Patent Abstract:
A method for randomly phase modulating a radar altimeter is described. The method includes momentarily applying a signal from a random noise source to an amplifier, applying an output of the amplifier to a voltage controlled oscillator (VCO), applying an output of the VCO to a transmitter and mixer of the radar altimeter to modulate transmissions of the radar altimeter and to demodulate reflected radar transmissions received by the radar altimeter and holding the output of the amplifier constant from before a radar altimeter transmission until after reception of the reflected radar signals from that transmission by the radar altimeter. The method further includes repeating the applying steps and the holding step.

Full Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to modulation techniques for radar, and more specifically to a radar altimeter which incorporates random noise modulation techniques. 
   A radar altimeter transmits pulses of radar energy and determines ranging information by measuring a delay time from transmission of the radar energy to receipt of an echo signal (the reflected pulses of radar energy). In known radar altimeters, to provide a continuous wave transmission capability, the transmitted radar signal is modulated and the delay in modulation is measured. For example, a frequency shift of the modulated, reflected pulses of radar energy is measured in order to obtain range information. 
   In military operations, reception of the transmitted radar signal not only provides an enemy with information concerning the existence of a source of a transmitted signal but also may include enough information to enable the enemy to provide false information to a radar altimeter. One such example is in the form of imitation echo signals. Imitation echo signals can be utilized, in one example, to cause a radar altimeter to provide an incorrect altitude. For this reason, various methods are used to reduce the power output of radar altimeters and to introduce pseudo-random noise (PRN) patterns to the radar-transmitted signal. One of the problems with pseudo-randomly modulated radar transmission signals is that the echo signals no longer fall within a very narrow frequency range, making a radar receiver that is designed only for receiving signals at a desired frequency to be used. In order to eliminate spurious signals from outside the very narrow frequency range, elaborate filtering techniques have been developed. When utilizing such techniques, the transmitted signal has to have sufficient amplitude to overcome problems caused by any spurious signals that are also being received. 
   Some known pulse Doppler radar altimeters incorporate bi-phase modulation techniques such as a pseudo-random noise (PRN) code which results in a finite code repeat interval. A finite code repeat interval provides predictable spectral line frequencies. One problem associated with predictable spectral line frequencies is that intercepting receivers (e.g., an enemy radar) can automatically search and acquire the transmit energy, detect a location, and jam the radar altimeter. The immunity to interception and jamming from such bi-phase modulation techniques realized by these altimeters is a direct function of the code word (PRN code) length. The transmitted signal is spread over a number of spectrum lines as a result of the PRN code. A 31 bit code word, for example, provides an intercept disadvantage in that the radar transmitted signal strength at the carrier frequency is reduced by the factor, 1/31, when compared with a radar altimeter that does not employ the PRN code. A received continuous wave jamming signal is spread over a number of spectrum lines. Therefore, an increase in jammer signal strength of 31 times is needed to jam the radar signal when compared with a radar altimeter that does not employ the PRN code. 
   The bandwidth of the radar PRN receiver should be narrow enough to integrate a period of time at least equal to the word length. The longer the modulation word for improved covertness and jam immunity, the narrower the bandwidth. Unfortunately, the receiver bandwidth has to be wide enough to process the Doppler shift caused by the platform velocity, resulting in a finite limit on the word length and accordingly on the level of covertness and jam immunity. 
   BRIEF SUMMARY OF THE INVENTION 
   In one aspect, a method for randomly phase modulating a radar altimeter is provided. The method comprises momentarily applying a signal from a random noise source to an amplifier, applying an output of the amplifier to a voltage controlled oscillator (VCO), applying an output of the VCO to a transmitter and mixer of the radar altimeter to modulate transmissions of the radar altimeter and to demodulate reflected radar transmissions received by the radar altimeter, and holding the output of the amplifier constant from before a radar altimeter transmission until after reception of the reflected radar signals from that transmission by the radar altimeter. The method also comprises repeating the applying steps and the holding step. 
