Spread spectrum radar

A spread spectrum radar apparatus wherein a plurality of carrier signals frequency modulated over predetermined time intervals and separated in frequency such that the spectra of the signals do not overlap are sequentially transmitted. Radar returns of this transmission sequence are coupled to a receiver which is frequency band activated in accordance with the transmission sequence and in a manner to minimize radar minimum detection range. Each radar echo is processed through a matched filter for pulse compression, delayed in accordance with the position of its transmitted signal, detected, and non-coherently summed with detections of returns from the same target of other interval transmissions.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to radar systems and more particularly to a radar 
system utilizing a spread spectrum and a combination of coherent and 
non-coherent signal processing techniques. 
2. Description of the Prior Art 
A conventional radar operates by transmitting a pulse modulated sine wave 
signal, detects returned pulses from reflecting objects in the 
transmission path, and determines the time interval between the 
transmission and reception of these signals. Maximum range for these 
radars is a function of the total energy transmitted with each pulse while 
range resolution capability is a function of the pulse width, increasing 
with decreasing pulse widths. Since the energy within each pulse is equal 
to the product of the peak power times the duration of the pulse, it is 
evident that a trade-off between maximum range and target resolution 
exists for radar systems that are peak power limited. The long pulse 
necessary to achieve long range capability for these radar systems need 
not, however, be incompatible with good range resolution. 
It is well known that radar range resolution is a function of the radar 
signal bandwidth, the resolving capability improving with increasing radar 
signal bandwidth. In a pulsed monochromatic system this bandwidth is 
substantially equal to the inverse of the pulse width, thus decreasing 
with increasing pulse width. Compensation for this reduction in bandwidth 
with increased pulse width may be realized by providing a frequency 
modulated carrier within the pulse. By appropriately processing the pulsed 
frequency modulated signal returned from a target the pulse may be 
compressed to form a pulse of a width corresponding to the transmitted 
bandwidth, thus providing a resolution capability which exceeds that 
achievable with a pulsed single frequency signal having the same pulse 
width. Maximum range for these pulse compression systems, as in the 
conventional single frequency pulse modulated systems, is a function of 
the total energy transmitted within each pulse and, for a peak power 
limited system is increased with increased pulse width. Since the receiver 
is generally disabled during transmission, this lengthening of the 
transmitted pulse adversely affects the minimum range of the system and 
may necessitate the utilization of a conventional pulsed radar system for 
coverage to the minimum range of the pulse compression system. 
Additionally, extremely high resolution and very long range operational 
characteristics necessitate very large time bandwidth products which 
require complex circuitry, are difficult to achieve, and are expensive to 
implement. 
SUMMARY OF THE INVENTION 
The present invention utilizes a combination of coherent and non-coherent 
signal processing techniques to achieve a spread spectrum radar system. In 
one preferred embodiment of the invention a transmitter is programmed to 
sequentiallly radiate a plurality of frequency modulated signals each 
having equal bandwidth but different carrier frequencies selected such 
that the spectra of adjacent signals in the sequence are substantially 
non-overlapping. Each of these frequency modulated signals extend over a 
time interval, of selected duration, in a sequence of contiguous time 
intervals, with the interval of shortest duration occurring as the last in 
sequence. At the beginning of this lateral interval, the system range gate 
is initiated and at its conclusion the receiver is activated. Thus, the 
minimum detection range is determined by the duration of the last time 
interval. 
Echo returns from a target of this sequence of frequency modulated signals 
are coupled to a bank of mixers in the receiver, each having a frequency 
band corresponding to a different one of the transmitted signals. These 
mixers provide a plurality of output signals which are frequency modulated 
within pulses of time duration that are substantially equal to the time 
durations of the plurality of contiguous time intervals. Each output 
signal is coupled to a corresponding matched filter-delay line combination 
to provide pulses of substantially equal width and in substantially time 
coincidence. These pulses are individually detected in a plurality of 
envelope detectors to derive video signals that are coupled to a summing 
network from which a non-coherent sum of the video signals is coupled to 
the radar processor. 
