Abstract:
In one aspect, a method of radar altimeter operation including a time dependent gain control is described. The method comprises triggering a Sensitivity Time Control (STC) gain control signal at a pulse repetition frequency (PRF) of a transmit pulse to attenuate interference from at least one of an antenna leakage signal and a signal reflected from equipment. The method also includes shaping the STC gain control signal from no attenuation at a first time, before a transmitter sends the transmit pulse, to a stable maximum attenuation at the time the transmitter sends the transmit pulse, to no attenuation at a second time, after the transmitter sends the transmit pulse. The method also includes matching a bandwidth of an intermediate frequency (IF) amplifier to the pulse width of a transmitted pulse.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to radar altimeters, and more specifically, to methods and systems that reduce interference caused by at least one of an antenna leakage signal and a signal reflected from other equipment mounted on an air vehicle. 
         [0002]    A radar altimeter typically includes a transmitter for applying pulses of electromagnetic energy, at a radio frequency (RF), and at regular intervals to a transmit 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 (e.g., the ground) which reflects (returns) the transmit beam. The reflected beam, sometimes referred to as a ground return, is received at a receive antenna of the radar altimeter. A signal from the receive antenna is processed to determine an altitude of the air vehicle incorporating the radar altimeter. 
         [0003]    At very low altitudes (generally defined as from 0 to about 50 feet), for example, during landing and take-off, altimeter performance may be impacted by leakage paths. Specifically, interfering signals may result from a leakage path between the transmit and receive antennas of the radar altimeter. In normal radar altimeter operations, as described above, a transmit antenna transmits a signal towards the ground which reflects the signal. The receive antenna receives the ground reflected signal for processing to determine air vehicle altitude. A leakage path exists when a portion of the transmitted signal is directly received by the receive antenna without having been reflected by the ground. 
         [0004]    Other interfering signals are transmit signals reflected from air vehicle surfaces and structures (e.g., wheel well doors and helicopter skids) and transmit signals reflected from other equipment mounted on air vehicle surfaces (e.g., communication antennas, forward looking infrared systems, and cameras). The receive antenna receives these types of signals after antenna leakage but before the ground return pulse returns from the ground. 
         [0005]    These unwanted signals may cause the altimeter to: (1) momentarily break track (i.e., the leakage signal periodically inhibits tracking the ground return signal such as during a hover phase cancellation mode), (2) lock on the leakage signal and always indicate a “zero” foot altitude regardless of the altitude of the air vehicle, or (3) oscillate between tracking the leakage signal, which indicates a “zero” foot altitude, and the ground return signal when the altitude is between 20 feet and 80 feet. 
         [0006]    In helicopters and other air vehicles with the ability to hover, return signals reflected from various types of terrain such as grass, foliage, or even water can either add to each other or subtract from each other (i.e., phase addition or cancellation). During cancellation, the ground return signal is attenuated and is then more susceptible to interference from a stronger antenna leakage signal. Also, with new air vehicles containing more externally mounted equipment, it is becoming more difficult to find an acceptable antenna mounting arrangement that will maintain antenna leakage signal attenuation. 
         [0007]    Solutions to interference signals may include different antenna placement, larger spacing between the antennas, different types of antennas, canting the antennas, or changing the location of other equipment that reflects signals back to the receive antenna. This can be very costly (e.g., changing the mounting or configuration of an air vehicle and potentially a number of the same model vehicles) as well as time consuming. Changing the mounting or configuration of an air vehicle can hold up shipment of new air vehicles or force users to ground air vehicles. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0008]    In one aspect, a method of radar altimeter operation including a time dependent gain control is provided. The method comprises triggering a Sensitivity Time Control (STC) gain control signal at a pulse repetition frequency (PRF) of a transmit pulse to attenuate interference from at least one of an antenna leakage signal and a signal reflected from equipment. The method also includes shaping the STC gain control signal from no attenuation at a first time, before a transmitter sends the transmit pulse, to a stable maximum attenuation at the time the transmitter sends the transmit pulse, to no attenuation at a second time, after the transmitter sends the transmit pulse. The method also includes matching a bandwidth of an intermediate frequency (IF) amplifier to the pulse width of a transmitted pulse. 
         [0009]    In another aspect, a radar altimeter including a time dependent gain control is provided. The radar altimeter comprises a transmit antenna configured to transmit radar signals toward the ground, a receive antenna configured to receive radar signals reflected from the ground, the receive antenna also receiving signals propagated along a leakage path from the transmit antenna, a receiver configured to receive signals from the receive antenna, at least two intermediate frequency (IF) amplifiers configured to receive a Sensitivity Time Control (STC) gain control signal at a pulse repetition frequency (PRF) of the transmitted radar signals to attenuate the leakage signals for a time period before and after the transmitted radar signals are sent, and a band pass filter (BPF) configured to receive a bandwidth control signal from a processor and control the bandwidth of the receiver. 
