Abstract:
Methods and apparatus for detecting obstacles in the flight path of an air vehicle are described. The air vehicle utilizes a radar altimeter incorporating a forward looking antenna and an electronic digital elevation map to provide precision terrain aided navigation. The method comprises determining a position of the air vehicle on the digital elevation map, selecting an area of the digital elevation map in the flight path of the air vehicle, based at least in part on the determined air vehicle position, and scanning the terrain representing the selected map area with the forward looking antenna. The method also comprises combining the digital elevation map data for the selected map area with radar return data for the scanned, selected area and displaying the combined data to provide a representation of the terrain and obstacles in the forward flight path of the air vehicle.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority of Provisional Application Ser. No. 60/560,292 filed Apr. 6, 2004. 

   BACKGROUND OF THE INVENTION 
   This invention relates generally to radar altimeters, and more specifically to a radar altimeter with a forward looking capability. 
   The proper navigation of an aircraft in all phases of its flight is based, to a large extent, upon the ability to determine the terrain over which it is passing, and further based on the ability to determine a position of the aircraft. In this regard, aircraft instrumentation, sensors, radar systems, and specifically, radar altimeters are used in combination with electronic terrain maps. The electronic terrain maps (sometimes referred to as digital elevation maps or DEMs) provide the height (elevation) of objects on the map, and together with the radar altimeter, aid in the flight and the planning of a flight path for the aircraft. 
   As such, radar altimeters are commonly implemented within aircraft. A radar altimeter typically includes a transmitter for applying pulses of electromagnetic energy at regular intervals to an antenna which then radiates the energy, in the form of a transmit beam, towards the earth&#39;s surface. A transmit beam from a radar is sometimes said to “illuminate” an area which reflects the transmit beam. 
   The radar altimeter further includes a signal receiver which receives return pulses, sometimes referred to as an echo or a return signal. Return pulses are received at an receive antenna, and constitute transmit beams that have been reflected from the earth&#39;s surface. It is known that some radar altimeters utilize the same antenna for both transmitting and receiving. A closed loop servo tracker for measuring a time interval between the transmitted pulse and its associated return pulse also forms a part of the radar altimeter. The time interval between the transmit pulse and the return pulse is directly related to the altitude of the aircraft. 
   However, problems still exist with flights into certain terrain. For example, aircraft, especially helicopters, are sometimes required to fly at very low altitudes. Flying at such low altitudes increases the probability that certain terrain features are in front of the aircraft, in the flight path, rather than safely below the aircraft, as is the case at normal flight altitudes. 
   Radar altimeters are generally incapable of detecting objects that are in a flight path. Examples of such objects include, for example, tall buildings, or the side of a cliff. While an aircraft equipped with a radar altimeter can determine an altitude, the aircraft is not able to determine the presence of objects in front of the aircraft if not equipped with, for example, a costly scanning laser radar. Problems also exist even when the scanning laser radar is implemented within an aircraft since they are sometimes rendered ineffective when encountering one or more of rain, fog, and smoke. 
   As described above, certain helicopter missions are flown at a very low altitude, for example, 20 to 100 feet. Such nap of the earth flights (e.g., low level contoured flights over the earth surface), may include flying around certain obstacles, to maintain as low a profile as possible in order to minimize detection by enemy forces. Medical emergency response missions also often require low altitude operations. Electronic digital elevation maps (DEMs) have been integrated with navigation systems, for example, radar altimeters and inertial measurement units, to provide a look ahead capability, based on the data in the DEM, that allows a pilot to see ahead through the weather, on a heads up or cockpit display. Such integrated systems therefore provide a display of obstacles recorded within the DEM based on a position as determined by the navigation systems. However, DEM data can be inaccurate, for example, due to an addition of new manmade structures after the DEM data was collected. These possible DEM inaccuracies have resulted in a lack of confidence to safely fly at the desired low altitudes during low visibility conditions. 
