46) $1,-1,-1,1,-1,-1,1,1\times \left[1,-1\right]\left[1,-1\right]\left[1,-1\right]\left[1,-1\right]=\hspace{1em}\hspace{1em}1,1,-1,-1,-1,1,1,-1$

(47) Thus we get:

(48) second substring 1, 1, −1, −1, −1, 1, 1, −1

(49) A full string may either be formed by concatenating the first substring and the second substring in either order and with either (or neither or both) being reversed prior to concatenation. Thus, for example:

(50) First substring concatenated with second substring gives:

(51) 1, −1, −1, 1, −1, −1, 1, 1 :: 1, 1, −1, −1, −1, 1, 1, −1

(52) Or, second substring concatenated with reversed first substring gives:

(53) 1, 1, −1, −1, −1, 1, 1, −1 :: 1, 1, −1, −1, 1, −1, −1, 1