Abstract:
A system and method for correcting for Doppler shift in transmitted and received electromagnetic wave, light wave, or acoustic wave signals between two platforms, where at least one of the platforms is moving relative to the other. The system involves determining a Doppler shift that affects the frequency of a signal being transmitted from a transmitting platform, as a result of motion of the transmitting platform, and adjusting the frequency of the transmitted signal to cancel out the determined degree of Doppler shift that will be experienced by the receiving platform. If the receiving platform is also moving, then a determination is made as to the Doppler shift that will be imparted to the signal being received because of motion of the receiving platform. A receiver on the receiving platform is controlled to account for this degree of Doppler shift. Therefore, the Doppler shift components attributable to the motion of each, or both, platforms is accounted for.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to communication systems, and more particularly to a system and method for removing the Doppler shift in the frequency of a signal transmitted from a mobile platform, and/or removing the Doppler shift from a signal received by a mobile receiving platform.  
       BACKGROUND OF THE INVENTION  
       [0002]     In a mobile communication network, Doppler shift occurs when the velocity vector of a transmitting mobile platform differs from the velocity of a receiving mobile platform. For example, when two platforms are stationary with respect to each other (or with respect to a common reference frame) and are communicating with each other, the frequency of the signal received by the receiving platform from the transmitting platform will be the same as the frequency transmitted by the transmitting platform. In this case no Doppler frequency shift exists since the distance between the two platforms remain constant. When the distance between the two platforms is reducing with time, the frequency of the signal received by the receiving platform from the transmitting platform will be higher than the frequency transmitted by the transmitting platform due to the Doppler affect. When the distance between the two platforms is increasing with time, the frequency of the signal received by the receiving platform from the transmitting platform will be lower than the frequency transmitted by the transmitting platform due to the Doppler affect.  
         [0003]     It is noted that distance between two platforms can also increase or decrease with time though one of the two platforms is stationary with respect to a common reference frame. This relative distance variation between the two platforms with respect to time will result in a Doppler frequency shift that needs to be accounted for by the receiving platform regardless whether it is the moving platform or the stationary platform with respect to a common reference frame.  
         [0004]     For a nominal frequency of f 0 , the actual frequency at the receiver is f 0 +Δf, where Δf is the Doppler shift. To accommodate this variation in the received frequency of an electromagnetic wave signal, previously developed systems call for the receiver electronics to accept a wider frequency bandwidth than the nominal frequency bandwidth of the signal. This increases the amount of noise entering the receiver, thereby reducing the signal-to-noise ratio. In addition, the variation in frequency means that the system must use larger guard bands, i.e., unused bands of frequency between each link&#39;s nominal frequency and the frequencies of other links. This arrangement wastes bandwidth. Furthermore, because the incoming frequency is not precisely known, the receiving modem must scan over a range of frequencies before it can lock onto the carrier. This reduces the time available for data to be received, especially in time division multiple access (TDMA) systems where the modem must resynchronize at the beginning of every time slot. The above described Doppler shift applies to acoustic signals having a frequency of possibly less than 1 Hz to hundreds of KHz, as well as to electromagnetic wave signals. The Doppler shift has the negative effect of increasing the time to establish a two-way communication link due to longer modem synchronization times, the drawback of necessitating the extra bandwidth needed for guard bands, and serves to increase noise present with the received signal as a result of the use of the guard bands.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention is directed to a system and method for compensating for the Doppler shift effect on a signal being transmitted from a transmitting platform to a receiving platform, where at least one of the two platforms is a mobile platform.  
         [0006]     In one preferred implementation the system of the present invention involves the use of a transmitter having an associated processor. The transmitter is located on a mobile platform. A receiver having its own processor is disposed on a separate platform. Signals, for example electromagnetic wave signals, are transmitted from the transmitting mobile platform to the receiving mobile platform. The transmitting mobile platform determines its velocity vector relative to a reference frame that is common to all platforms in the system, as well as a unit vector that represents the direction of the receiving platform relative to the transmitting platform. The processor associated with the transmitter uses the velocity vector and the unit vector to calculate a Doppler shift that would be experienced by the receiving platform if the receiving platform were stationary with respect to a common reference. The processor then alters the transmitted frequency of the signal from the transmitter as necessary to cancel the Doppler shift that the receiving platform would experience.  
