METHOD AND DEVICE FOR MONITORING VARIATIONS IN TERRAIN

The invention relates to the field of aircraft navigation, with or without a pilot.

It also relates to the navigation of airplanes, helicopters or even of any carrier including civilian drones, and applies particularly to terrain tracking flight phases. The altitude of the aircraft above the ground may be fairly low.

In this field, one of the objectives is to be able to anticipate with precision the changes in the terrain in order to adapt the navigation of the aircraft. The objective of the terrain tracking is to maintain a certain altitude between the aircraft and the overflown ground.

One of the major problems relates to the adaptation of the navigation in the presence of fixed above-ground elements such as buildings, pylons, hangars, cliffs, etc. ... it needs to be possible to detect them, to localize them and to characterize them with precision. Depending oh the type of above-ground element (punctual or extensive), the adaptation of the navigation may be different.

In the extreme cases of low-altitude flight, it is then a matter of detecting the routes of collision with the above-ground elements, and of triggering a warning procedure and changing the direction of the aircraft in order to avoid the collision.

This problem is notably critical for the operation of radio altimeters (RA). Figure 1 illustrates the case of a radio altimeter 1 in a position PRAW, said radio altimeter being disturbed by the presence of an above-ground element 2: since the distance -Hi between the radio altimeter and the above-ground element is shorter than the distance Ho between the RA and the ground at the instant t, the conventional radio altimeter provides the value Hi instead of H0 as the altitude at the instant t. Firstly, the RA does not have the precise vertical altitude at the instant t, and secondly it is hot capable of predicting the exact position of the frontal above-ground element nor of characterizing it, notably of estimating its altitude and of knowing whether it is a punctual or extensive element. Figure 2 schematically shows the altitude H provided by the RA as a function of time in the presence of an above-ground element 2.

A first approach from the prior art consists in equipping the aircraft with a function that is known from synthetic aperture radars, more commonly known by the abbreviation "SAR", which allows the true altitude H0 at the instant t to be extracted by reducing the spot Si illuminated on the ground (figure 1 corresponding to the area S2 on the ground as seen by a sensor equipped with an SAR function). However, this technique does not permit anticipation of the change of terrain,.detection of the routes of collision or characterization of the above-ground elements.

This problem of anticipating the change of terrain is equally critical for allowing the development of drone flights in civilian airspace.

The general technical problem thus relates to anticipation of the change in the relief of terrain and the characterization of the ground and in an above-ground element and/or in the above-ground. In extreme cases, the problem involves avoiding frontal collisions.

The prior art known from the applicant for anti-collision systems for aircraft comprises:
- cooperative alarm systems such as TCAS, the abbreviation for "Traffic Alert and Collision Avoidance System", especially dedicated to detecting moving obstacles,
- dedicated forward point radars (radar illuminating a cone situated in front of the airplane in the axis of travel of the carrier), often in millimeter waves,
- multisensory systems (visible or infrared radar and cameras) with data fusion.

The patent FR 2 913 775 describes an obstacle detection system for taxiing aircraft, with at least two radars.

The patent US 6,885,334 relates to a radio altimeter operating on a basis of an SAR principle to which an anti-collision function with a complementary forward point radar at 35 GHz has been added. This patent US 6,885,334 describes a method implementing a navigation assistance system PTAN (Precision Terrain Aided Navigation) augmented by a forward point radar. The PTAN consists in implementing Doppler processing on a radio altimeter allowing a reduction, in the axis of travel of the carrier, of the spot seen on the ground (figure 1). The extraction of the zero Doppler frequency makes it possible to provide a better estimation of the altitude below the aircraft and not to be disturbed by frontal above-ground elements. In figure 1, for example, the distance Hi between the radio altimeter and the above-ground element is less than the distance Ho between the radio altimeter and the ground at the instant t. The conventional radio altimeter provides the value Hi as altitude at the instant t, while the RA equipped with an SAR function will provide the altitude H0. The patent US 6,885,334 resorts to the use of a dedicated forward point radar for providing the anti-collision function.

