Passive ranging method and apparatus

An improved passive ranging method of the class which includes measuring the ambiguous change in phase of an emitter's signal occurring between a plurality of points along a path separated by arbitrary periods of time and distance with a Long Baseline Interferometer (LBI) mounted on a platform moving between said points, wherein the improvement comprises:

• measuring said phase unambiguously and sequentially at said points with a Short-Baseline Interferometer (SBI) mounted on said platform;
• measuring the wavelength and unambiguous phase difference of said signal at said plurality of points with said Short Baseline Interferometer (SBI);
• computing the unambiguous angular position of said emitter relative to said path at said points from said SBI measurements;
• predicting the range within which the unambiguous LBI phase difference must lie from said computed angular positions;
• computing the integer number of 2.pi. rotations occurring in said signal between said points from said predicted range and said ambiguous measurements;
• and
• computing the unambiguous LBI change in phase between said points from said ambiguous change and said computed number of 2.pi. rotations.

A method for passive ranging from a moving platform to a stationary, non-cooperative emitter of a pulsed signal, comprising the steps of:

• measuring the wavelength of at least a portion of at least one pulse of said signal at first and second points along said platform's path;
• measuring the unambiguous phase difference of at least a portion of at least one pulse of said signal at said first and second points with a Short Baseline Interferometer (SBI);
• measuring the ambiguous phase difference of at least a portion of at least one pulse of said signal at said points with a Long Baseline Interferometer (LBI);
• measuring the position and attitude of said platform at and between said points with a Navigation system (NAV);
• computing the length and direction of a straight line between said points from said NAV measurements;
• computing the unambiguous angular position of said emitter relative to said straight line, corrected for roll, pitch and heading of said platform, at said first and second points, from said SBI and NAV measurements;
• computing the ambiguous change in phase difference of said signal between said points from said LBI measurements;
• computing the unambiguous change in phase difference of said signal between said points, corrected for roll, pitch and heading of said platform at said points, from said computed angular positions, said NAV measurements, and said computed ambiguous change;
• and
• computing the slant range and angular position of said emitter relative to the midpoint of said straight line from said measured wavelengh of said signal, said computed length of said straight line, said computed angular positions of said emitter relative to said straight line at said points and said computed, unambiguous change in phase difference between said points, whereby the present angular position and range of said emitter relative to any other point in space may be computed.

A method for passive ranging from a moving platform to a stationary, non-cooperative emitter of a pulsed, frequency-agile signal, comprising the steps of:

• detecting that said signal is frequency-agile;
• measuring the wavelength and unambiguous phase difference of said signal over at least a portion of at least one pulse of said signal at first and second points along said platform's path with a Short Baseline Interferometer (SBI);
• measuring the position and attitude of said platform at and between said points with a Navigation system (NAV);
• computing the length and direction of a straight line between said points from said NAV measurements;
• computing the unambiguous angular position of said emitter relative to said straight line, corrected for roll, pitch and heading of said platform, at said first and second points from said SBI and NAV measurements;
• computing the unambiguous angular position of said emitter relative to the midpoint of said straight line from said computed, unambiguous angular positions at said first and second points;
• measuring the frequency, wavelength and ambiguous phase difference of at least a portion of at least N pulses of said signal at each of said first and second points with a Long Baseline Interferometer (LBI) and storing said values;
• comparing said stored values of frequency of said N pulses measured at each said point and pairing said pulses into at least M pairs of said pulses, each said pulse pair having one pulse from said N pulses measured at said first point and one pulse from said N pulses measured at said second point whose frequencies are within a given bandwidth of each other, and wherein N.gtoreq.M;
• computing the ambiguous change in phase difference between said pulses in each said M pulse pair from said stored, measured values of ambiguous phase difference of said pulses;
• computing the ambiguous phase correction due to difference in cable length in said LBI, if any, between the pulses in each said M pulse pair from said stored, measured values of frequency of said pulses;
• computing the ambiguous phase correction due to frequency difference between the pulses in each said M pulse pair from said computed angular position of said emitter relative to said midpoint of said straight line of said stored, measured values of said wavelength of said pulses;
• computing the corrected, ambiguous change in phase difference between the pulses in each said M pulse pair, corrected for said cable length and frequency differences, for each said pulse pair;
• averaging said corrected, ambiguous change in phase difference computed for each said pulse pair over said M pairs of said pulses;
• computing the unambiguous change in phase difference of said signal between said points, corrected for roll, pitch and heading of said platform at said points, from said computed angular positions of said emitter, said NAV measurements, and said computed, corrected, averaged ambiguous change in phase difference;
• and
• computing the slant range and angular position of said emitter relative to the midpoint of said straight line from said measured wavelength of said signal, said computed length of said straight line, said computed angular positions of said emitter relative to said straight line at said points and said computed, unambiguous change in phase difference between said points, whereby the present angular position and range of said emitter relative to any other point in space may be computed.