Since the range gate is initiated at the commencement of the last signal 
interval in the sequence and the receiver is opened at its conclusion, the 
signal returns from all the preceding intervals for ranges corresponding 
to the time duration of the final interval would have arrived prior to the 
receiver activation, thus at short ranges only returns of the final 
interval signal are receivable. As the elapsed time from the activation of 
the receiver increases, signal returns from targets at longer ranges 
resulting from transmissions during prior intervals become available until 
the elapsed time from the activation of the receiver equals the total 
transmission time interval, i.e., the sum of the plurality of time 
durations of the intervals in the sequence, after which signal returns 
from targets at ranges greater than that corresponding to this total 
transmission interval are available for all transmission intervals. Signal 
returns from these long range targets are summed as described above to 
derive the signal coupled to the radar processor. 
In a second embodiment of the invention, the first transmission interval is 
of the shortest duration in the sequence. A multiplicity of mixers, each 
tuned to receive signal returns resulting from signals transmitted within 
a selected one of the sequence of time intervals, are coupled through a 
switch bank to the input terminals of the receiver. At the conclusion of 
the first transmission interval, a mixer tuned to receive echoes of the 
signal transmitted therein is switchably coupled to the receiver input 
terminal, thereby permitting return signals to couple thereto. When the 
second transmission interval is complete, a second mixer is switchably 
coupled to the receiver input terminal. With the completion of each 
succeeding transmission interval, another mixer is coupled to the input 
terminal of the receiver until all the transmission intervals are 
completed. Each of the output signals from the mixers are coupled to 
matched filter-delay line combinations and processed as previously 
described.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the block diagram of FIG. 1, a spread spectrum radar 
system 10 may include a multiplicity of frequency modulators 12.sub.1 
through 12.sub.N respectively coupled to a multiplicity of transmitters 
11.sub.1 through 11.sub.N. A selector 13 is coupled to transmitters 11 and 
frequency modulators 12, while an antenna 15 is coupled to transmitters 11 
via a frequency multiplexer 14 and a transmit-receive circuit 16. Antenna 
15 also may be coupled via transmit-receive circuit 16 to a mixer 17 which 
may be coupled through a bandpass amplifier 18 to a demultiplexer 19. 
Output terminals 19.sub.1 through 19.sub.N of the demultiplexer 19 are 
coupled to corresponding input terminals of a matched filter-delay line 
unit 23 having output terminals 23.sub.1 through 23.sub.N coupled 
respectively to envelope detectors 24.sub.1 through 24.sub.N. Matched 
filter-delay line unit 23 may be of the surface acoustic wave type 
described by Van deVaart and Solie in Applied Physics Letters, Volume 31, 
No. 1, July 1, 1977. Output terminals of each of the envelope detectors 
24.sub.1 through 24.sub.N are coupled via single pole double throw 
switches 25.sub.1 through 25.sub.N, which are controllable by selector 13, 
to a summing network 26, the output terminal of which is coupled to a 
radar processor 27. 
In operation, selector 13 sequentially activates transmitter-frequency 
modulator combinations 11.sub.1, 12.sub.1 through 11.sub.N, 12.sub.N to 
couple a sequence of signals to the frequency multiplexer 14. This 
sequence, comprising carrier frequencies f.sub.1 through f.sub.N frequency 
modulated with substantially equal bandwidths over time intervals of 
generally unequal duration, the last interval in the sequence being of the 
shortest duration, is coupled from the frequency multiplexer 14 to the 
antenna 15 via TR circuit 16 for transmission as the radar signal. FIG. 1A 
depicts a generalized sequence of transmitted signals. A frequency 
modulated signal S.sub.1 with carrier frequency f.sub.1 and bandwidth 
.DELTA.f is transmitted during the interval between 0 and .tau..sub.1, a 
frequency modulated signal S.sub.2 with carrier frequency f.sub.2 and 
bandwidth .DELTA.f is transmitted during the interval between .tau..sub.1 
and .tau..sub.2, with other frequency modulated signals of substantially 
equal bandwidth existing in subsequent intervals of the sequence, the last 
signal S.sub.N in the sequence being transmitted during the interval 
.tau..sub.N-1 and .tau..sub.N, which is the shortest in the series. The 
carrier frequencies f.sub.1 through f.sub.N of signals S.sub.1 through 
S.sub.N and the bandwidth .DELTA.f are chosen to preclude spectra 
overlapping. FIG. 2 is an example of a transmission sequence containing 
five transmission intervals, the first interval having a duration of ten 
microseconds, the second, third, and fourth each having a duration of five 
microseconds, and the fifth having a duration of one microsecond. Within 
each interval a linear fm signal with a 200 megahertz bandwidth is 
generated about a center frequency that is 400 megahertz from the center 
frequency of the fm signal in an adjacent interval, thus providing 
frequency separations between interval spectra of substantially 200 
megahertz. At the time .tau..sub.N-1 selector 13 couples a pulse via line 
28 to trigger the range gate in the radar processor 27 and at .tau..sub.N 
couples a pulse via line 31 to activate the switch 32 thereby enabling 
mixer 17 to receive reflections of the transmitted signal sequence. 
