Title of the invention: Method for detecting by radar the distance and relative velocity
of an echo with respect to the carrier of the radar, and radar implementing such a
method.
[0001] The invention relates to the field of airborne radars and especially of
helicopter-borne radars, for example for assisting with landing. In this technical field,
the invention more particularly relates to linear-frequency-modulation continuouswave (LFM/CW) radars allowing the velocities of the carrier along three axes, and
distances in radar beams, to be measured.
[0002] A LFM/CW radar must necessarily work in a wide frequency band to obtain the
best possible distance resolution, this resolution r being given by the relationship r =
c / 2.B where c and B are the speed of the transmitted wave and the bandwidth,
respectively.
[0003] A transposition between the transmitted wave and the received wave is
necessary to convert the delay between the transmitted wave and the received wave
into a signal the frequency of which is proportional to this delay, and therefore to the
distance. This transposition technique is known as “deramping”. This expression will
be used below.
[0004] In addition to distance, it is necessary to measure velocity. To measure
velocity via the Doppler effect, the radar must be equipped with a function for
detecting amplitude and phase to form two I and Q channels allowing the sign of the
Doppler shift to be preserved when the signal is transposed to baseband. The
importance of the preservation of this sign is greater if the applications in question
(assistance with aircraft landing in particular) may involve small distances and low
velocities, able to randomly alternate positively or negatively about zero. This
detecting function will be referred to as the APD below (APD being the acronym of
amplitude/phase detector).
[0005] One problem inherent to this type of solution is that it is very difficult to
preserve a quadrature between and to balance the gains of the two channels of a
wideband microwave APD. It is therefore necessary to perform carrier coding to
achieve a digital APD. [0006] However, carrier coding requires the transmitted band to be coded and
therefore a high number of samples to be processed. A more general problem is
therefore that of obtaining these distance- and velocity-measuring capabilities, while
preserving a good selectivity (which cannot be that of the transmitted band), with a
digital APD requiring only a low number of samples.
[0007] The solutions of the prior art do not allow this objective to be achieved.
Conventionally, deramping is carried out from the first transposition, but this does not
allow the sign of the Doppler shift to be preserved, this possibly being useful in
air/ground applications that involve only aircraft approaches
[0008] Navigation radars for example process only Doppler shifts and do not carry out
deramping, since there is no need to measure distances. In this case, an APD is
achieved directly at microwave frequencies. One solution allowing both types of
measurements (distance and velocity) to be addressed would be to code the entire
band received about an intermediate-frequency (IF) signal and to perform the APD
function and deramping digitally. However, this solution would require digital
processing of a high number of samples (spanning at least two times the received
band) and, in addition, this solution decreases reception selectivity since the IF filter
must pass at least the received band. Specifically, receiver selectivity is a criterion
that is important to correct operation, because it allows the receiver to be protected
from the out-of-band electromagnetic environment.
[0009] By way of example, to obtain a resolution of one metre, it is necessary to
transmit in a band of 150 MHz, this involving processing the signal at at least 300
mega-samples per second, whereas, if the deramping is carried out from the first
mixing operation, the useful band to be processed is decreased to a few tens of kHz
depending on the distance and the frequency-modulation ramp transmitted K = B / TE ,
B being the transmitted or received band and TE being the transmission time of the
frequency ramp.
[0010] Other solutions employ FFT processing operations that allow information on
distance and velocity to be acquired using a homodyne reception mode (without
intermediate frequency), but the processing operations are more computationally
expensive. [0011] One aim of the invention is especially to address the aforementioned general
problem, namely that of obtaining these distance- and velocity-measuring capabilities,
while preserving a good selectivity, with a digital APD requiring only a low number of
samples. To this end, one subject of the invention is a method for detecting by radar
the distance and relative velocity of an echo with respect to the carrier of the radar,
comprising a transmitting step, a receiving step and a processing step that are such
that:
- in said transmitting step, at least one local transmission wave OLE is
generated with a periodic frequency ramp, said wave being intended to be
transmitted;
- in said receiving step, a local reception wave OLR is generated, said wave
OLR being a replica of said local transmission wave OLE transposed to an
intermediate frequency IF, said local reception wave OLR being mixed with each
received signal SR, the mixed signal being filtered to give a signal the frequency
component of which is the frequency difference ΔF between the wave OLR and the
received signal SR;
- said frequency difference ΔF being digitized, the processing step measures
said velocity on the basis of information contained in said frequency difference.