         [0007]     In another preferred implementation, the receiving platform operates to cancel the Doppler shift on a signal that it is receiving from a transmitting platform. In this embodiment the receiver determines a vector of velocity relative to the common reference frame, as well as a unit vector in the direction from itself to the transmitting platform. A processor associated with the receiver uses this information to determine a Doppler shift that would be experienced by the receiving platform when receiving the signal from the transmitting platform, if the transmitting platform was stationary. The process to adjust the frequency of the receiver is needed to cancel the Doppler shift in the receive signal that is attributable to movement of the receiving platform. In another alternative preferred embodiment, both the receiving and transmitting platforms perform the above-mentioned Doppler shift determinations. The transmitting platform removes the Doppler shift that would be imparted to the transmitted signal as a result of motion of the transmitting platform, while the receiving platform similarly determines and removes the Doppler shift component that is attributable to its own motion, relative to the transmitting platform.  
         [0008]     Another alternative preferred embodiment is designed for two-way communications where each platform includes both a receiver and a transmitter that perform the above-mentioned Doppler shift determinations. The transmitter on each platform removes the Doppler shift that would be imparted to the transmitted signal (and experienced by the other mobile platform), as a result of its own motion, while the receiver on each platform similarly determines and removes the Doppler shift component that is attributable to its own motion, relative to the other platform.  
         [0009]     The present invention is not limited to electromagnetic wave signals but can be applied to acoustic signals, including sonar signals, as well as optical signals including laser signals. Furthermore, the present invention can be used with electromagnetic wave or light signals transmitted through the atmosphere or between spacecrafts, or to electromagnetic wave or acoustic signals between two underwater vessels communicating with each other while submerged underwater. The present invention is equally applicable to situations where one platform is stationary and the other one is mobile, or where both platforms are mobile.  
         [0010]     The features, functions, and advantages can be achieved independently in various embodiments of the present inventions or may be combined in yet other embodiments.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0012]      FIG. 1  is a simplified schematic diagram of two mobile platforms communicating with each other and further illustrating, in simplified fashion, the unit vectors that each makes use of in determining a direction from it to the other platform;  
         [0013]      FIG. 2  is a simplified block diagram of the components on the transmitting platform and the receiving platform;  
         [0014]      FIG. 3A  is a flow chart illustrating the steps performed by a transmitting mobile platform, in accordance with a preferred implementation of the present invention; and  
         [0015]      FIG. 3B  is a flow chart illustrating the steps performed by a receiving mobile platform, in accordance with a preferred implementation of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0017]     Referring to  FIG. 1 , there is shown a system  10  that includes a preferred implementation of the present invention. The system  10  generally includes a transmitting mobile platform  12  having a transmitter  14  carried thereon, and a receiving mobile platform  16  having receiver  18  located thereon. Assuming for the moment that receiving platform  16  is stationary or moving toward mobile platform  12 , then electromagnetic wave signals transmitted from transmitter  14  of transmitting platform  12 , as represented by signal line  20 , would be increasing slightly in frequency since platform  12  is moving towards platform  16 . This increase in frequency represents the Doppler shift that the receiving platform  16  “sees.” If receiving platform  16  is moving towards transmitting platform  12 , then an additional Doppler shift component will be experienced by receiving platform  16  due to its own movement.  
         [0018]     With brief reference to  FIG. 2 , the transmitter  14  on the transmitting platform  12  is in communication with a processor  22 . An antenna  24  is used for transmitting electromagnetic wave signals. The receiving platform  16  similarly includes a processor  26  which is in communication with the receiver  18 . An antenna  28  is used for receiving electromagnetic wave signals from the receiver  18 . Antennas  28  and  24  may be located on an exterior surface of each mobile platform  12  and  16  or at a suitable location internal to a fuselage of each mobile platform  12  and  16 . Furthermore, while the transmitting platform  12  and the receiving platform  16  are each illustrated as aircraft, it will be appreciated that the present invention can be implemented with any mobile platform such as trains, busses, ships, satellites, spacecrafts, missiles, submarines, torpedoes, other airborne vehicles, other undersea vessels, other land vehicles, or other space vehicles. Furthermore, each mobile platform could have a bi-directional transceiver for both transmitting and receiving electromagnetic wave signals. In this instance, as will be explained in greater detail in the following paragraphs, each mobile platform will determine the necessary Doppler shift corrections needing to be applied, on both transmit and receive operations, as a result of its own motion.  