The patent US 5,736,957 discloses a method allowing the variation in the surface R of the ground and/or the presence of above-ground elements to be tracked.

The disadvantages of the prior art for the anti-collision systems are notably as follows:
- the cooperative systems by definition rely on external information to work, said information being provided by stations on the ground or by other aircraft, for example;
- the use of forward point radars necessitates the implementation of a radar dedicated to the anti-collision function, which is expensive. Moreover, the angular performance is generally low;
- the use of optical systems is limited by climatic conditions. Furthermore, the field of vision is poor;
- the implementation of multisensory systems with data fusion is complex and expensive.
In conclusion, the systems and methods described in the prior art do hot allow the use of a system for anticipating terrain tracking and an

anti-collision system for an aircraft in flight having all of the following properties:
- autonomy (not relying on a cooperative technique),
- use of a single sensor/radio frequency line,
- low-complexity mode of operation,
- operation at high resolution in terms of distance and at high resolution along the axis of travel of the carrier,
- low-cost operation.

Definitions

In the description that follows, the notations below will be used
H: altitude below aircraft,
Hj: altitude of the above-ground element j,
dj: distance from the above-ground element j to the aircraft,
0j: angle of sight, determined in relation to the x axis, for the object j,
x axis: horizontal axis in the direction of travel of the aircraft, at the start of
the observation,
y axis: horizontal axis perpendicular to the direction of travel of the aircraft, at the start of the observation,
z axis: vertical axis, directed downward.

The x, y and z axes form a direct trihedron. The notion of distance is relative to the distance between the aircraft and the relief and/or between the aircraft and the above-ground element.

The word aircraft is used to denote any carrier that moves by traveling.

The word "chirp" is defined as a signal that is frequency-modulated about a carrier frequency.

The word deramping is known to a person skilled in the art as a technique implementing demodulation of the received signal using a replica of the transmitted wave that is itself delayed.

Spectral analysis generally denotes the analysis of frequency components of a signal.

Unfocused processing effects processing suited to constant Doppler frequencies over the observation period. This processing assumes that

Doppler variations in signals reflected by the above-ground elements are low over the observation period.

Focused processing takes into account variations, over the analysis period, in the Doppler frequencies of reflected signals.

The subject matter of the present invention relates to a method allowing the variation in the surface R of the ground and/or the presence of above-ground elements to be tracked, said method being implemented on a carrier travelling at the speed V = (Vx, Vy, Vz), said carrier comprising at least one transmission/reception antenna, a signal processing means, said method comprising at least the following steps:

a) determination of a waveform h(t), made up of a succession of pulses or a wave train,
b) definition of a range of distance to be monitored,
c) transmission of said wave h(t) of "chirp" type, FMCW ramps, or a pseudo-random signal, of bandwidth B, recording of the signals reflected by the ground and received by the carrier,
d) execution of coherent processing within the element of the wave train in order to obtain the initially fixed distance resolution, the method measuring the distance dj by means of an adaptive distance filtering method or deramping within each reflected pulse/ramp, and
e) for a set of N signals reflected at N different instants, for a range of distance and for each angle of sight {9c1}, performance of processing for the set of signals received by applying the formula translating the Doppler frequency of the reflected signals:
2 (v, cos 0C' + vz sin 0C') 2 (v, sin 0C' - vz cos 0C')2
M)" Pi 1D0 {)
where D0 belongs to the processed range of distance, A is the wavelength of the transmitted wave, i is an index for an angle of sight,
{0c1} is a given value for an angle of sight belonging to the set of angles explored by the beam,
f) for each pair of values (dj, 0j) to be monitored, application of an energy detection criterion Ej by using a chosen threshold value Es, preserving only the pairs (dj, 0j) that go beyond the chosen threshold value. These pairs of values (dj, 0j) correspond to the presence of above-ground elements or variation in ground level,
g) determination of the value of the altitude H below the carrier,
h) estimation of the values of the altitudes Hj of the above-ground elements by using the altitude below the aircraft H, the values (dj, 0j), for example using a trigonometric calculation,
i) knowing these values (dj, 0j, Hj), determination of the features of the above-ground elements, the position pj thereof and the variations Ar in the level of the ground or above-ground.