[0012] In one particular mode of implementation, in said transmitting step, a second
local transmission wave OLE1 is generated and transmitted with a periodic frequency
ramp of different slope to the slope of the wave OLE, a second local reception wave
OLR1 being generated via replication of said wave OLE1 and transposition to said
intermediate frequency IF, said second wave OLR1 being mixed with said received
signal SR, the mixed signal being filtered to give a signal the frequency component of
which is the frequency difference ΔF1 between the wave OLR1 and the received
signal SR, said frequency difference ΔF1 being digitized, the processing step
measures said distance and said velocity on the basis of information contained in the
frequency difference ΔF1 and the frequency difference ΔF.
[0013] Said method is for example applied to assist with landing said carrier, said
carrier for example being an aircraft having the ability to hover, a helicopter for
example.
[0014] Another subject of the invention is a radar implementing such a method. Said
radar for example comprises:
- a first waveform generator that generates said local transmission wave; an antenna intended to transmit said local transmission wave;
- a microwave mixer;
- a circulator that guides said local transmission wave from said first waveform
generator to said antenna, and that guides a signal received from said antenna to
said mixer;
- a second waveform generator that generates said local reception wave
intended for said mixer with a view to mixing said local reception wave with said
received signal;
- a filter that filters said intermediate frequency IF from the mixed signal output
from said mixer;
- a receiver that receives the signal output from said filter;
- an analogue-digital converter that converts said signal output from said
receiver;
- processing means;
- a synchronizing system that synchronizes said waveform generators.
[0015] Other features and advantages of the invention will become apparent with the
aid of the description which follows, made in relation to the appended drawings which
show:
[0016] [Fig.1] Figure 1, an illustration of an overview of the principle of transmission
and reception according to the invention;
[0017] [Fig.2] Figure 2, an illustration of the waveform of a local transmission wave
used by the invention;
[0018] [Fig.3] Figure 3, an illustration of the waveform of a corresponding received
wave;
[0019] [Fig.4] Figure 4, an illustration of a band of frequencies occupied by fixed
echos and the corresponding aliased frequency band.
[0020] Figure 1 illustrates the principle of transmission and reception according to the
invention. This overview shows an example of a system implementing the
transmitting step, the receiving step and the processing step.
To obtain:
- preservation of the sign of the Doppler shift; optimization of selectivity; and
- limitation of the number of samples to be processed,
the invention deramps an intermediate-frequency IF carrier and implements a digital
APD using, in the transmitting step, a local transmission wave (OLE). This local
transmission wave is produced by a first waveform generator 1 (WFG). It is
transmitted to the transmitting and receiving antenna 2 via a circulator 3. The wave
OLE is then radiated by the antenna 2 with the following characteristics:
=

0,

()
In this relationship, K is the slope B/TE and t time.
[0021] Figure 2 illustrates the waveform of the local transmission wave OLE, which
varies with the periodic frequency ramp 21 (value K) as a function of time t, and
which is repeated periodically every TE. In this illustration, the frequency OLE varies
between 0 and B (bandwidth) with the ramp 21, the carrier frequency having been
subtracted for the purpose of making illustration easier. The value of TE may be
about 150 ms for example.
[0022] With reference once again to figure 1, a return signal SR is received with a
delay τ proportional to the distance d of the echo:
τ = 2.d / c
The received signal also exhibits a frequency shift FD related to the Doppler effect:
FD = 2V / λ
V being the relative radial velocity between the beam and the target, which is the
ground in the application of assistance with landing for example.
The following is the relationship between this radial velocity and the velocity of the
carrier:
Vp = α V where α is a trigonometric coefficient related to the projection of the vector
Vp of the velocity of the carrier onto the axis of irradiation of the antenna 2.
[0023] Figure 3 illustrates the waveform of the received signal SR, the principle of the
illustration being the same as that of figure 2. The modulation of the received wave SR follows a periodic frequency ramp 31 that is shifted in time by τ and shifted in
frequency by FD with respect to the transmission ramp 21 of the wave OLE.
