Method for monitoring the earth surface

The invention relates to a method for monitoring the earth's surface with a moving aircraft using a radar sensor with a synthetic aperture. In order to produce high resolution radar images, the flight parameters of the aircraft are also required. According to the inventive method, key parameters, such as speed and acceleration, are detected in the sight line of the radar sensor from the radio signals received. One of the advantages of the invention is that an inertial navigation system (INS) is no longer required for this purpose.

BACKGROUND OF THE INVENTION
 The invention is directed to a method for monitoring the earth's surface
 using synthetic aparture radar.
 Monitoring of the surface of the earth is required for many applications. A
 moving aircraft, e.g. an airplane or a dirigible or a satellite, may be
 used, if the monitoring for the most part must be independent of the
 influence of weather conditions, e.g. clouds and/or fog, it makes sense to
 provide an aircraft with a radar sensor having a viewing direction toward
 the earth's surface. In the event that high spatial resolution must be
 achieved during the monitoring, it makes sense to use a radar sensor with
 a synthetic aperture. This radar sensor is operated, for example, in the
 so-called SAR mode or the DBS mode (operating mode). This type of radar
 monitoring requires that the flight parameters in particular are always
 known, e.g. flight elevation, flight path, and flight speed of the
 aircraft. Only if these are known can radar images, recorded successively
 in time with the synthetic aperture method, be evaluated with
 predetermined accuracy, without the occurrence of errors and/or
 interferences such as fuzzy images. The aforementioned flight parameters
 can be determined, for example, with an INS (inertial navigation system).
 Such a method has the disadvantage of being technically involved and not
 cost-effective because an autonomous device, the INS device, must be used
 to determine the parameters, required in particular for SAR radar images.
 SUMMARY OF THE INVENTION
 It is thus the object of the invention to specify a method of the type
 discussed above, which permits in a cost-effective manner an automatic
 focusing of radar images, generated with the aid of a radar sensor with a
 synthetic aperture.
 This object is solved with a monitoring system which is characterized in
 that the transmitting/receiving antenna of the radar sensor is designed
 such that in the region to be monitored, an illuminated spot for the
 antenna is generated, which is adapted to a specifiable spatial
 resolution; a radar image is generated from the received radar signals
 through superimposing Fourier spectra, which have been processed
 block-by-block with respect to the range and/or the Doppler signal. By
 correlating such Fourier spectra, which succeed each other in time, in the
 azimuth and elevation dimensions, it is possible to determine the
 parameters acceleration on the line of sight (B.sub.los), as well as
 ambiguity errors on the line of sight (M.sub.los). The remaining speed on
 the line of sight (R.sub.los) is determined within an ambiguity range by
 estimating the echo center of gravity of the Doppler signal within the
 Fourier spectra. A definite speed is determined on the line of sight
 (V.sub.los) in accordance with the following formula:
EQU (V.sub.los)=(M.sub.los).multidot..lambda.B.sub.m /2+R.sub.los,
 wherein
 .multidot.=multiplication operator
 .lambda.=radar wave length; and
 B.sub.m =measuring band width of the radar sensor in Doppler.
 The parameters phase correction range (.phi..sub.r) and phase correction
 Doppler (.phi..sub.d), which are necessary to focus the radar images, are
 determined from the acceleration on the line of sight (B.sub.los) and the
 speed on the line of sight (V.sub.los).
 BRIEF DESCRIPTION OF THE DRAWINGS

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The invention is based on the realization that it is possible to determine
 from the radar signal, analyzed by the radar sensor, the speed and
 acceleration on the line of sight, meaning the connecting line between the
 radar sensor and the illuminated spot for the antenna on the earth
 surface. These develop as a result of apparent motions of the radar sensor
 relative to the earth surface, particularly for a fast-moving aircraft.
 Such parameters, obtained during the analysis of the radar signals, are
 used to determine the parameters that are necessary for focusing the
 image.
 For this, the radar sensor analyzes a predetermined region of the earth's
 surface to be monitored in the SAR scan mode, the spotlight mode and/or
 the MTI mode. An angle tracking of a detected target is possible if the
 radar sensor is operated in the monopulse mode. In that case, it is
 particularly advantageous if the radar sensor can process the received
 signals in real-time.