The signals reflected and received by said carrier are, by way of example, recorded following the step of distance processing over a few pulses or a wave train or in the course of step c).

According to one implementation variant, the aircraft travels horizontally at a uniform speed and the spectral analysis of the signals received in the course of time over the N pulses is carried out for the Doppler frequency fs'
2V />/o'=f-cos0c'

In the case of a nonzero vertical speed of travel of the aircraft, the Doppler frequency of interest is the Doppler frequency fs'
2(vxcosflc<+vzsinflc<)

For a carrier having arbitrary movement, the change in the Doppler frequency is thus dependent on the variations in speed of the carrier, and the spectral analysis step is replaced by the implementation of an adaptive filter such as: h(t) = s\-t), where
-4;-£>(o
s(t) = e x ,re[0,reJ with D(t) being the relief/aircraft distance in the course of time and Te being the analysis period.

According to one implementation variant, the altitude H below the carrier is determined by using the zero Doppler frequency.

The method may have a step allowing determination of the extent or length L of an above-ground element along the axis of travel of the aircraft, comprising at least the following steps:

preservation, for each angle of sight value, of the minimum value of distance dj for which the energy value Ej is above the detection threshold Es, on the basis of adjacent angle of sight values, determination of the position pj at the start of the above-ground element, then the position pj+L of the end of said above-ground element and deduction of an approximate length of the above-ground element by implementing the following steps:

• in the event of the sampling of the angles of sight being fine, that is to say less than or equal to the angular resolution, construction of a distance profile on the basis of the angle, then, by means of geometric calculation, construction of an altitude profile on the basis of the angle, and

• in the event of the sampling of the angles of sight being broad, that is to say greater than the angular resolution, above-ground elements are detected for angles of sight adjacent to 6C', then the length of the object is estimated.

According to one implementation variant, the method is used for tracking the change in the aircraft/object distance in the course of time, and for triggering a warning when the distance becomes less than a threshold ds, said method having at least the following steps: - transmission of a waveform of chirp, FMCW or pseudo-random type,

- recording of the signals reflected by the objects and received by the
carrier,
for a set of N signals reflected at N different instants,
for each angle of sight 9C' and for a range of aircraft/relief or
aircraft/above-ground element distance,
- use of adaptive distance filtering in each pulse in order to optimize the signal-to-noise ratio,
- performance of adaptive Doppler filtering from pulse to pulse by implementing the following steps:
o calculation of the spectral component f's of the reflected
signals, o in the event of extension to the carrier in non-URM movement, adaptive filtering,
- use of an energy-type method to detect the possible presence of an above-ground element at the angle of sight 0c1 and at a distance d1,
- recording of the distance d'(t) corresponding to the minimum distance dmin for which there is detection for each value of 9c1,
- if 9c corresponds to the angle of sight at the vertical, calculation of the altitude HO below the carrier corresponding to the minimum distance for which there is detection for 9c -rc/2,
- otherwise,
o if 9c1 corresponds to the minimum angle defined by the aperture of the antenna:

■ for t > T0, comparison of d'(t) with {d'^ant)}, tant < t, - if d'(t) < {d'(tant)}, tant < t and dj(t) < threshold, triggering of a warning,

• the triggering of the alarm may also be dependent on the characterization of the objects, as follows: o characterization of the extension of the object:

■ for angles of sight 0cvi adjacent to 0c1, performance of the comparison of the distances dvl and estimation of the extension of the object along the axis of travel of the carrier. The invention also relates to a carrier or a device mounted on a carrier travelling at a speed V allowing implementation of the method of detection and tracking of variation in relief of the ground and/or of detection of obstacles, characterized in that it has at least one transmission/reception antenna, a signal-processing means, a speed-awareness module.