   BRIEF SUMMARY OF THE INVENTION 
   In one aspect, a method for detecting obstacles in the forward flight path of an air vehicle is provided. The air vehicle utilizes a radar altimeter and an electronic digital elevation map for precision terrain aided navigation, and the radar altimeter incorporates a forward looking antenna. The method comprises determining a position of the air vehicle on the digital elevation map, selecting an area of the digital elevation map in the flight path of the air vehicle, based at least in part on the determined air vehicle position, and scanning the terrain representing the selected map area with the forward looking antenna. The method also comprises combining the digital elevation map data for the selected area with radar return data for the scanned, selected area and displaying the combined data to provide a representation of the terrain and obstacles in the forward flight path of the air vehicle. 
   In another aspect, a radar altimeter is provided. The radar altimeter comprises a precision terrain aided navigation (PTAN) processor configured to process interferometric radar altimeter data, a terrain correlation processor configured to correlate data from the PTAN processor to a present vehicle location on a digital elevation map (DEM), and a forward map scanning processor configured to receive an altitude from the PTAN processor and determine a position on the DEM to scan which is forward of the vehicle. The radar altimeter further comprises a forward looking processor configured to receive data relating to a scan of the terrain corresponding to the position on the DEM that the forward map scanning processor is configured to scan, and a display processor configured to process and reconcile data from the forward map scanning processor and the forward looking processor. 
   In still another aspect, a processing unit for a radar altimeter configured to provide an altimeter function and a forward looking obstacle avoidance function is provided. The processing unit is configured to receive and process interferometric radar altimeter data, correlate processed interferometric radar altimeter data to a present vehicle location on a digital elevation map (DEM), and determine a terrain position to scan which is forward of the vehicle based at least in part on a position generated from the correlated interferometric radar altimeter data. The processing unit is further configured to receive data relating to a scan of the terrain corresponding to the correlated interferometric radar altimeter data and combine the data relating to the scan of the terrain with data from the DEM. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an elevation view of a nap of the earth flight path over a terrain. 
       FIG. 2  is a block diagram of a radar altimeter incorporating a forward looking antenna. 
       FIG. 3  is an illustration of one possible scanning pattern for a forward looking antenna 
       FIG. 4  is a block diagram illustrating a multiple receive antenna system included a forward looking antenna incorporated into a radar altimeter. 
       FIG. 5  is a block diagram illustrating utilization of data from a PTAN processor and a forward looking processor. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A radar altimeter which provides a forward looking capability is herein described. Precision terrain aided navigation (PTAN) allows correlation of radar ground return data with a digital elevation map (DEM), resulting in a position update to a navigation system. Such navigation systems typically will incorporate at least two sources of navigation information in providing a position solution. For example, an inertial navigation unit, for example, along with a radar altimeter incorporating PTAN capability, will provide a navigation solution. In such a scenario, position updates based on radar are used to subtract out drift errors generated by an inertial sensor of the inertial navigation unit, resulting in highly accurate navigation position capability. The success of poor visibility, low flying missions depends not only on a system like a radar altimeter with PTAN capability to provide an exact vehicle location on the DEM, but also the ability to determine the existence of obstacles in front of the aircraft not shown on the DEM. 
     FIG. 1  is an elevation view of a nap of the earth flight path over terrain (a nap of the earth flight is a low level contoured flight over the earth surface). As illustrated, flight path  2  is thirty feet above ground level (AGL)  4 . By following flight path  2 , helicopter  6  is on a collision course with a man made obstacle  8 . A present location of helicopter  6  on a DEM is provided by a PTAN radar system mounted in helicopter  6 . In the example illustrated by  FIG. 1 , obstacle  8  is not including in the DEM data, and represents a critical hazard to helicopter  6  during poor visibility conditions. The systems and methods described herein provide an ability to detect unmapped obstacles similar to obstacle  8 . Further, the described systems and methods, provide a safe path for navigating around such unmapped obstacles, by adding a forward looking narrow beam-scanning antenna (not shown in  FIG. 1 ) to the PTAN radar system. 
