Abstract:
A radar processor for controlling detonation of a munition and operable to receive a detonation altitude from an external source is provided. The radar processor is configured to set a first range gate and a reference range gate based on the received detonation altitude, and cause a radar transmitter to operate in a continuous wave mode, for a predetermined period, upon receipt of radar return signals through the first range gate. The radar processor calculates a velocity of the munition from continuous wave return signals, and calculates a time delay for outputting a detonation signal based on the received detonation altitude, the calculated velocity, and a reference altitude of the munition, the altitude of the munition calculated based upon receipt of radar return signals through the reference range gate. The radar transmitter operates in a pulse mode while the munition is outside the reference range gate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. patent application Ser. No. 11/330,027 (U.S. Pat. No. 7,352,319), filed Jan. 11, 2006 and entitled “METHODS AND SYSTEMS UTILIZING DOPPLER PREDICTION TO ENABLE FUSING” (the &#39;027 Application). The &#39;027 Application is incorporated herein by reference. 
   This application claims the benefit of priority to U.S. provisional application No. 60/689,369 filed Jun. 10, 2005, the disclosure of which is incorporated herein by reference. 

   BACKGROUND 
   This invention relates generally to radar systems, and more particularly to methods and systems that utilize Doppler prediction to set a height of fusing, for example, for a radar based weapon system. 
   A radar based weapon system is configured to fuse the weapon of the system at various low altitudes so that the weapon will have optimum impact. A dual antenna (i.e. one transmit antenna and one receive antenna) radar system installation within the weapon system that has sufficient separation between the two antennas is not practical for smaller weapon systems. One reason separation between the antennas is important is to reduce an antenna leakage signal that propagates between the two antennas. Sufficient antenna separation to measure elevations near or at ground level such that antenna leakage signals do not interfere with altitude measurements typically cannot be efficiently incorporated into smaller weapon systems. In these smaller weapon systems there is simply not enough room to adequately separate the two antennas. 
   In smaller weapons systems that incorporate two antennas, the two antenna apertures are typically located near one another, significantly increasing the antenna leakage signal. Therefore, with a dual antenna weapon system it is difficult to accurately measure certain low altitudes because the antenna leakage signals interfere with time coincident ground return signals. 
   A single antenna installation using a duplexer is even more difficult to implement in smaller weapons. The reason is that one antenna systems do not even exhibit some of the inherent antenna leakage isolation found in the dual antenna systems. Rather, signals similar to leakage signals are internal to the radar. As a result, it is also difficult for a one antenna radar sensor on a small weapon system to measure altitudes near or at ground level. 
   SUMMARY 
   In one aspect, a radar processor for controlling detonation of a munition is provided which is configured to receive a detonation altitude from an external source. The radar processor is configured to set a first range gate and a reference range gate based on the received detonation altitude, and cause a radar transmitter to operate in a continuous wave mode, for a predetermined period, upon receipt of radar return signals through the first range gate. The radar processor calculates a velocity of the munition from continuous wave return signals, and calculates a time delay for outputting a detonation signal based on the received detonation altitude, the calculated velocity, and a reference altitude of the munition, the altitude of the munition calculated based upon receipt of radar return signals through the reference range gate. The radar transmitter operates in a pulse mode while the munition is outside the reference range gate. 

   
     DRAWINGS 
       FIG. 1  is a block diagram of a radar system for fusing weapon systems at or near ground level altitudes. 
       FIG. 2  is an illustration of transmit and receive spectrums for the system of  FIG. 1 . 
       FIG. 3  is an illustration of timing signals present in the system of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   The methods and systems described herein provide a solution for weapon system fusing at low altitudes by measuring a precision reference altitude where antenna leakage interference is not present and measuring the velocity of the vehicle as it approaches the ground. Once velocity of the munition is determined, the munition continues toward the ground until it encounters a ground return that is time coincident with a preset reference altitude gate, establishing the precision reference altitude. From these two measurements, a time delay can be used to accurately predict when the weapon system will attain an altitude at or near ground level, and a timing sequence is started. As a result, antenna leakage signals do not interfere with the operation of the sensor. 