Other features and advantages of the preset invention will better appear upon reading the description that follows of examples provided by way of example and in entirely non-limiting fashion that are annexed in the figures, in which:

• figure 1 shows an illustration of the principle of a radio altimeter operating on the basis of an SAR principle,
• figure 2 shows the schematic altitude provided by a conventional RA in the presence of above-ground elements,
• figure 3A shows an illustration of the problem of tracking terrain in the presence of above-ground elements, with figure 3B showing the Doppler distance grid,
• figure 4 shows the monitoring of a set of angles of sight {9C'} toward the front,
• figure 5 shows the resolution performance curves along the axis of travel of the carrier for what is known as "unfocused" Doppler processing,
• figure 6 shows an illustration of the capability of characterizing the type of above-ground elements - the case of a punctual above-ground element,
• figure 7 shows an illustration of the capability of characterizing the type of above-ground element - the case of an extensive above-ground element,
• figure 8 shows the use of an antenna misaimed toward the front,
• figure 9 shows an illustration of the problem of tracking terrain and of collision avoidance on an aircraft, and
• figure 10 shows anticipation of the tracking of terrain/detection of routes of collision.

The examples that will be given to illustrate the method and the system according to the invention relate to an aircraft 10 or a carrier equipped with at least one transmission/reception antenna 12 illuminating a portion of the ground situated toward the front of the carrier, a transmission module 13 for a wave h(t), for example a radio frequency line, a processor 14 suitable for executing the steps of the method according to the invention and a memory 15 allowing storage of the received signals and the information pertaining to distance, angle and altitude that result from implementing the method. The antenna 12 transmits a beam of given physical aperture that will allow monitoring of terrain and above-ground elements in front of the moving aircraft.

The aircraft may also be equipped with a means for displaying the results, such as the position of the above-ground elements, the altitude thereof or even the variation in level or in the relief R of overflown terrain. This display means 16 is linked to the processor. In some cases, the processor 14 will transmit these results to a module 17 generating an audible or visual alarm, for example in the case of the anti-collision application. The aircraft generally comprises a module 18 allowing its speed V of travel to be known and transmitting this information to the processor 14. This module may be an inertial unit that generally equips airplanes, a satellite positioning system known by the abbreviation GNSS (Global Navigation Satellite System), such as the GPS (global positioning system), GLONASS or Galileo system, etc.

The aircraft may be equipped with a radar or with a radio altimeter, the function of which is notably to measure the altitude of said aircraft relative to the ground.

Figure 3A shows the problem of tracking terrain in the presence of above-ground elements 2 by an aircraft 10 that is travelling at an arbitrary speed V. The relief of the terrain is shown by the reference R and the above-ground element by the reference 2. The speed vector V is broken down into Vx, Vy and Vz in the example along the axes x, y and z that are shown in figure 1. On the basis of the definition of the axes, Vy is zero at the start of observation and constantly zero in the case of a constant speed. Figure 4 also shows an example for a set of directions of sight {0c1} allowing forward monitoring of the variation in the relief R of the ground and of the presence of above-ground elements.

The proposed solution is based on the implementation on the flying aircraft 10 of a function for anticipating terrain tracking that works on the basis of an SAR radar principle and that monitors a set of directions of sight situated toward the front of the carrier. This function is implemented in the processor of the aircraft.

By way of example, the sensor used is a radar in which the antenna 12 illuminates a portion S of the ground situated in front of the moving aircraft 10.-

The radar works on the basis of an SAR principle, transmitting a signal, recording the signals received (in terms of amplitude/phase) in the course of time and processing them in coherent fashion (using the phase information) on the basis of a principle that is outlined below.

The solution notably involves analyzing the Doppler frequencies fD of the signals reflected at a plurality of angles of sight {0c'}. The measured values comprise the angle nil (in radians), corresponding to the vertical. The minimum value of the angle 0c' is limited by the physical aperture of the antenna. The spacing between the measured angles 0c' is taken to be less than or equal to the angular resolution (which may result in measurements that are not regularly spaced in terms of angle). The monitoring of this set of angles of sight will allow the knowledge of the terrain to be anticipated.

