Communications: directive radio wave systems and devices (e.g. – Determining distance – Altimeter
Reexamination Certificate
1991-10-28
2001-07-03
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Determining distance
Altimeter
C342S192000
Reexamination Certificate
active
06255982
ABSTRACT:
BACKGROUND OF THE INVENTION
DISCUSSION OF BACKGROUND
The present invention relates to the field of navigation aids for aircraft.
More specifically, the present invention proposes a novel method for utilizing the signals provided by an airborne radio altimeter of the FM/CW type, which are generated when an aircraft flies over a ground area which is to be analyzed.
FM/CW radio altimeters have traditional applications in the field of altimetry. They are transceivers whose carrier wave is transmitted at a frequency of, for example, a few gigahertz to a few tens of GHz and which is cyclically modulated by a frequency-modulated signal. The measurement of the frequency of the beat between the transmitted and received waves provides directly an information representative of the distance separating the transceiver from the obstacle having reflected the signal.
Traditionally, the processing of the beat signal from FM/CW radio altimeters has been performed by analog circuitry.
Now, the power density of the beat signal produced by such a system exhibits a more or less spread frequency spectrum
11
, as schematically shown in FIG.
1
. But, in fact, for the height measurement performed in conventional altimetry, only a small portion of the spectrum is useful and consequently used (for example, in the simple case where the attitude of the airplane is parallel to the ground, and in the absence of Doppler shift, it is the beat frequency f
O
corresponding to the point
10
of maximum value A
m
of power density A which is directly proportional to the height of the aircraft above ground).
As a consequence, in conventional altimetry with analog processing, the beat signal is very highly filtered so as to eliminate the non-utilized portions of the spectrum, in particular in the decay region
13
.
SUMMARY OF THE INVENTION
An object of the present invention is to utilize all of the information contained in the power density spectrum of the beat signal by assuming that this spectrum is also representative of the nature of the overflown ground. As a matter of fact, the power density spectrum depends on the reflectivity characteristics of the ground returning the signal.
More specifically, a first object of the present invention is to use a FM/CW radio altimeter to perform the identification of the overflown ground with reference to a few standard types previously cataloged.
Another object of the present invention is to provide such a method allowing the recognition of outstanding elements of the ground, in particular outstanding lines, such as water areas, roads, railways, buildings, forests, high-voltage power lines, etc. This makes possible, for example, re-alignment operations in navigation by correlation with a stored map, or “map-matching guidance”, even in the absence of relief.
A supplementary object of the present invention is to provide such a method giving information about the condition of the overflown ground, for example the determination of the state of sea (which affects its reflectivity) in order to infer from it the optimum flying altitude of an aircraft. More generally, an object of the method according to the present invention is to provide aircraft guidance information while furnishing decision elements for navigation (for example, a system of the type with a Kalman filter).
These objects, as well as others which will become apparent later, are achieved by means of a method of characterization of an overflown ground, in particular to recognize a predetermined type of ground, or outstanding transitions or sequences of transitions on said ground, consisting in transmitting in the direction of the ground a signal by means of a radio altimeter of the FM/CW type, and in analyzing, in the frequency domain, the shape of the spectrum of the beat signal generated by said radio altimeter to characterize said ground. According to the present invention, the method consists in extracting from the shape of the spectrum of said beat signal information relating to the reflectivity of said ground.
Preferably, the present invention consists in generating a quantity representative of the reflectivity of the overflown ground from at least one of the two following measurements:
measurement of the amplitude of the spectrum of the beat signal, as being representative of the reflectivity of the ground straight under the aircraft;
measurement of the frequency spreading (inverse of the range above threshold of the spectrum of the beat signal, as being representative of the backscatter diagram of the overflown ground.
Advantageously, the reference values are first generated so as to categorize spectra of the beat signal in accordance with at least three categories of ground:
specular ground;
averagely scattering ground;
higly scattering ground.
In an advantageous embodiment of the present invention, the method consists in generating a signature of the overflown ground, for one thing, from said reflectivity information and, for another thing, from the information of height above ground provided by the radio altimeter;
The method of the invention allows also to use as a criterion for the recognition of the spectrum of the beat signal a model of the decay of said spectrum.
In this case, the model of the slope of the beat signal spectrum is advantageously implemented by means of a function of the type:
y=
1/
u
2N−1
,
where:
p: antenna lobe index
q: ground backscatter index
N=p+q+2
u=f/f
O
(beat frequency r reduced by the reference frequency f
O
corresponding to a reflection normal to the ground, in the absence of Doppler shift:
f
O
=2(
h
O
/c
) (
F/T
).
The coefficient N derived from the model gives then a value representative of q that can be multiplied with the measures of amplitude and spectrum spreading to form a composite coefficient of signature analysis.
Recognition of the shape of the beat signal spectrum may also take into account at least one of the following criteria:
maximum amplitude of the main peak of the spectrum;
measurement of contrast (maximum amplitude/average amplitude);
steepnes of the spectrum leading edge;
number of secondary peaks.
For analyzing the beat signal spectrum, it is also advantageous to weight the recognition criteria of the beat signal spectrum with at least one of the following additional criteria:
attitude of the aircraft;
Doppler shift due to the velocity relative to ground;
radiation pattern of the radio altimeter antenna.
According to the present invention, the characterization may also be peformed by taking into account the time-related fluctuations of the spectrum of the beat signal.
In a further embodiment of the present invention, the recognition of said ground is performed by combining the information extracted from two beat signal spectra from two radio altimeters operating in different frequency bands.
REFERENCES:
patent: 3213451 (1965-10-01), Alpers
patent: 3344419 (1967-09-01), Lund
patent: 4161731 (1979-07-01), Barr
patent: 4185285 (1980-01-01), Bose
patent: 4620192 (1986-10-01), Collins
patent: 4766436 (1988-08-01), Crepin et al.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Sotomayor John B.
Thomson TRT Defense
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