         [0019]     Referring further to  FIG. 1 , the transmitting platform  14 , to remove the Doppler effect that will be experienced by the receiving platform  16  due to the motion of the transmitting platform  12 , must determine 1) its own velocity relative to a common, pre-established reference frame, and 2) a direction to the receiving platform  16  relative to the common reference frame. If the receiving platform  16  is a mobile platform such as illustrated in  FIG. 1 , then the receiving platform also must determine its velocity relative to the same (i.e., common, pre-established) reference frame, and also its direction relative to the transmitting platform  12  relative to the same common, pre-established reference frame. The scenario with both platforms  12  and  16  moving towards each other will be assumed for the purpose of the following description.  
         [0020]     The total Doppler shift of the electromagnetic wave signal  20  received at the receiver  18  is due to two velocities: the velocity of the transmitting platform  12 , as well as the velocity of the receiving platform  16 . Assuming for the moment that platform  16  is stationary, the received frequency at receiver  18  can be represented by the following equation:  
         [0021]     Δf T =f 0  r TR ·v T /C, where f T  is the frequency emitted by the transmitter; “c” is the speed of light; “·” represents a vector dot product; where Δf T  is the frequency shift due to the transmitting platform&#39;s motion; and where f 0  represents the carrier frequency of the transmitted signal  20  for the stationary platform.  
         [0022]     With the present invention, each mobile platform  12 ,  16  performs local adjustments to eliminate the part of the frequency shift that is due to its own motion. For example, if the transmitting platform  12  and the receiving platform  16  were each stationary, the transmitting platform  12  would transmit at f 0  and the receiving platform  16  would receive the electromagnetic wave signal  20  at f 0 . However, since the transmitting platform  12  is moving relative to the receiving platform  16 , the transmitting platform  12  first determines its velocity vector v T  relative to an agreed upon common reference frame, for example, Local Earth Coordinates or common stellar coordinates. In this regard, it will be appreciated that a common coordinate frame of reference for position and velocity in a three dimensional coordinate system will be necessary in most instances. Three dimensional coordinate systems can be obtained by the aircraft&#39;s on-board navigation system which provides longitude, latitude, altitude, direction, and velocity with respect to Local Earth Coordinates. This can be provided by multiple earth stations or multiple GPS satellites (GPS constellation) to correlate a position and determine a velocity. This can be accomplished for a spacecraft by referencing to multiple heavenly bodies (sun, planets, stars, constellations, etc.) to triangulate a position and determine a velocity. There are numerous other common coordinate navigation systems that can be used to determine position and velocity such as Long Range Radio Aid to Navigation (LORAN), VHF Omnidirectional Range navigation system (VOR), etc.  
         [0023]     The transmitting platform  12  must then determine a unit vector r TR  in the direction from itself to the receiving platform  16  with respect to the common reference frame. The precise mechanism or system for determining the unit vector r TR  depends on the system in which the invention is used. For example, and with further reference to  FIG. 1 , if the satellite  30  is employed with the system  10 , then each platform  12  and  16  can continuously report its position to other platforms operating in a given region via the satellite&#39;s  30  transponder. This would be assuming that the satellite is a geosynchronous satellite whose location is constant relative to a position on Earth. Other means of establishing a common reference frame could involve having each transmitting platform continuously track the direction of its link partners using synchronous beam cloning, using a signal strength indication in combination with pointing a directional antenna, or other suitable means.  
         [0024]     Referring further to  FIG. 1 , since the transmitting platform  12  knows its approximate position relative to the satellite  30 , and since the transmitting platform is able to retrieve approximate position information on the position of the receiving platform  16  via the satellite  30 , a unit vector  32  (r TR ) can be determined. Unit vector  32  represents the direction from the transmitting platform  12  to the receiving platform  16 . Similarly, the receiving platform  16  determines a unit vector  34  that represents the direction from it to the transmitting platform  12 . The unit vector  34  (r RT ) is also determined using position information of the position of the transmitting platform  12  that is obtained via the satellite  30 . Receiving platform  16  obtains its own position information via any suitable means, such as its inertial navigation system or from satellite  30 .  
         [0025]     The processor  22  on the transmitting platform  12  determines the Doppler frequency shift (Δf T ) that a stationary receiver in direction r TR  would observe. The processor  22  adjusts the frequency at which transmitter  14  transmits, thus changing its emitted frequency from f 0  to (f 0 −Δf T ). This cancels the Doppler shift due to the transmitting platform&#39;s  12  motion so the actual Doppler shift experienced by a stationary receiver in direction r TR  would be zero.  
         [0026]     The receiving platform  16  performs similar steps. The processor  26  initially obtains a velocity of the receiving platform  16  (v R ) using the same reference frame as the transmitting platform  12  (i.e., the common reference frame). The receiving platform  16  also determines the unit vector r RT    34  representing the direction from itself to the transmitting platform  12 . The processor  26  computes the Doppler frequency shift (Δf R ) that the receiver would observe for a signal transmitted at frequency f 0  from a stationary transmitter along the unit vector  34  (i.e., r RT ). For electromagnetic wave signals, this frequency shift can be given by the equation: 
 
