Abstract:
A method of compensating for component errors within a radar altimeter is described. The method includes periodically switching transmit pulses from a transmit antenna to a programmable delay device, calculating an altitude based on a transmit pulse received from the programmable delay device, comparing the calculated altitude to an expected altitude, the expected altitude based on a pre-set delay through the programmable delay device, and compensating an altitude measured by the radar altimeter, based on transmit pulses output through the transmit antenna, by an error correction amount based on a difference between the calculated altitude and expected altitudes.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 11/462,901, filed on Aug. 7, 2006 and entitled “HIGH ACCURACY RADAR ALTIMETER USING AUTOMATIC CALIBRATION” (the &#39;901 application). The &#39;901 application is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to radar altimeters, and more specifically, to methods and systems that improve radar altimeter accuracy. 
     The proper navigation of an aircraft in all phases of its flight is based to a large extent upon the ability to determine accurately the height above 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 accurate electronic terrain maps. The electronic terrain maps provide the height 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” or “paint” an area which reflects the transmit beam. Based on a configuration of the antenna, the transmit beam includes a main lobe, and one or more side lobes which are separated from the main lobe by an angle. 
     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 a receive antenna, and constitute the transmitted 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 the 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. 
     Many aircraft require better accuracy from a radar altimeter than presently exists. Generally, the accuracy becomes more important at low altitudes where aircraft require controlled flight into and just above terrain, for example, during landing, low altitude equipment drops, precision hovering, detection avoidance, and nap of the earth flying. Some of these applications include unmanned vehicles where landing is controlled remotely and there is little room for error. The low altitude region of a radar altimeter, where the accuracy becomes more important, is usually defined as from 0 to 50 feet. Laser systems have been proposed but problems, for example, with weather, errors relative to aircraft attitude with a collimated beam, and inability to see through dust, rain, fog and other environments have negated their use for critical radar altimeter applications. 
     The total accuracy of a radar altimeter system is a function of sensor accuracy and ground return signal accuracy. Sensor accuracy is diminished by variations due to environmental changes, including but not limited to changes in temperature and humidity, and affected by variations in signal amplitude, risetime, bandwidths, pulse or gate widths, and clock frequencies. 
     In contrast to sensor accuracy where the error is caused by variations within the radar altimeter system, ground return signal accuracy is a function of the radar signal from when it leaves a transmit antenna to when it is received at a receive antenna. Ground return signal errors are caused by vehicle attitude, the external environment including but not limited to rain, fog, and dust, and terrain characteristics and associated reflection coefficient characteristics including shaping functions. The above described errors are difficult to detect and correct in a radar altimeter. As a result, wide accuracy tolerances are utilized to account for the various error sources. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, a method of compensating for component errors within a radar altimeter is provided. The method comprises periodically switching transmit pulses from a transmit antenna to a precision programmable delay device, calculating an altitude based on a transmit pulse received from the programmable delay device, comparing the calculated altitude to an expected altitude, the expected altitude based on a pre-set delay through the programmable delay device, and compensating an altitude measured by the radar altimeter, based on transmit pulses output through the transmit antenna, by an error correction amount based on a difference between the calculated altitude and expected altitudes. 
     In another aspect, a radar altimeter is provided. The radar altimeter comprises a programmable delay device configured to periodically delay a received signal by a set time delay. The radar altimeter further comprises a switching device configured to switch transmit pulses of the radar altimeter between a transmit antenna and the programmable delay device. The radar altimeter still further comprises a processing device configured to: (1) control operation of the switching device, (2) calculate an altitude based on the transmit pulse received from the programmable delay device and compare the calculated altitude to an expected altitude, the expected altitude based on a pre-set delay through the programmable delay device, and (3) compensate an altitude measured by the radar altimeter, based on transmit pulses output through the transmit antenna, based on a difference between the calculated altitude and expected altitudes. 
     In another aspect, a calibration unit for an RF transmitting and receiving device is provided. The calibration unit comprises a programmable delay device configured to periodically delay a received signal by a set time delay. The calibration unit further comprises a switching device configured to switch RF pulses between a transmit antenna and the programmable delay device. The calibration still further comprises a processing device configured to: (1) control operation of the switching device, (2) calculate a result based on the RF pulse received from the programmable delay device and compare the result to an expected result, the expected result based on a set time delay through the programmable delay device, (3) compensate a result measured by said RF transmitting and receiving device, based on RF pulses output through the transmit antenna, based on a difference between the calculated result and expected results, and (4) communicate with systems external to the RF transmitting and receiving device. 