   In one embodiment, the radar altimeter provides a down looking altitude function, as is known in the art, based on transmissions from and reflections received at one or more radar altimeter antennas. The altimeter also provides a forward terrain or obstacle warning function, based on transmissions from and reflections received at a forward looking antenna. 
     FIG. 2  is a block diagram illustrating one embodiment of a radar altimeter  10  which incorporates the above described forward looking capabilities. Radar altimeter  10  includes a transmitter  12  and a receiver  14 . In one embodiment, an output of transmitter  12  is routed through a transmit switch  16  to one of a up/down converter  18  or a transmit antenna  20 , which transmits pulses towards the ground as part of an altimeter function. Receiver  14  receives its inputs from a receiver switch  22  which switches between up/down converter  18  and a receive antenna  24  that receives radar pulses reflected from the ground that originated from transmit antenna  20  as another part of the radar altimeter function. To provide the PTAN capability, receive antenna  24  and receiver switch  22  are representative of a multiple receive antenna system which provides highly accurate altitude measurements. 
   Transmitter  12  further receives pulse modulation and phase modulation data originating from radar processor  30 . Receiver  14  forwards radar return data received at antenna  24  to radar processor  30 . Radar processor  30  determines an altitude of the vehicle in which it is installed, and forwards the altitude data directly or indirectly to displays and other functions which utilize altitude data within an avionics system of an air vehicle. 
   As described above, radar altimeter  10  includes an up/down converter  18  which is coupled to both transmitter  12  and receiver  14  through the respective transmit and receiver switches  16  and  22 . Up/down converter  18  is also coupled to a forward looking antenna  34 . Up/down converter  18  functions to convert a normal altimeter frequency, for example, 4.3 GHz, up to a higher frequency, for example, 35 GHz, for transmission from forward antenna  34 . Up/down converter  18  is also utilized to down convert the 35 GHz forward radar return, received at forward looking antenna  34 , down to 4.3 GHz for processing by receiver  14  of radar altimeter  10 . Up/down converter  18  includes a circulator which functions to circulate energy originating at its antenna port  36  through its down conversion channel to its receive port  38 , and energy originating at its transmit port  40  to antenna port  36  allowing a single antenna (e.g., antenna  34 ) to both transmit and receive. Radar processor  30 , receives data from an inertial navigation system (INS) within the vehicle which provides data relating to, for example, vehicle attitude and velocity. Further, radar processor  30  receives an antenna position  42  from forward looking antenna  34  which is utilized as feedback in a closed loop scan control servo  44  for movement of antenna  34  as further described below. 
   While example frequencies of 4.3 Ghz for radar altimeter operation, and 35 Ghz for the forward looking radar function, are described herein, it is to be understood that such frequencies are examples only, and that other operating frequencies are contemplated. The example 4.3 GHz radar altimeter operating frequency allows for a large transmit beam, for example, 40 degrees. The radar altimeter frequency of 4.3 Ghz is up converted to 35 Ghz for the forward looking radar function to allow generation of a narrow beam, on the order of a couple of degrees. Antenna size related to a particular frequency increases as beam width is decreased. Therefore, up converting to a frequency on the order of 35 GHz, for example, allows for a forward looking antenna (antenna  34  in FIG.  1  and antenna  144  in  FIG. 4 ) of reasonable size while still providing a two or three degree beam width for the forward looking function. 
