Laser telemetry and Doppler measurement system with pulse compression

Laser telemetry and Doppler measurement apparatus using pulse compression has a transmitter for providing a periodic pulsed transmission laser wave having a pair of pulses, one pulse being frequency modulated on one side of a main frequency F.sub.E, and the other pulse being frequency modulated on the other side of the main frequency F.sub.E. The transmitter also provides a reference laser wave having a frequency F.sub.L. A photomixer is adapted for superheterodyne reception of the reference laser wave and a return laser signal which has been reflected from a target. The return laser signal has a Doppler shift frequency F.sub.D. The photomixer provides a beat signal having a frequency F.sub.I plus F.sub.D, where F.sub.I is an intermediate frequency. A Doppler aquisition loop transposes the beat signal frequency and provides a transposed signal to compensate for the Doppler shift. The Doppler acquisition loop provides a coarse compensation signal having a frequency near the frequency F.sub.D. Under target tracking conditions, the Doppler acquisition loop then provides an automatic fine compensation signal which compensates for the Doppler difference .DELTA.F.sub.D which exists between the compensation signal frequency and the Doppler shift frequency. Receiving and processing means then receive the transposed signal and provide an output signal indicative of the distance to the tracked target and the Doppler shift frequency of the target.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to improvements in pulse-compression systems 
for laser telemetry and Doppler measurement which are primarily intended 
to equip fire control systems. 
More specifically, the invention is concerned with a system of this type in 
which means are provided for acquisition and tracking of the Doppler 
frequency shift generated by the radial velocity component of the moving 
target that is being tracked. 
2. Description of the Prior Art 
In the case of a radar system, the Doppler frequency shift hardly produces 
any disturbances, taking into account the wavelength values of the radar 
system and the medium-frequency passband of the receiving circuits. 
On the other hand, in the case of laser telemeters or lidar systems, the 
wavelength is very short and the Doppler frequency which is given by the 
expression F.sub.D =2V.sub.R /.lambda. becomes very high in respect of 
small variations in the radial velocity V.sub.R. By way of example, in the 
case of a lidar having a wavelength .lambda.=10.59 .mu.m, the Doppler 
frequency shift varies by 0.19 MHz each time the velocity varies by one 
meter per second. In consequence, the limits of the medium-frequency 
passband of the receiver are reached in a very short time. For example in 
the case of a passband of .+-.12.5 MHz on each side of an intermediate 
center frequency of 150 MHz, these limits are reached in respect of a 
radial velocity of the order of .+-.240 km/hr. This results in narrow 
operating limits beyond which there is a loss of information of the signal 
which is no longer processed by the receiver. In consequence, the system 
presents difficulties arising from its basic design concept. One solution 
would be to increase the bandwidth downstream of the photomixer with all 
the disadvantages attached to this solution. 
French patent Application No. 79 19 970 of Aug. 3rd, 1979 granted under No. 
FR-A-2 462 717 disclosed a system for laser telemetry and Doppler 
measurement with pulse compression, which permits measurement of both 
distance and radial velocity and is more particularly adapted to 
unambiguous discrimination of a plurality of detected targets. In this 
system, provision is made at the transmitter for two pairs of delay lines 
in which the time delays are variable as a function of the frequency and 
which have slopes +K and -K, +K' and -K' respectively, the same set of 
lines being employed in conjugate relation at the receiver. By means of 
these delay lines, two transmission pulses can be produced periodically by 
employing a different pair of lines from one period to the next and a 
reliable measurement of the distance and radial velocity of each detected 
target can be deduced from the instants of reception of echos by means of 
a simple calculation with removal of ambiguity. 
In practice, however, this solution is applicable only to targets which are 
traveling at a relatively moderate speed. 
By reason of the dispersive lines, the process employed is in fact suitable 
only within a range limited to approximately .+-.2 MHz. This corresponds 
to radial velocities which do not exceed .+-.10 m/sec, that is to say 
velocities in the vicinity of .+-.36 km/hr. This system is therefore more 
particularly applicable to surface-to-surface systems and not to 
surface-to-air or air-to-air systems in which high-speed targets may 
travel at speeds of the order of several Mach numbers. 