     In another aspect, a method of reducing ground return signal errors affecting an accuracy of a radar altimeter is provided. The method comprises determining errors in an altitude output of the radar altimeter based on time variations of received ground return pulses and adjusting operating parameters of the radar altimeter to reduce errors, adjusting based on at least one of pitch, roll, and pulse width. 
     In still another aspect, a radar altimeter is provided. The radar altimeter is configured to adjust operating parameters of said radar altimeter, based on at least one of varying pulse widths of transmit pulses, varying vehicle attitudes, and varying vehicle altitudes, to reduce ground return signal errors. 
     In still another aspect, a processing device is provided. The processing device is configured to reduce ground return signal errors of a radar altimeter and adjust operating parameters of the radar altimeter based on at least one of varying pulse widths of transmit pulses, varying vehicle attitudes, and varying vehicle altitudes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a radar altimeter that includes a delay device. 
         FIG. 2  is a more detailed block diagram of a radar altimeter that includes a delay device. 
         FIG. 3  is a plot of the amplitude over time of two return signals. 
         FIG. 4  is a plot of ground return signal errors as a function of altitude for various pulse widths and attitudes (roll). 
         FIG. 5  is a plot of ground return signal error as altitude, pulse width, and attitude change, with ground return signal error compensation activated. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides methods and systems that improve radar altimeter accuracy. In one embodiment, to increase sensor accuracy, a programmable delay device is employed that is configured to automatically provide common mode compensation for any variations or errors in the sensors. To increase ground return signal accuracy, the radar altimeter incorporates additional characteristics which provide compensation as a function of antenna attitude. 
     Referring now to the drawings,  FIG. 1  is a block diagram of a radar altimeter  10 . Radar altimeter  10  includes a transmitter  12  and a receiver  14 . Transmitter  12  is connected to a transmit antenna  16  through a switch  17 , and receiver  14  is connected to a receive antenna  18  through a switch  19 . A controller  20  controls switches  17 ,  19 ,  23 , and  25  within radar altimeter  10  and also provides a delay control to a programmable delay device  30  according to instructions from a tracker/processor  34 . Tracker/processor  34  receives signals from receiver  14  and is programmed to provide receiver data to external systems  36 . In one embodiment, external systems  36  includes display instruments. 
     Referring now to the drawings,  FIG. 1  is a block diagram of a radar altimeter  10 . Radar altimeter  10  includes a transmitter  12  and a receiver  14 . Transmitter  12  is connected to a transmit antenna  16  through a switch  17 , and receiver  14  is connected to a receive antenna  18  through a switch  19 . A controller  20  controls switches  17 ,  19 ,  23 , and  25  within radar altimeter  10  and also provides a delay control to a programmable delay device  30  according to instructions from a tracker/processor  34 . Tracker/processor  34  receives signals from receiver  14  and is programmed to provide receiver data to external systems. 
     Delay device  30  is part of a system that provides compensation for any variations or errors within the sensors, for example, receiver  14 , antenna  18 , and the interconnections therebetween. To achieve the compensation, transmitter  12  is configured to periodically send a transmit signal to delay device  30 , rather than transmit antenna  16 . The transmit signal is delayed by a known time interval to simulate normal operation of radar altimeter  10 . More specifically, the time the signal is delayed is a simulation of the time interval between when a signal leaves transmit antenna  16 , is reflected off a surface, and received at receive antenna  18 . Although this is a simulation of the normal operation of radar altimeter  10 , delay device  30  eliminates all sources of errors other than sensor errors. The time interval between when a transmit signal leaves transmit antenna  16  and is received by receive antenna  18  corresponds to a specific altitude. If the time interval is set and accurately reproduced by delay device  30 , but radar altimeter  10  does not display the altitude that should correspond with that set time interval, there are one or more sensor errors within radar altimeter  10 . In one specific embodiment, if programmable delay device  30  is set by tracker/processor  34  for a delay of 9.6 nanoseconds (nsec), since a transmitted radar signal takes 2.0334 nsec to travel one foot, this delay represents a simulated altitude of 4.72 feet. 
     The sensor errors discussed above may cause the radar altimeter to display an altitude that does not correspond to the actual altitude that, according to the mathematics of radar altimeter operation, should be displayed for a set delay interval. Delay device  30 , in combination with processor  34 , compensates for the sensor errors. In one embodiment, a calibration algorithm within processor  34  compensates for the sensor errors at a multitude of calibration altitudes. Radar altimeter  10  uses the calibration algorithm to adjust a measured altitude to remove the sensor errors. In one specific embodiment, delay device  30  is programmed in 0.010 nsec increments that can range from delay intervals of 9.6 nsec to 90 nsec. These delay intervals correspond to altitudes from 4.72 feet to 44 feet which simulate the low altitude region of a radar altimeter. The 0.010 nsec delay increments correspond to 0.0049 foot altitude increments (i.e. simulated altitudes every 0.0049 feet). Sensor errors are determined at each simulated altitude, stored in a memory, and in combination with the sensor errors determined at the other simulated altitudes a calibration algorithm is created that is continuous throughout the low altitude range. 
