Communications: directive radio wave systems and devices (e.g. – Synthetic aperture radar
Reexamination Certificate
2002-07-01
2004-01-13
Gregory, Bernarr E. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Synthetic aperture radar
C244S158700, C342S147000, C342S156000, C342S175000, C342S190000, C342S191000, C342S195000, C342S417000, C342S422000, C342S423000, C342S424000
Reexamination Certificate
active
06677884
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
Applicants claim priority under 35 U.S.C. §119 of German Application No. 101 32 723.4 filed Jul. 5, 2001.
FIELD OF THE INVENTION
The invention relates to a satellite configuration for interferometric and/or tomographic remote sensing by means of synthetic aperture radar (SAR).
BACKGROUND OF THE INVENTION
In a configuration involving a cluster of radar satellites, maintaining a baseline important for interferometry as stable as possible over a complete orbit is a problem. Baseline is termed the separation between two receiver satellites, a distinction being made between across-track baseline and along-track baseline. The first one is perpendicular to the velocity vector, serves to survey the ground level elevation and contributes by its component standing normal to the line connecting antenna and target point to sensing the ground level elevation. The along-track baseline designates the separation of two receiver satellites in the direction of the velocity vector (R. Schreiber et al., “Overview of interferometric data acquisition and processing modes of the experimental airborne SAR system of DLR”, Proc. IGARSS'99, Hamburg, Germany, June 1999, pp. 35-38).
The following is an introduction to SAR systems, subsequently extended to cover interferometric SAR systems. Synthetic aperture radar (SAR) is a remote sensing instrument finding ever-increasing application in terrestrial and extraterrestrial mapping, surveillance and inspection. One such system has a platform moving at constant speed, an antenna looking downwards to the imaged scene and a coherent radar system which transmits periodic electromagnetic pulses. The direction of movement of the platform is termed the azimuth direction, the direction slanting downwards to the scene is termed the range direction. Shown in
FIG. 1
is an interferometric SAR system including two satellites S
1
and S
2
with the baseline B. &THgr; designates the viewing angle. A conventional SAR system consists of one platform only, e.g. a satellite. In
FIG. 1
a SAR satellite S
1
is depicted flying over a swath to be mapped. In this arrangement, a high-resolution map in the azimuth and range direction of the backscatter coefficient of the swath is generated by signal processing the raw data sensed on fly-over.
The SAR system consisting of a platform (satellite, e.g. S
1
) is configured by a second satellite (S
2
) into an interferometric system. The gist of interferometry is by measuring the phase difference of two SAR images obtained from differing perspectives to obtain additional information with which, for instance, an indication as to the relative difference in elevation of all targets in the swath can be derived. This phase difference is a result of the slight differences in range between target and the two antennas.
For the accuracy in elevation sensing it is the so-called baseline that is deciding. In
FIG. 1
the baseline is illustrated as the line connecting S
1
and S
2
.
FIG. 1
shows precisely the case in which the baseline is perpendicular to the velocity vector of the satellite S
1
and normal to the line connecting the satellite S
1
and a target on the ground. This baseline is termed normal baseline, it being decisive for elevation sensing. When two satellites orbit at precisely the same altitude, in the same azimuth position and on parallel orbits, it is only the resulting normal baseline that contributes towards elevation sensing. The same applies to the case of two satellites orbiting precisely one above the other in thus forming a vertical baseline. Here too, it is only the resulting normal baseline that contributes towards elevation sensing. The end product of sensing the elevation is a so-called digital elevation model (DEM).
Referring now to
FIG. 2
there is illustrated an interferometric system for along-track interferometry. In this arrangement the two satellites S
1
and S
2
are on the same orbit but slightly shifted in the flight direction. If the satellites are on differing orbits, along-track interferometry is likewise possible, it being, however, only the shift of the two satellites in the flight direction that is decisive for each constellation. Along-track interferometry is used for sensing the velocity and detecting moving targets. A typical example application of along-track interferometry is surface flow observation.
There are basically two possibilities for configuring an interferometric cluster of at least two satellites, namely multi-pass interferometry and single-pass interferometry. In multi-pass interferometry the site under observation is flown over with a SAR sensor temporally delayed with a slightly differing flight path depending on the requirement for along-track or across-track interferometry. Multi-pass interferometry is implemented successfully with the ERS-1 satellite, for example.
For single-pass interferometry at least two SAR sensors are needed and the site under observation is mapped by both sensors simultaneously. The advantage of single-pass interferometry is that the site under observation does not change between the individual SAR maps, ensuring high coherence between the two sets of interferometric data.
The disadvantage of single-pass interferometry is that several SAR sensors are needed which, as a rule, adds to the costs. Instead of two satellites a rigid design having a sufficiently long baseline between the SAR antennas may be used. This achievement was made use of in the SRTM mission.
A further achievement of the interferometric principle is the interferometric cartwheel (WO 99/58997 by D. Massonnet “Roue interferométrique”) in which a cluster of satellites describes a revolution about a virtual cartwheel center by using slightly elliptical orbits with different arguments of perigee. The configuration provided consists of, for example, three receiver satellites forming together a cartwheel and SAR transmitter which, more particularly, may be an already existing SAR sensor.
Referring now to
FIG. 3
there is illustrated one such cartwheel center, where &THgr;
sq
designates the bistatic squint angle. During an orbit the individual cartwheel satellites describe a complete ellipse about the cartwheel center, both across-track (vertical) and along-track baselines occuring between the individual cartwheel satellites. Depending on the particular application the satellites most favorable for the application can be selected from the SAR receiver satellites.
For across-track interferometry, for example, the satellites always selected are those having the largest vertical (across-track) baseline. However, all along-track baselines may also be combined to enhance the performance of the along-track interferometry, the same optimization applying likewise to across-track interferometry for terrain topographie. (For along-track interferometry, for example, the satellites always selected are those having the optimum along-track baseline e.g. for water current mapping.)
The disadvantage of the cartwheel is that it cannot be optimized at the same time for along-track interferometry and across-track interferometry. The advantage is that maximum baselines (along or across, depending on the cartwheel expression) selected from a set of receiver satellites are highly stable over the complete orbit, i.e. vary little in length. A further disadvantage is that for safety reasons a large separation needs to be maintained between the cartwheel and the transmitter satellites. This separation results in a large bistatic squint and thus in a high Doppler centroid which makes signal processing very difficult and complicated.
In addition to along/across-track interferometry the cartwheel concept also offers the possibility of a super-resolution by making use of angular differences resulting from the local offset of the receiver satellites in along-track and across-track at which the target area is observed. These angular differences result in a shift of the corresponding spectra in the azimuth and range direction. Joining the two spectra in azimuth and range produces
Krieger Gerhard
Mittermayer Josef
Moreira Alberto
Collard & Roe P.C.
Deutsches Zentrum fur Luft-und Raumfahrt e.V.
Gregory Bernarr E.
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