Moreover, French patent Application No. 82 00238, published as French Pat. 
No. 2 519 771 relates to a pulse-compression lidar which is equipped with 
means for acquisition and tracking of moving targets and retains the 
characteristics of matching and sensitivity of the receiver. These means 
involve frequency transposition of a local oscillation laser wave and 
locking of the transposed local frequency when beating by superheterodyne 
mixing with the reception light signals produces an electric signal at the 
intermediate frequency F.sub.I. After acquisition, the transposition 
frequency F.sub.T is controlled automatically in dependence on the Doppler 
shift. The target acquisition and tracking means comprise an 
acoustooptical delay-line device controlled by sawtooth signals via a 
frequency synthesizer. At the time of acquisition, the receiver initiates 
locking of the local beat frequency F.sub.L +F.sub.T, then produces 
automatic adjustment of the transposition frequency F.sub.T to the 
instantaneous value of F.sub.D (Doppler tracking). 
The major drawback of this solution lies in the length of time of the 
search and acquisition stage during which a frequency excursion is carried 
out by scanning of the synthesizer. This frequency excursion in fact makes 
it possible to cover the entire processing range of Doppler frequencies 
and the search time is therefore related to the location of the Doppler 
value to be found within this range. This search time is longer as 
coincidence takes place nearer the end of the frequency excursion. 
SUMMARY OF THE INVENTION 
The object of the invention is to overcome the disadvantages mentioned 
above by equipping the laser telemetry and Doppler measurement system with 
means for acquisition and tracking of moving targets which permit rapid 
acquisition followed by automatic compensation for the Doppler shift 
produced by the moving target, said compensation being produced 
electrically downstream of the receiving photomixer. 
In accordance with the invention, it is proposed to construct a laser 
telemetry and Doppler measurement system with pulse compression, 
comprising means for transmission and local oscillation provided with a 
laser generator for producing a first laser wave frequency-modulated on 
each side of a mean value F.sub.E, said first laser wave being intended 
for transmission, and for producing a second unmodulated laser wave having 
a frequency F.sub.L =F.sub.E -F.sub.I which is intended for 
superheterodyne reception, where F.sub.L is the local frequency and 
F.sub.I is the intermediate center frequency. The system further comprises 
a photomixer for mixing said local wave with the received light signals 
and producing a electric beat signal at the intermediate frequency, said 
signal being affected by a Doppler shift F.sub.D at the time of detection 
of a moving target. A receiver supplied by said beat signal comprises 
processing and computing circuits for measuring the distance and Doppler 
frequency of detected targets. The system also comprises means for 
acquisition and Doppler tracking of a moving target. These means are 
interposed on the electric signal path downstream of the photomixer in 
order to produce a frequency transposition from (F.sub.I =F.sub.D) to 
F.sub.I of the beat signal, where F.sub.D in the foregoing expression 
corresponds in value and in sign, at the time of acquisition proper, to 
the instantaneous Doppler shift F.sub.Do. This acquisition stage is 
immediately followed by a tracking stage during which a signal delivered 
by the receiver produces automatic control of the signal thus transposed 
to the intermediate frequency F.sub.I by compensating for the 
instantaneous variation in the difference value .DELTA.F.sub.D of the 
Doppler shift with respect to the acquisition value F.sub.Do.

DETAILED DESCRIPTION OF THE INVENTION 
In the general diagram of FIG. 1, the block 1 represents means for 
transmitting two light waves from at least one laser generator 2, a first 
laser wave having a frequency F.sub.L and frequency-modulated on each side 
of a mean value F.sub.E. This first light wave is preferably modulated in 
accordance with a linear sawtooth law of increasing then decreasing 
magnitude, which is renewed periodically. Said light wave is transmitted 
to a radiating device 3 constituted by a suitable optical system such as, 
for example, a catadioptric optical system of the Cassegrain type. The 
transmission means 1 also generate a second unmodulated laser wave having 
a frequency which is given by the expression F.sub.L =F.sub.E -F.sub.I. 