 FIG. 1 shows a diagram of the design for a transmitting/receiving unit of a
 radar sensor in the millimeter wave range (mmw). The following features
 are used in this case for the radar sensor:
 radar frequency range 35 Ghz (K.sub.a band);
 modulation method: linear FM-CW;
 full inter-ramp coherence;
 narrow beam transmitting/receiving antenna of the Cassegrain type;
 antenna on a stabilized platform;
 4-channel monopulse system; and
 two orthogonal received polarizations that are coupled with the two angular
 planes (azimuth and elevation).
 According to FIG. 1, a FM modulation signal is generated with digital
 direct synthesis (DDS) and is fed via an up converter (UP) to a HF
 transmitter. The echo signal from the four receiving channels is
 respectively mixed (MIX) with the decoupled transmitting signal and,
 following a low-pass filtering (TP), is scanned or sampled (digitized)
 (A/D) (homodyne method). A cardanic system with position stabilization of
 the antenna (platform) permits an orientation of the antenna in the
 azimuth range over more than 180.degree. (parallel to the earth surface),
 symmetrical to the flight direction (meaning a so-called "forward looking"
 and "side looking" to the right or left is possible), as well as over more
 than 90.degree. in the elevation range (perpendicular to the earth
 surface).
 As a result of the selected antenna design, a narrow antenna characteristic
 is produced, which is necessary for a possible target tracking, as well as
 a good output balance. A high spatial resolution is achieved through a
 correspondingly selected band width for the transmitting signal, as well
 as a coherent integration time that is adapted to this. The illuminated
 antenna spot on the earth surface has dimensions of, for example, 100 m to
 150 m which, together with the previously mentioned position stabilized
 platform, makes it possible to conduct, for example, a meandering scanning
 in a specifiable azimuth/elevation range (FIG. 2). The dwell time for a
 (radar) target to be detected and/or classified in the antenna lobe is
 essentially determined by a specifiable antenna search pattern, e.g. a
 meandering scanning. In that case, an increase in the Doppler resolution
 requires a reduction in the otherwise selected, specifiable search strip
 width or an interruption over a predetermined period of time. Such
 interruptions are necessary, e.g. for an exact analysis of a detected
 target (classification). In dependence on the available target scenario,
 this occurs in a so-called "spotlight mode" or a search sweep with a very
 low, specifiable sweeping speed.
 The so-called MTI mode of the radar sensor is used for the azimuth sweep
 symmetrical to the flight direction. In that case and with unchanged
 transmitting modulation and signal pre-processing, a possibly existing
 ground clutter is imaged in this viewing direction on just a few
 resolution cells of the Doppler spectrum because of the selected narrow
 Doppler band width. Driving (moving) targets consequently appear primarily
 outside of this range, in the thermal noise. The existing high range
 resolution is maintained and is used for the target recognition.
 FIG. 3 shows an arrangement for analyzing the received signals (echo
 signals). In that case, the four received signals are initially digitized
 (A/D) and parallel-processed (processed). Initially, the analysis window
 for the "range" (distance) is selected via a digital synthesizer, as well
 as a FIR filter. Prior to the further signal processing, the signal is
 subsampled or sub-scanned, based on the band width reduction of the FIR
 filter.
 Following a phase compensation, the existing, small dimension of the
 illuminated antenna spot on the earth surface permits a simple
 range-Doppler-processing (distance Doppler processing) of the signals, a
 method known per se, since the objects (targets) of interest are located
 within the depth of sharpness range ("depth of focus"), even with a high
 lateral resolution of, for example, less than 1 m. Individual, so-called
 footprint images are generated for this with a block processing method in
 a fixed, predetermined time grid. These images strongly overlap in the
 aperture, as specified, and are assembled on the basis of the so-called
 multilook principle to form larger images, depending on the search
 pattern. The previously mentioned high range resolution (distance
 resolution) of less than 1 m requires an very exact determination of the
 speed V.sub.los on the line of sight. The same is true for the
 low-frequency share of the acceleration B.sub.los on the line of sight
 where the gravity contribution cannot be compensated exactly, owing to the
 unavoidable uncertainty when directing the antenna.