The method implemented by the invention notably comprises the following steps:

a) determination of a waveform h(t), made up of a succession of pulses or a wave train,

b) definition of a range of distance to be monitored, the distance corresponding to the aircraft/relief and/or aircraft/above-ground element distance,

c) transmission of said wave h(t) of "chirp" type, FMCW ramps, or a pseudo-random signal, of bandwidth B, and recording of the signals reflected and received by the carrier. The signals can also be recorded following distance processing over a few pulses or a wave train (step 4),

d) execution of coherent processing within the element of the wave train in order to obtain the initially fixed distance resolution, the method measuring the distance dj, for example by means of an adaptive distance filtering method or "deramping" within each reflected pulse/ramp,

e) for a set of N signals reflected at N different instants, for a range of distance and for each angle of sight {9c1}, performance of processing for the set of signals received by using the formula translating the Doppler frequency (first-order approximation in t) of the reflected signals:

where D0 belongs to the processed range of distance. The calculations can be carried out for a set of values Do belonging to the processed distance range,

A is the wavelength of the transmitted wave, is an index for an angle of sight,

{9c1} is a given value for an angle of sight belonging to the set of angles explored by the beam. The order in which steps d) and e) (distance processing and Doppler processing) are carried may be reversed. These processing operations can be carried out jointly.

Note: For optimum processing of all the above-ground elements, it is also necessary to take into account the position of the elements along the y axis. The preceding formula corresponds to above-ground element positioned at y=0,

f) for each pair of values (dj, 9j) to be monitored, application of an energy detection criterion Ej by using a chosen threshold value Es, preserving only the pairs (dj, 0j) that go beyond the chosen threshold value. These pairs of values (dj, 9j) correspond to the presence of above-ground elements or variation in ground level. Energy-type detection is implemented in order to detect the presence of an above-ground element at the angle of sight 9c' and for a certain aircraft/relief or aircraft/above-ground element distance range.

g) determination of the value of the altitude H below the carrier,

h) estimation of the values of the altitudes Hj of the above-ground elements (2j) by using the altitude below the aircraft H, the values (dj, 9j), for example using a trigonometric calculation,

i) in light of these values (dj, 9j, Hj), determination of the features of the above-ground elements (2j), the position pj thereof and the variations Ar in the level of the ground or above-ground.

In the case of a zero vertical carrier speed Vz, the unfocussed processing of the zero Doppler frequency signals makes it possible to extract the vertical altitude H of the carrier.

Determination of the length of the above-ground element or extent of the above-ground element - figure 7

According to one variant embodiment, the method may have a step k) in which the extent L or the length of the above-ground element L is estimated along the axis of travel of the carrier, in order to determine the type of above-ground element, i.e. punctual or extensive, as shown in figure 7.

For that purpose, it is possible to interpret the energy measurements taken for various distance values dj and angle values, and to preserve, for each angle value, the minimum value of dj for which the energy value Ej is above the detection threshold, and then, on the basis of the corresponding values of angles of sight, determine the position pj of the start of the above-ground element, then the position pj+L of the end and to deduce therefrom an approximate length of the above-ground element.

In the event of the sampling of the angles of sight being fine, the method will build a distance profile on the basis of the angle and then, by means of geometric calculation, an altitude profile on the basis of the angle.

In the event of the sampling of the angles of sight being broad, above-ground elements are detected for angles of sight adjacent to Gc', and then the length of the object is estimated.

The waveform used by the radar consists, by way of example, in a train of pulses of chirp type or of continuous wave ramps in which its frequency Of transmission varies linearly as a function of time, better known by the abbreviation "FMCW" meaning Frequency Modulation Continuous Wave. The object/carrier distance is obtained, by way of example, by means of a conventional deramping (term defined previously) or adaptive filtering calculation that is known to a person skilled in the art. Case of a carrier in uniform rectilinear movement, or URM, having a nonzero vertical speed

In the case of a nonzero vertical speed, V=(Vx, 0, Vz). The method calculates the appropriate Doppler frequencies, which are based on the scale product of the speed V of the aircraft and the direction of the object, for extraction in order to obtain the desired altitudes, that is say the altitude H below the aircraft as a function of a given vertical speed.