This second wave can be constituted by a fraction of the laser wave 
F.sub.L and constitutes a local wave to be applied to the mixer 4 in order 
to achieve a superheterodyne reception and to produce the intermediate 
center frequency F.sub.I by heterodyning or beating with a reception wave. 
The photomixer device 4 receives the reception light signals via an 
optical receiving system 5 which is coaxial with the optical transmission 
system 3. The beat signal S4 at the frequency F.sub.I +F.sub.D, where 
F.sub.D is the Doppler shift of the illuminated target, is transmitted to 
the receiver 6. The transmitting means 1 and receiving means 6 are so 
arranged as to produce the pulse-compression operation. 
In accordance with known techniques, the means for transmission and 
reception in a pulse-compression lidar system utilize the properties of 
dispersive delay lines. It is also known that, when the target has a 
radial velocity to which corresponds a Doppler shift F.sub.D, it is 
demonstrated that the compressed pulse is displaced in time by a value 
equal to -KF.sub.D in an algebraic value, where K is the characteristic 
constant of the dispersive delay line (K higher than zero for a modulation 
in which the frequency increases with time, K lower than zero when this is 
not the case). The Doppler-distance uncertainty is removed by successively 
transmitting two pulses, one pulse being frequency-modulated with 
increasing frequencies and the other pulse being frequency-modulated with 
decreasing frequencies. If t.sub.1 and t.sub.2 designate the instants of 
occurrence of echos relating to the lines having slopes -K and +K 
respectively (as shown in FIG. 2D), if t.sub.o is the time interval which 
elapses between the instants of transmission and reception of the same 
pulse and if T.sub.o is the time interval which elapses at the time of 
transmission between two pulses of the same pair, there can accordingly be 
deduced the following relations (zero on the time coordinate at the 
instant of the first pulse): 
EQU t.sub.1 =t.sub.o -KF.sub.D 
EQU t.sub.2 =t.sub.o +T.sub.o +KF.sub.D 
the values of t.sub.o and FD: 
##EQU1## 
from which are derived the values of the distance D.sub.o and of the 
radial velocity V.sub.R by: 
##EQU2## 
In short, D.sub.o and V.sub.R are deduced from the values of t.sub.1 and 
t.sub.2. 
Since the values of t.sub.1 and t.sub.2 are known with a degree of accuracy 
which depends on the signal-to-noise ratio, it is necessary over a long 
range to perform a post-integration with a maximum time-duration which is 
compatible with the performances of the pointing or angle-tracking device. 
In accordance with the invention, the transmission means comprise a local 
oscillator 10 and a pulse generator 11 for alternately switching the 
output of the oscillator via a switching circuit 12 to a first dispersive 
delay line 13, then to a second delay line 14, the respective slopes of 
which are +K and -K. The signals designated by the references S0, S1 and 
S2 are indicated in FIG. 2. The local signal S0 considered at the output 
of the switching circuit 12 is controlled by the two pulses S1 at each 
transmission period TR. The signal S2 is obtained after passage through 
the dispersive delay lines 13 and 14. The slope indicated represents a 
linear frequency variation (Chirp function) which is respectively 
increasing and decreasing. The video signal S3 is the signal collected at 
the output of the processing circuits 15 in which compression and video 
detection take place. 
The signal S2 is applied to an acoustooptic modulator 16 for 
frequency-modulation of the laser beam. In regard to the operation of the 
acoustooptic modulator, relevant information can be obtained from many 
technical publications. Worthy of particular mention on this subject is 
the article by Robert Adler published in the IEEE Spectrum Review, May 
1967 issue, pages 42 to 47, entitled "Interaction Between Light and 
Sound". 
The mixer 4 consists of a photodetector which receives a light signal at 
the frequency F.sub.L at one input, said signal being derived from the 
wave emitted by the laser via an optical separator, which receives the 
light signal at the frequency F.sub.E +F.sub.D at the other input, and 
which delivers the beat signal S4 at the output. 