         [0010]    In yet another aspect, a radar receiver unit is provided. The radar receiver unit comprises at least two intermediate frequency (IF) amplifiers, a time shaping gain control generator configured to provide a time dependent gain control signal to at least one of the at least two IF amplifiers, and a band pass filter (BPF) configured to receive a bandwidth control signal from a processor and control the bandwidth of the receiver unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0011]      FIG. 1  is a block diagram of a radar altimeter positioned at an altitude above a surface. 
           [0012]      FIG. 2  is a plot comparing the signal strength of a ground return signal, an antenna leakage signal, and a gain control signal as altitude changes. 
           [0013]      FIG. 3  is a plot illustrating the relationship as a function of time between a transmit pulse, two ground return pulses, two antenna leakage pulses, and a track gate signal. 
           [0014]      FIG. 4  is a block diagram of a radar altimeter utilizing a time sensitivity gain control signal, referred to as sensitivity time control (STC). 
           [0015]      FIG. 5  is a plot of an STC signal. 
           [0016]      FIG. 6  is a plot illustrating the relationship over time between a transmit pulse, two ground return pulses, two antenna leakage pulses, a track gate signal, and a sensitivity time control (STC) signal. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    Methods and systems that reduce interference caused by at least one of an antenna leakage signal and a signal reflected from other equipment mounted on an air vehicle are described herein. Such interference can be reduced by utilizing, for example, a gain control signal that is a function of time, or more specifically, a Sensitivity Time Control (STC) signal. 
         [0018]    Referring now to the drawings,  FIG. 1  is a block diagram of a radar altimeter  10  positioned at an altitude of H 1  above a surface  12 . Radar altimeter  10  is connected to a transmit antenna  14  and a receive antenna  16 . Transmit antenna  14  radiates a transmit pulse  18  toward surface  12 . A portion of transmit pulse  18  is reflected off of surface  12  and received at receive antenna  16 . This reflected signal is sometimes referred to as a ground return signal  20  and is utilized by radar altimeter  10  to calculate an altitude. However, a portion of transmit pulse  18  leaks directly to receive antenna  16 . This is referred to as an antenna leakage signal  22 . Another portion of transmit pulse  18  is reflected from equipment mounted on the air vehicle surface, in the example of  FIG. 1 , the equipment is a forward looking infrared system  24 . The portion of transmit pulse  18  reflected from forward looking infrared system  24  is a reflected interference signal  26 . Antenna leakage signal  22  and reflected interference signal  26  interfere with the ability of radar altimeter  10  to determine an altitude of the air vehicle. At low altitudes it is hardest for radar altimeter  10  to distinguish between interference signals  22  and  26  and ground return signal  20 . This is because at low altitudes there is less time between when interference signals  22  and  26  reach receive antenna  16  and when ground return signal  20  reaches receive antenna  16 . Interference signals  22  and  26  and ground return signal  20  can be time coincident or in proximity to each other and radar altimeter  10  distinguishes between ground return signal  20  and interference signals  22  and  26 . This is typically accomplished utilizing amplitude gain control. 
         [0019]      FIG. 2  is a plot comparing the signal strength of a ground return signal  30 , an amplitude gain control signal  32  (e.g., radar altimeter break track or sensitivity range control (SRC)), and an antenna leakage signal  34  as altitude changes. More specifically,  FIG. 2  is a plot of the change in signal strength as altitude increases for ground return signal  30 , SRC signal  32 , and antenna leakage signal  34 . Programmed analog automatic gain control signals are known in the prior art and referred to as SRC signals. SRC signals are used to control the gain function of a radar altimeter throughout the low altitude region. 
         [0020]    The maximum sensitivity of a radar altimeter is very high. In one numerical example, the maximum sensitivity is −130 dB. At low altitudes the ground return space attenuation is low, which results in a very large ground return signal. Therefore, a gain control must reduce the sensitivity at low altitudes.  FIG. 2  illustrates antenna leakage signal  34  attenuated by SRC signal  32  to remain below ground return signal  30  throughout the low altitude region. 