In the case of a carrier having a nonzero vertical speed, the Doppler frequency is estimated (first-order approximation in t) by

For unfocussed processing, the Doppler frequency fs' of interest becomes:

Case of a carrier in uniform rectilinear movement, or URM, having a zero vertical speed

In this case, V=(Vx, 0, 0). For unfocussed processing, the second term in equation (1) is ignored. The method performs spectral analysis of the signals received in the course of time (by way of N received pulses) for the Doppler frequency fs'

This example corresponds to what is known as "unfocussed" SAR processing, which is known to a person skilled in the art. The resolution obtained may be sufficient for it not to be necessary to take into account the variation in the Doppler frequency fo in the course of the analysis period. Figure 5 provides examples of performance curves for the resolution obtained as a function of the altitude of the carrier and the angle of sight. Case of a carrier in uniform nonrectilinear movement

By way of example, this may be uniformly accelerated movement or arbitrary movement. In all cases, the changes in speed and acceleration are presumed known.

In the case of a carrier having arbitrary movement, the change in the Doppler frequency fD is thus dependent on the variations in speed of the carrier, which need to be taken into account in order to have processing having good angular resolution. The spectral analysis step, analysis of the frequency components of the signal, is replaced by the implementation of an adaptive filter such that:
h(t) = s\-t) inverse conjugate waveform over time (Doppler frequency change taken into account) where
-4j-D{t) r n
s(t)-e x ,/e[0,reJ with D(t) being the object/aircraft distance in the course of time and Te being the analysis period.

Note: This involves the implementation of what is known as "focused" SAR processing.

According to one embodiment, it may be advantageous to misaim the antenna toward the front, according to the diagram described in figure 8, in order to increase the period available for anticipating the change in terrain. For that purpose, the idea is to transmit an antenna beam of aperture 0 using a misaim angle 6d for the antenna counted in relation to the vertical considered in relation to the ground.

Figures 9 and 10 schematically show an embodiment allowing the change in the aircraft/object distance to be tracked in the course of time and a warning to be triggered when the distance becomes less than a threshold, which may correspond to the presence of one or more above-ground elements. The threshold ds is dependent on the application.

The method has at least the following steps:
- transmission of a waveform of chirp, FMCW or pseudo-random type,
- recording of the signals reflected by the objects and received by the carrier,
for a set of N signals reflected at N different instants,
for each angle of sight 9c' and for a range of aircraft/relief or
aircraft/above-ground element distance,

- adaptive distance filtering in each pulse in order to optimize the signal-to-noise ratio according to a method that is known to a person skilled in the art,
- adaptive Doppler filtering from pulse-to-pulse
o calculation of the spectral component // of the reflected
signals,
* o in the event of extension to the carrier in non-URM
movement, performance of adaptive filtering
- energy-type detection, which indicates the possible presence of an object at the angle of sight 8C' and at a distance d',
- recording of the distance d'(t) corresponding to the minimum distance for which there is detection for each value of 0c1,
- if Gc1 corresponds to the angle of sight at the vertical, then calculation of the altitude HO below the carrier corresponding to the minimum distance for which there is detection for BC-TI/2,
- otherwise,
o if 0c' corresponds to the minimum angle defined by the aperture of the antenna:
■ for t > T0, comparison of d'(t) with {d'(tant)}> tant < t, .
« if d'(t) < {d'(tant)}; tant < t and d'(t) < threshold, triggering of a warning,
• the triggering of the alarm may also be dependent on the characterization of the objects, see below, o characterization of the extension of the object
■ for angles of sight 0cvl adjacent to 0c1, comparison of the distances dvl and estimation of the extension of the object along the axis of travel of the carrier.

The analysis of the change in the aircraft/object distances for each monitored angle of sight thus makes it possible to detect routes of collision. When the distance between the aircraft and the ground or an above-ground element drops below a certain threshold ds, a warning is triggered and a command for changing direction is transmitted to the pilot or to the control system of the aircraft.

The triggering of the alarm may likewise be dependent on the characteristics of the above-ground elements (punctual/extensive). By means of complementary processing, for example that described in order to determine the length or extent of an object, the method moreover provides characterization of the above-ground elements. The aircraft is thus able to know whether the frontal object is punctual, of tree, pylon or building type, or extensive, of hangar or cliff type. The carrier will thus be able to adapt its flight strategy as a function of the characteristics of the ground and the above-ground.

