Radar method used in a motor vehicle

The invention relates to a radar method for an automatic intelligent traffic control (AICC) in a motor vehicle. The use of a frequency modulation continuous wave method (FM-CM) is suggested in order to securely detect the distance to, relative speed and angle of a preceding motor vehicle. It is furthermore suggested according to the invention that when using an A/D converter 5 with 8-bit resolution, the necessary dynamics are generated by means of a level switchover, that the R, V information is generated in FFTs [Fast Fourier Transformations] 6 with blocked R and V-FFTs, that the useful signals are separated from the noise in a detection device 7 by means of a R-dependent adaptive CFAR threshold, that in a track formation 8, the detection is directly assigned to the tracks and that the association of a detection i to a track j is in the process computed as probability r (i, j).

CROSS-REFERENCE TO RELATED APPLICATION
 This application claims the priority of German Patent Application No. 198
 22 622.5 filed May 20, 1998.
 BACKGROUND OF THE INVENTION
 The invention relates to a radar method used in a motor vehicle for an
 automatic, intelligent traffic flow control (AICC).
 The characteristics and design of known radar devices in motor vehicles are
 essentially defined by the use of pulse radar. The known systems make use
 of involved and cost-intensive multipath systems to detect several
 measuring variables,.
 A high-resolution radar method is described in the previously filed German
 patent application 197 32 509.2, which is based on a frequency-modulated,
 continuous-wave (FM-CW) method with saw-tooth modulation to achieve a
 simultaneous resolution of the radar scene with respect to distance and
 relative speed. With this method, the radar echo is mixed at the
 transmitting reference to form the baseband signal. The method is
 distinguished in that:
 The baseband signal of each modulation period is digitized in an
 analog/digital converter and is converted to distance/time values by means
 of a N-point Fourier Transformation;
 this conversion is repeated for M modulation periods that follow
 successively in time;
 the determined distance values for each modulation period are stored in a
 line, assigned to the respective modulation period, of a two-dimensional
 data memory;
 the stored distance/time values are subjected in columns to an M-point
 Fourier Transformation to form relative speed values and that a
 distance/speed matrix of the observed radar scene is generated in this
 way.
 This method provides for a frequency-dependent and thus also
 distance-dependent level compensation in the analog portion of the radar
 sensor, or a frequency limitation of the baseband signals to higher
 frequencies by utilizing an anti-aliasing filter that is connected in
 series before the A/D converter. One embodiment of the method provides
 that of the N distance/time values, determined with the N-pointed Fourier
 Transformation, only N.sub.R &lt;N/2 values are stored and processed for the
 subsequent signal processing. The Fourier Transformations used (e.g. the
 Fast Fourier Transformation (FFT)) operate with suitable data windows to
 reduce minor lobes to a preset value for the resolution cells of the
 distance/speed matrix.
 SUMMARY OF THE INVENTION
 It is the object of the invention to create a cost-effective radar method
 for an AICC in a motor vehicle, which makes it possible to have radar
 systems using little space. The radar method is designed to detect the
 distance, the relative speed and the angle to preceding motor vehicles and
 to process these values for use in an AICC.
 The solution according to the invention is provided by a radar method used
 in a motor vehicle for an automatic, intelligent communication control
 (AICC), characterized in that a FM-CW method is used, that when using an
 A/D converter 5 with 8-bit resolution, the necessary dynamic is generated
 with a level switchover, that the R, V information is generated in FFTs 6
 with blocked R and V FFTs, that the useful signals are separated from the
 noise in a detection 7 by means of a R-dependent, adaptive CFAR threshold,
 that in a track formation 8 the detection is assigned directly to the
 tracks, and that in the process, the assigning of a detection i to a track
 j is computed as probability r(i,j). The crosstalk noise from the
 transmitting range and the receiving range, which is standard for short
 distances, is compensated in two stages.