Δf r =f 0    r   RT   ·v   r   /c,  where “ v   r ” is the velocity of the receiving platform. 
 
         [0027]     The processor  26  then adjusts the frequency setting of receiver  18 , changing its nominal frequency from f 0  to f 0 +Δf R . Given that the transmitting platform  12  has removed the frequency shift component (i.e., Δf T ) due to the transmitting platform&#39;s  12  motion, the frequency (f R ) of the signal arriving at the receiving platform  16  can be given by the equation: 
 
 f   R   =f   0   +Δf   r   +Δf   T   −Δf   T  
 
= f   0   +Δf   R . 
 
         [0028]     The term f R  thus represents exactly the frequency at which the receiving platform&#39;s receiver  18  expects to receive the electromagnetic wave signal  20 . This substantially eliminates the problems that arise from frequency mismatch due to the Doppler shift of the signal  20 .  
         [0029]     In practice, the remaining frequency mismatches after the above-described Doppler shift corrections are applied are typically about 1.5-2.0 orders of magnitude smaller than the Doppler mismatch remaining when using many previous developed correction systems. Many remaining frequency mismatches are largely due to measurement errors in speed and direction, and the time lag between these measurements. Additionally, errors in frequency adjustment (i.e. imprecise voltages used to adjust frequency, or imprecise voltage-to-frequency shift coefficients) also can affect the degree of frequency mismatch reduction achieved with the present invention. However, for equipment typically used on aircraft, these errors are typically on the order of one percent to a few percent that of the Doppler shifts encountered without the benefit of the present invention. Thus, the noise entering the receiver, the frequency scan time and the guard bandwidth can all be reduced significantly compared to what is needed with previous correction systems that do not achieve the significant degree of Doppler mismatch reduction that the present invention achieves.  
         [0030]     With brief reference to  FIGS. 3A and 3B , a simplified flowchart setting forth the basic steps of one preferred implementation of the present invention  10  will be described. This implementation assumes that both the transmitting platform  12  and the receiving platform  16  are moving relative to the common reference frame. At operation  50 , transmitting platform  12  obtains a velocity vector (v T ) representing its speed, relative to the common reference frame. At operation  52 , the transmitting platform  12  next obtains the unit vector (r TR ) for the direction from it to the receiving platform  16 . At operation  54 , the processor  22  determines, from the velocity vector (v T ) and the unit vector (r TR ), the Doppler shift (Δf T ) affecting the signal  20  being transmitted as a result of the motion of the transmitting platform  12 . In operation  56 , the processor  22  then controls the transmitter  14  in a manner so that the frequency of the signal  20  transmitted is adjusted as needed to cancel out the Doppler shift (Δf T ) that will be experienced by the receiving platform  16 . These operations are periodically repeated to adjust for changes in v T  and r TR . The update rate can be as low as 10 ms for fast aircraft or missiles for which the transmitting platform  12  is relatively close to the receiving platform  16  (i.e., quick dynamic shifts in Doppler). The update rate can be several seconds or longer for submarines with slow maneuvers while receiving ultra low frequency signals (slow dynamic shifts in Doppler). The update rate will depend on how fast the Doppler shift changes. Doppler shift dynamics depend on 1) how fast the relative velocity changes between the receiver and the transmitter and 2) the relative difference in platform velocity with respect to the signal velocity.  
