Methods and systems for enhancing accuracy of terrain aided navigation systems

A navigation system is described which includes a navigation processor, an inertial navigation unit configured to provide a position solution to the navigation processor, and a digital elevation map. The described navigation system also includes a radar altimeter having a terrain correlation processor configured to receive map data from the digital elevation map and provide a position solution based on radar return data to the navigation processor. A map quality processor within the navigation system is configured to receive map data from the digital elevation map and provide a map quality factor to the navigation processor which weights the position solution from the terrain correlation processor according to the map quality factor and determines a position solution from the weighted terrain correlation processor position solution and the position solution from the inertial navigation unit.

BACKGROUND OF THE INVENTION

This invention relates generally to navigation of air vehicles, and more specifically, to methods and systems for enhancing accuracy of terrain aided navigation systems.

Precision terrain aided navigation (PTAN) correlates interferometric Doppler radar ground return data with a digital elevation map (DEM), resulting in position updates that are provided to a navigation system. Typical navigation systems incorporate at least two sources of navigation data to provide a total position solution. For example, position data from an inertial navigation system (INS), can be combined with position data from PTAN to provide a total position solution. The radar updates from PTAN are utilized to subtract out drift errors that occur within inertial sensor systems like the INS.

Accuracy of the PTAN system relies somewhat on the type of terrain features over which the aircraft is flying. For example, PTAN provides a very low accuracy update over featureless terrain such as water, or flat desert since it is difficult to correlate the featureless terrain with the data stored in the DEM. Conversely, high levels of accuracy are provided over mountainous terrain by a PTAN system because of the ability to correlate the rapidly changing terrain features with the DEM data. Urban areas are similar to mountainous terrain with respect to accuracy of the PTAN system, specifically, the existence of terrain elevation changes as the aircraft moves along its flight path allows for easy correlation with data stored in the DEM.

BRIEF SUMMARY OF THE INVENTION

In one aspect, a terrain aided navigation system is provided which comprises a navigation processor, an inertial navigation unit, a digital elevation map, a radar altimeter, and a map quality processor. The inertial navigation unit is configured to provide a position solution to the navigation processor, and the radar altimeter comprises a terrain correlation processor configured to receive map data from the digital elevation map and provide a position solution based on radar data to the navigation processor. The map quality processor is configured to receive map data from the digital elevation map and provide a map quality factor to the navigation processor. The navigation processor is configured to weight the position solution from the terrain correlation processor according to the map quality factor and determine a position solution from the weighted terrain correlation processor position solution and the position solution from the inertial navigation unit.

In another aspect, a method for navigating a vehicle is provided which comprises receiving a position solution from an inertial navigation unit, and receiving a terrain correlated position solution from a terrain aided navigation system which correlates radar altimeter data with digital elevation map data. The method further comprises weighting the terrain correlated position solution based on a map quality factor, the map quality factor based at least partially on digital elevation map data, and combining the position solution from the inertial navigation unit with the weighted terrain correlated position solution into a navigation position solution.

In still another aspect, a processor programmed to determine a quality of data stored within a digital elevation map is provided. The processor is configured to receive heading and altitude from an inertial navigation system, receive map data from a digital elevation map, and calculate a map quality factor that is based at least partially on the map data.

In yet another aspect, a navigation processor programmed to determine a navigation position solution is provided. The navigation processor is configured to receive a position from an inertial navigation system, receive a position from a terrain correlated radar altimeter, and receive a map quality factor from a map quality processor. The navigation processor is programmed to weight the position received from the terrain correlated radar altimeter based on the map quality factor.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1illustrates an aircraft10flying over a terrain12with abundant features. A radar altimeter (not shown) within aircraft10transmits a beam14towards terrain12and receives the reflected beam14for processing. The widely varying features of terrain12allow for easy correlation of the altitudes provided by the radar altimeter with a digital elevation map (not shown). Correlation of the altitudes provided by the radar altimeter with the digital elevation map provide a mechanism for determining a location of aircraft10with respect to the digital elevation map.

FIG. 2illustrates aircraft10flying over a featureless terrain20. Examples of featureless terrains similar to terrain20include bodies of water or a flat plain or desert. Again, the radar altimeter (not shown) within aircraft10transmits a beam22towards terrain20and receives the reflected beam22for processing. The non-varying features of terrain20make it difficult to correlate the altitudes provided by the radar altimeter with the digital elevation map since the radar altimeter will continually provide the same altitude data. As an example, a lake surface could be several thousand acres in area, all having the same altitude. In addition, depending on resolution, the digital elevation map may include multiple map entries representative of lake altitude data stored therein. In such a scenario, radar altimeter determined altitudes cannot be correlated with a particular map entry for the lake since all altitudes are the same.

