Abstract:
A method of delaying propagation of a radio frequency (RF) signal through a circuit is described. The method comprises receiving data that represents a delay time interval, providing an RF signal when a start pulse triggers a memory device, initiating a count through the delay time interval based on receipt of a start pulse, and outputting the RF signal after the delay time interval has expired.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to delay devices used in electronic circuits, and more specifically, to a radio frequency (RF) delay device and system for radar altimeter calibration. 
         [0002]    Many aircraft require better accuracy from a radar altimeter than presently exists. Generally, the accuracy becomes more important at low altitudes where aircraft perform controlled flight into and just above terrain. For example, accuracy becomes more important 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. 
         [0003]    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. 
         [0004]    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. 
         [0005]    Radar altimeters are currently tested for accuracy by either acoustic or optical delay lines. These delay lines are external of the radar altimeter and used mostly in production testing rather than while a radar altimeter is in use. These delay lines are also very large, often larger than a radar altimeter itself, and expensive. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    In one aspect, a method of delaying propagation of a radio frequency (RF) signal through a circuit is provided. The method comprises receiving data that represents a delay time interval, providing an RF signal when a start pulse triggers a memory device, initiating a count through the delay time interval based on receipt of a start pulse, and outputting the RF signal after the delay time interval has expired. 
         [0007]    In another aspect, a programmable radio frequency (RF) delay device is provided. The programmable RF delay device comprises a frequency synthesizer that includes an input register and a reference input. The input register is configured to receive data relating to a delay time interval. The programmable RF delay device further comprises an RF signal source and a logic circuit configured to logically combine an output of the RF signal source and a start pulse to initiate a count through the delay time interval. An output of the logic circuit is coupled to the reference input of the frequency synthesizer. The frequency synthesizer provides an output of the RF signal when the count reaches the set delay time interval. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0008]      FIG. 1  is block diagram of one embodiment of a delay device. 
           [0009]      FIG. 2  is a block diagram of a radar altimeter that includes the delay device of  FIG. 1 . 
           [0010]      FIG. 3  is a frequency chart showing a simulated return pulse output by the delay device of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    A radio frequency (RF) delay device is described. In one specific embodiment, the delay device is implemented within a radar altimeter to significantly improve the accuracy of the radar altimeter. 
         [0012]    Referring now to the drawings,  FIG. 1  is a block diagram of one embodiment of a programmable RF delay device  30 . Programmable RF delay device  30  includes two phase locked loop (PLL) frequency synthesizers  50  and  52 . Frequency synthesizers  50  and  52  are used in modern coherent radar systems to maintain a stable operating frequency and phase. In one embodiment, synthesizers  50  and  52  are fabricated using the ADF4106 6 GHz PLL Frequency Synthesizer, manufactured by Analog Devices, Inc. of Norwood, Mass. It is also possible to create frequency synthesizers similar to synthesizers  50  and  52  from discrete components. 
         [0013]    In one specific embodiment, synthesizers  50  and  52  are configured to perform different functions. Synthesizer  50  provides a stable operating frequency for delay device  30  while synthesizer  52  is utilized in providing the delay function. In the illustrated embodiment, synthesizer  50  provides a 4.3 GHz output signal (VCOout)  54  to synthesizer  52 . Synthesizer  50  along with a voltage controlled oscillator (VCO)  58 , and a crystal controlled oscillator  60  form a PLL circuit. Crystal controlled oscillator  60  may be, for example, a temperature compensated oscillator. VCO  58  provides VCOout  54  which is fed back into an RF input  62  of synthesizer  50 . VCOout  54  is also provided as the operating frequency for synthesizer  52 . The accuracy of delay device  30  is a function of the stability of the operating frequency provided by synthesizer  50 . The PLL configuration along with a temperature compensated crystal controlled oscillator provide this accurate operating frequency. VCOout  54  can be programmed and set to various frequencies for a frequency agile radar system, which is often utilized to reduce the intercept probability of the radar. 
         [0014]    A modulation pulse  106 , for example, as received from a radar transmitter, starts the timing within delay device  30 . The leading edge of modulation pulse  106  triggers a switch or memory device, for example, a flip-flop  107 , which also receives a fed-back output  108  of synthesizer  52 . The signal from flip-flop  107  is input into a logic gate  110  along with VCOout  54 . The operation performed at logic gate  110  determines when VCOout  54  is provided to synthesizer  52 . A delay count  112  is set by a system processor  113  at a serial data input  116 . Delay count  112  is then loaded into an “A” counter  122  and a “B” counter  124 . VCOout  54  is used to count down from the preset delay  112  and at the end of the count down, a delayed signal is output at MUX Out pin  108 . 