[0024] The received signal SR respects the following relationship:
= ( − ) ± = ( − ) ±
; +
(#)
It will be noted that the minimum measurable frequency is equal to 1/TE.
In relationship (2), as in the other relationships, the signal is expressed in terms of
frequency, independently of amplitude.
[0025] With reference once again to figure 1, the receiving step will now be described
in more detail. A local reception second wave is generated by a second waveform
generator 4. This local reception wave (OLR) is used as intermediate frequency to
transpose the received signal. To this end, it is combined, in a conventional way, with
the signal SR output from the circulator 3, by a microwave mixer 5.
[0026] Figure 4 illustrates the waveform of the local reception wave OLR, which has
been represented by a time-dependent periodic frequency ramp 41. This OLR is a
replica of the transmitted signal OLE, representing the first ramp 21 transposed to a
frequency IF with a view to deramping the carrier wave at this frequency IF. As
illustrated in figure 1, this signal OLR is mixed with the signal SR. It is then filtered by
a filter 6 in order to preserve only the frequency component ΔF corresponding to the
frequency difference between the local reception wave OLR and the received signal
SR, i.e., with reference to figure 4:
ΔF = OLR - SR
the frequency difference expressing the fact that, at any given time, the frequency ΔF
is the difference between the frequency of the local reception signal OLR and the
frequency of the received signal SR.
[0027] Written otherwise:
∆ = . ± + &
0, + '()
(*)
τmax being the maximum delay generated by the measurement of the maximum
distance [0028] The IF (intermediate frequency) frequency band depends on the maximum
velocity and maximum distance to be measured, and on the slope K of the
modulation ramp of the transmitted signal. It is therefore chosen accordingly.
[0029] Relationship (3) demonstrates that ΔF contains Doppler information, i.e.
information on FD, and therefore velocity information, the distance being obtained
from the delay , which is moreover measured.
[0030] By way of example, the case of a helicopter having a maximum flight altitude
of 4500 metres and a maximum velocity of 125 m/s, corresponding to a Doppler
frequency FD equal to 10 kHz, may be taken. With a specification requiring a
measurement of a minimum frequency of 10 Hz and a distance resolution of 1 metre,
the band B of variation in the ramp of the wave OLE is B = 150 MHz. It then follows:
- as regards the ramp time, TE = 0.1 second;
- i.e. a ramp K = 1.5 GHz/s.
[0031] The maximum delay corresponding to the maximum distance of 4500 metres
is τmax = 30 µs, corresponding to a received signal SR such that:
'() = 1.5 -
∗ 30 01 + 10
2 = 55 345 (4)
[0032] With reference once again to figure 1, the filter 6 is followed by a receiver 7
(reception-end amplifier), which receives the signal ΔF as input. The latter therefore
has, in the present example, a 55 kHz band centred on the frequency IF. In known
techniques, the filter 6 at the output of the mixer 5 takes account of these
characteristics. The receiver 7 is a conventional receiver the function of which is in
particular to scale the signal ΔF to the scale of the analogue-digital converter 8, i.e. to
the scale able to be converted thereby.
[0033] Specifically, the filter 7 is followed by the converter 8, which converts ΔF into a
digital signal. The signal ΔF is coded at the rate of a clock Hcod the frequency of
which is very much higher than the frequencies IF and ΔFmax (maximum value of the
frequency ΔF) - more particularly Hcod >> 2.(IF + ΔFmax).
[0034] The digitally converted frequency-component signal ΔF is transmitted to digital
processing means 9. The latter then carry out conventional time-domain digital
processing allowing two digital-data channels to be obtained for the baseband I and
Q components from the digitized signal ΔF, after amplitude and phase detection, digital filtering and decimation, the processing means 9 implementing the APD
function in a conventional manner.