FIG. 3illustrates aircraft10flying over having area of abundant terrain features30, while still resulting in a poor correlation with the digital elevation map. The poor correlation, even though terrain30has abundant features30, is due to two equal altitude ridges32and34, which results in a position solution halfway between two ridges32and34, based on the digital elevation map. To further explain the poor correlation between the radar altimeter and the digital elevation map, even with abundant terrain features30, a short explanation of operation of the radar altimeter follows. A radar altimeter provides cross-track and vertical distance to the highest object below aircraft10in down-track swaths, which are bounded in the cross-track direction by an antenna pattern. Beams36and38illustrate one embodiment of the bound of the cross-track pattern. As used herein, “Down-track” means in the direction of travel and “Cross-track” means perpendicular to the direction of travel. The downtrack width of a swath varies with the altitude of aircraft10.

A digital elevation map is comprised of resolution cells, each of which has an associated elevation representing the highest terrain elevation in that cell. Position updates are derived by correlation of the radar altimeter derived elevation associated with the cell of generally, highest elevation with the map elevation data. Accurate correlation requires elevation changes, and changes in cross track position of the cell of highest elevation within a certain swath down track resolution size as the aircraft advances down its flight path. Referring again toFIG. 3, equal altitude ridges32and34, while providing elevation changes in the cross-track pattern, do not provide changes in cross track position of the cell having the highest elevation.

FIG. 4is a block diagram of a radar altimeter50having a position output52to a terrain correlation processor (shown inFIG. 5). In one embodiment, radar altimeter50is incorporated in an air vehicle, for example, aircraft10(shown inFIGS. 1-3). Radar altimeter50includes three channels: phase ambiguity channel60, phase A channel62and phase B channel64. Phase ambiguity channel60includes antenna70, receiver72and digitizer74. Phase A channel62includes antenna80, receiver82and digitizer84. Phase B channel64includes antenna90, transmit/receive switch92, receiver94and digitizer96. In one embodiment, receivers72,82and84each include a low noise amplifier, mixer and intermediate frequency (IF) amplifier (none shown). Transmit/receive switch92in channel64allows channel64to operate in either a transmit mode or a receive mode.

Radar altimeter50transmits a radar signal toward the ground which is generated as set forth herein. Specifically, clock generator106operates at a frequency and provides a clock signal to IF offset generator116. IF offset generator116generates an offset signal for the transmitted radar signal. As an example, for a clock generator106frequency of 120 MHz, IF offset generator116divides the clock signal from clock generator106by four, and outputs a clock signal at 30 MHz. SSB mixer114mixes the 30 MHz clock signal from IF offset generator116with an RF signal from RF oscillator104, resulting in a 30 MHz offset of the RF signal. SSB mixer114outputs the offset signal to modulator112. An example RF oscillator104operates at about 4.3 GHz, and modulator112receives transmit code data from range processor120, and pulse modulates and phase modulates the signal received from SSB mixer114and outputs the modulated signal to power amplifier110. Power amplifier110amplifies the received signal and outputs the amplified signal to antenna90through transmit/receive switch92. Antenna90transmits the modulated signal toward the ground.

After a radar signal is transmitted by channel64, the signal reflected from the ground is received by antennas70,80, and90and is processed by the components within each of channels60,62, and64. Further, each of channels60,62, and64performs the same functions as the other channels. Therefore, only the functions performed by channel64will be described.

The return signal received by antenna90passes through transmit/receive switch92to receiver94. Within receiver94the return signal is amplified, mixed, down to an IF offset signal, amplified again, and output to digitizer96. Digitizer96digitizes the received signal and outputs the digitized signal to DSP100for further processing. The frequency of clock generator106determines the rate that the incoming analog signals on channels60,62, and64are sampled and digitized by digitizers74,84, and96respectively.

For each channel60,62, and64, plus a range channel120including a range processor122, DSP100includes range gate/correlators130,132,134, and136, word integration band pass filters (BPFs)140,142,144, and146, image reject mixers150,152,154, and156, and doppler band pass filters (BPFs)160,162,164, and166. Range processor120receives the output from doppler BPF166to determine an altitude. Coarse phase processor170, coordinate location processor172and fine phase processor174, are sometimes collectively referred to as a phase processor.

When a radar signal is transmitted down to the ground, the return signal comes back at the same frequency as the transmitted signal plus (or minus) a doppler shift. If the radar altimeter is transmitting signals towards the ground forward of air vehicle10, the return signals will be doppler shifted up in frequency. If the radar is transmitting signals towards the ground behind air vehicle10, the return signal will be doppler shifted down in frequency.