   In operation, radar altimeter  10  combines the transmit, receive, and their associated signal processing functions of known radar altimeters with forward looking antenna  34  through up/down converter  18  to provide a forward obstacle avoidance function. In one embodiment, forward looking antenna  34  and up/down converter  18  provides, at least in part, a 40 degree in azimuth by 20 degree in elevation field of view by having its transmit beam moved in a raster scanning motion (further described with respect to FIG.  3 ). The raster scanning motion is controlled via the above described antenna position functionality provided through radar processor  30  and switching between the forward obstacle avoidance function and radar altimeter function through transmit switch  16  and receiver switch  22 . Further control of the forward obstacle avoidance function is provided by the scan control functionality also via radar processor  30 . 
   Radar altimeter  10  further includes a voltage controlled oscillator (VCO)  50 , a clock  52 , sequencer  58 , an intermediate frequency (IF) amplifier-filter  62 , digitizer  64 , and memory  66 . 
   Transmitter  12  transmits pulses of RF energy, via transmit switch  16 , towards the ground through antenna  20 . The RF energy is modulated, in one embodiment, with a pulse compression bi-phase coded format produced by sequencer  58  resulting in: modulated radar signals. The output power of transmitter  12  is controlled in a closed loop fashion by processor  30 . The output power of transmitter  12  is minimized by processor  30  for a low probability of detection. 
   Antenna  24  receives the modulated radar signals reflected from the ground. The received signals are routed through receiver switch  22 , amplified and mixed down to an IF by receiver  14 , and further amplified and band limited by IF amplifier-filter  62 . Digitizer  64  digitizes the received signal, and outputs the digitized samples to memory  66 . 
   Sequencer  58  selects ground return samples corresponding to a present altitude delay (as determined by processor  30 ) and communicated to sequencer  58  on an internal range line) and shifts the selected samples from memory  66  to processor  30 . Processor  30  then determines if the next set of samples should be taken closer in or further out in range, and generates a new internal range command. The result is a closed-loop altitude tracking servo, such that as the altitude changes, processor  30  generates a measure of range tracking error which is used to change the internal range command fed back to sequencer  58 . Processor  30  generates an output altitude from the internal range. 
   Circulator  18  is coupled to transmitter  12  through transmit switch  16  and to receiver  14  through receiver switch  22 . Operation of forward looking antenna  34  is controlled through antenna positioning data  42  and scan control functions  44 , for example, through couplings to processor  30 . Transmissions transmitted by forward looking antenna  34  may reflected by an object and subsequently received by forward looking antenna  34 . Circulator  18  takes the RF energy received at forward looking antenna  34  and outputs the energy to receiver  14  through receiver switch  22 . 
   In another embodiment of a radar altimeter (not shown) that incorporates a forward looking antenna, a circulator (not shown) is utilized so that a single antenna can be utilized for both the radar altimeter transmit and receive functions. In the embodiment, the circulator receives outputs from transmitter  12  (through transmitter switch  16 ) and provides that signal to the single antenna for transmission of radar pulses. The circulator also receives the reflected pulse received at the single antenna, and directs those signals to receiver  12  (through receiver switch  22 ). Therefore, the circulator is coupled to both transmitter  12  and receiver  14  and further configured to alternate the single antenna between transmit and receive modes. 
     FIG. 3  is an illustration of one possible embodiment of a scanning pattern for forward looking antenna  34  (shown in FIG.  2 ). In the embodiment shown, forward looking antenna  34  provides a ±10 degree (in elevation) by ±20 degree (in azimuth) field of view by moving its transmit beam  100  in a raster scanning motion. At ends  102  of the horizontal scans  104 , and during a fly back portion  106  of the scan, collectively referred to as turnaround portion of the scan, the forward looking function within radar altimeter  10  is turned off, by radar processor  30 , for example, and the altimeter function is activated. Once the altimeter function is completed, and forward looking antenna  34  is again scanning horizontally, radar altimeter processing is halted, and the above described forward looking function is again activated. 
