Method and system for measuring the velocity of a carrier with respect to the ground

1. A method for measuring the velocity of a carrier vehicle with respect to the ground, the carrier vehicle moving parallel to the ground and being equipped with a radar or sonar having an emission and reception cone about a line of sight, the method comprising the following steps:

(a) orienting the line of sight of the radar or sonar toward the ground, such that a footprint on the ground of said emission and reception cone is passed through by the projection onto the ground of the velocity vector of the carrier vehicle;

(b) using the radar or sonar to emit a plurality of signals that are directed toward the ground, and to acquire respective echo signals at a plurality of echo delay time values;

(c) processing the acquired echo signals so as to obtain, for one or more of the plurality of echo delay time values, a corresponding Doppler spectrum;

(d) for said one or more echo delay time value, identifying an uppermost, in absolute value, frequency value of the corresponding Doppler spectrum; and

(e) computing the velocity of the carrier vehicle with respect to the ground as a function of: its height with respect to the ground, said one or more uppermost frequencies and the corresponding echo delay time values.

2. The method as claimed in claim 1 , wherein:

step c) comprises processing the acquired echo signals so as to obtain a said Doppler spectrum for each of the plurality of echo delay time values;

step d) comprises determining a plurality of said maximum frequencies for a plurality of said Doppler spectra; and

step e) comprises computing the velocity of the carrier vehicle with respect to the ground as a function of its height with respect to the ground and of a plurality of said maximum frequencies.

3. The method as claimed in claim 2 , wherein step e) also comprises computing an index of confidence in the velocity measurement as a function of a plurality of said maximum frequencies.

4. The method as claimed in claim 2 , wherein step c) comprises constructing a delay-Doppler matrix and step d) comprises applying an edge-detection algorithm to said matrix.

5. The method as claimed in claim 1 , wherein, in step d), said or each maximum frequency is determined by identifying a maximum of the derivative of the corresponding Doppler spectrum with respect to frequency.

6. The method as claimed in claim 1 , wherein step e) comprises, for said one or more echo delay time value, designated T R , computing a velocity given by:

V r/s,R =( c·f DR )/(2 f em ·cos(α R ))

where:

c is the propagation speed of the signal emitted by the radar or sonar;

f DR is the maximum frequency of the Doppler spectrum corresponding to the delay T R ;

f em is the frequency of the signal emitted by the radar or sonar;

α R =sin −1 (2h/c·T R ), h being the height of the carrier vehicle with respect to the ground; and

wherein said velocity of the carrier vehicle with respect to the ground is determined depending on one or more of the velocities v r/s,R thus computed.

7. The method as claimed in claim 1 , wherein the emission and reception cone has an aperture angle in a plane perpendicular to the ground and containing the line of sight comprised between 40° and 80°.

8. The method as claimed in claim 1 , also comprising a step d′) comprising processing the acquired echo signals so as to determine a maximum and minimum value of the echo delay time values of said echo signals, and to deduce therefrom an estimation of the height h of the carrier vehicle with respect to the ground and of the angle made by the line of sight and the ground.

9. The method as claimed in claim 1 , wherein, in step a), the beam is directed obliquely toward the ground.

10. A system for measuring the velocity of a carrier vehicle with respect to the ground by means of a method as claimed in claim 1 , comprising:

a radar or sonar having an emission and reception cone about a line of sight that is configured to be oriented toward the ground; and

a device comprising at least one of a processor or a dedicated electronic circuit for processing echo signals acquired by said radar or sonar;

wherein said device is configured to:

process the acquired echo signals so as to obtain, for said one or more echo delay time values, the corresponding Doppler spectrum;

for said or at least one said echo delay time value, determine the maximum, in absolute value, frequency value of the corresponding Doppler spectrum; and

compute the velocity of the carrier vehicle with respect to the ground as a function of a datum indicative of its height with respect to the ground, of said one or more maximum frequencies and of corresponding delay time values.