[0035] It will be noted that the signal at the frequency ΔF contains a velocity and
distance component (see relationship (3)). It may therefore be necessary to
discriminate between these two elements if the delay giving the distance has not
been exploited. This discrimination is for example achieved by transmitting another
signal with a second slope, in order to obtain a system of two equations with two
unknowns for ΔF and ΔF1. To this end, with respect to the first local transmission
wave, the first slope may be OLE = Kt, and, with respect to the second local
transmission wave, the second slope may then be OLE1 = -Kt + B. It is then possible
to extract the velocity and distance from the system of equations by solving ΔF and
ΔF1. The principle of the discrimination between distance and velocity is especially
described in patent application FR 1800838.
[0036] The frequency difference ΔF1 is obtained in the same way as the frequency
difference ΔF, on the basis of the local transmission wave OLE1, then of the local
reception wave OLR1, which is obtained via transposition to the frequency IF, as
illustrated in figure 4, and by mixing 5 and filtering 6 as for ΔF.
[0037] A synchronizing system 10 generates a synchronization signal intended for the
two waveform generators 1, 4 and for the analogue-digital converter 8 (the clock Hcod
then being synchronous with the synchronization signal). All the signals generated by
the WFGs 1, 4 are then coherent in phase.
[0038] The overview of figure 1 is given by way of example - other arrangements are
possible provided that they allow the method according to the invention to be
implemented. In particular, a plurality of wave-generating modes are possible.
[0039] The method advantageously optimises the architecture of the radar by
allowing a frequency component related to distances and another related to velocities
to be measured, while guaranteeing a very high selectivity and preservation of the
Doppler sign with a very high level of rejection of image frequencies, while limiting
the flow of data to be processed.
[0040] The invention is especially applicable to carriers such as helicopters; however,
it is advantageously applicable to any aircraft able to hover.

CLAIMS
1. Method for detecting by radar the distance and relative velocity of an echo with
respect to the carrier of the radar, comprising a transmitting step, a receiving step
and a processing step, characterized in that:
- in said transmitting step, at least one local transmission wave OLE is
generated (1) with a periodic frequency ramp (21), said wave being intended to be
transmitted;
- in said receiving step, a local reception wave OLR (41) is generated (4), said
wave OLR being a replica of said local transmission wave OLE transposed to an
intermediate frequency IF, said local reception wave OLR being mixed (5) with each
received signal SR, the mixed signal being filtered (6) to give a signal the frequency
component of which is the frequency difference ΔF between the wave OLR and the
received signal SR;
- said frequency difference ΔF being digitized, the processing step measures
said velocity on the basis of information contained in said frequency difference.
2. Detecting method according to Claim 1, characterized in that, in said
transmitting step, a second local transmission wave OLE1 is generated and
transmitted with a periodic frequency ramp of different slope to the slope of the wave
OLE, a second local reception wave OLR1 being generated via replication of said
wave OLE1 and transposition to said intermediate frequency IF, said second wave
OLR1 being mixed (5) with said received signal SR, the mixed-signal being filtered (6)
to give a signal the frequency component of which is the frequency difference ΔF1
between the wave OLR1 and the received signal SR;
said frequency difference ΔF1 being digitized, the processing step measures said
distance and said velocity on the basis of information contained in the frequency
difference ΔF1 and the frequency difference ΔF.
3. Method according to any one of the preceding claims, characterized in that it is
applied to assist with landing said carrier.
4. Method according to Claim 3, characterized in that said carrier is an aircraft
having the ability to hover.
5. Method according to Claim 4, characterized in that said aircraft is a helicopter.
6. Radar, characterized in that it is able to implement the method according to
any one of the preceding claims.
7. Radar according to Claim 6, characterized in that it comprises:
- a first waveform generator (1) that generates said local transmission wave;
- an antenna (2) intended to transmit said local transmission wave; - a microwave mixer (5);
- a circulator (3) that guides said local transmission wave from said first
waveform generator (1) to said antenna, and that guides a signal received from said
antenna to said mixer;
- a second waveform generator (4) that generates said local reception wave
intended for said mixer (5) with a view to mixing said local reception wave with said
received signal;
- a filter (6) that filters said intermediate frequency IF from the mixed signal
output from said mixer;
- a receiver (7) that receives the signal output from said filter;
- an analogue-digital converter (8) that converts said signal output from said
receiver;
- processing means (10);
- a synchronizing system (10) that synchronizes said waveform generators (1,
4).