Selector 13 also couples a signal via line 29 to the switching unit 25 to 
activate the switch 25.sub.N thereby coupling the envelope detector 
24.sub.N to the summing network 26. This condition prevails for a time 
period substantially equal to the time duration .DELTA..tau..sub.N 
=.tau..sub.N -.tau..sub.N-1 of the N.sup.th transmission interval in the 
sequence. During this period, video signals derived from transmitter 
11.sub.N radar returns from targets within the range limits 
(C.DELTA..tau..sub.N)/2 .ltoreq.R.sub.1 .ltoreq.C .DELTA..tau..sub.N, C 
being the velocity of light, are coupled to sum network 26 and therefrom 
to radar processor 27. 
At the completion of the time period, .DELTA..tau..sub.N switch 25 
.sub.(N-1) is activated by a signal from the selector 13 thereby coupling 
the envelope detector 24.sub.(N-1) to the summing network 26 expanding the 
system's range beyond c.DELTA..tau..sub.N. At this time the video signals 
from the envelope detectors 24.sub.N and 24.sub.(N-1) are coupled to the 
summing network 26 and summed therein. This condition prevails for an 
elapsed time substantially equal to the (N-1).sup.th transmission interval 
.DELTA..tau..sub.N-1 =.tau..sub.N-1 .sup.-.tau..sub.N-2 during which video 
signals derived from transmitter 11.sub.N-1 radar returns and video 
signals derived from transmitter 11.sub.N radar returns from targets 
within the range limits c.DELTA..tau..sub.N and (c.DELTA..tau..sub.N-1)/2 
are coupled to and summed in summing network 26, wherefrom the signal 
resulting from the sum of these two video signals are coupled to the radar 
processor 27. Switches 25 are sequentially activated after elapsed times 
that are substantially equal to the next preceding interval in the 
sequence until all the switches 25.sub.1 through 25.sub.N in the switching 
network 25 have been activated and video signals derived from all 
transmitter returns are coupled to the summing network 26 from which a 
signal that is representative of the total of these video signals returned 
from a distant target is coupled to the radar processor 27. This 
sequential switch activation causes the number of video pulses 
non-coherently summed in the summing network 26 to increase from one 
during the first elapsed time interval to N video pulses after the final 
switch 25.sub.1 is activated, which condition exists through the maximum 
range of the radar system. 
Still referring to FIG. 1, radar returns are received by the antenna 15 and 
coupled via TR 16 to mixer 17 from which a down converted signal is 
coupled to a bandpass amplifier 18. Down converted signals from bandpass 
amplifier 18 are coupled to mixers 33.sub.1 through 33.sub.N from each of 
which a frequency modulated signal emerges that is representative of one 
of the radar returns of the transmitted signals S.sub.1 through S.sub.N. 
These pulsed frequency modulated signals are coupled to bandpass 
amplifiers 34.sub.1 through 34.sub.N to the one combination of matched 
filter and delay line of the matched filter-delay line combinations 
35.sub.1, 36.sub.1 through 35.sub.N, 36.sub.N corresponding thereto in the 
matched filter-delay line circuit 23. After pulse compression and delay, 
available signal returns from a common target are in time coincidence at 
the output terminals 23.sub.1 through 23.sub.N and coupled therefrom to 
envelope detectors 24.sub.1 through 24.sub.N, wherefrom time coincident 
video signals, each derived from a return of one of the transmitted 
waveforms, are coupled to the summing network 26 nd non-coherently 
integrated therein. 
