Method for determining the position, notably in terms of elevation, of a target flying at very low altitude

1. Method for determining by an electromagnetic detection system an elevation of a target close to a ground, comprising the steps of:

receiving, by a reception antenna, a first signal emitted directly by a target;

receiving, by the reception antenna, a second signal emitted by the target towards the ground then reflected by the ground toward the electromagnetic detection system, wherein the reception antenna comprises a plurality of sub-arrays of radiating elements;

sampling an interference signal produced by the first signal and by the second signal at the level of the sub-arrays, to produce a sampled interference signal; and

determining the elevation of the target from an amplitude of the sampled interference signal,

wherein a frequency f of the sampled signal is a value which minimizes a function:

J

⁡

(

U

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=

∑

i

,

k

⁢



r

i

+

k

+

r

i

-

k

-

2

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r

i

⁢

cos

⁡

(

2

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π

⁢

⁢

Uk

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Δ

)



wherein

r i+k is a subtraction between a value of a signal received at sub-array i+k and the value of the signal received at sub-array i+k+1,

r i−k is a subtraction between the value of the signal received at sub-array i−k and the value of the signal received at sub-array i−k−1, and

Δ is a distance between the phase centers of two consecutive sub-arrays.

2. The method of claim 1 , further comprising the steps of:

emitting a detection signal towards the target, wherein the signal re-emitted directly by the target is the first signal, and the signal re-emitted by the target towards the ground then reflected by the ground is the second signal.

3. The method of claim 1 , wherein the sub-arrays comprise a line of radiating elements disposed in a substantially horizontal, parallel and uniform manner over the reception antenna as a whole.

4. The method of claim 1 , wherein the sub-arrays are situated in a substantially vertical plane.

5. The method of claim 1 , further comprising the following step:

calculating a ratio between an height h 2 of the target relative to the ground and a distance d projected on the ground between the electromagnetic detection system and the target according to the relationship: h 2 /d=λf/2 where λ is a wavelength of the first signal and f is a frequency of the interference signal.

6. The method of claim 5 , further comprising the following step:

obtaining the coordinates (x R , z R ) of a reflection point R and an angle α R between the tangent plane to the ground at the point R and the horizontal on the basis of a cost function:



C

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(

r

)



=

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h

2

-

z

R

+

x

R

×

α

R

x

R

-

h

2

d

-

x

R



wherein:

h 1 represents the height relative to the ground of the antenna,

h 2 represents an estimation of the height calculated in the case of plane and horizontal ground,

d represents the distance projected on the ground of the target from the antenna, and

the values (x R , z R , α R ) minimizing the cost function are the coordinates of the reflection point R.

7. The method of claim 6 , further comprising the following steps:

correcting the height of the target relative to the tangent plane to a value H 2 , using the value of the angle α R between the tangent plane at R to the terrain and the horizontal, as:

H 2 =h 2 +z R ( d−x R )×α R .

8. The method of claim 5 , further comprising the following step:

determining the height h 2 for a non-plane and non-horizontal ground on the basis of a point of reflection R of a signal re-emitted by the target towards the ground on non-horizontal ground, or of a height determined for plane and horizontal ground.

9. The method of claim 1 , further comprising the following step:

correlating an amplitude and a phase of the signal received with a predefined replica.

10. The method of claim 1 , wherein the antenna used includes a sparse antenna.

11. The method of claim 1 , wherein the target comprises an active emitter.

12. The method of claim 1 , wherein the target includes a passive reflector.

13. The method of claim 1 , wherein the target includes a drone in the landing phase.

14. The method of claim 1 , further comprising the step of determining a height h 2 of the target by a monopulse-type process if the value of the amplitude of the reflection coefficient is less than a predetermined threshold.