   In another aspect, a method for randomly phase modulating a radar altimeter which includes a VCO having an output which modulates transmissions of the radar altimeter and demodulates reflected radar transmissions received by the radar altimeter is provided. The method comprises configuring the VCO to provide a random phase modulation source and holding the phase of the modulation source substantially constant from a time when the radar altimeter transmits a radar signal until a time when a reflection of the transmitted radar signal is received by the radar altimeter. 
   In still another aspect, a radar altimeter is provided which comprises a VCO for modulating transmissions of the radar altimeter and demodulating reflected radar transmissions received by the radar altimeter. The radar altimeter also comprises a random noise source, a holding circuit configured to sample a voltage from the random noise source and hold the voltage constant at an input to the VCO from the time a signal is transmitted by the radar altimeter until a reflected radar return signal is demodulated by the radar altimeter. 
   In yet another aspect, a circuit for randomly phase modulating transmissions of a radar altimeter and demodulating reflected radar transmissions received by the radar altimeter is provided. The radar altimeter includes a VCO and the circuit is connected to an input of the VCO. The circuit comprises a random noise source, an amplifier comprising an input and further comprising an output connected to the VCO, and a switch. The switch is between the random noise source and the input of the amplifier. The switch is configured to be open from the time a signal is transmitted by the radar altimeter until a reflected radar return signal is demodulated by the radar altimeter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a radar altimeter which incorporates a random phase modulation source. 
       FIG. 2  is a block diagram of the random phase modulation source of  FIG. 1 . 
       FIG. 3  illustrates a pulse repetition interval contrasted against a word repetition interval. 
       FIG. 4  illustrates a frequency spectrum for a radar altimeter modulated with bi-phase non-random modulation. 
       FIG. 5  illustrates a frequency spectrum for a randomly modulated radar altimeter of  FIG. 1 . 
       FIG. 6  illustrates the received signal spectrum after coherent demodulation by a mixer within the radar altimeter of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The below described apparatus and methods incorporate random phase modulation into a radar altimeter while also providing an infinite phase resolution resulting in no code repeat intervals. Known pseudo-random noise (PRN) modulation systems incorporate bi-phase non-random modulation, for example, of either 0 or 180 degrees. The phase resolution methods described herein result in an infinite word length, and therefore provide greatly improved covertness and jam immunity over known radar altimeters. In addition, since a bandwidth of the radar receiver is narrow enough to integrate the period of time between individual radar pulses, as opposed to integrating the period of time between entire words, the level of covertness and jam immunity is not affected by platform Doppler velocity as are known radar altimeter systems. 
   Referring to  FIG. 1 , a simplified block diagram of a radar altimeter  10  includes a phase modulated voltage controlled oscillator (VCO)  12  providing a frequency source for a radar transmitter  14  and a mixer  16 . Mixer  16  demodulates the signals received at receive antenna  18  with the frequency from VCO  12  to provide a radar return signal at a Doppler frequency to range gate  20 . 
   Mixer  16  down converts received radar signals to a base band frequency. Such a down conversion is part of a processing sequence for a radar return signal received by radar altimeter  10 , and is sometimes referred to as decimation to a base band frequency. For example, in known radar altimeters, down conversion to base band frequency is normally accomplished in a processor by sampling the radar return signal at a period of length that is an integer multiple of the period of the return signal. In some altimeters, the base band frequency is referred to as a Doppler frequency, as the base band frequency is the result of a Doppler shift in the received radar signal. 
   Range gate  20  is configured to pass the Doppler frequency signal at a set time after a transmission from radar transmitter  14  through transmit antenna  22 . The set time is dependent on a range to a target. The Doppler frequency signal is then is sampled at narrow band filter  24 , and processed at radar processor  26  in order to generate radar data that can be utilized by other systems or displayed, for example, on an aircraft display (not shown). Radar processor  26  further generates timing signals  28  which are utilized to control passage of the Doppler frequency signals through range gate  20 . Radar processor  26  also provides timing signals to radar transmitter  14  to enable transmission of radar signals through transmit antenna  22 . 