   A programmable sequencer provides the capability to program a desired detonation altitude, which is sometimes referred to as a height of fuse (HOF), prior to launch.  FIG. 1  is a block diagram of a radar based fusing system  10  incorporating such a sequencer. Referring specifically to  FIG. 1 , an RF oscillator  12  provides the frequency source for transmission through transmit antenna  14  and for down conversion of the radar return pulse received at receive antenna  16 . The frequency is down converted at mixer  18  and the frequency from RF oscillator  12  is provided to mixer  18  through power divider  20 . Modulation switch  22  provides pulse modulation of the signals to be transmitted. A buffer amplifier  24  provides isolation for RF oscillator  12  from the impedance variations caused by modulation switch  22  to reduce oscillator frequency pulling during transmission to a tolerable level, keeping the return frequency within a pass band of the receiver portion of system  10 . 
   The down converted radar return frequency output by mixer  18  is at a Doppler shift proportional to the downward velocity of the vehicle and is sometimes referred to as the Doppler frequency. The Doppler frequency, f d , is equal to 2×velocity/wavelength. In one embodiment, the transmitter frequency (f o ) is at 4.3 GHz and therefore, the wavelength is c/f o , where c is the speed of light. For example, at a velocity of 2461 Ft/sec (i.e. 750 m/sec), and 4.3 GHz oscillator frequency (i.e. wavelength of 0.229 feet), the Doppler shift is f d =2×2461/0.229, or 21,493 Hz at an output of the mixer  18  as illustrated in  FIG. 2 . Since the trajectory could be 10° from Nadir, then the maximum closing velocity would be 738 m/sec and the resulting Doppler frequency would be 21,149 Hz. 
   The low end of the Doppler bandwidth is determined when the velocity of the vehicle is at its minimum, for example, 820 ft/sec (i.e. 250 m/sec), which results in a Doppler frequency of 7,161 Hz. Again, with trajectory angle of 10°, the minimum closing velocity would be  246  and the minimum Doppler frequency is 7,046 Hz. The maximum Doppler bandwidth is therefore 21,493-7,046=14,447 Hz. 
   Referring back to  FIG. 1 , system  10  includes a velocity detector  30 . A Doppler gate  32  is set at a higher altitude (i.e. the first altitude) and the Doppler frequency is measured. This Doppler frequency is used by velocity detector  30  to calculate the velocity. For low altitudes or penetration detection, a time delay can be calculated by time delay controller  33 , based on the velocity along with knowing when the munition is at the reference altitude, and then used to accurately generate the height of fuse (HOF) signal at the desired detonation altitude. 
   Also, the Doppler frequency signal is peak detected with peak detector  34 , integrated at integrator  35 , and threshold detected at comparator  36  so that when the ground return signal passes through Doppler gate  32 , at a time coincident with a reference HOF gate, then a fusing signal  40  can be generated. Doppler gate  32  can be pre-set with a reference HOF gate at any time that is consistent with a position within the fusing range of the weapon to be detonated. 
     FIG. 3  is a timing diagram  100  illustrating a sequence of events which occurs within system  10  (shown in  FIG. 1 ) after launch of such a system. For example, if the vehicle is at an altitude of greater than 200 feet (i.e. a radar ground return time of 406.7 ns), the transmitter of system  10  is operating in a pulse mode. When operating in the pulse mode, the pulse width is narrow, for example, about 25 nanoseconds, and at a pulse repetition frequency (PRF) of about 40 KHz. This PRF is about twice the maximum Doppler frequency. The ground return pulse  102  is received at receive antenna  16  (shown in  FIG. 1 ) in excess of 406.7 nanoseconds from the time of transmission. 