In accordance with the invention, the system is arranged with means for 
acquisition and tracking of the Doppler frequency shift F.sub.D associated 
with the radial velocity of a moving target. These acquisition and 
tracking means are essentially constituted by means for transposing the 
frequency F.sub.I +F.sub.D of the beat wave S4 emerging from the 
photomixer 4 to the intermediate value F.sub.I in order to compensate for 
the Doppler shift F.sub.D which is present. These transposition means 
comprise the block 20 within the receiver and the switching circuit 21 and 
the connection 22 at the transmitter. During the search and Doppler 
acquisition stage, the switching circuit 21 directly transmits the local 
wave S0 via the connection 22 to the acoustooptic modulator 16 in order to 
produce a continuous transmission mode or so-called CW transmission of the 
laser wave F.sub.L which is modulated without interruption at the constant 
frequency of the oscillator 10. The block 20 is composed of a Doppler 
acquisition loop 23 provided with a spectrum analyzer which covers the 
processed Doppler-frequency band for rapidly detecting the Doppler 
frequency of the beat signal S4 with a limited degree of accuracy and for 
producing coarse compensation. This loop supplies via its output S5 a 
local voltage-controlled oscillator 24 (VCO), the output S6 of which is 
applied to a mixer 25 which also receives the signal S4. There corresponds 
to the voltage S5 a frequency F.sub.Do of the local signal S6 which is 
sufficiently close to the incident Doppler shift (which is the usual case 
since there is only a slight probability of exact correspondence), with 
the result that the output S7 of the mixer 25 is included in the reception 
band. 
From this instant, the acquisition phase is ended, the action of the loop 
is automatically blocked, its output S5 remains in the same state and it 
delivers a control signal S8 which activates the switching circuit 21 of 
the transmitter. The operation of the system then changes over to the 
tracking mode in order to carry out, with modulated-frequency transmission 
and matched reception, the compensation for end of Doppler shift 
.DELTA.F.sub.D corresponding to the difference value between the 
instantaneous Doppler frequency F.sub.D and the value F.sub.Do of 
acquisition locking-on. The corresponding means comprise a second loop 
which has the function of initiating the operation of a computation 
circuit 17 for calculating the differential Doppler value .DELTA.F.sub.D 
from the detected video signal S3, and a control circuit 26 for carrying 
out Doppler tracking by delivering to the local transposition oscillator 
24 a control voltage S9 corresponding to said instantaneous difference 
value .DELTA.F.sub.D. The computation circuit 17 normally serves to 
produce the data relating to distance D.sub.o and radial velocity V.sub.R 
(or Doppler frequency F.sub.D) of the detected target in accordance with 
the relations given earlier for the purpose of ancillary processing of 
said data in a visual display device 30, for example. Since the receiver 
is controlled in dependence on the Doppler frequency, F.sub.D represents 
(in the formulae given above) only the difference value .DELTA.F.sub.D 
between the intermediate center frequency F.sub.I of the receiver and the 
center frequency of the received signal, these values being collected 
downstream of a Doppler compensation mixer 25. In order to compute the 
real value of the Doppler frequency, it is necessary to take into account 
the frequency of the VCO 24. The output S6 of this oscillator gives said 
real value with respect to a reference frequency of the VCO (zero 
Doppler). 
The laser telemeter can be combined with an ancillary device 31 such as a 
radar system, for example, which is capable of providing beforehand a 
measurement of the Doppler frequency F.sub.D with a lower degree of 
accuracy. The signal S10 corresponding to this measurement can accordingly 
be employed in order to produce coarse locking-on as desired. To this end, 
use is made of a spectrum analyzer, the passband of which is of 
appreciably smaller width, subject to the need for a frequency 
transposition of the signal S4 derived from the local mixer 4 as will 
hereinafter be more clearly understood. 
FIG. 3 relates to one example of construction of the pulse-compression 
laser telemetry and Doppler measurement system equipped with acquisition 
and tracking means. 