This solution can notably be integrated into a radio altimeter in order to enhance the operation thereof. Numerical example

The numerical example that follows is that of a helicopter in the extreme case of low-altitude anti-collision. By way of example, the matter under consideration is a helicopter in penetration phase flying at an altitude of 100 m, a speed of 10 m/s, and an antenna with a physical aperture of 90° (half-angle cone of 45° at the top) oriented vertically downward, and for a monitored angle equal to 45° toward the front of the aircraft.

An integration period of 0.27 s provides a resolution of 2.7 m along the axis of travel of the carrier, while an integration period of 0.03 s provides a resolution of approximately 13 m, which is still advantageous because this resolution value is compatible with the dimensions of a cell of , a digital terrain map.

The period between the instant of first detection and the instant of collision is 10 s, which leaves time for detecting the route of collision and for triggering a warning procedure and for changing the direction of the aircraft.

The solution provides an autonomous anticipation and terrain-tracking system, that is to say without the need for external cooperation.

A single sensor is necessary in order to implement the anticipation function, and it is moreover possible to use the antenna and the radio frequency line of a radio altimeter already installed on the aircraft to implement the method. The terrain-anticipation and anti-collision-alarm functions are obtained by virtue of the monitoring of a set of angles of sight toward the front, with a single sensor.

The method is based on processing operations that have low complexity and therefore low cost.

The method has the advantage of providing a capability of characterizing the ground and the above-ground, notably by providing an estimation of the type of above-ground elements (punctual of tree/pylon type or extensive).

Finally, the method can be integrated in a radio altimeter operating on the basis of an SAR principle, thus providing a complementary terrain-anticipation function for the radio altimeter.

The solution is a low-cost solution because:

• the processing operations implemented are of low complexity. For acceptable resolutions, simple spectral analyses by means of Fourier transform calculation are sufficient;

• a single sensor is necessary, the solution moreover being able to be integrated into a radio altimeter.

The high resolution along the axis of travel of the carrier is ensured by the coherent processing of the signals reflected by the objects over a certain analysis period (analysis over a certain number of reflected pulses/ramps). The detected objects are positioned finely in terms of distance and along the axis of travel of the carrier.

CLAIMS

1. A method suitable for tracking the variation in the surface R of the ground and/or the presence of above-ground elements (2j), said method being implemented on a carrier traveling at the speed V = (Vx, Vy, Vz), said carrier (10) comprising at least one transmission/reception antenna (12), a signal-processing means (14), said method comprising at least the following steps:
a) determination of a waveform h(t), made up of a succession of pulses or a wave train,
b) definition of a range of distance to be monitored,
c) transmission of said wave h(t) of "chirp" type, FMCW ramps, or a pseudo-random signal, of bandwidth B, recording of the signals reflected by the ground and received by the carrier,
d) execution of coherent processing within the element of the wave train in order to obtain the initially fixed distance resolution, the method measuring the distance dj by means of an adaptive distance filtering method or deramping within each reflected pulse/ramp, and
e) for a set of N signals reflected at N different instants, for a range of distance and for each angle of sight {0c1}, performance of processing for the set of signals received by applying the formula translating the Doppler frequency of the reflected signals:
2(vc'-vzcos6>c*)^
M)" X XD0 {)
where Do belongs to the processed range of distance,
A is the wavelength of the transmitted wave,
i is an index for an angle of sight,
{0c1} is a given value for an angle of sight belonging to the set of angles
explored by the beam, said method being characterized in that it has the
following steps:
f) for each pair of values (dj, 0j) to be monitored, application of an energy detection criterion Ej by using a chosen threshold value Es, preserving only the pairs (dj, 9j) that go beyond the chosen threshold value,
g) determination of the value of the altitude H below the carrier,
h) estimation of the values of the altitudes Hj of the above-ground elements (2j) by using the altitude below the aircraft H, the values (dj, 0j), for example using a trigonometric calculation,
i) in light of these values (dj, 9j, Hj), determination of the features of the above-ground elements (2j), the position pj thereof and the variations Ar in the level of the ground or above-ground.