By properly adjusting doppler band pass filters160,162,164, and166, any point on the ground can be selected and bounded. Therefore, only return signals from the one selected ground swath are processed. The horizontal location of the highest point within a particular swath is determined by performing phase comparisons of the return signals. If the highest point being illuminated by radar is directly below air vehicle10, then the return signal will come back at the same time to antennas80and90. Alternatively, if the highest point is off to one side of air vehicle10, the return signal will be received by one antenna (e.g., antenna90) before it is received by the second antenna (e.g., antenna80), because the path is longer from and to second antenna80. The phase or the time of arrival of the return signals at each of the antennas is compared. The greater the distance between the two antennas80and90, the more accurate the measurements will be. However, as the distance between antennas80and90increases, one or more phase ambiguities may result. At a typical antenna separation, three or four phase ambiguities may occur. Such antenna separation is sometimes referred to as multiple wavelength antenna separation.

The phase ambiguity problem associated with multiple wavelength antenna separation is solved through the addition of a third antenna70spaced from antennas80and90such that the combination of the three phase comparisons eliminates the ambiguity. The third antenna70is referred to as an ambiguity antenna. The ambiguity antenna70is positioned closer to one of the other two antennas80or90, such that there are no phase ambiguities between the ambiguity antenna70and the antenna closest to the ambiguity antenna. Because of the small baseline or distance separation between the ambiguity antenna70and the antenna closest to the ambiguity antenna, accuracy is lost. Therefore, the widely spaced antennas80and90are used to provide the necessary accuracy, and the two closely spaced antennas are used to eliminate the phase ambiguities.

Radar altimeter50outputs target position vectors identifying the position of the highest point within particular regions or “swaths” on the ground, and also outputs above ground level (AGL) altitude data that identifies the vehicle altitude. The target position vectors are output to a terrain correlation processor and utilized for terrain correlation as further described below.

FIG. 5is a block diagram of a PTAN/IMU navigation system200incorporating a map quality processor202, a PTAN system204, an inertial measurement unit (IMU)206(sometimes referred to as an inertial navigation unit or inertial navigation system), and a navigation processor208. PTAN system204includes radar altimeter50as described above, a terrain correlation processor210and a digital elevation map (DEM)212. DEM212includes data relating to the elevation of different portions of a geographic area. As described above, radar altimeter50outputs target position vectors to terrain correlation processor210which correlates the target position vectors with data from DEM212, relating to a specific geographic area, to determine a radar position, which is provided to navigation processor208. IMU206provides a position to navigation processor208based on measurements made by the inertial sensors within IMU206. IMU206also provides a heading and altitude to map quality processor202.

Navigation processor208utilizes a present navigation position solution and estimates a path ahead of aircraft10by extending previous navigation position solutions. Map quality processor202includes a random distribution measurement algorithm which provides a map quality factor (e.g. radar position update quality) that is utilized by navigation processor208to weight the affect of radar position updates from terrain correlation processor210. The radar position and map quality factor are utilized in combination to address a drift in the IMU position update from IMU206which naturally occurs sometimes within IMU206. In one embodiment, the random distribution measurement algorithm utilizes heading and altitude from IMU206in determining the map quality factor as shown.

In one embodiment, the map quality factor is calculated by map quality processor202through an analysis of approaching terrain features stored in digital elevation map212. In the embodiment, the random distribution measurement algorithm is applied both in cross track position and elevation to digital elevation map212data along a projected flight path to calculate the map quality factor.

As further described below, digital elevation map212includes a plurality of map cells, and weighting the terrain correlated position solution includes determining a locus of map cells having the highest elevation which swings back and forth a cross track swath and providing an elevation mapping which varies in elevation for the cross track swath. A width of the cross track swath is determined utilizing a field of view of the radar altimeter and a current altitude.

FIG. 6is a flowchart250which illustrates a method for navigating a vehicle incorporating PTAN/IMU navigation system200. The method includes receiving252a position solution from an inertial navigation system, for example, IMU206. A terrain correlated position solution is received254from a terrain aided navigation system which correlates radar altimeter data with digital elevation map212data. The terrain correlated position solution is weighted256based on a map quality factor which is at least partially based on digital elevation map212data. The position solution from the inertial navigation system is combined258with the weighted terrain correlated position solution into a navigation position solution.

FIG. 7is a grid300representing map cells302within DEM212which are utilized in map quality processing. A flight path304, including present position “A” and extending along an expected path through position “B” overlays the map cells302. Radar altimeter50shown inFIG. 4includes, for example, a cross-track swath of terrain of width “W”. Each map cell302on the map (e.g., grid300) has an associated elevation representing the highest terrain elevation in that cell. Radar position updates are derived by correlation of the radar derived elevation (altitude) from processor100(shown inFIG. 5) with an individual map cell302having generally, the highest elevation data. As described above, accurate correlation requires elevation changes, and changes in cross track position of map cell302of highest elevation within a particular swath down track resolution size as aircraft10advances down flight path304. Thus, a locus of map cells of highest elevation which swings back and forth in cross track, and at the same time providing a elevation mapping varying in elevation, both in a random fashion, will provide a very high quality radar position update. Thus, application of a random distribution measurement algorithm both in cross track position, and elevation along flight path304, results in a high quality measure of the map quality on flight path304ahead of aircraft10.