The operation of the dispersive delay lines is equivalent to an 
autocorrelation and the presence of a small Doppler component results in a 
drift in time of the autocorrelation peak which can be employed for the 
measurement. At high values of shift of the Doppler frequency F.sub.D, it 
is readily demonstrated that the quality of the autocorrelation function 
which constitutes the compressed pulse undergoes rapid degradation by 
flattening of the main lobe and upward displacement of the side lobes. It 
is further apparent that drift of a power oscillator which produces the 
wave F.sub.E with respect to a local oscillator which would be employed to 
produce the wave F.sub.L would have the same effect with, in addition, the 
introduction of a systematic error in the measurement of F.sub.D. All 
these problems are solved in a laser system equipped in accordance with 
the invention, that is, in the design solution illustrated in FIG. 3. 
In this embodiment, the system comprises only one laser generator which 
constitutes a power master oscillator 40. The modulator 16 of the 
acoustooptic type is located outside the laser cavity in this 
configuration. Said modulator may be followed if necessary by a power 
amplifier 42 and is preceded by a Faraday isolator 43 which serves to 
reduce parasitic back-couplings caused by residual reflections and 
scattering within the modulator. The partially-transparent mirror 44 
serves to reflect part of the energy of the unmodulated transmission beam 
of frequency F.sub.L in order to constitute the local wave which is 
directed to the mixer 4. The modulation circuits connected as shown in the 
figure mainly comprise the following elements: a 75-MHz quartz oscillator 
10; a modulator 12A controlled by a train of two pulses of small width 
(for example of the order of 50 nsecs) at the transmission recurrence 
frequency TR; a switching device 12B for directing signals to the 
dispersive delay lines having opposite slopes ; a pulse generator 11 for 
delivering among others the distance-time measurement start pulse S20 to 
the computation circuit 17 and controlling the switching devices 12A and 
12B; a pair of dispersive delay lines 13, 14 having opposite slopes +K and 
-K and their power amplifiers 51 and 52; an amplifier chain 53 with 
limiter 54 for delivering the modulation signal to the acoustooptic 
modulator 16; and a diode switching device 21A, 21B which permits either 
continuous transmission at the intermediate frequency of 150 MHz during 
acquisition or frequency-modulation transmission. 
While tracking is in progress, the signal applied to the acoustooptic 
modulator 16 is thus linearly frequency-modulated on each side of a center 
frequency F.sub.I and alternately with two symmetrical dispersion slopes 
by employing the passive generation mode comprising the two 
electroacoustic delay lines 13 and 14. The pairs of pulses thus modulated 
are separated by a time interval which is compatible with an unambiguous 
variation in target distance. 
The receiver subassembly mainly comprises the following elements connected 
as shown in FIG. 3: a low-noise preamplifier 60 for the output signal of 
the photodetector 4, which is a wideband function; a wideband linear 
amplifier chain 61; a BLU mixer 25; a voltage-controlled oscillator 80 
(VCO) controlled from the spectrum analyzer 72 or from the differential 
Doppler signal .DELTA.F.sub.D ; a power amplifier 62; a pair of 
amplitude-weighted dispersive delay lines 63, 64 having opposite slopes; a 
summing amplifier 65 for summation of the two channels; an envelope 
detector 66; a video amplifier 67; a processing unit comprising a 
post-integration function 68; a threshold comparator 69, the threshold 
level of which establishes the probability of false alarm and detection; a 
digital module 17 for computing the distance and radial velocity; and a 
Doppler-tracking control interface 76 of the transposition oscillator 80. 
At the receiver, after optical heterodyning at 4, there is thus collected a 
medium-frequency (MF) signal which is centered on F.sub.I +F.sub.D, where 
F.sub.D is the Doppler frequency. Medium-frequency processing entails the 
use of a filtering system matched with the useful signal to be received 
and composed periodically of the two electroacoustic delay lines 63 and 64 
which are conjugate to the transmission delay lines. Two compressed pulses 
(shown in FIG. 2D) are collected on the sum channel at each repetition. 
Said pulses have a width of the order of 1/.DELTA.F (.DELTA.F=modulation 
bandwidth) and are displaced by -KF.sub.D with respect to the position 
occupied by these pulses in the case of a stationary target (K=dispersion 
slope in seconds per Hertz). 
