CLAIMS
1. Method for determining in real time the distance, d, from the ground of an aircraft by radar measurements, the said distance d being determined, for a span of distances extending up to a given distance Umax, with a given constant resolution Rdo, characterized in that it implements the following operations:
- the emission of at least two successive frequency ramps and the reception of the signals reflected by the ground subsequent to these emissions;
- the construction, on the basis of the reflected signals received of two signals Eo(t) and Ei(t) each corresponding to the reflected signal ro(t) or ri(i) originating from the successive emissions of two waveforms eo(t) and ei(t), after demodulation of the signals ro(t) and n(t) by the corresponding wave eo(t) or ei(t) and sampling of the demodulated signal at a given frequency Fech,
the said waveforms eo(t) and ei(t) being represented in a "time-frequency" space by two frequency ramps exhibiting respective slopes of variation Ko and Ki, on two frequency bands Bo and Bi whose ratio BilBo is a given integer number, the band Bo being determined as a function of the distance resolution Rdo by the relation:
B° = 72Rd0' the slope Ki being determined as a function of the distance dmax by the relation:
TV- £ . Fech
2 2a-dimax
the slopes Ko and Ki being respectively given by the relations
K0 = B°/T and K± = 5I/T
' 1E0 lEl
where TE0 and TE1 represent the respective durations of emission of the waveforms e0(t) and e^t);
- the application of an FFT, on TVsamples, to the signals Eo(t) and Ei(t)
and determination of the spectral components Eo and Ei corresponding to
the said signals, Zib corresponding to the component of lowest frequency
of the spectrum of Eo(t)
- the determination, on the basis of the spectral components Eo and Eit
of the distances measured do and d\ by means of the waveforms eo(t)
and ei(t) corresponding respectively to the spectral components Eo and
Ei, doand di being given by the relations:
d0 = -■ —- and dx=---L-
u 2 CCK0 1 2 aK-L
- the determination, on the basis of do and di, of the distance dto be
measured, dbe\ng given by the relation:
d = M ■ d0max + d0 with d0max = -■ ^^ = c-.hcK_
untax 2 aRo 2 2aKo
and M = INT (-^-) = INT(2 ■ E1 ■ T6ch).
2. Method according to Claim 1, characterized in that it comprises the following steps:
- a first step (31) during which are determined the slopes Ko and Ki,
and frequency bands Bo and Bi characterizing the two signals eo(t) and
ei(t) as well as the durations of emission Tm and TEi of these two
signals; this first step being carried out while taking into account the
following parameters (35):
- maximum distance measured: dmax,
- distance resolution required: Rdo,
- frequency of sampling of the signals received by the radar: Fech,
- number N of points on which the spectral decomposition of the signals must be performed;
- a second step (32) of emission during which:
- the radar emits at least two waves corresponding a chosen type of emission (36), at least one of these waves consisting of a frequency ramp of band Bo and of slope Ko,
- the radar receives the reflected signals resulting from the reflection by the ground of the emitted waves;
- the signals Eo(t) and Ei(t) are formed on the basis of the reflected signals received;

- a third step (33) during which are carried out the spectral decomposition by FFT on Appoints of the signals Eo(t) and Ei(t) and the determination of the spectral components Eo and Ei,
- a fourth step (34) during which is carried out the calculation of the distance don the basis of E0 and Ex.
3. Method according to Claim 2, characterized in that the fourth step 34
comprises:
- a first sub-step (341) of calculating the distance di defined by:
d1=-- —*- where Gi represents the integer part of the spectrum of El(t)
2 CcKi
- a second sub-step (342) of calculating the scale factor M defined by:M = /ivr(i£L);
- a third sub-step (343) of calculating the distance do defined by: d0 = --—2-where Ho represents the integer part of the value of the first
2 CCKQ
spectral line, Eo, of the spectrum of EO(t)- a fourth sub-step (344) of calculating the distance d, defined by:
d = M ■ d0max + a0.
4. Method according to one of Claims 2 or 3, characterized in that the waveforms emitted by the radar in the course of the second step (32) of the method consist of a first frequency ramp eo(t) (61) of band Bo and of slope Ko followed by a second frequency ramp ei(t) (62) of band Bi and of slope Ki.
5. Method according to Claim 4, characterized in that the signal eo(t) and the signal ei(t) have distinct durations of respective emission TEo and TEI and occupy one and the same frequency band B, so that their respective slopes Ko and Ki are expressed by:
KQ= — and K-y = —.
TE0 TE1
6. Method according to one of Claims 4 or 5, characterized in that the
signals Eo(t) and Ei(t) are obtained after demodulation of the signals
received ro(t) and n(t) by the signals emitted eo(t) (61) and ei(t) (62),

the signals Eo(t) and Ei(t) being thereafter sampled in the course of the third step (33) at the frequency Fech so as to obtain the signals £"o(t) and E{(i), the FFT spectral analysis of which is carried out.
7. Method according to one of Claims 2 or 3, characterized in that the waveforms emitted by the radar in the course of the second step (32) of the method consist of a recurrent string of TV identical frequency ramps eo(t-xftm) (71), mutually shifted in time by a duration TEo equal to the emission time of a ramp, of band #and of duration TEo.
8. Method according to Claim 7, characterized in that for each of the N frequency ramps emitted (71), the signal Eo(t) obtained by demodulation of the signal received by the signal emitted is sampled to obtain the corresponding signal £0*(t), and then a signal E^(t) is constructed by considering a sample of the signal £"o(t) formed for each of the TV ramps emitted, the signal E^(t) consisting of the TV samples thus tapped off.
9. Method according to Claim 8, characterized in that for the nth frequency ramp e0(t) emitted the signal received ro(t) is demodulated by applying a phase shift equal ton- —to the local oscillator of the radar
receiver.
10. Method according to one of Claims 8 or 9, characterized in that, for
each of the N signals E*0(t) formed, one taps off the sample
corresponding the instant tn determined by the expression:
ln — n ' l EO "+" n ' ~>
in which n represents the rank, in the string of frequency ramps emitted, of the frequency ramp n corresponding to the signal Elit) considered.