DETAILED DESCRIPTION OF THE INVENTION
 The block diagram shown in FIG. 1 contains the analog signal modules,
 standard for a FM-CW radar sensor, comprising a HF block 1 and a video/mod
 printed circuit board 2. The digital signal processing occurs on a DSP
 circuit board 3. The signal input of this board is supplied via a cable
 connection 4 with the analog signal of the radar scene, which is
 determined by the radar sensor. On the DSP circuit board 3, the analog
 signal is digitized in an A/D converter 5.
 The method according to the invention uses a radar sensor with
 cost-effective A/D converter 5 with 8-bit resolution for a scanning rate
 of 2 Mhz in order to convert the analog measured values from the radar
 sensor. A level switchover for the A/D converter is implemented into the
 method to ensure the necessary dynamic of more than 50 db with this low
 bit number. This level switchover is triggered by the signal amplitude
 behind the A/D converter 5. If a threshold value is exceeded within a
 specified time interval of a predetermined number of values, then an
 analog amplification switchover is triggered on the input side. At the
 same time, the digital signal is amplified by the same factor, so that the
 level ratio of the digital processing remains unchanged. The level
 switchover of the A/D converter 5 is not explained in further detail in
 FIG. 1 for reasons of clarity.
 As is known, the R-/V gates are generated with the FM-CW method via a
 frequency analysis of the baseband. In the most simple case, the R-/V
 information is obtained with an M*N points FFT 6, following resorting of
 the frequency gates. This refers to M ramps and N scanning values for each
 ramp. The invention, however, calls for a blocked processing with M pieces
 N-points R-FFTs and N.sub.r pieces M-points V-FFTs The R-FFTs are real and
 form N/2 valid values; the V-FFTs are complex. The advantage of this
 processing is that the R gates can be processed selectively (N.sub.r &lt;N/2)
 and that less storage space is needed for the synchro factors of the FFTs
 6. The FFTs 6 are realized twice because of the clock pulse switching
 known in the FM-CW method.
 Furthermore, a signal is present in principle for short distance in the
 FM-CW method, which signal represents the crosstalk noise from the
 transmitting branch and the receiving branch and is the result of the
 simultaneous transmitting and receiving operation. The signal normally
 blocks the gates 1 and 2 (0-10m), or an object is simulated in this way at
 this location.
 According to the invention, this crosstalk noise is compensated in two
 stages. For V.sub.inherent &gt;15 km/h, the crosstalk signal (amount) is hard
 compensated; for V.sub.inherent &lt;15 km/h, the crosstalk signal (amount) is
 estimated, and the change in the signal is observed to be able to
 distinguish slow-moving, real objects at R-gate 1 or 2, e.g. driving in
 the city, at a traffic light or in a backup, from the crosstalk noise
 signal.
 The R-gates and V-gates from the two FFTs are arranged into a matrix in the
 detection 7. The power distribution within this R, V matrix is used for
 the detection of existing objects and the determination of distance and
 speed. With the method according to the invention, the detection occurs by
 means of an adaptive CFAR threshold, which separates useful signals from
 the noise. The threshold is designed to be R-dependent, in order to do
 justice to the R-dependent noise level. The threshold value for a specific
 distance for the following cycle is formed through averaging via the V
 cells, which are below the threshold in the current cycle.
 The method according to the invention includes a track formation 8,
 designed to track individual objects over time and, if they exist, to
 select objects that are relevant to the problem definition. Several
 process steps are planned for this:
 The initialization of tracks;
 The assigning of completed detections to existing tracks;
 The filtering/updating of the characteristic values (distance, speed and
 angle) of a track;
 The deleting of tracks, which are no longer supplied with new data; and
 The selection/prioritizing of specific tracks based on suitable criteria.
 The special feature of the implemented track formation 8 is that it assigns
 the detection directly to the tracks, without intermediate step. Depending
 on the method according to the invention, up to nine detections exist even
 for a point target. Real objects are therefore expanded in the R, V
 matrix.
 Owing to this expansion of real objects, an intermediate step is carried
 out in known methods, the so-called object formation. This object
 formation combines related detections and computes respective average
 values of the characteristic variables for the related object. The tracks
 are then formed from these objects.
 The difficulty with this known track formation in certain situations is
 that it must be determined what "related" stands for. Complicated methods
 are required in part to identify as such closely related detections, e.g.
 belonging to two real objects, since the detections can fuse to form one
 object.
 Detections can also be assigned simultaneously to several tracks for the
 assigning. According to the invention, the assigning of a detection i to a
 track j is computed in the track formation 8 as probability r(i,j), which
 depends on a generalized distance measure.