         [0015]    The delayed signal triggers a switch or memory device, for example, a flip-flop  132 , which drives an isolator/switch  134 . A feedback circuit on flip-flop  132  controls the output pulse width. As a result, delayed pulse  108  modulates VCOout  54  producing a simulated delay. In a specific embodiment, the delayed pulse  108  is provided to a radar altimeter receiver and used as a calibration circuit. 
         [0016]      FIG. 2  is a simplified block diagram of an RF portion of a radar altimeter  200 . Radar altimeter  200  includes a transmitter  210  and a receiver  214 . Transmitter  210  is connected to a transmit antenna  218  through a switch  222 , and receiver  214  is connected to a receive antenna  226  through a switch  230 . Transmitter  210  is also connected to delay device  30  through a switch  234 , and delay line  30  is connected to receiver  214  through a switch  236 . A controller  238  controls switches  222 ,  230 ,  234 , and  236  within radar altimeter  200  and also provides a delay control to programmable delay device  30  according to instructions from a system processor  242 . System processor  242  receives signals from receiver  214  and is programmed to provide receiver data to external systems. 
         [0017]    In the embodiment of  FIG. 2 , delay device  30  is utilized in a radar altimeter system that provides compensation for any variations or errors within the transmitter  210  and the sensors, for example, receiver  214 , antenna  226 , and interconnections therebetween. To achieve the compensation, transmitter  210  is configured to periodically send a transmit signal to delay device  30 , rather than to transmit antenna  218 . Controller  238 , by changing the state of switches  222 ,  230 ,  234 , and  236 , chooses between normal radar altimeter operation where a transmit signal is transmitted from transmit antenna  218  and received by receive antenna  226 , and a simulated test mode of operation where a transmit signal is sent to delay device  30 , delayed for a set time interval, and sent to receiver  214 . 
         [0018]    When in the simulated test mode of operation where transmitter  210  is connected to delay device  30 , the transmit signal is delayed within delay device  30  by a known time interval, relevant to a simulated altitude, to simulate normal operation of radar altimeter  200 . More specifically, the time the signal is delayed is a simulation of the time interval between when a signal leaves transmit antenna  218 , is reflected off a surface, and received at receive antenna  226 . The time the signal is delayed, like the time between when a signal leaves transmit antenna  218 , is reflected off a surface, and received at receive antenna  226 , is a function of altitude. Although the delayed transmit signal is a simulation of the normal operation of radar altimeter  200 , delay device  30  eliminates all sources of errors other than sensor errors. The time interval between when a transmit signal leaves transmit antenna  218  and is received by receive antenna  226  corresponds to a specific altitude. If the time interval is set and accurately reproduced by delay device  30 , but radar altimeter  200  does not display the altitude that should correspond with that set time interval, there are one or more sensor errors within radar altimeter  200 . In one numerical example, if programmable delay device  30  is set by system processor  242  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. 
         [0019]    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  242 , compensates for the sensor errors. In one embodiment, a calibration algorithm within processor  242  compensates for the sensor errors at a multitude of calibration altitudes. Radar altimeter  200  uses the calibration algorithm to adjust a measured altitude to remove the sensor errors. In one numerical example, 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 a low altitude range. 
         [0020]    In one embodiment, 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. 
         [0021]    In one specific numerical example, if delay device  30  is set to provide a delay of 20.334 nsec, with no errors, the radar altimeter should display a corresponding 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. Continuing the example, when delay device  30  is not connected, and a return signal is being received at receiver  214 , 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  214  and processor  242 , 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. 
         [0022]    Delay device  30  provides a very accurately delayed pulse to receiver  214 . 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. 
         [0023]      FIG. 3  is a frequency chart showing a simulated return pulse output by the delay device of  FIG. 1 . In one specific embodiment, controller  238  disconnects transmitter  210  from transmit antenna  218  utilizing switch  222 , and connects transmitter  210  to delay device  30  utilizing switch  234 . A leading edge of a transmit pulse  300 , as shown in  FIG. 3 , begins the timing of a delay interval  310 . Delay device  30  utilizes an operating frequency, which for example, can be created within delay device  30  or provided to delay device  30  by an external system, to count down from a preset delay. When the count down is completed, delay device  30  outputs a simulated signal return  320 . In the embodiment of  FIG. 2 , controller  238  opens switch  230  and closes switch  236  to connect delay device  30  to receiver  214 . 
         [0024]    Delay device  30  fulfills a need for a small, inexpensive, and accurate programmable RF delay device. The above described delay device can be utilized to significantly improve the accuracy of a radar altimeter through periodic calibration made possible by background testing, and also be utilized within other circuits where a small, inexpensive, and accurate programmable RF delay device would be beneficial. 
         [0025]    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.