Abstract:
A method for detecting precipitation using a radar locating device for motor vehicles, that is designed to locate objects in the surroundings of the vehicle, in which method a locating signal ( 42 ), which is an index for the received power density as a function of the distance (R), is integrated over a first distance range (INT 1 ), which is below a limit distance (R lim ) for the detection of precipitation, wherein the locating signal ( 42 ) is additionally integrated over a second distance range (INT 2 ), which is above limit distance (R lim ), and for the detection of precipitation, the integrals over the first and second distance range are compared with each other.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for detecting precipitation using a radar locating device for motor vehicles that is designed to locate objects in the surroundings of the vehicle, in which method a locating signal, which is an index for the received power density as a function of the distance, is integrated over a first distance range that is below a limit distance for the detection of precipitation. 
         [0003]    2. Description of Related Art 
         [0004]    Radar locating devices are frequently used in driver assistance systems for motor vehicles, for example, for an automatic distance regulation or for the early detection of a risk of collision. 
         [0005]    Precipitation in the form of rain or also spray spattering up from the road may reflect a portion of the emitted radar radiation and thus cause a reduction of the range of the radar radiation and thus a reduction of the locating depth of the radar sensor. For reasons of traffic safety, it is important that it is possible to detect such a restriction of the function of the radar locating device preferably without delay. 
         [0006]    Published German patent application document DE 10 2006 054 320 A1 describes a method of the type mentioned at the outset for a multibeam radar, in particular for an FMCW radar locating device. In this method, the locating signals of a plurality of radar beams are respectively separately integrated, and the resulting integrals are compared to one another. 
         [0007]    The functional principle of an FMCW radar locating device (Frequency Modulated Continuous Wave) consists in the radar signal being continuously emitted; however, the frequency of this signal is periodically modulated with rising and falling ramps (in this instance, the term “ramp” is not to mean that the frequency modulation within the “ramp” necessarily has to be linear.) A mixer mixes a portion of the transmission signal with the signal received by the antenna and thus generates a mixed product whose frequency corresponds with the difference between the frequency of the current transmission signal and the frequency of the received signal. 
         [0008]    If a radar echo from a located object is received, the frequency of the mixed product is thus a function of the signal propagation time and thus the distance of the object, but, due to the Doppler effect, it is also a function the relative speed of the reflecting object. Thus, in the spectrum formed from the mixed product, each located object emerges on each modulation ramp as a peak in the frequency that is a function of the distance and the relative speed. The distance and the relative speed of the object may then be determined by comparing the frequency positions of peaks that derive from the same object, on modulation ramps having different slopes. 
         [0009]    Rain drops or spattering spray also constitute “objects” in this sense, which leave a weak but still detectable peak in the spectrum when the radar sensor is not at too great a distance, at distances up to approximately 10-50 m, for example. In the event of heavy precipitation, these peaks in the frequency range, which corresponds to the above-mentioned distance range, add up to a background signal, the so-called rain clutter. The power of this rain clutter is thus an index for the presence and the strength of precipitation. 
       BRIEF SUMMARY OF THE INVENTION  
       [0010]    The objective of the present invention is to indicate a method for detecting precipitation, which method is more robust with regard to interference influences. 
         [0011]    According to the present invention, this objective is achieved in that the locating signal is integrated additionally over a second distance range that is above the limit distance, and in that for the detection of precipitation the integrals over the first and second distance range are compared to one another. 
         [0012]    The method according to the present invention has the advantage that the precipitation detection is less significantly impaired by interference influences, for example, by temperature or aging effects of the electronic components of the radar locating device, effects of the installation location and the installation conditions of the radar locating device in the vehicle, special properties of the locating device itself, and the like. Thus, in total, an improved mass production suitability and robustness of the radar locating system is achieved. In addition, in an FMCW radar, an additional advantage is that the method is also independent of the special selection of the parameters of the modulation ramps. 
         [0013]    The interference influences mentioned influence the integrals formed over the first and second distance range in the same manner, while the rain clutter increases only the value of the first integral, but not the value of the second integral. The interference influences mentioned may be largely eliminated in that the two integrals are compared to one another to detect precipitation. 
         [0014]    Advantageous embodiments of and further refinements of the present invention are delineated in the dependent claims. 
         [0015]    In one advantageous specific embodiment of the method according to the present invention, real objects such as preceding vehicles and the like are effectively masked through a filter procedure, so that the subsequent integrations essentially record only those portions of the signal that are caused by noise and rain clutter. 