 Signals for which the highest frequency is lower than the inverse value of
 the coherent integration time, explained in more detail in the following,
 are herein referred to as low-frequency signals. Low-frequency signals can
 be found, for example, in a frequency range of 1 Hz to 10 Hz.
 The auto-focusing carried out with this is incorporated into the block
 processing of the signals in such a way that time-consuming iterations are
 advantageously avoided. For the signal processing, the method known as
 "mapdrift" from the literature reference "Spotlight Synthetic Aperture
 Radar: Signal Processing Algorithms, Walter G. Carrara, Ros S. Goodman and
 Ronald M. Majewski, Artech House, 1995," is modified according to FIG. 4
 in such a way that the speed V.sub.los and the low-frequency share of the
 acceleration B.sub.los can be determined simultaneously.
 The incoherently composed radar images are used as basis for detecting a
 target and supporting the aircraft navigation, whereas the complex
 individual images from the four receiving channels in two polarizations
 are available for the target analysis.
 In the following, the determination of the speed V.sub.los and the
 acceleration B.sub.los on the line of sight is explained in further
 detail.
 The SAR and the DBS technology can supply the required resolution. For this
 technology, a radar signal with wide band width B.sub.r is transmitted
 (transmitting signal S.sub.r) with suitable modulation within a short
 frame cycle T.sub.r, and its echo is received (E.sub.r), is scanned with
 the clocking frequency T.sub.s and is processed (N.sub.s scanning values
 within the clocking frequency T.sub.s).
 The transmitting signal S.sub.r is repeated periodically via a specifiable
 number N.sub.r of frame cycles T.sub.r. The coherent processing of the
 received signal E.sub.r leads to a two-dimensional spectrum of the "range"
 and "Doppler" dimensions, wherein with a suitably selected measuring
 geometry, the Doppler dimension provides the local resolution lateral to
 the range axis ("azimuth").
 The following correlations apply:
 Range resolution:
 ##EQU1##
 Measuring band width for Doppler:
 ##EQU2##
 Doppler resolution:
 ##EQU3##
 Azimuth resolution:
 ##EQU4##
 c=speed of light; .lambda.=radar wave length;
 .OMEGA.--angle difference to the target object within the integration time
 T.sub.int =N.sub.r T.sub.r.
 The processing of the N.sub.r received signals E.sub.r for the herein
 described method occurs as two-dimensional Fourier transformation of the
 matrix.
 E.sub.tt =[E.sub.r (T.sub.r) . . . E.sub.r (N.sub.r T.sub.r)] after the
 matrix E.sub.tt phase has been corrected as specified. In this as well as
 the following formulas, letters with double underlining designate a matrix
 and letters with single underlining designate a vector. This so-called
 range Doppler processing ensures that the radar images are focused within
 the depth of focus .DELTA.R, resulting from the following formula:
 ##EQU5##
 A method for determining the correction phases of the matrix E.sub.tt is
 explained in the following, which can be used for sequentially generated
 range Doppler images.
 Marginal Conditions
 A coherent integration time T.sub.int is selected, which cannot last any
 longer than approximately 1/2 the target illumination time TOT ("time on
 target"). The DBS image generating process is repeated at timed intervals
 T.sub.image (image cycle), wherein the (not necessarily constant) time
 interval T.sub.image is measured such that several images can be computed
 during the time on target TOT, which together contain echo signals from
 the total time on target TOT (FIG. 5). These procedural steps are in the
 following called block processing, wherein each block is defined by the
 matrix E.sub.tt.
 Signal Processing
 The signal processing process is shown diagrammatically in FIG. 6. The
 sequence in time is as follows:
 a) Generating the E.sub.tt spectrum:
EQU E.sub.tt
 =&gt;e.sup.i.phi.R.smallcircle.=&gt;FFT"range"=&gt;e.sup.
 i.phi.R.smallcircle.FFT"Doppler"=&gt;E.sub.RD
 or
EQU E.sub.tt
 =&gt;e.sup.i.phi.R.smallcircle.e.sup.
 i.phi.R.smallcircle.=&gt;FFT"range"=&gt;FFT"Doppler"=&gt;E.sub.RD,
 wherein
 E.sub.RD is the spectrum belonging to the matrix E.sub.tt and .smallcircle.
 represents the linking operator.