     Because of limited processing time, a select number of simulated altitudes are chosen. In one specific embodiment, eight calibration points are processed to provide five foot increments. Curve fitting utilizing, for example, linear or quadratic algorithms, provides a very accurate calibration algorithm in the low altitude region. 
     In one specific embodiment, if delay device  30  is set to 20.334 nsec, with no errors, the radar altimeter should display an altitude of ten feet. If, after receiving a pulse delayed by 20.334 nsec the radar altimeter displays an altitude of nine feet, sensor errors are causing a one foot variance. When delay device  30  is not connected, and a return signal is being received at receiver  14 , if the radar altimeter measures that it is at an altitude of ten feet, the calibration algorithm will be applied, removing the one foot of sensor errors, and the radar altimeter will display an altitude of eleven feet. The one foot difference is caused by errors or variations in receiver  14  and processor  34 , for whatever reason. The errors are common mode compensated because a known precision signal is periodically measured and any variance will also be in the actual altitude measurement since they are both processed in the same circuitry. 
     Delay device  30  provides a very accurately delayed pulse to receiver  14 . In one specific embodiment, the accuracy of delay device  30  is less than 0.7 nsec from −40° C. to 85° C. This accuracy corresponds to a variance of 0.35 feet. 
       FIG. 2  is a more detailed block diagram of radar altimeter  10  including programmable delay device  30 . As shown in  FIG. 2 , transmitter  12  includes a radio frequency (RF) oscillator  50 , a power divider  52 , and a buffer amplifier  54 .  FIG. 2  shows that receiver  14  includes a low noise amplifier  60 , a mixer  62 , and an amplifier  64 . RF oscillator  50  and power divider  52  provide mixer  62  with a RF frequency to demodulate received radar signals. Isolator/switch  70  periodically switches transmit pulses from transmitter  12  from antenna  16  to delay device  30 . Isolator/switch  70  switches the destination of the transmit pulses at a pulse repetition frequency. In one specific embodiment, the pulse repetition frequency is 100 KHz. At the pulse repetition frequency, the transmit pulse is switched from antenna  16  (i.e. normal operation) to delay device  30  for operation of radar altimeter  10  in the simulated mode. Delay device  30  stays at a first set delay value for a fixed number of pulses, changes to a second set delay value for a fixed number of pulses, and continues this sequence until the maximum delay is reached. After the fixed number of pulses at the maximum delay, delay device  30  changes back to the first set delay value. The fixed number of pulses at each delay value is a function of the integration period needed to track and measure the simulated altitude at that particular delay value. 
     Delay device  30  delays the transmit pulses for a number of clock pulses set by tracker/processor  34 . The number of clock pulses set by tracker/processor  34  is varied to provide data over a range of times corresponding to a range of altitudes within the low altitude portion of radar altimeter  10 . Delay device  30  sends the delayed transmit pulse to isolator/switch  76  and to receiver  14 . Receiver  14  provides the delayed pulse to tracker/processor  34 . From the collected data, tracker/processor  34  creates a compensation algorithm, using for example, linear or quadratic algorithms, which includes error correction values for all altitudes within the low altitude region of the altimeter. These algorithms are utilized to compensate the radar altimeter&#39;s readings, essentially making the radar altimeter independent from errors in the sensors caused by temperature and humidity changes. Tracker/processor  34  provides an automatic gain control (AGC)  78 , which is discussed further below. 
     The second component affecting the total accuracy of radar altimeter  10  is ground return signal accuracy. This accuracy component is a function of the radar signal from when it leaves a transmit antenna  16  to when it is received by a receive antenna  18 . In one embodiment, a computer is programmed to utilize a radar return integral to analyze the ground return signal and calculate the desired loop sensitivity as well as determine the accuracy of radar altimeter  10  for various configurations of altitudes, attitudes, reflection coefficients, pulse widths, and antenna patterns. Signal loss, and therefore altimeter sensitivity requirements, and predicted accuracy can be obtained from this integral, which follows: 
             Pr   =         Pt   ⁢           ⁢       Ga   2     ⁡     (   0   )       ⁢     λ   2     ⁢     σ   ⁡     (   0   )               (     4   ⁢   π     )     3     ⁢     H   2         ⁢       ∫   0     β   ⁡     (     ct   /   H     )         ⁢       ∫   0     2   ⁢   π       ⁢         [     fant   ⁡     (       θ   ⁢           ⁢   p     ,     θ   ⁢           ⁢   r       )       ]     2     ⁢   σ   ⁢           ⁢     o   ⁡     (   β   )       ⁢     sin   ⁡     (   β   )       ⁢   cos   ⁢           ⁢     (   β   )     ⁢     ⅆ   ψ     ⁢       ⅆ   β     .                   