   PTAN capability is at least in part provided by a multiple receive antenna system which provides highly accurate altitude measurements.  FIG. 4  is a block diagram illustrating one embodiment of such a multiple receive antenna system  120  that can be incorporated into radar altimeter  10  (shown in FIG.  2 ). For transmission of radar pulses, a local oscillator  122  (similar to VCO  50  shown in  FIG. 2 ) provides a signal to a modulator  124  and a receiver  126 . The modulated signal is output from modulator  124  to a power amplifier  128  and on to a transmit/receive switch  130  for transmission from right antenna  132  as part of the radar altimeter function. Signals transmitted from right antenna  132  are reflected off the earth&#39;s surface and received, at slightly different times at right antenna  132 , center antenna  134 , and left antenna  136 , resulting in phase differences between the received signals due to the cross track spacing of the antennas. Antennas  132 ,  134 , and  136  provide the received signals to receiver  124  which forwards the received signals to data acquisition bank  138  (which is representative of IF amplifier-filter  62 , digitizer  64 , and memory  66  (all shown in FIG.  2 )) for sampling. PTAN processor  140  provides the Doppler filtering to limit the ground illumination area to a very narrow cross track swath within the antenna illuminated area, at a known filter center frequency or resulting angular position with respect to the line of flight of the aircraft. PTAN processor further provides for tracking of the nearest return, generally the highest point in elevation on the ground, and utilizes the phase relation between the tracked radar return signals of the three channels to determine a cross track angle to the nearest (highest in elevation) position within the illuminated swath. The measured radar delay to this tracked target provides a slant range to the target. Horizontal and vertical position of this highest point is then calculated from the measured slant range, cross track angle, and angle to the Doppler swath. 
   Modulator  124  also provides a 4.3 GHz modulated signal to 35 GHz up/down converter  142  for up conversion and transmission of 35 GHz radar pulses forward of the vehicle from forward looking antenna  144 . Radar pulses are transmitted forward of the vehicle, as described herein, reflected off any obstacles forward of the vehicle, and are received at forward looking antenna  144 . These received signals are routed through up/down converter  142 , where the received return is down converted to 4.3 GHz, and routed to receiver  126 . From receiver  126  the signals are sampled at data acquisition bank  138  and the samples are sent to forward looking processor  146  for processing as described below. PTAN processor  140 , forward looking processor  146 , and other processors described below, in one embodiment, are processing functions incorporated within processor  30  of radar altimeter  10  (shown in FIG.  1 ). 
     FIG. 5  is a block diagram  150  illustrating utilization of data from PTAN processor  140  and forward looking processor  146  (both also shown in  FIG. 4 ) to provide warning of forward obstacles in a flight path of a vehicle. PTAN capabilities are provided, at least in part, by PTAN processor  140  processing interferometric Doppler radar altimeter data received from data acquisition bank  138 , DEM  154 , terrain correlation processor  156 , inertial navigation unit  158 , and navigation processor  160 . In operation, a radar altimeter (e.g. radar altimeter  10  shown in  FIG. 2 ) transmits (and receives for eventual processing at PTAN processor  140 ), a Doppler swath towards the ground. An area on the ground which is processed by the PTAN processing function  140  is bounded by the antenna beam and further bounded by a narrow bandpass Doppler filter (within processing function  140 ) providing a narrow in down track, and wide in cross track Doppler swath. The resulting return from this bounded area is further processed with a closed loop tracker, with its range gate passing (sometimes referred to as tracking), only the return from the nearest point on the ground to the radar, and within the area on the ground bounded by the Doppler swath within the bounds of the antenna beam. A location of the locus of tracked surface of received radar returns of the nearest, or generally highest, points on the ground within the crosstrack bounds of the antenna is generated in local coordinates by PTAN processor  140 , including elevation, for correlation with DEM  154  in terrain correlation processor  156 . Correlation processor  156  provides an aircraft location on DEM  154 , which is utilized to update navigation processor  160  which also receives location information from inertial navigation unit  158 . An output of navigation processor  160  is therefore a combined PTAN/inertial navigation position  162 . 