   VCO  12  receives a modulation signal (e.g., a voltage) from a random phase modulation source  30  and switching of the modulation signal from random phase modulation source  30  is also controlled by timing signals from radar processor  28 . The phase modulation initiated by random phase modulation source  30  is not bi-phase, but is a random phase relation which results in an infinite phase resolution. In one embodiment, the phase from random phase modulation source  30  is held constant from the time transmitter  14  transmits pulses of RF energy (e.g., the radar signal) towards the ground through transmit antenna  22  until the time the reflected radar signal is received and processed through mixer  16 . Holding the phase constant from the time of transmission to the time of reception of the range delayed radar return allows proper demodulation which then results in an in-phase signal for filtering and eventual processing at the output of mixer  16 . 
   As described above, modulation of VCO  12  is provided by random phase modulation source  30 .  FIG. 2  is a block diagram of random phase modulation source  30 . Random phase modulation source  30  includes, in the embodiment shown, a random noise source  32  whose output is received by a band pass filter  34 . The output of band pass filter  34  is switched through switch  36  to an input of amplifier  38 . An output of amplifier  38  is the VCO control voltage  40  that is input into VCO  12 , as described above. 
   Switch  36  is open from a beginning of a radar transmission until the reflected radar return signal is received at mixer  16  (shown in  FIG. 1 ). Operation of switch  36  in random phase modulation source  30  provides coherent operation of radar altimeter  10  (shown in  FIG. 1 ) from pulse-to-pulse. Switch  36  is then closed for a sufficient time to randomly phase modulate VCO  12 . Capacitor C 1  provides additional filtering for VCO control voltage  40 . Capacitor C 2  is utilized to hold the voltage at the input of amplifier  38  constant during the time that switch  36  is open. Band-pass filter  34  is used to control the random noise frequency spectrum. For example, low frequencies being applied to the input of amplifier  38  could change a center frequency of VCO  12 . Combinations of switch  36 , amplifier  38 , and capacitors C 1  and C 2  provide a hold device that is configured to hold a voltage applied to VCO  12  until the radar return signal has been received at antenna  18  and phase demodulated by mixer  16 . 
   In one embodiment, random noise source  32  includes a noise diode and an amplifier. A noise diode is a solid state noise source where a voltage potential applied to the noise diode results in an excess noise ratio from the noise diode. In another embodiment, random noise source  32  includes a high ohm resistor (i.e., in excess of one megohm) connected to an input of a high gain (&gt;1000) amplifier. The values for the resistor and gain for the amplifier are typical values and produce about 12 milli-volts RMS noise in a 1.0 MHz bandwidth for input into band pass filter  34 . 12 milli-volts RMS noise is representative of an input level to specific voltage controlled oscillators. Other resistor values and amplifier gains can be utilized in conjunction with other voltage controlled oscillators. Thermal noise produced by the resistor provides a random noise voltage. 
   Other embodiments for random phase modulation source  30  also exist. For example, by removing switch  36  and connecting the output of band pass filter  34  to an input of amplifier  38 , and adjusting band-pass filter  34  to provide an almost constant VCO phase during a given pulse repetition interval of radar altimeter  10 . Another method of achieving random phase modulation of radar altimeter  10  is to remove DC power (not shown) from VCO  12  for a sufficient time for the VCO output signal supplied to mixer  16  and transmitter  14  (both shown in  FIG. 1 ) to decay. When DC power is restored to VCO  12 , a phase of VCO  12  is determined by thermal noise generated within an amplifier that is internal to VCO  12 . 
   Still another method for achieving random phase modulation includes biasing the amplifier internal to VCO  12  to an off condition after receiving radar return signal at mixer  16 . By applying an impulse voltage to the amplifier internal to VCO  12  or applying an impulse voltage to a frequency determining resonant circuit random phase modulation can be attained. However, in such an embodiment, timing of the impulse voltage cannot be derived from a frequency of VCO  12 . Specifically, a timing of the impulse is random with respect to a phase of VCO  12 . 