The signal then undergoes an envelope detection followed by a 
post-integration process. After digital computation, the values of the 
distance D.sub.o and of the radial velocity V.sub.R of the target are 
displayed. 
Since the Doppler bandwidth of the receiver is limited to a few MHz, a 
frequency transposition is carried out at 25 prior to MF amplification. 
After determination of the Doppler frequency F.sub.D, this permits very 
rapid recentering of the received signal spectrum on the operating center 
frequency F.sub.I of the receiver (in accordance with the Doppler 
acquisition process). For the purpose of Doppler measurement, the laser 
beam undergoes a fixed-frequency translation and the form factor can 
attain 100%. 
The chief elements employed in the Doppler acquisition chain are the 
following: a band-stop filter 70 centered on the frequency F.sub.I (case 
of zero Doppler); a wideband linear amplifier 71 for adapting the signal 
derived from the detection to the spectrum analyzer; a high-speed spectrum 
analyzer 72 for measuring the Doppler frequency within a band .DELTA.F 
with a resolution of the order of I/T, where T is the portion of the 
continuous signal analyzed, this resolution being adapted on the one hand 
to the stability of the laser source and on the other hand to the Doppler 
selectivity band of the distance receiver; high-rate digital conversion 
circuits 73 for quantizing the analog output of the spectrum analyzer for 
processing in a digital and management processing unit 74; a time base 75 
for initiating the operating cycles of the spectrum analyzer as well as 
sampling the analog signal derived from the frequency analyzer; and 
circuits for analog conversion of the control voltage V.sub.D 
corresponding to the detected Doppler shift F.sub.Do. 
In the system under consideration, the Doppler frequency of the received 
signal can vary within the range of -20 MHz to +240 MHz, for example. In 
order to ensure that the equivalent band of the reception matched filter 
is equal to that of the distance acquisition modulation signal, the 
Doppler selectivity of the distance receiver must be limited to a few MHz. 
The use of a high-speed spectrum analyzer permits acquisition of the 
Doppler frequency of the received signal before it is possible to employ 
the distance acquisition chain with acceptable time intervals in the case 
of an air-to-air system. 
Considering that an ancillary radar 31 cooperates and provides a 
predetermination of the Doppler frequency to within approximately 10 MHz, 
for example, and that a frequency translator 78-79 controlled by the 
management unit 74 is placed upstream of the spectrum analyzer 72, it is 
possible to limit the bandwidth of this latter with a resolution which is 
compatible with the Doppler selectivity of the distance receiver. The 
signal S10 delivered by the radar is transmitted to the management unit 74 
which generates a corresponding control signal S11 to be applied to the 
voltage-controlled oscillator (VCO) 78. The output of this VCO is applied 
to the mixer 79 conjointly with the signal S4 produced by the filter 70 
The practical application of heterodyne detection makes it possible to 
obtain a signal at a frequency containing the Doppler velocity 
information. In the case of the system proposed, it is considered that the 
selected intermediate frequency is 150 MHz. In the case of Doppler 
frequencies varying between -20 MHz and +240 MHz (relative target 
velocities within the range of 100 m/sec to 1200 m/sec), the frequency of 
the received signal will vary between 130 and 390 MHz. 
Frequency analysis of the signal is performed by means of a spectrum 
analyzer 72 of the electroacoustic type which constitutes a known means 
and the principle of operation of which is recalled hereinafter with 
reference to FIGS. 4 and 5. 
The electroacoustic spectrum analyzer makes it possible to obtain, 
practically in real time, the Fourier transform of a time-interval portion 
T of a signal which can be continuous at the input. 
Frequency resolution of a system of this type is limited by the time 
interval T during which the signal undergoes physical analysis. This time 
interval is equal to 1/T, which corresponds to the width of the Fourier 
transform of a monochromatic signal having a time-duration T. The 
frequency scale is converted linearly to a time scale and the spectral 
band B under analysis (B being dependent on the characteristics of the 
analyzer) extends at the output over a time interval T. 
