Integrity monitoring of radar altimeters

1. A method for radar altimeter integrity monitoring, the method comprising:

obtaining one or more global navigation satellite system (GNSS) measurements, or one or more hybridized GNSS/inertial navigation system (INS) measurements, in an earth-centered-earth-fixed (ECEF) coordinate frame for a vehicle;

obtaining one or more altitude measurements from one or more radar altimeters on the vehicle;

transforming the one or more altitude measurements into the ECEF coordinate frame using a terrain map and a GNSS or hybridized GNSS/INS based position estimate with ensured integrity, wherein the transformed one or more altitude measurements are given by the following equation:

{tilde over (ρ)} RA,transformed ={tilde over (ρ)} RA +{circumflex over (ρ)} TERMAP

 where {tilde over (ρ)} RA is the radar altimeter measurement, and {tilde over (ρ)} TERMAP is an estimate of orthogonal distance between an ECEF frame origin and terrain at a point given by a horizontal position of the vehicle and the terrain map;

computing one or more characteristics of the transformed one or more altitude measurements, including orthogonal distance ρ TERMAP moments, wherein a mean value of the orthogonal ρ TERMAP distance moments is computed as:

{circumflex over (ρ)} TERMAP =E [ρ TERMAP ]=function 1 ({circumflex over (ρ)} HOR ,map),

 where {circumflex over (ρ)} HOR is a horizontal position estimate of the vehicle and map is the terrain map;

determining a full solution estimate of position for the vehicle based on the transformed one or more altitude measurements, and the one or more GNSS or hybridized GNSS/INS measurements;

determining one or more sub-solution estimates of position for the vehicle based on a subset of the transformed one or more altitude measurements, and the one or more GNSS or hybridized GNSS/INS measurements;

comparing the full solution estimate with the one or more sub-solution estimates using statistical analysis; and

determining an altitude protection level for the vehicle based on a probability of hazardous misleading information and a probability of false detection.

2. A navigation system, comprising:

a processor in a vehicle;

a memory unit operatively coupled with the processor, wherein a terrain map is stored in the memory unit;

a global navigation satellite system (GNSS) receiver operatively coupled with the processor; and

one or more radar altimeters operatively coupled with the processor;

wherein the processor is configured to execute processor readable instructions to perform the method for radar altimeter integrity monitoring according to claim 1 .

3. The navigation system of claim 2 , further comprising a plurality of inertial sensors operatively coupled with the processor and configured to provide inertial data to the processor.

4. The navigation system of claim 3 , wherein the inertial sensors are implemented in an inertial measurement unit (IMU).

5. The navigation system of claim 2 , further comprising a barometric altimeter operatively coupled with the processor.

6. A computer program product, comprising:

a non-transitory computer readable medium having instructions stored thereon executable by a processor to perform the method for radar altimeter integrity monitoring according to claim 1 .

7. A method for radar altimeter integrity monitoring, the method comprising:

obtaining one or more global navigation satellite system (GNSS) measurements, or one or more hybridized GNSS/inertial navigation system (INS) measurements, in an earth-centered-earth-fixed (ECEF) coordinate frame for a vehicle;

obtaining one or more altitude measurements from one or more radar altimeters on the vehicle;

determining a first plurality of sub-solution estimates of altitude above terrain with ensured integrity for the vehicle, based on subsets of the one or more GNSS or hybridized GNSS/INS outputs and a terrain map, each sub-solution estimate in the first plurality of sub-solution estimates determined based on a respective sub-set of the one or more GNSS or hybridized GNS SANS outputs;

computing a variance of the estimates of altitude as:

var[ρ TERMAP ]=σ MAP 2 +σ MAP,HPint 2

 where σ MAP 2 is given by accuracy of the terrain map, and σ MAP,HPint 2 describes additional uncertainty given by a GNSS horizontal position protection area, wherein σ MAP,HPint 2 is computed on the basis of terrain map evaluation at multiple points within the protection area given by a horizontal protection level (HPL) as:

σ MAP,HPint 2 =function 2 ({circumflex over (ρ)} HOR ,HPL,map);

determining a second plurality of sub-solution estimates of altitude above terrain with ensured integrity for the vehicle, based on a subset of the one or more altitude measurements, each sub-solution estimate in the second plurality of sub-solution estimates determined based on a respective subset of the one or more altitude measurements; and

merging the first plurality of sub-solution estimates and the second plurality of sub-solution estimates to determine a full solution or global estimate of altitude above terrain, which provides an altitude protection level for the vehicle, wherein a global altitude estimate mean is computed as a weighted average of local mean estimates, and a global covariance matrix is computed as a weighted sum of local variances, wherein a global protection level is determined as the protection level overbounding all local protection levels, in that:

p (HMI fused )≤ p (HMI estim )× p (HMI RAmeas,1 )× . . . × p (HMI RAmeas,n )

 where p(HMI) is the probability of hazardous misleading information.

8. A navigation system, comprising:

a processor in a vehicle;

a memory unit operatively coupled with the processor, wherein a terrain map is stored in the memory unit;

a global navigation satellite system (GNSS) receiver operatively coupled with the processor; and

one or more radar altimeters operatively coupled with the processor;

wherein the processor is configured to execute processor readable instructions to perform the method for radar altimeter integrity monitoring according to claim 7 .

9. The navigation system of claim 8 , further comprising a plurality of inertial sensors operatively coupled with the processor and configured to provide inertial data to the processor.

10. The navigation system of claim 9 , wherein the inertial sensors are implemented in an inertial measurement unit (IMU).

11. The navigation system of claim 8 , further comprising a barometric altimeter operatively coupled with the processor.

12. A computer program product, comprising:

a non-transitory computer readable medium having instructions stored thereon executable by a processor to perform the method for radar altimeter integrity monitoring according to claim 7 .