2. The method as claimed in claim 1, characterized in that the signals reflected and received by said carrier are recorded after the distance processing step over a few pulses or wave trains or in the course of step c).

3. The method as claimed in claim 1, characterized in that the aircraft travels horizontally at a uniform speed and in that the spectral analysis of the signals received in the course of time over the N pulses is performed for the Doppler frequency fs':
2V />/„'=—cos0c'

4. The method as claimed in claim 1, characterized in that for a nonzero
vertical speed of travel of the aircraft, the Doppler frequency of interest is
the Doppler frequency fs':
. 2(vxcosflc'+vzsinflc')

5. The method as claimed in claim 1, characterized in that in the case of
a carrier having arbitrary movement, the change in the Doppler frequency
is thus dependent on the variations in speed of the carrier, and the spectral analysis step is replaced by the implementation of an adaptive filter such that:
h(t) = s\-t), where
s(t) = e x ,te[0,Te\ with D(t) being the relief/aircraft distance in the course of time and Te being the analysis period.

6. The method as claimed in one of claims 1 to 5 characterized in that the altitude H below the carrier is determined by using the zero Doppler frequency.

7. The method as claimed in one of claims 1 to 6 characterized in that it has a step allowing determination of the extent or length L of an above-ground element along the axis of travel of the aircraft, comprising at least the following steps:
preservation, for each angle of sight value, of the minimum value of distance dj for which the energy value Ej is above the detection threshold Es, on the basis of the adjacent angle of sight values, determination of the position pj at the start of the above-ground element (2j), then the position pj+L of the end of said above-ground element and deduction of an approximate length of the above-ground element by implementing the following steps:
• in the event of the sampling of the angles of sight being fine, that is to say less than or equal to the angular resolution, construction of a distance profile on the basis of the angle, then, by means of geometric calculation, construction of an altitude profile on the basis of the angle;
• in the event of the sampling of the angles of sight being broad, that is to say greater than the angular resolution, above-ground elements are.detected for angles of sight adjacent to 6c', then the length of the object is estimated.

8. The method as claimed in claim 1 for tracking the change in the aircraft/object distance in the course of time, and for triggering a warning
when the distance becomes less than a threshold ds, said method having at least the following steps: transmission of a waveform of chirp, FMCW or pseudo-random type, recording of the signals reflected by the objects and received by the carrier, for a set of N signals reflected at N different instants, for each angle of sight 9C' and for a range of aircraft/relief or aircraft/above-ground element distance,
- use of adaptive distance filtering in each pulse in order to optimize the signal-to-noise ratio,
- performance of adaptive Doppler filtering from pulse to pulse by implementing the following steps:
o calculation of the spectral component fs of the reflected
signals, o in the event of extension to the carrier in non-URM movement, use of adaptive filtering,
- use of an energy-type method to detect the possible presence of an above-ground element at the angle of sight 0C' and at a distance d',
- recording of the distance d'(t) corresponding to the minimum distance dmin for which there is detection for each value of 9c',
- if 9c' corresponds to the angle of sight at the vertical, then calculation of the altitude HO below the carrier corresponding to the minimum distance for which there is detection for 0c'=rc/2,
- otherwise,
o if 9c' corresponds to the minimum angle defined by the aperture of the antenna:
■ for t > T0, comparison of d'(t) with {d'(tant)}, tant < t
■ if d'(t) < {d'(UnO>. '-W < t and d'(t) < threshold, triggering of a warning.

9. The method as claimed in claim 8, characterized in that the triggering of a warning is dependent on the characterization of the extension of the object as follows: for angles of sight 9CVI adjacent to 9c', comparison of the distances dvl and estimation of the extension of the object along the axis of travel of the carrier.

10. A device or carrier traveling at a speed V suitable for implementing the method for detecting and tracking variation in relief of the ground and/or for detecting obstacles as claimed in one of the preceding claims, characterized in that it has at least one transmission/reception antenna (12), a means for processing the signals (14), a speed awareness module.