The integral illustrates that the power at the receiver, Pr, is a function of β(ct/H), and where Pt is a transmit power, H is height (i.e., altitude), β is an angle of incidence, σ(0) is a scattering coefficient at vertical incidence, σo(β) is a normalized scattering coefficient, [σ(β)=σ(0)σo(β)], fant(θp,θr) is a normalized antenna pattern where receive and transmit antennas are assumed to be identical and collimated, i.e., the antenna spacing is ignored relative to H, Ga(0) is an antenna boresight gain, ψ is an azimuth location angle, λ is a wavelength, c is a velocity of propagation, and t is a time from the very first nadir return&#39;s arrival at the receive antenna.
 
     Simulating the radar return signal utilizing the radar return integral enables a higher accuracy to be obtained within an altimeter, specifically when the altimeter is configured to optimize the characteristics identified by the simulations as leading to the highest accuracy. Specifically, the operating frequency is chosen to minimize environmental effects on the transmit and return signals. For example, the proper operating frequency can result in a reduction in the effects of reflections from rain, fog, and dust. In one specific embodiment, 4.3 GHz is utilized as the operating frequency. This operating frequency provides good weather penetration and is a high enough frequency to allow practical antenna aperture sizes. 
     Computer simulations of the radar return integral illustrate that narrow transmit pulses result in altitude determinations having a high degree of accuracy. In an example embodiment, transmit pulses of 10 nsec are utilized in low altitude regions (i.e., 0 to 50 feet). Since bandwidth is proportional to the inverse of pulse width, when a narrower transmit pulse is transmitted, it is desirable for a receiver to be able to support a wider bandwidth. For a high accuracy radar altimeter, the bandwidth of the receiver is configured to support narrow transmit pulses so that the leading edge of the return pulse is not distorted and result in inaccuracies. 
     The radar return integral also illustrates that the best accuracy from a radar altimeter is obtained when an antenna beamwidth is matched to the vehicle attitude requirements. Wide beam antennas are utilized to obtain high accuracy altimeter readings in vehicles subject to large rolling or pitching. Narrow beam antennas allow for high accuracy altimeter readings in vehicles with minimal roll or pitch. Accuracy is most important in the low altitude region where generally, roll and pitch are held to a minimum. In an example embodiment, either switchable beamwidth or steerable beamwidth antennas are utilized where the beamwidth is switched or programmed as a function of vehicle roll or pitch requirements. Incorporating these types of antennas keeps the maximum gain toward the ground and will improve accuracy and increase the ground return signal amplitude. 
     The radar return integral is utilized to solve for the power at receiver  14  as a function of transmit power, altitude, antenna characteristics, scattering coefficient, and aircraft attitude, which includes roll and pitch conditions. The radar return integral is also utilized to solve for loop attenuation between the signal received and the signal transmitted. 
       FIG. 3  is a plot of the amplitude over time of two return signals. The amplitude of a return signal  80  is sufficient for operation of AGC  78  (shown in  FIG. 2 ). The amplitude of a return signal  84  is attenuated below the AGC  78  threshold, but is of sufficient amplitude to be above the track threshold  86  of the receiver.  FIG. 3  shows return signal  84  reaching track threshold  86  at a later time than return signal  80 . Therefore, radar altimeter  10  will convert the travel time of return signal  84  to a higher altitude than that of return signal  80 , and return signal  84  will include an outbound error. 
     The radar return integral illustrates that the highest accuracy from a radar altimeter is obtained when the ground return signal has sufficient amplitude for AGC  78  to function. AGC  78  maintains constant amplitude regardless of amplitude variations caused by space loss, antenna characteristics, and varying ground reflection coefficients. The constant amplitude minimizes variances in the ground return leading edge risetime. The leading edge is the part of the pulse used to measure the time delay between transmit pulse and the ground return pulse. This time delay is converted to altitude in the radar altimeter. 