 The phase correction .phi..sub.r amounts to:
 ##EQU6##
 V.sub.los =up-to-date measured value for the line of sight speed.
 The phase correction .phi..sub.d is:
 ##EQU7##
 B.sub.LOS =up to date measured value for the line of sight acceleration.
 b) Obtaining V.sub.LOS and F.sub.DOT from the spectrum E.sub.RD :
 i) Measuring the echo center of gravity in "range" J.sub.r and "Doppler"
 J.sub.d
 ii) Correlation of .vertline.E.sub.RD.vertline.(t.sub.0) and
 .vertline.E.sub.RD.vertline.(t.sub.0 -mT.sub.image) along the "range" and
 the "Doppler,"
 wherein the total number m is selected such that
 .vertline.E.sub.RD.vertline.(t.sub.0) and .vertline.ERD .vertline.(t.sub.0
 -mT.sub.image) is measured within the time on target TOT of a point
 target.
 The correlation occurs at a section of the range that is symmetrical to the
 average value from J.sub.r (t.sub.0) and J.sub.r (t.sub.0 +mT.sub.image).
 The correlation is cyclical along the "Doppler" dimension.
 The maxima of the correlation function for positions K.sub.r and K.sub.d
 are located in the "range" or "Doppler" dimension.
 iii) Determination of V.sub.LOS from K.sub.r and J.sub.d :
 The echo center of gravity in "Doppler" measures the frequency:
 ##EQU8##
 respectively the speed
 ##EQU9##
 The correlation measures the speed:
 ##EQU10##
 The ambiguity share in the Doppler frequency M.sub.los M.sub.los.epsilon.Z
 (total number), and thus also V.sub.LOS, is determined with:
 ##EQU11##
 iv) Determination of F.sub.DOT from K.sub.d :
 ##EQU12##
 c) Filtering V.sub.LOS and F.sub.DOT :
 The measuring band width for V.sub.LOS and F.sub.DOT is
 1/mT.sub.image.apprxeq.1/T.sub.integrate. A specifiable, non-linear filter
 (Median or Sigma filter) eliminates interferences occurring as a result of
 poor measuring quality (lack of inhomogeneity of the radar echo for the
 correlation).
 The contrast of the correlation maxima at K.sub.r or K.sub.d controls an
 adaptive low-pass filter, so that the resulting measuring band
 width&lt;1/mT.sub.image.
 d) Generate new image:
EQU t.sub.0.fwdarw.t.sub.0 +T.sub.image ; see section a).
 Testing the Method
 The method was used successfully with measured data from a Ka-band radar
 sensor, wherein resolutions below 1 m.times.1 m were achieved. Depending
 on the measuring scenario, there is a "transient phase" of 2 to 15 image
 cycles until focused images are achieved. If necessary, this phase can be
 shortened if supported by INS information. The 2-dim correlation can be
 taken back to two 1-dim correlations if the Doppler correlation is
 initially carried out and the Doppler offset K.sub.d is then taken into
 account for the range correlation (see FIG. 7).
 With such a method, for example, it is possible to produce a radar sensor
 for the following operating conditions:
 for manned or unmanned subsonic aircraft;
 for a sensor range of several kilometers;
 for a search strip width of several kilometers;
 for an autonomous target recognition and target tracking within the search
 area, e.g. for an unmanned aircraft;
 for an exact target survey with a point of impact selection;
 for a reliable monitoring operation, even if weather conditions are
 unfavorable, for example if weather conditions are poor (rain, snow and/or
 clouds).
 The invention is not limited to the above-described examples, but can be
 used in many different ways, e.g. for the detection, classification and
 tracking of stationary as well as moving objects, such as land vehicles
 and/or oceangoing vehicles (ships).