Present-day driver assistance systems frequently operate with radar sensors. These radar sensors are now used for distance measurements and to determine relative speeds of observed objects. The azimuth angle under which an object is observed can also be determined with ordinary radar sensors.
Current and future comfort and safety systems for automobiles are described in the document “Optimized Transmitted Signal Proposal for Automobile Radar”, M. M. Meinecke, Shaker Verlag 2001, ISBN 3-8265-9223-9. Different radar sensor techniques, with their properties, are also presented.
Linear frequency modulated continuous wave sensors (LFMCW sensors) are often used, whose essential design is shown in FIG. 4. Very precise distances can be determined with these sensors. This type of LFMCW radar sensor is characterized by its range resolution ΔR, its maximum range Rmax and the speeds Vrel,min to vrel,max. The relations of radar parameters to transmitted signal relevant for the sensors are given by:             Δ      ⁢                           ⁢      R        =          c              2        ⁢                  f                      Hub            ,            Radar                                ,          ⁢           ⁢            Δ      ⁢                           ⁢      v        =          λ              2        ⁢                  T                      Chirp            ,            Radar                              Hub=swingin which the radar signal can consist of a sequence of linear chirps (linear frequency modulated transmitted signals) with different slopes. The radar signal is also modulated with a specific frequency swing. The slope mi of an individual chirp i is defined by       m    i    =            f              Hub        ,        Chirp                    f              Hub        ,        Radar            Hub=swing
The radar chirps are reflected by observed objects. The reflected radar chirps are represented as frequency peaks in the receiver in the frequency spectrum, for example, in an FFT analysis. The position of the peak then specifies the target coordinates in range or speed of the observed object.
A frequency spectrum of a typical point target is shown as an example in FIG. 1. In this example an object is situated at a range R=100 m and has a relative speed of v=20 m/s. At a modulation swing of fswing,radar=150 MHz and a measurement time of Tchirp,radar=2.5 ms, the spectrum shown in FIG. 1a is obtained for an upchirp (linear frequency up-modulated transmitted signal) and for a downchirp in FIG. 1b. As is apparent from the figures, the frequency peaks 1 are found at the frequencies κ1=185 (FIG. 1a) and κ2=129 (FIG. 1b).
The information of an individual chirp is ambiguous and restricts the target coordinates of the target only with respect to degrees of freedom, and the following applies       v          rel      ,      f        =                                          -            Δ                    ⁢                                           ⁢          v                          Δ          ⁢                                           ⁢          R                    *              m        1            *              R        j              +          Δ      ⁢                           ⁢      v      *              κ        1            in which m1 is the normalized slope of chirp 1 and κ1 is the corresponding measured normalized frequency in spectrum 1 (see FIG. 1a).
To eliminate ambiguity, several chirps of different slope can be used, in order to achieve an unambiguous measurement at the intersection of their lines.
Calculation of the intersection points in the range-relative speed diagram (R-vrel diagram) occurs by calculating all ideal intersection points of all lines from the up- and downchirps from all found frequency positions κ1 (corresponding measured normalized frequency from spectrum 1 of chirp 1) and κ2 (corresponding measured normalized frequency from spectrum 2 of chirp 2) according to the relation:             R      f        =          Δ      ⁢                           ⁢      R      ⁢                           ⁢                                    κ            2                    -                      κ            1                                                m            1                    -                      m            2                                ,          ⁢            v              rel        ,        i              =          Δ      ⁢                           ⁢      v      ⁢                           ⁢                                                  m              1                        ⁢                          κ              2                                -                                    m              2                        ⁢                          κ              1                                                            m            1                    -                      m            2                              
It is also possible that additional linear chirps are evaluated to improve the results. It is also possible to carry out measurements according to linear frequency-modulated shift keying (LFMSK method, see “Optimized Transmitted Signal Proposal for Automobile Radars”, M. M. Meinecke) or the frequency shift keying (FSK) method or according to pulse radar techniques. Other methods are also conceivable.