         [0021]    The SRC signal is an analog signal generated from an internal range signal (not shown). The internal range signal is an analog signal that varies with altitude. It varies, for example, from 0 to 10 volts representing an altitude from 0 to 5,000 feet. The resulting SRC signal is relatively flat (i.e., constant gain) from, for example, 0 to 30 feet. It controls antenna leakage signal  34  at a fixed level (e.g., −78 dB). For example, at 30 feet, SRC signal  32  allows the altimeter receiver to increase in gain because at high altitude there is more space attenuation of ground return signal  30  and a higher receiver sensitivity is desired. SRC signal  32  becomes non-controlling in this example at approximately 150 feet. The Automatic Gain Control (AGC) (not shown) and Noise Gain Control (NAGC) (not shown) take over and control the receiver gain as a function of the signal strength of ground return signal  30  and system noise. 
         [0022]      FIG. 3  is a plot illustrating the relationship over time between a transmit pulse  100 , two ground return pulses  102  and  104 , two antenna leakage pulses  106  and  108 , and a track gate signal  110 .  FIG. 3  shows transmit pulse  100  with a pulse width of 25 nsecs. Ground return pulse  102  is received by the radar altimeter at 0 nsecs which corresponds to a radar altimeter located at an altitude of 0 feet. Antenna leakage pulse  106  has the same pulse width as transmit pulse  100  and ground return pulse  102 . However, the amplitude of antenna leakage pulse  106  is attenuated by an SRC signal (not shown) to be lower than ground return pulse  102 . This attenuation enables a radar altimeter receiver to distinguish between ground return pulse  102  and antenna leakage pulse  106 . 
         [0023]    Ground return pulse  104  is sensed at a radar altimeter receiver approximately 60 nsec after it was sent from a transmitter. This return period corresponds to an altitude of approximately 30 feet. As air vehicle altitude increases, track gate  110  moves out in time. As a result, track gate  110  ignores near in signals such as an antenna leakage signal which does not change with altitude. 
         [0024]    However, when the altitude increases, the SRC signal (not shown) no longer attenuates antenna leakage signal  108 . As described above, the SRC signal allows the receiver to increase in gain at approximately 30 feet and becomes non-controlling at about 150 feet. As a result, the amplitude of the antenna leakage signal increases. In one specific example, the pulse width of transmit pulse  100  is 25 nsecs and therefore the pulse width of antenna leakage signal  108  is also 25 nsec. If an intermediate frequency (IF) amplifier bandwidth is not matched to this pulse width (i.e., the IF bandwidth is 30 MHz rather than 1/25 nsec or 40 MHz) but is less than what will support the pulse width, the pulse width will stretch in width. In this example, instead of a received antenna leakage signal that has a pulse width of 25 nsec, the signal as viewed at a video amplifier will stretch to 40 nsec. This aggravates the interference problem. 
         [0025]      FIG. 3  illustrates the problem a widened antenna leakage pulse width will cause. When at an altitude of 60 feet, the SRC signal allows the entire signal bus to increase which allows the amplitude of antenna leakage signal  108  to increase. Track gate signal  110  may not only overlap ground return signal  104  at the leading edge of ground return signal  104  as is desired, but the leading edge of track gate signal  110  may also overlap the trailing edge of increased antenna leakage signal  108 . When this occurs, the internal control loops can not control track gate  110  to be coincident with the leading edge of ground return signal  104  and therefore, break tracks, altitude errors, or oscillation problems occur. 
         [0026]    It is desirable to increase the gain at low altitudes quickly to compensate for space losses of the ground return signal. However, as shown in the above example, the rate that the gain can increase, without causing interference, is limited. The rate that the gain can increase is limited because the leakage signal will also increase as the gain is increased, and at a higher altitude, will interfere with the ground return signal. 
         [0027]      FIG. 4  is a block diagram of a radar altimeter  150  utilizing STC. Radar altimeter  150  includes an RF oscillator  152  that provides a frequency for transmission and for down conversion of radar return pulses. More specifically, and with respect to transmission, RF oscillator  152  provides an RF signal  154  to a power divider  156 . Power divider  156  outputs an RF signal  158  to a buffer amplifier  160 , which outputs an amplified RF signal  162  for transmission. The amplified RF signal  162  for transmission is provided to a modulator switch  164 , which, depending on a state of modulator switch  164 , modulates amplified RF signal  162  and routes a modulated output signal  166  to a transmit antenna  168  for transmission as a radar signal towards the ground. 
         [0028]    With respect to reception, RF oscillator  152  provides an RF frequency signal to a mixer  172 . Transmitted pulses are received at receive antenna  174  and amplified by a low noise amplifier  176 . Mixer  172  demodulates the received signals with the frequency from RF oscillator  152  after the received signals are amplified by low noise amplifier  176 . The received signals are further amplified by a first intermediate frequency (IF) amplifier  178 . First IF amplifier  178  is provided with a gain control signal  180  from a time shaping gain control generator  182 . Time shaping gain control generator  182  receives a pulse repetition frequency (PRF) of a transmitter pulse  183  from a processor  184  and provides a time sensitivity gain control signal to first IF amplifier  178  and a second IF amplifier  186  at times corresponding to the PRF and timed with reference to the transmit pulse. Second IF amplifier  186  provides the signal to a video amplifier  188  that amplifies the detected signal from IF amplifier  186 . A gate switch  190  receives a track gate signal  191  from processor  184 . Gate switch  190  is utilized to “track” a return pulse. 