       FIG. 4  is a plot of ground return signal errors as a function of altitude for various pulse widths and attitudes (roll).  FIG. 4  illustrates the results of ground return signal error analysis, using the return integral simulation, to reflect accuracy of radar altimeter data as a function of both transmitter pulse width and vehicle roll or pitch. To form the plots of  FIG. 4 , multiple plots of the type shown in  FIG. 3  are derived utilizing the ground return integral computer simulations. The plots of the type shown in  FIG. 3  illustrate the shape of the ground return pulse. Based on the plots of  FIG. 3  and knowing tracking thresholds, errors are determined based on time variations of the return ground pulse at the tracking threshold. The errors calculated from multiple plots showing the shape of the ground return pulse (i.e., plots according to  FIG. 3 ) are compiled in  FIG. 4 . These plots do not include all of the system errors. They represent the errors relative to the ground return signal. Plot  100  shows the error in feet at varying altitudes in an embodiment where the transmitter pulse width is 10 nsec and the vehicle roll is zero degrees. Plot  102  shows the error in feet at varying altitudes in an embodiment where the transmitter pulse width is 50 nsec and the vehicle roll is zero degrees. Plot  104  shows the error in feet at varying altitudes in an embodiment where the transmitter pulse width is 10 nsec and the vehicle roll is 30 degrees. Plot  106  shows the error in feet at varying altitudes in an embodiment where the transmitter pulse width is 50 nsec and the vehicle roll is 30 degrees. 
     The low altitude errors, more specifically, the ground return signal errors in the altitudes from 0 to 50 feet, are very low. In this embodiment, the accuracy with a narrow transmit pulse width (i.e. 10 nsec) is less than 0.6 feet with no roll and less than 1.5 feet at a 30 degree roll. In the low altitude region, rolls or pitches to 30 degrees are unlikely to occur. 
     Referring once again to  FIG. 2 , in one specific embodiment, radar altimeter  10  is configured to receive inertial data from an inertial measurement unit (IMU)  150 . Additional compensation is made by inputting information from IMU  150  to radar altimeter  10 . As shown in  FIG. 4 , ground return signal errors will always be outbound (i.e. indicate a higher altitude than the actual altitude). The errors will always be outbound because either the ground return signal rise time is close to the transmit signal rise time, which occurs when a vehicle is over a surface with high reflectivity, or the ground return signal rise time increases due to integration of the pulse due to reflection off a surface with less reflectivity. For example, a surface with high reflectivity is water, and a surface with less reflectivity is farm land. 
       FIG. 5  is a plot of the ground return signal errors as altitude, pulse width, and attitude change, with ground return signal error compensation activated.  FIG. 5 , where IMU compensation is included, illustrates the reduction in ground return signal errors as compared to the ground return signal errors shown in  FIG. 4 . The radar altimeter is compensated as a function of the roll or pitch measured by IMU  150  to reduce the uncompensated errors. Plot  200  shows the error in feet at varying altitudes in an embodiment where the transmitter pulse width is 10 nsec, the vehicle roll is zero degrees, and ground return signal error compensation is activated. Plot  202  shows the error in feet at varying altitudes in an embodiment where the transmitter pulse width is 50 nsec, the vehicle roll is zero degrees, and ground return signal error compensation is activated. Plot  204  shows the error in feet at varying altitudes in an embodiment where the transmitter pulse width is 10 nsec, the vehicle roll is 30 degrees, and ground return signal error compensation is activated. Plot  206  shows the error in feet at varying altitudes in an embodiment where the transmitter pulse width is 50 nsec, the vehicle roll is 30 degrees, and ground return signal error compensation is activated. In one embodiment, with a narrow pulse (i.e. 10 nsec), the errors at 30 degrees are substantially reduced (i.e. from approximately 1.5 feet to 0.8 feet) at a 50 foot altitude and the errors are even lower at lower altitudes. 
     The computer simulations utilizing the radar return integral further illustrate that the highest accuracy from a radar altimeter is obtained when there is a significant sensitivity margin throughout the low altitude region. Antennas with good antenna isolation and adequate antenna separation minimize the leakage signal and provide a significant sensitivity margin. In one specific embodiment, a sensitivity margin of at least 10 dB is utilized. 
     By applying the above described delay device and producing and operating a radar altimeter with the above described characteristics, the accuracy in the low altitude region (i.e., 0 to 50 feet) is less than 0.6 feet (as shown in  FIG. 5 ). Therefore, the overall accuracy is less than 1 foot with less than 10 degrees of vehicle pitch and/or roll and less than 1.5 feet at 30 degrees pitch and/or roll which is significantly higher (i.e., more than twice as accurate) than current radar altimeters. This is exceptional accuracy compared to existing radar altimeters and meets the demanding requirements for critical applications as discussed above. 
     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.