| CPC G01S 13/781 (2013.01) [G08G 5/0008 (2013.01); G08G 5/0013 (2013.01)] | 23 Claims |
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1. A method for determining a current position of an airborne vehicle equipped to receive signals from at least one secondary surveillance radar (SSR), the method comprising:
(a) determining a first pulse repetition frequency (PRF) pattern for staggered interrogation pulses (P1, P2, P3) of a first SSR, including determining an interrogation mode of the first SSR and duration of the first PRF pattern;
(b) determining a first distance from the first SSR to the airborne vehicle, comprising:
(i) at the airborne vehicle, at a first reception time instant, receiving an interrogation signal, comprising at least one of P1, P2 or P3 pulses, from the first SSR;
(ii) estimating a first transmit time instant of said at least one of P1, P2 or P3 pulses, based on the first reception time instant and the first PRF pattern, comprising:
at a prior reference reception time instant of receiving a reference interrogation signal from the first SSR at the airborne vehicle, recording a reference position of the airborne vehicle from a navigation instrument and a position of the first SSR, thereby determining a distance between the SSR and the airborne vehicle, and calculating a reference time instant when the reference interrogation signal has been sent by the first SSR;
at consecutive time instants, receiving consecutive interrogation signals from the first SSR at the airborne vehicle when the airborne vehicle is within a main lobe of the first SSR;
determining an integer number of first PRF patterns fitting into respective time intervals between said consecutive time instants and the reference time instant, comprising determining respective remaining time intervals which are shorter than the duration of the first PRF, the remaining respective time intervals defining a fractional section of the first PRF pattern, including matching the fractional section of the first PRF pattern with the first PRF pattern;
determining the first transmit time instant based on the integer number of first PRF patterns and said matched fractional section of the first PRF pattern; and
determining said first distance, at said current position, from said first SSR according to said first transmit time, said first reception time, and the speed of light;
(c) determining other measurements required for determining the current position; and
(d) determining the current position using said first distance and said other measurements.
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