   As the vehicle continues toward the ground, the ground return signal will become coincident with a programmed ground pulse return time within Doppler gate  32  (shown in  FIG. 1 ), and illustrated in  FIG. 3  as Doppler gate pulse  104 . When the ground return pulse occurs  102  within the gated time (i.e., during the Doppler gate pulse  104 ), the transmitter of system  10  is configured to revert from a gated or pulsed transmission to a continuous wave (CW) transmission for about 25 milliseconds, shown in  FIG. 3  as  106 . During this time, the Doppler frequency based on the CW transmission is accurately measured within system  10 . 
   At a minimum velocity (i.e. about 250 meters/sec) of the vehicle (e.g., weapon), the Doppler frequency is about 7 KHz, as described above. During the 25 millisecond period, 176 cycles will be sampled with a distribution error of about 0.56% or about 4.6 ft/sec. The distance traveled by the vehicle at this velocity and time is 20.5 feet. At a maximum velocity of the vehicle (i.e. 750 meters/sec), the Doppler frequency is about 21.5 KHz. During the 25 millisecond period of CW transmission, 537 cycles will be sampled with a distribution error of 0.186% or 4.6 ft/sec. The distance traveled by the vehicle at this velocity and time is 61.25 feet. At the end of the 25 milliseconds, the Doppler frequency will be measured and the transmitter will revert back to the pulse mode. 
   As the vehicle continues toward the ground, the return signal will eventually be coincident with a reference altitude HOF gate  108  that has been programmed into system  10 , for example, programmed into Doppler gate  32 . Reference altitude HOF gate  108  is precisely placed in time, for example, at 203.34 nanoseconds after transmission of a radar pulse, which is representative of the return time for a radar pulse from a vehicle at about 100 feet in altitude. By knowing the precise reference altitude of the vehicle, based on a ground return pulse being received when HOF gate  108  is active, and the velocity of the vehicle relative to the ground based on the Doppler measurement during the CW transmission, then delays, for example, using time delay controller  50  (shown in  FIG. 1 ) can be established. The delay is the time that it will take the vehicle to travel from the altitude that is coincident with the reference altitude HOF gate  108  to the desired detonation altitude, based on the vehicle velocity. Programming of such a delay results in generation of a fusing signal which occurs at the programmed detonation altitude setting (i.e. between 0 and 25 meters). 
   With an upper cutoff frequency of the Doppler frequency of 21,493 Hz, the pulse repetition frequency (PRF) should be set at about two times the Doppler frequency or about 40 KHz. This PRF will result in a first spectral line that is comfortably outside the Doppler bandwidth. If the transmitter pulse width is 25 nanoseconds, then the duty cycle is only 0.1%. With a low duty cycle, the gated system should provide additional jam immunity. 
   The above described gating and ground return signal processing configuration allows reliable operation, for example programmable detonation altitudes, down to zero feet altitude with a much lower antenna spacing than in previous systems. If there is no motion of the vehicle (munition), the output of the mixer will be 0 Hz (DC). This is the case for antenna leakage interference. Varying the phase of the local oscillator signal of mixer  18  signal results in the DC level of the antenna leakage signal varying. The DC level is set to zero by fixing the line length from power divider  20  to mixer  18  to provide a 180 degree shift with respect to the antenna to antenna leakage path length. 
   The above described system satisfies the need for a height of fuse (HOF) radar system for small munitions that operates at selected altitudes near and at ground level. Both pulse gating and Doppler frequency measurements are used to provide fusing of the munition at accurate altitudes. Fusing can be obtained down to zero feet with much closer antenna spacing or even with a single antenna by using coherent processing, Doppler isolation, and Doppler velocity prediction by avoiding altitude measurements that may be coincident with the receiving of an antenna leakage signal. The result is a low cost and small size sensor through incorporation of a single-frequency-oscillator design which provides both the transmit source and receiver down conversion source. A small, low cost, and low transmitter power is achievable through the incorporation of the signal oscillator, radar receiver processing technique, which takes advantage of the positive ground return Doppler shift associated with the downward motion. 
   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.