Velocity and drift angle tracking system using altimetry signals

A system for determining the drift angle of a fluid-born vehicle, said drift angle being defined as the angle between the vehicle heading direction and the direction said vehicle is moving relative to an adjacent stationary reflecting surface, said system comprising, in combination:

• transmiting means provided on said vehicle for illuminating said surface with a train of wave energy pulses;
• receiving means on said vehicle for receiving said wave energy after reflection from said surface, said receiving means having forward and aft energy-receiving phase centers spaced apart from one another in said vehicle heading direction;
• offset means for electronically intermittantly shifting said forward and aft phase centers by selected amounts in directions lateral to said heading direction;
• correlating means responsive to reflected wave energy pulses received at said forward and aft pulse centers for processing selected pairs of said forward and aft received pulses to generate a crosscorrelation output indicative of the crosscorrelation of a plurality of said selected pairs, said correlating means further operating to generate a crosscorrelation difference output indicative of the difference between crosscorrelation outputs generated for different lateral positions of said phase centers and to generate an offset control signal in response to said crosscorrelation difference outputs;
• electronic control means responsive to said offset control signal for controlling said offset means to alter the amount of lateral displacement of said phase centers so as to minimize said crosscorrelation difference output;
• calculating means responsive to the amount of said phase center displacement for generating a representation of drift angle;
• wherein said correlating means further includes means for generating a correlation output e.sub.vi in accordance with the algorithm
• e.sub.vi = 1/2[A.sub.N+i - F.sub.i)F.sub.i-2 - F.sub.i+2)+(A.sub.N+i+2 - F.sub.i+2) (F.sub.i - F.sub.i+4)]
• where A.sub.N+i and A.sub.N+i+2 represent two consecutively received aft pulses, F.sub.i and F.sub.i+2 represent the forward pulses received immediately before and after said F.sub.i pulse and F.sub.i and F.sub.i+4 represent the forward pulses received immediately before and after said F.sub.i+2 pulse, the subscript i denoting a progression of alternate odd integers to indicate the chronological occurrence of said pulses and the subscript N denoting that each aft pulse of said pair is received N pulse repetition intervals later in time than its associated forward pulse;
• means responsive to said correlation output for controlling said transmitting means to adjust the repetition frequency of said train of wave energy pulses such that said correlation output is maintained at a minimum value;
• and
• means for determining said heading velocity based on the quotient of said phase center separation distance and N multiplied by the duration of the interval between pulses in said transmitted pulse train, and wherein said calculating means includes means responsive to said representations of heading velocity and drift angle for generating a representation of the drift velocity of said vehicle.

A system for determining the drift angle of an aircraft relative to the ground, said drift angle being defined as the angle between the vehicle heading direction and the direction said vehicle is moving relative to an adjacent stationary reflecting surface, said system compromising, in combination:

• transmitting means provided on said aircraft for illuminating said surface with a train of rf pulses;
• receiving means on said aircraft for receiving said rf pulses after reflection from said surface, said receiving means having forward and aft energy-receiving phase centers spaced apart from one another in said vehicle heading direction, wherein said receiving means further comprises a pair of forward antennas spaced apart from one another in said lateral direction;
• a pair of aft antennas spaced apart from one another in said lateral direction;
• and hybrid waveguide means for combining the rf energy received at said respective pairs of antennas such that said forward phase center is located on a phase center line joining the phase centers of said pair of forward antennas and said aft phase center is located on a phase center line joining the phase centers of said aft antennas;
• offset means for electronically intermittantly shifting said forward and aft phase centers by selected amounts in directions lateral to said heading direction;
• correlating means responsive to reflected rf pulses received at said forward and aft phase centers for processing selected pairs of said forward and aft received pulses to generate a crosscorrelation output indicative of the crosscorrelation of a plurality of said selected pairs, said correlating means further operating to generate a crosscorrelation difference output indicative of the difference between crosscorrelation outputs generated for different lateral positions of said phase centers and to generate an offset control signal in response to said crosscorrelation difference output;
• electronic control means responsive to said offset control signal for controlling said offset means to alter the amount of lateral displacement of said phase centers so as to minimize said crosscorrelation difference output;
• and
• calculating means responsive to the amount of said phase center displacement for calculating said drift angle.