         [0016]    For example, the integrand is a detection threshold that is obtained through a noise estimation and at the same time may be used to differentiate the real object peaks from the noise background (and, if applicable, the rain clutter). Such a noise estimation may be implemented by a hierarchy filter and is known in the literature under the name “ordered statistic constant false alarm rate” (OS-CFAR) (A. Ludloff: Praxiswissen—Radar und Radarsignalverarbeitung [Practical Knowledge—Radar and Radar Signal Processing], 3rd Edition, Vieweg Publishing House 2002). Due to its adaptivity, the detection threshold adjusts to the respective strength of the rain clutter; however, on the other hand it is not influenced by narrow-band individual peaks caused by “compact” radar targets such as vehicles, for example. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0017]    An exemplary embodiment of the present invention is illustrated in the drawing and explained in greater detail in the following description. 
           [0018]      FIG. 1  shows a block diagram of a radar locating device by which the method according to the present invention may be implemented. 
           [0019]      FIG. 2  shows a schematic sketch regarding the detection of precipitation. 
           [0020]      FIG. 3  shows an example of a radar cell for the detection of precipitation. 
           [0021]      FIGS. 4 and 5  show examples for the evaluation of locating signals in accordance with the method according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    The radar locating device illustrated in  FIG. 1  has an antenna  10 , which is supplied via a mixer  12  with a transmission signal (FMCW radar) that is generated by an oscillator  14  and frequency-modulated by a modulator  16 . The signal emitted by antenna  10  is bundled by a radar lens  18 . If an object (not shown in  FIG. 1 ) is located by the radar locating device, then the signal reflected by the object is bundled again by the lens onto the same antenna  10  that emitted the transmission signal (monostatic antenna concept). In mixer  12 , the received signal is mixed with a portion of the transmission signal, and a time-dependent signal (intermediate frequency signal) is obtained as a mixed product, which is sampled and digitized in an analog/digital converter stage  20 . In a transformation stage  22 , the digitized signal is converted, for example, through fast Fourier transformation (FFT) into a spectrum, in the following labeled locating signal  24 , that indicates the dependency of the amplitude of the mixed product on frequency f. When the Doppler effect is disregarded, frequency f is proportional to distance R of the respective reflection target. In terms of its absolute value, the relative speed of the rain drops is approximately equal to the velocity of the vehicle that is equipped with the radar system. This relative speed therefore results in a shift in frequency, which may be determined by way of calculation. At least within certain limits, it may thus be said that locating signal  24  also constitutes indirectly an index for the received power density as a function of distance R. Thus, in the following, the locating signal is to be considered, in somewhat simplifying fashion, as a function of the distance. In an evaluation stage  26 , locating signal  24  is further evaluated, in order to determine the distances and relative speeds of the located objects and—in the case of a multibeam radar—also their azimuth angles. 
         [0023]    In  FIG. 2 , precipitation in the form of drops  28  is shown, which drops fill up the entire space between radar lens  18  and an object  30  to be located. Each of drops  28  acts as a small reflection target that leaves behind a small peak in the spectrum, that is, in locating signal  24 . These peaks are scattered across a broad frequency band and contribute to the background as so-called rain clutter; however, only up to a specific frequency limit, which corresponds to limit distance R lim , in which a radar echo from the individual rain drops is still detectable (in practice, approx. 10 to 50 m). On the other hand, the reflections by individual drops  28  weaken the radar signal that reaches object  30  and after being reflected is received again by the locating device. This may cause the sensitivity of the locating device to be significantly restricted, for example, in the event of very heavy rain. 
         [0024]    The radar locating device therefore has a detection device  32  that allows for the detection of the presence of precipitation and the quantitative evaluation of the strength of the precipitation, so that it is possible to estimate the extent to which the functionality of the locating system is restricted by precipitation. For this purpose, detection device  32  evaluates locating signal  24 , as described in the following in more detail. 
         [0025]      FIG. 3  illustrates schematically a radar cell  34 , within which rain clutter may be detected. The horizontal of radar cell  34  extends across a specific azimuth angle range and the vertical extends across a specific elevation angle range. These angle ranges are determined by the form of the radar lobes emitted from the radar locating device. In the radial direction, radar cell  34  extends from a certain minimum distance R min  below which a reliable locating of reflection targets is no longer possible, up to limit distance R lim  mentioned, up to which the radar echo of individual drops  28  may still be measured as rain clutter. 