         [0029]    IF amplifiers  178  and  186  are configured to attenuate, as a function of time only, an antenna leakage signal, a reflected interference signal, and a ground return signal. 
         [0030]    Radar altimeter  150  also includes a switchable multi-section Band Pass Filter (BPF)  192  that receives a signal from first IF amplifier  178 , filters the signal, and provides the signal to second IF amplifier  186 . BPF  192  matches the IF bandwidth to the transmit pulse width to prevent the return pulse width from stretching. Transmit pulses typically increase in pulse width at higher altitudes to provide adequate system sensitivity. As a result, to prevent additional interference from leakage signals that stretch in pulse width at low altitudes, the bandwidth of the IF amplifiers is able to support the wider transmit pulse widths utilized at higher altitudes. To achieve this support, the bandwidth of the IF amplifiers is programmed, by processor  184 , to match the transmit pulse width. In one numerical example, at a low altitude a transmit pulse is sent with a pulse width of 10 nsec. In this example, the IF amplifier bandwidth should be approximately 100 MHz. In another numerical example, at a higher altitude, a transmit pulse has a pulse width of 100 nsec. In this example, the IF amplifier bandwidth should be approximately 10 MHz. 
         [0031]      FIG. 5  is a plot of a time sensitivity gain control signal  194 , referred to as Sensitivity Time Control (STC). STC signal  194  is produced by time shaping gain control generator  182  and provided to first IF amplifier  178  and second IF amplifier  186 .  FIG. 5  shows how STC signal  194  changes over time. 
         [0032]    To increase receiver gain at low altitudes quickly while continuing to reduce the interference of the antenna leakage pulse and reflected interference signals, STC signal  194  is implemented. STC signal  194  is triggered each PRF and timed with reference to transmitter pulse  183 . In one specific example, STC signal  194  transitions from no control to −78 dB starting approximately 300 nsec before transmitter pulse  183  is sent to modulator switch  164 . This time period provides sufficient time to stabilize and provide attenuation to −78 dB, as shown in  FIG. 5 . STC signal  194  is shaped to increase the gain as a function of time (i.e., as opposed to the SRC in current radar altimeters which is shaped to increase the gain as a function of an analog signal that is a function of altitude.) 
         [0033]      FIG. 6  is a plot illustrating the relationship over time between a transmit pulse  200 , two ground return pulses  202  and  204 , two antenna leakage pulses  206  and  208 , a track gate signal  210 , and an STC signal  212 . STC signal  212  maintains attenuation of antenna leakage pulses  206  and  208  at all times and will increase the gain for a ground return signal at higher altitudes as shown. Unlike other radar applications where distant objects are what is of interest and sensitivity is kept low until returns from close-in clutter have been received, in a radar altimeter at a low altitude, it is not only distant objects that are of interest. The vehicle&#39;s altitude is of interest at low altitudes. STC signal  212  increases the gain of the receiver after the transmit pulse is emitted, allowing return pulses to be identified even at low altitudes. 
         [0034]    It is also desirable at low altitudes to employ narrow transmit pulses, for example, 10 nsec. This will allow STC signal  212  to increase gain rapidly without antenna leakage pulses  206  and  208  interfering with tracking gate  210 . 
         [0035]    The methods and apparatus described above facilitate attenuation of antenna leakage signals and other reflected signal interference to prevent momentary breaks in tracking, lockup on the leakage signal, oscillation between the tracking and leakage signals, and inaccuracies due to gate slide or hover fading. This is achieved by attenuating the antenna leakage signals and other reflected signal interference with an STC signal every PRF. So as not to interfere with the tracking gate or ground return pulse, the STC signal is shaped to increase the system gain as a function of time and improve the amplitude of ground return signals at altitudes greater than 10 feet. This mechanization can provide excellent hover performance when narrow transmit pulses are utilized by spreading the spectrum due to narrow transmit pulses that minimize signal cancellation. This mechanization allows the system to operate properly even in air vehicles with marginal antenna installations, providing air vehicle manufacturers and users more latitude in locating the antennas as well as other equipment on the surface of the vehicle. This mechanization also provides significant cost and schedule savings along with customer satisfaction, and eliminates the potential for costly troubleshooting and air vehicle changes. 
         [0036]    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.