The operation of an analyzer of this type makes use of dispersive delay 
lines (of the type shown in FIG. 4). These delay lines have the property 
of delaying the different spectral components of a signal by different 
lengths of time. This is achieved by propagation of surface acoustic waves 
on a monocrystalline substrate 83 on which are etched two semi-reflecting 
arrays of grooves 84 and 85 having a continuously variable pitch. 
The acoustic waves SR are obtained from the input signal SE by means of an 
input transducer 86 and propagate on the substrate along the array of 
grooves. 
A spectral component of the acoustic wave will undergo a 90.degree. 
reflection from the top array only at the point at which its wavelength 
corresponds to the pitch of the array (point B in the diagram). 
Constructive interference is in fact produced by the waves reflected by 
those grooves of the array which are adjacent to the point B. 
The same will apply to said spectral component at the point C at which this 
component is reflected from said second array to the output transducer 87 
and is reconverted by this latter to an electric signal SR. 
A spectral component at a different frequency will follow another path such 
as, for example, the path AB' C' D having a different length, which 
accordingly entails a different transit time (since the propagation 
velocity of acoustic waves is not very high). 
An effective time separation has thus been achieved between the different 
spectral components. 
In its most simple design, the analyzer comprises two dispersive delay 
lines 91-92 and a multiplier 90 as illustrated schematically in FIG. 5. 
The multiplier 90 forms the product of the input signal S21 times a 
linearly frequency-modulated signal S22 of duration T or so-called Chirp 
signal obtained as response to a Dirac pulse S23 applied to the first 
delay line 91. 
The signal S24 thus obtained drives the second delay line 92 which produces 
signal convolution with a Chirp function. The output S25 directly supplies 
the Fourier transform of the input signal, that is, to within the nearest 
phase factor but this is unessential when consideration is given solely to 
the amplitude of the Fourier transform as in this instance. 
The frequency resolution is 1/T in the frequency scale. This corresponds to 
a physical time duration at the output of the analyzer of 1/B in the time 
scale, where B is the band being scanned. 
The time reference for the output is given by the Dirac pulse which drives 
the first delay line. 
The received signal of duration T.sub.1 is sampled by the spectrum analyzer 
at a period T. At each interval T, the spectrum analyzer delivers a signal 
which is characteristic of its frequency. Each signal can then be 
converted to digital form in the conversion circuit 73 and stored in the 
unit 74 (shown in FIG. 3). 
If the ratio T1/T=K is higher than 1, K signals are stored and can undergo 
incoherent addition within the storage unit. This method permits 
post-integration at 74, thereby producing a very substantial improvement 
in the signal-to-noise ratio and therefore in the range of the device. 
The spectrum analyzer is characterized by the frequency band B to be 
analyzed and the frequency resolution F=1/T, where T is the time of 
analysis of the spectrum. 
The product B.multidot.T is also characteristic of the analyzer since it 
represents the number of analyzable points in a frequency band B. 
The laser telemetry and Doppler measurement system described in the 
foregoing extends to alternative embodiments in accordance with the 
distinctive features disclosed and included within the scope of the 
invention. In particular, consideration can be given to the use of two 
lasers, namely a power laser for producing the frequency-modulated wave at 
the frequency F.sub.E and a second laser for constituting a local 
oscillator and delivering the unmodulated wave at the frequency F.sub.L. 
The power laser can produce either internal or external modulation. In the 
case of external modulation, the parasitic transmission/reception coupling 
arising from residual reflections and scattering at the level of the 
modulator is attenuated to the maximum extent, subject to a disadvantage 
arising from the need to employ a second laser and a frequency control 
loop in order to maintain the difference F.sub.E -F.sub.L =F.sub.I. This 
loop and likewise the control circuit 26 employed for Doppler tracking can 
be formed by connecting in series a preamplifier, a limiter amplifier, a 
frequency discriminator and a filtering and matching network (or 
integrator) which delivers the fine control for frequency locking of the 
element to be controlled (power laser or local transposition oscillator 
24).