         [0026]      FIG. 4  shows a typical curve of locating signal  24  for the case where no precipitation is present. In the example shown, a peak  36 , which clearly stands out from noise background  38 , represents a located object whose distance is smaller than the limit distance R lim  for the detection of precipitation. For all remaining frequencies and distances, the average height of locating signal  24  is provided by the level of noise background  38 , which is approximately constant over the entire distance range. 
         [0027]    For comparison,  FIG. 5  illustrates the curve of locating signal  24  with precipitation. In this instance, a peak  36 ′ represents an object (vehicle) whose distance is greater than limit distance R lim . The weakening of the radar signal through the reflection by drops  28  initially results in the reduction of the height of peak  36 ′. On the other hand, the reflection by these drops results in the rain clutter mentioned, which may be recognized in  FIG. 5  outside of peak  36  in the form of a background signal  40 , which is higher than noise background  38  and apart from that decreases as distance increases. Starting from limit distance R lim , rain clutter can no longer be differentiated from the normal noise background and is thus no longer detectable. 
         [0028]    A detection of precipitation is now in principle possible in that the power density of locating signal  24  is integrated over a distance interval INT 1  that lies between minimum distance R min  and limit distance R lim . Since the normal noise background  38  for a specific type of the radar locating device is known, the value to be expected for this integral in the precipitation-free case is known, and if the current value of the integral is significantly greater, then this indicates the presence of precipitation. 
         [0029]    However, if at least one object (peak  36 ) is located in distance interval INT 1 , over which integration is to occur in the event of precipitation detection, as in the case illustrated in  FIG. 4 , then peak  36  would result in an increase in the value of the integral in the precipitation-free case as well, and it would not be possible to determine whether the increase of the integral was brought about by the object alone or additionally by precipitation. 
         [0030]    In the method provided here, locating signal  24  therefore is not integrated immediately, but rather this locating signal is first subjected to a filter procedure, for example, a noise estimation, which determines for each frequency value or distance value a detection threshold  42 , which a peak  36 ,  36 ′ caused by an object must at least surpass, in order for the corresponding object to be detected with certainty. 
         [0031]    In  FIG. 1 , detection device  32  accordingly includes a filter stage  44  for the noise estimation and an integration stage  46  for the integration of the filtered signal, that is, of detection threshold  42 . 
         [0032]    As may be seen in  FIGS. 4 and 5 , in each instance detection threshold  42  extends approximately in parallel to noise background  38  and to background signal  40  (more specifically: at an approximately constant distance above this background signal), while it effectively “masks” each peak  36 ,  36 ′ deriving from an object. Now if detection threshold  42  is integrated over interval INT 1 , then the integral obtained (in  FIGS. 4 and 5  represented by a surface  48  and  48 ′) is independent of whether or not an object is located in interval INT 1 . 
         [0033]    In order to also achieve a greater independence with regard to other interference influences, in particular, with regard to temperature and aging effects that influence the sensitivity of the radar locating device, special installation conditions of the radar locating device, and the like, in the method provided here detection threshold  42  is not only integrated over interval INT 1 , which lies beneath limit distance R lim , that is, in the ranges in which rain clutter may occur, but also over a second distance range INT 2  (for example, of the same size), which lies above limit distance R lim , that is, in the range in which the power density outside of object peaks is determined only by noise background  38 . The corresponding integral is represented in  FIGS. 4 and 5  by a surface  50  and  50 ′. 
         [0034]    In the case of  FIG. 4 , that is, without precipitation, approximately the same value is obtained for the integrals represented by surfaces  48  and  50 . As a rule, temperature influences and similar interference influences affect both integrals in the same manner. 
         [0035]    In contrast, in  FIG. 5 , that is, in the event of precipitation, the value of the integral (surface  48 ′) formed over first distance range INT 1  is increased by the rain clutter, while the value of the integral (surface  50 ′) formed over second distance range INT 2  essentially remained the same. 
         [0036]    Detection device  32  shown in  FIG. 1  has a comparison stage  52  that is used to compare the integrals formed over first and second distance ranges INT 1  and INT 2  to each other and to determine the ratio of these two integrals to each other, for example. This ratio immediately provides a reliable index for the strength of the precipitation, a ratio in the proximity of one representing the precipitation-free state. Temperature drift and similar interference influences influence this ratio fundamentally significantly less than the absolute height of the integrals themselves. In this manner, an increased robustness is achieved with respect to the interference influences. 
         [0037]    While in the example shown the spectrum of the intermediate frequency signal is evaluated, in a modified specific embodiment it is also possible to evaluate in a corresponding manner the time signal received in the analog/digital converter stage  20 .