Method and radar system for determining road conditions

1. Method for determining a condition of a road by using a radar system,

wherein the radar system comprises a radar transmitter unit configured to transmit radar waves having two different polarizations and a radar receiving unit configured to receive radar waves having two different polarizations, and

wherein the radar system is configured to provide transmit signals and receive signals indicating an intensity of the respective transmitted and received radar waves,

the method comprising:

determining, via a processing device, co-polarized backscattering coefficients and at least one cross-polarized backscattering coefficient based on the transmit signals and the receive signals provided by the radar system,

if the cross-polarized backscattering coefficient is greater than or equal to a predetermined cross-polarization threshold, determining, via the processing device, the condition of the road based on a ratio of the co-polarized backscattering coefficients and based on a difference of one of the co-polarized backscattering coefficients and the cross-polarized backscattering coefficient, and

if the cross-polarized backscattering coefficient is smaller than the cross-polarization threshold, determining, via the processing device, the condition of the road based on the ratio and a difference of the co-polarized backscattering coefficients.

2. Method according to claim 1 , wherein

the radar transmitter unit is configured to transmit radar waves having horizontal polarization and radar waves having vertical polarization, and the radar receiving unit is configured to receive radar waves having horizontal polarization and radar waves having vertical polarization,

the ratio of the co-polarized backscattering coefficients is determined by dividing a backscattering coefficient for horizontally polarized transmitted radar waves and horizontally polarized received radar waves (HH backscattering coefficient) by a backscattering coefficient for vertically polarized transmitted radar waves and vertically polarized received radar waves (VV backscattering coefficient).

3. Method according to claim 2 , wherein

the cross-polarized backscattering coefficient is the backscattering coefficient for horizontally polarized transmitted radar waves and vertically polarized received radar waves (HV backscattering coefficient).

4. Method according to claim 3 , wherein

if the HV backscattering coefficient is greater than or equal to the predetermined cross-polarization threshold, the difference of one of the co-polarized backscattering coefficients and the cross-polarized backscattering coefficient is determined as the difference between the VV backscattering coefficient and the HV backscattering coefficient.

5. Method according to claim 3 , wherein

if the HV backscattering coefficient is smaller than the predetermined cross-polarization threshold, the difference of the co-polarized backscattering coefficients is determined as the difference between the VV backscattering coefficient and the HH backscattering coefficient.

6. Method according to claim 3 , wherein

if the HV backscattering coefficient is smaller than the predetermined cross-polarization threshold, a plurality of HH backscattering coefficients and a plurality of VV backscattering coefficients are determined at different elevation angles, and wetness of the road is determined based on a variance of the ratio with respect to the difference.

7. Method according to claim 6 , wherein

the condition of the road includes a wet surface if the variance is greater than a predetermined variance threshold, and the condition of the road includes a dry surface if the variance is equal to or smaller than a predetermined variance threshold.

8. Method according to claim 6 , wherein

determining the condition of the road additionally includes determining a material of the road based on one of the co-polarized backscattering coefficients.

9. Method according to claim 8 , wherein

the material of the road is determined based on the VV backscattering coefficient.

10. Method according to claim 3 , wherein

if the HV backscattering coefficient is smaller than the predetermined cross-polarization threshold, a plurality of HH backscattering coefficients and a plurality of VV backscattering coefficients are determined, and

if an average of the ratio of the HH backscattering coefficients and the VV backscattering coefficients is smaller than or equal to 1 and an average of the difference between the VV backscattering coefficients and the HH backscattering coefficients is smaller than or equal to 0, the condition of the road is determined based on the average of the ratio with respect to the average of the difference.

11. Method according to claim 10 , wherein

the condition of the road includes a high irregularity due to vertical objects if the average of the ratio of the HH backscattering coefficients and the VV backscattering coefficients is close to 1 and the average of the difference between the VV backscattering coefficient and the HH backscattering coefficient is close to 0,

the condition of the road includes snow on the road if the average of the ratio of the HH backscattering coefficients and the VV backscattering coefficients is smaller than 1 and the average of the difference between the VV backscattering coefficient and the HH backscattering coefficient is smaller than 0.

12. Method according to claim 1 , wherein

if the cross-polarized backscattering coefficient is greater than or equal to the predetermined cross-polarization threshold, the condition of the road is determined based on thresholds for the ratio of the co-polarized backscattering coefficients and for the difference of one of the co-polarized backscattering coefficients and the cross-polarized backscattering coefficient.

13. Method according to claim 12 , wherein

for a plurality of predefined road conditions, the thresholds define respective ranges for the ratio of the co-polarized backscattering coefficients and respective ranges for the difference of one of the co-polarized backscattering coefficients and the cross-polarized backscattering coefficient, and

a respective combination of a range for the ratio of the co-polarized backscattering coefficients and of a range for the difference of one of the co-polarized backscattering coefficients and the cross-polarized backscattering coefficient is uniquely related to one of the plurality of predefined road conditions.

14. Radar system configured to be installed at a vehicle, the radar system comprising:

a radar transmitter unit configured to transmit radar waves having two different polarizations and to provide transmit signals indicating an intensity of the transmitted radar waves,

a radar receiving unit configured to receive radar waves having two different polarizations and to provide receive signals indicating an intensity of the received radar waves, and

a processing device configured to:

determine co-polarized backscattering coefficients and at least one cross-polarized backscattering coefficient based on the transmit signals and the receive signals,

if the cross-polarized backscattering coefficient is greater than or equal to a predetermined cross-polarization threshold, determine the condition of the road based on a ratio of the co-polarized backscattering coefficients and based on a difference of one of the co-polarized backscattering coefficients and the cross-polarized backscattering coefficient, and

if the cross-polarized backscattering coefficient is smaller than the cross-polarization threshold, determine the condition of the road based on the ratio and a difference of the co-polarized backscattering coefficients.

15. Non-transitory computer readable medium comprising instructions for carrying out the computer implemented method of claim 1 .