FIG. 3 illustrates an embodiment of the invention wherein the initial 
interval in the sequence of transmitted signals is of the shortest 
duration. A spread spectrum radar system 40 having a first transmission 
interval shorter than subsequent transmission intervals, in a sequence of 
transmission intervals, may include a multiplicity of transmitters 
41.sub.1 through 41.sub.N with carrier frequencies f.sub.1 ' through 
f.sub.N ' frequency modulated by frequency modulators 42.sub.1 through 
42.sub.N. Transmitters 41 and frequency modulators 42 are sequentially 
activated by a selector 43 while the output terminals of the transmitter 
41 are coupled to antenna 43 via a frequency multiplexer 44 and a 
circulator 5. Antenna 43 is also coupled via circulator 45 to a 
demultiplexer 47. Demultiplexer 47 possesses a multiplicity of output 
terminals 47.sub.1 through 47.sub.N respectively corresponding to target 
signal returns of signals generated by transmitters 41.sub.1 through 
41.sub.N, each of which is coupled to the one matched filter-delay line 
combination of the matched filter-delay line combinations 60.sub.1, 
61.sub.1 through 60.sub.N, 61.sub.N in a matched filter-delay line circuit 
52. Matched filter-delay line unit 23 may be of the surface acoustic wave 
type described by Van deVaart and Solie in Applied Physics Letters, Volume 
31, No. 1, July 1, 1977. The output terminal 52.sub.1 through 52.sub.N of 
each matched filter-delay line combination 60.sub.1, 60.sub.1 through 
60.sub.N, 61.sub.N is coupled via a corresponding envelope detector 
53.sub.1 through 53.sub.N to a summing network 54, the output terminal of 
which is coupled to a radar processor 55. Selector 43 sequentially 
activates transmitter frequency modulator combinations 41.sub.1, 41.sub.2 
through 41.sub.N, 42.sub.N to couple a sequence of signals with carrier 
frequencies f.sub.1 ' through f.sub.N '. Each carrier is frequency 
modulated with substantially equal bandwidths over time intervals of 
generally unequal duration, the first interval in the sequence being of 
the shortest duration. FIG. 3A depicts a generalized sequence of 
transmitted signals. A frequency modulated signal S.sub.1 ' with carrier 
frequency f.sub.1 ' and bandwidth .DELTA.f' is transmitted during the 
interval between zero and .tau..sub.1 ', a frequency modulated signal with 
carrier f.sub.2 ' and bandwidth .DELTA.f' is transmitted during the 
interval between .tau..sub.2 ' and .tau..sub.1 ' with other frequency 
modulated signals of substantially equal bandwidth existing in subsequent 
intervals of this sequence. The carrier frequencies f.sub.1 ' through 
f.sub.N ' and the bandwidths .DELTA.f' are chosen such that the spectra 
within adjacent transmission intervals do not overlap. FIG. 4 is an 
example of a transmission sequence containing five transmission intervals, 
the first interval having a duration of one microsecond, the second, third 
and fourth each having durations of five microseconds and the fifth having 
a duration of ten microseconds. Within each interval a linear fm signal 
with a 200 megahertz bandwidth is generated about a center frequency that 
is 400 megahertz from the center frequency of the fm signal in adjacent 
intervals, thus providing frequency separations of the transmitted signals 
of substantially 200 megahertz. The sequence of pulsed fm signals 
generated by the transmitters 41.sub.1 through 41.sub.N is coupled to the 
frequency multiplexer 44 from which it is coupled to the antenna 43 via 
the circulator 45 and radiated therefrom. Target returns for each signal 
within the sequence are received by the antenna 43 and coupled to 
demultiplexer 47 via the circulator 45, wherein they are down converted to 
establish a sequence of signals corresponding to the transmitted sequence 
having corresponding time intervals, substantially equal bandwidths within 
each interval, and down converted carrier frequencies. 
