Passive radar responder

The invention concerns beacons associated to radars. It consists in modulating the re-emitted signal, in phase by the frequency of a vocal signal, and in amplitude by the envelope of this signal. It allows vocal transmission between such a signal and a Doppler radar.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention concerns passive radar responders or answering 
equipment that allow to send back towards a radar, especially a Doppler 
radar, a much wider fraction of the signals emitted by said radar than 
that due to natural reflection on the most reflecting objects. 
2. Description of the Prior Art 
It is known to utilize as a passive reflector a rectangular trihedron of 
the "cube corner" type, or a Luneburg lens. 
In a more complex manner, it is known to use a network of dipoles connected 
together to be in phase and connected to a short-circuit. The radar signal 
received is re-emitted at the pace of this manipulation in a direction 
determined by the antenna or aerial equivalent to the dipoles network. 
This network can be constituted for very short frequencies by a simple 
triplate circuit on which the dipoles are photoengraved. Thereby in band X 
with 8.times.8 dipoles disposed on a plate of about 30.times.30 cm.sup.2, 
a principal response lobe having an opening of 8.degree. is obtained. Such 
directivity is particularly useful when it is required to localize a point 
with discretion. 
When the fixed echoes are eliminated, especially by using the Doppler 
effect, it is necessary to modulate the re-emission in order to simulate 
movement of the responder. 
SUMMARY OF THE INVENTION 
In order to achieve this aim a system similar to that illustrated 
schematically in FIG. 1 is used. 
The wave received from the radar by the aerial 11 is reflected by a 
short-circuit 12 towards this same aerial through a controllable phase 
shifter 13. The wave is then returned by the aerial towards the radar with 
a phase that is dependent upon the state of the phase shifter. This state 
is controlled by a control circuit 14 that operates from a reference 
oscillator 15. 
The phase shifter 13, is, for example, of the type represented in FIG. 2. 
The connection with the aerial 11 is made through the intermediary of a 
guide element 21. This is provided with two guide portions 22 of length 1 
and 23 of length L that are at right angles to it and are connected to the 
ground respectively by two diodes 24 and 25 connected in opposite 
directions. The junction of these three guides is connected to the output 
of an amplifier 26 that is mounted peak limiter-wise so as to supply an 
output signal capable of assuming one of two values +V and -V allowing to 
saturate respectively the two diodes. To achieve this result, the 
amplifier receives on its input E, for example, a binary signal at the 
frequency Fo, of which one of the states corresponds to +V and the other 
to -V. 
Under these conditions, when a diode is saturated, the corresponding guide 
is short-circuited at the ground, and it is disconnected in the opposite 
direction. Therefore, the phase shift of the re-emitted ultra high 
frequency (UHF) wave re-emitted between these two states is equal to 2.pi. 
(L-1)/.lambda., .lambda. being the length of the hyperfrequency wave. The 
maximum signal detected by the Doppler effect in the radar will be 
obtained for a phase shift equal to .pi., that therefore corresponds to 
L-1=.lambda./2. In fact, as represented in the top part of FIG. 3, after 
discrimination the obtained signal thus comprises impulses at recurrence 
frequency F.sub.R, of maximal amplitude and reversed polarities at 
frequency F.sub.o. The output Doppler signal will thus be, as represented 
at the bottom of this FIG. 3, a square signal of frequency F.sub.o that 
reproduces the modulation signal at input E of the phase shift 13. 
It is easy to take for this frequency F.sub.o an audible value, 
particularly easy to exploit. This value will thus be that supplied by 
generator 15. 
In order to individualize further the responder, this audible frequency can 
be hatched, for example, through the use of logic circuits contained in 
the modulator 14 and also operating from the generator 15, in order to 
represent a particular signal. FIG. 4 represents, for example, a 
modulation extending over 2.75 seconds and corresponding to the letter A 
in Morse Code. On this figure, the hatched parts correspond to the audible 
frequency, and the others to silences during which the phase shifter 13 is 
in a fixed state. 