An improved ranging system of the class having a Long Baseline Interferometer (LBI) mounted on a moving platform for measuring the precise but ambiguous change in phase of an emitter's signal occurring between two or more points along a path separated by arbitrary periods of time and distance, wherein the improvement comprises:

• a Short Baseline Interferometer (SBI) mounted on said platform for sequentially measuring said phase unambiguously at said points;
• means for computing the unambiguous angular position of said emitter relative to said path at said points from said SBI measurements;
• means for predicting the range within which the unambiguous LBI phase difference must lie from said computed angular positions;
• means for computing the integer number of 2.pi. rotations occurring in the signal between said points from said predicted range and said ambiguous measurements;
• and
• means for computing said unambiguous LBI change in phase between said points from said ambiguous change and said computed number of 2.pi. rotations.

An apparatus for passive ranging from a moving platform to a stationary, non-cooperative emitter of a pulsed signal, comprising:

• a Short Baseline Interferometer (SBI) mounted to said platform for measuring the wavelength and unambiguous phase difference of at least a portion of at least one pulse of said signal at first and second points along said platform's path;
• a Long Baseline Interferometer (LBI) mounted to said platform for measuring the ambiguous phase difference of at least a portion of at least one pulse of said signal at said points, said LBI having a longer baseline than said SBI;
• navigation (NAV) means for measuring the position and attitude of said platform at and between said points;
• and
• signal processing means, including:
• means for computing the length and direction of a straight line between said points from said NAV measurements;
• means for computing the unambiguous angular position of said emitter relative to said straight line, corrected for roll, pitch and heading of said platform at said first and second points, from said SBI and NAV measurements;
• means for computing the ambiguous change in phase difference of said signal between said points from said LBI measurements;
• means for computing the unambiguous change in phase difference of said signal between said points, corrected for roll, pitch and heading of said platform at said points, from said computed angular positions, said NAV measurements, and said computed ambiguous change;
• and
• means for computing the slant range and angular position of said emitter relative to the midpoint of said straight line from said measured wavelength of said signal, said computed length of said straight line, said computed angular positions of said emitter relative to said straight line at said points and said computed, unambiguous change in phase difference between said points, whereby the present angular position and range of said emitter relative to any other point in space may be computed.

An improved passive ranging method of the class which includes measuring the phase of an emitter signal from a moving platform, comprising the steps of:

• sequentially measuring the precise but ambiguous changes in phase of said emitter signal occurring between a plurality of points along a path using a Long Baseline Interferometer (LBI) mounted on said moving platform to establish the difference in angle toward said emitter from said points;
• sequentially measuring the unambiguous phase of said emitter signal occurring between said points along said path using a Short Baseline Interferometer (SBI) mounted on said moving platform to establish the angles toward said emitter from said points;
• using said SBI angle measurements to establish an estimated range to said emitter;
• using said estimated range to said emitter to resolve the ambiguity in said LBI changes in phase;
• and
• using said resolved LBI phase to refine said range estimate.

An improved passive ranging system of the class which includes measuring the phase of an emitter signal from a moving platform, comprising:

• a Long Baseline Interferometer (LBI) mounted upon said moving platform for sequentially measuring the precise but ambiguous changes in phase of said emitter signal occurring between a plurality of points along a path to establish the precise difference in angles towards said emitter from said points;
• a Short Baseline Interferometer (SBI) mounted upon said moving platform for sequentially measuring the unambiguous phase of said emitter occurring between said points along said path to establish the angles toward said emitter from said points;
• means for using said SBI angle measurements to established an estimated range to said emitter;
• means for using said estimated range to said emitter to resolve the ambiguity in said LBI changes in phase;
• and
• means for using said resolved LBI phase to refine said range estimate.