         [0031]     At operation  58 , the receiving platform  16  obtains velocity vector (v r ) information relative to the common reference frame. At operation  60 , the receiving platform  60  obtains unit vector (r RT ) for the direction from it to the transmitting platform  12 . At operation  62 , the receiving platform  16  determines the Doppler shift (Δf T ) affecting signal  20  as a result of its own motion, and assuming that transmitting platform  12  is stationary. Finally, at step  64 , the processor  26  adjusts the receive frequency of receiver  18  to cancel out the Doppler shift that is attributable to the motion of the receiving platform  16 . These operations are periodically repeated to adjust for changes in v R  and r RT .  
         [0032]     For a two-way communications link between platform  12  and platform  16 , there would be a receiver and a transmitter on each platform. The receiver and transmitter on platform  12  will have a common platform velocity vector (v T12 =v R12 ) and a common direction vector (r T12R16 =r R12T16 ). The resulting transmitter and receiver Doppler frequency adjustments made on platform  12  will be equal and opposite (Δf T12 =−Δf R12 ). Platform  16  would also have a receiver and a transmitter that will have a common platform velocity vector (v T16 =v R16 ) and a common direction vector (r T16R12 =r R16T12 ). The resulting transmitter and receiver Doppler frequency adjustments made on platform  16  will be equal and opposite (Δf T16 =−Δf R16 ). Thus, each mobile platform would be performing a Doppler shift correction during both transmitting and receiving operations.  
         [0033]     The formula for Doppler shift of sound waves varies depending on whether the source is moving, the observer is moving, or both. If the observer (receiver) is moving, the formula for frequency f′ that the receiver hears is: 
 
 f ′=( v+v   observer )/ v    Formula 1 
 
 where v is the speed of sound in the medium, v observer  is the speed of the observer (receiver) toward the source, and f is the frequency in the absence of any Doppler shift. Note that v observer  is negative if the observer (receiver) is moving away from the source. If the source (transmitter) is moving, the formula for frequency f′ that a stationary receiver hears is: 
 
 f′=fv /( v−v   stationary receiver )   Formula 2 
 
         [0034]     where v and f are the same as before and v stationary receiver  is the speed of the transmitter toward the receiver.  
         [0000]     The two formulae combine when both transmitter and receiver are in motion: 
 
 f′=f ( v+v   observer )/( v−v   stationary receiver ). 
 
         [0035]     For the present system and method, the receiver would use the Formula 1 above to correct the incoming frequency and the transmitter would use the Formula 2 to correct the outgoing frequency.  
         [0036]     The present invention thus offers a means to reduce the waste of transmitter to receiver frequency synchronization time and bandwidth that is currently caused by Doppler shift in mobile communication systems. This waste is most significant in systems that use fast moving nodes like aircraft, missiles and spacecraft. The Doppler shift has less impact on slow moving systems such as automobiles, vans, trucks or land vehicles, and watercraft. With such slow moving vehicles, the signal velocity is very high compared to the speed of the vehicle as is typically the case when electromagnetic radio frequencies or light frequencies are used. Doppler shift can be significant for relatively slow moving nodes if the signal velocity is comparatively slow, as can be the case when acoustic signals are used.  
         [0037]     While various preferred embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the inventive concept. The examples illustrate the invention and are not intended to limit it. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.