   In known systems which utilize bi-phase coded modulation, the filter (similar to narrow band filter  24  shown in  FIG. 1 ) is set to a bandwidth, B, which is equal to or less than the reciprocal of a word repetition interval (WRI) or, B≦1/WRI for the bi-phase coded modulation. The restriction on bandwidth, as described further below with respect to  FIG. 4 , provides the result that the filter passes only a carrier frequency Doppler signal and rejects range side-lobes spaced at 1/WRI intervals from the carrier frequency. As described below with respect to  FIG. 6 , random phase modulation source  30  allows the bandwidth of filter  24  to be increased to be equal to or less than the reciprocal of a pulse repetition interval (PRI) or, B≦1/PRI. This increase in bandwidth of filter  24  reduces the Doppler bandwidth restriction associated with present PRN modulated radar altimeters. The radar return signal received at receive antenna  24  and forwarded to mixer  16  is demodulated by mixer  16  resulting in a pulse train of equal phase. In one embodiment, filter  24  integrates the pulse train to a continuous wave signal which is processed by radar processor  26  utilizing normal radar altimeter processing algorithms. 
     FIG. 3  illustrates a pulse repetition interval  50  and a pulse width  52  which are contrasted against a word repetition interval  54 , which, as described above, is utilized in known bi-phase non-random modulation. As pulse repetition interval  50  has a shorter time of transmission than word repetition interval  54 , the bandwidth of a filter (e.g., filter  24 ) is increased accordingly according to, B≦1/PRI as opposed to, B≦1/WRI. 
     FIG. 4  illustrates a transmitted PRN spectrum  100  for a radar altimeter which utilizes bi-phase non-random modulation at a word repetition interval (WRI). Such modulation results in spectral line spacing, or a spectral width, that is about the reciprocal of the word repetition interval, or about 1/WRI. The spectral line spacing is sometimes referred to as pseudo-random noise word lines  102 . As described above, bandwidth of a band pass filter is restricted to pass only a carrier frequency Doppler signal and to reject range side-lobes spaced at 1/WRI intervals from the carrier frequency. 
     FIG. 5  illustrates a transmitted random noise spectrum  150  for random noise modulated radar altimeter  10  (shown in  FIG. 1 ). Noise spectrum  150  has a sin (x)/x spectral width of about the reciprocal of pulse width, or about 1/PW. Referring to the transmitted random phase modulation described herein, there are no repeated, evenly spaced transmitted spectral lines as there is no repetition of the modulation words. The sin x/x amplitude envelope in both the bi-phase non-random modulation of known systems and the random phase modulation described herein, is shown, which is the energy content of the transmitted spectrum. 
     FIG. 6  illustrates a received signal spectrum  200  for randomly modulated radar altimeter  10  (shown in  FIG. 1 ) after coherent demodulation by mixer  16  (shown in  FIG. 1 ). Spectrum lines  202  occur with a spacing of 1/PRI, rather than with a spacing of 1/WRI (as shown in  FIG. 4 ) as in PRN modulated radar altimeters. As described above, random phase modulation source  30  allows the bandwidth of filter  24  to be increased to be equal to or less than the reciprocal of a pulse repetition interval (PRI) or, B≦1/PRI. This increase in bandwidth of filter  24  reduces the Doppler restriction associated with present PRN radar altimeters. 
   Capabilities which allow random phase modulation of radar altimeter signals provide infinite phase resolution and no modulation code repeat intervals which heretofore have not been previously attained in radar altimeters. The above described improvements over known bi-phase modulation techniques (e.g., PRN modulation) allows radar altimeters to provide the above described improved capabilities without addition of costly and complex circuitry. 
   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.

Technology Classification (CPC): 6