At time zero selector 43 couples a signal to radar processor 55 via line 56 
to activate a range gate therein and at time .tau..sub.1 ' couples a pulse 
to the demultiplexer which activates switch 57.sub.1 that couples a 
bandpass filter 58.sub.1 to a mixer 59.sub.1. Bandpass filter 58.sub.1 and 
mixer 59.sub.1 are tuned to down convert signals within the first 
transmission interval to a frequency modulated signal with bandwidth 
.DELTA.f' and center frequency f.sub.0. This down converted signal may be 
coupled through bandpass amplifier 64.sub.1 to the demultiplexer output 
terminal 47.sub.1, wherefrom it is coupled to matched filter 60.sub.1 in 
the matched filter-delay line circuit 52, compressed therein and delayed 
in delay line 61.sub.1 for a time .tau..sub.N-1 '. After this delay, the 
signal is coupled via output terminal 52.sub.1 to the envelope detector 
53.sub.1, wherefrom a video pulse representative of an echo return of the 
signal transmitted during the interval of zero and .tau..sub.1 ' from 
targets at ranges greater than (c.tau..sub.1 ')/2 is coupled to the 
summing network 54. At .tau..sub.2 ', the conclusion of the second 
transmission interval, selector 43 couples a signal to the demultiplexer 
41 that activates a switch 57.sub.2 to couple a second bandpass filter 
58.sub.2 to a second mixer 59.sub.2, Bandpass filter 58.sub.2 and mixer 
59.sub.2 are tuned to down convert signals in the second transmission 
interval to a frequency modulated signal of bandwidth .DELTA.f' and center 
frequency f.sub.0. This frequency modulated signal may be coupled to a 
bandpass amplifier 64.sub.2 wherefrom a signal representative thereof may 
be coupled to the output terminal 47.sub.2 of the demultiplexer 47. The 
pulsed fm signal at terminal 47.sub.2 is coupled to matched filter 
60.sub.2 of the matched filter-delay line circuit 52 wherein it is 
compressed and thereafter delayed in delay line 61.sub.2 for a time 
(.tau..sub.N-1 '-.tau..sub.1 ') to couple a pulse to the output terminal 
52.sub.2 that is representative of the returns of the signal transmitted 
within the interval .tau..sub.2 ' and .tau..sub.1 ' from targets at ranges 
greater than (c.tau..sub.2 ')/2. Since switch 57.sub.1 remains closed 
during the transmission interval .tau..sub.2 ' and .tau..sub.1 ', 
compressed signals representative of target returns from targets at ranges 
greater than (c.tau..sub.2 ')/2, of the signal transmitted between the 
interval .tau..sub.1 ' and zero are present at output terminal 52.sub.1 
substantially in time coincidence with the signal returns at terminal 
52.sub.2. The effective ranges for each of the transmitted signals S.sub.1 
through S.sub.N and the ranges after which time coincidence of the echo 
signals occur are shown in FIG. 5. 
The compressed signals at terminals 52.sub.1 and 52.sub.2 are respectively 
coupled to envelope detectors 53.sub.1 and 53.sub.2 wherefrom video 
signals are coupled to summing network 54 integrated therein and a signal 
representative of the integration thereof is coupled to radar processor 
55. The process of activating a switch to couple a filter to pass a band 
of frequencies transmitted during a transmission interval to a mixer tuned 
to the same frequency band, compressing the received signal, appropriately 
delaying the compressed signal, envelope detecting the delayed signal, and 
integrating the detected signal with detected target returns of signals 
transmitted during previous transmission intervals, continues until the 
bandpass filter 58.sub.N is coupled to the mixer 59.sub.N permitting the 
reception of signal returns of the signals transmitted in the final 
transmission interval of the sequence. After all the switches have been 
activated, the receiver remains open for a time necessary to receive 
returned signals from targets at the maximum range of the system. At the 
conclusion of the time for receiving returned signals from targets at the 
maximum range R.sub.M of the system, the selector 43 couples a pulse to 
the demultiplexer 47 to deactivate the switches 57.sub.1 through 57.sub.N 
decoupling the bandpass filters 58.sub.1 through 58.sub.N from the mixers 
59.sub.1 through 59.sub.N. The receiver remains disabled until the next 
transmission sequence is initiated after which the switch activation 
process and signal reception operation is repeated. 
While the invention has been described in its preferred embodiment, it is 
to be understood that the words which have been used are words of 
description rather than limitation and that changes may be made within the 
purview of the appended claims without departing from the true scope and 
spirit of the invention in its broader aspects.