Such a beacon thus allows to indicate a particular point. It is desirable 
that an operator using the beacon be able to transmit messages that are 
non-determined in advance. To achieve this aim can be used, for example, a 
Morse key as switch between the generator 15 and the phase shifter 13. 
Such a process is long and difficult to operate. 
In order to transmit especially vocal messages, the invention proposes to 
modulate the re-emitted signal, in phase by the frequency of the vocal 
signal and in amplitude by its envelope.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
To transmit vocal data with a passive responder the diagram represented in 
FIG. 1 is modified in order to achieve the diagram represented in FIG. 5. 
The basic system always comprises aerial 11, the short-circuit 12, the 
phase shifter 13, the modulator 14 and the generator 15. 
The vocal data are applied to a microphone 16 that supplies a signal, which 
is thereafter filtered by a low pass filter 17. The cut-off frequency of 
this filter is lower than F.sub.R /2 in order to prevent any folding 
problems of the spectrum. This presumes that the repetition frequency 
F.sub.r of the radar is sufficiently high to maintain good intelligibility 
of speech. This is generally the case. 
The signal S at the output of the filter is represented on the first line 
of FIG. 6. It can be described as an alternating signal of variable period 
and amplitude. The amplitude variation is low compared to the period and 
thus determines an envelope that approximately resembles an amplitude 
modulation. 
This signal S is first of all applied to the modulator 14 in which it is 
clipped in order to obtain a rectangular control signal of the phase 
shifter 13. This clipped signal S becomes a signal F represented in the 
second line of FIG. 6. The transitions of F corresponds to the passages at 
zero of S, and F is thus similar to a PDM type modulated signal. 
The signal re-emitted by the responder is thus modulated in phase by the 
signal F and the radar reconstitutes this latter by demodulation of the 
re-emitted signal. Such a signal could be intelligible, but with 
difficulty and with poor sonority. 
In order to improve this intelligibility, the amplitude of S is furthermore 
transmitted with amplitude modulation of the signal retransmitted by the 
responder. 
To maintain simplicity of this responder, a real modulation of the 
retransmitted signal is not carried out, but rather a simple attenuation 
approximately proportional to the envelope of S. 
This amplitude A is represented in the third line of FIG. 6 and can be 
obtained very simply by an envelope detector 18. 
To modulate the re-emission signal is inserted between the dephaser 13 and 
the aerial 11 a variable attenuator 19 of which the attenuation is an 
inverse function, substantially linear, of the control signal; this latter 
being the signal A supplied by the detector 18. Such an attenuator can be 
formed by a diode controlled in current and of which the UHF attenuation 
curve is represented by the graphic of FIG. 7, in which it will be 
observed that this attenuation D is an approximately reverse function of 
the direct current I. 
In function of the signal A, the attenuation D of the re-transmitted signal 
is represented by the last line of FIG. 6 where it is seen that it 
reproduces fairly accurately the inverse of A with a dynamic of 70 dB. 
Since it involves an attenuation, the retransmitted signal will have an 
envelope substantially represented by A. 
At the reception in the radar, it will thus only be necessary to extract 
the amplitude of the signal received, available for example, on an AGC 
output, and to modulate in amplitude the signal F, reconstituted for 
example, with a variable gain amplifier, in order to obtain a signal that 
reproduces the vocal signal S with a very sufficient fidelity. 
In a variant of the invention, is used, furthermore, a generator 15 of 
square fixed frequency signal that allows to modulate in a constant manner 
the re-emission of the responder apart from the moments where the micro 16 
is used. 
In another variant a receiver 20 is also used in order to determine if an 
emission of the radar is well received; this is a way to overcome speaking 
in the micro in pure loss. 
This receiver can be a simple rectifier diode supplying an alarm signal. It 
can also be more complex and comprise, for example, a decoder allowing to 
decode a coded interrogation emitted by the radar. A more elaborate 
version comprises a receiver similar to that utilized in the radar in 
order to demodulate the re-emission of the responder. In this case, the 
radar can emit an emission modulated in the same manner as in the 
responder, which allows a bi-directional vocal link between the radar and 
the responder. Only very simple material will be used in this latter and 
only the energy supplied by the radar is used for the transmission in both 
directions.