   As described above, manmade structures built after the DEMs are generated, and any other errors in the maps, provide a hazard to low flying vehicles relying on these maps for navigation and obstacle avoidance. The combined navigation position  162  provides a reference on DEM  154 , representing a present location of the vehicle, which is provided to forward map scanning processor  164 . Forward map scanning processor  164  also receives return data received at forward looking antenna  144   a  velocity and heading from inertial navigation unit  158 , map data from DEM  154 , and altitude from PTAN processor  140 . Forward map scanning processor  164  utilizes the inputs to determine a position on DEM  154  to scan that is forward of the vehicle. In one embodiment, forward map scanning processor  164  scans a horizontal field of view of DEM  154  that is approximately 40 degrees, for example, and forward of vehicle position in range a distance, for example, that is dependent on vehicle velocity. In one embodiment, and by way of example, the forward scan is positioned such that a pilot is provided about seven seconds warning time for an obstacle forward of the vehicle based on a current vehicle velocity. 
   Elements of the terrain ahead of the vehicle, including man made structures in terms of range, heading, and elevation, all in vehicle body coordinates, are provided by forward looking processor  146 , are processed and reconciled with similar data from forward map scanning processor  164 , by display processor  168  for display on display  170 . In such a system, obstacles not recorded in DEM  154 , but detected by the above described forward looking portion of radar altimeter  10  (shown in FIG.  2 ), can be safely avoided by the vehicle. 
   In one embodiment, forward looking radar antenna  144  (shown in  FIG. 4 ) scans a field of view approximately ±20 degrees in azimuth by about ±10 degrees in elevation, with a narrow beam of about 2 degrees in width. The radar altimeter forward scans out a distance, dependent on vehicle velocity, as described above, providing range, heading, and elevation of obstacles all in body coordinates to display processor  168 . Display processor  168  combines the DEM data from forward map scanning processor  164  and data from forward looking processor  146  to provide a user with a true picture of what is ahead of the vehicle. 
   The two sources of data (forward map scanning processor  164  and forward looking processor  146 ) also compliment one another. The forward looking function of radar altimeter  10  cannot see through obstacles (i.e. a building on the far side of a hill), as it is blinded by the hill. By contrast, forward map scanning processor  164  will know that the building is on the other side of the hill (assuming the map data includes the building). Structures built after map generation will not be seen by forward map scanning processor  164 , whereas forward looking processor  146  will be able detect the structure based on the signals received at antenna  144 . Combination of the data by display processor  168  results in radar data used to update map generated data before it is displayed. Obstacles detected by the forward looking function of radar altimeter  10 , but not stored in DEM  154  are incorporated into the display data to be displayed on display  170  by display processor  168 . 
   Radar altimeter  10  incorporating PTAN capabilities, forward looking radar antenna  144 , along with DEM  154 , provides a pilot with an awareness of such unmapped obstacles at the desired low altitudes and a capability to calculate paths around such obstacles. Further, addition of forward looking radar antenna  144 , provides a safety factor resulting in an increased level of confidence, when pilots are required to fly along a low altitude flight path during poor visibility conditions. 
   The method for updating DEM generated data is also applicable to a radar altimeter with a forward looking antenna where a single antenna is switched between transmit and receive modes via a circulator to provide the altimeter (altitude) function, although such an altimeter is limited to specific operating conditions. A single radar antenna time sharing transmit and receive functions via a circulator or other transmit/receive switch can be used if the desired minimum radar range capability is somewhat greater than zero, and is directly dependent on a transmit pulse width. For example, a pulse width of 40 nanoseconds (which corresponds to the time it takes a radar transmission to travel about 20 feet) limits the altimeter range to about 20 feet (20 feet each for transmission and reflection) when a signal settling time is considered. However, where certain aircraft, for example helicopters, utilize altimeter functionality down to zero feet, separate transmit and receive antennas are utilized for the altitude function. 
   While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.