High precision surveillance system by means of multilateration of secondary surveillance radar (SSR) signals

• US 7,570,194 B2
• Filed: 05/10/2005
• Issued: 08/04/2009
• Est. Priority Date: 05/17/2004
• Status: Active Grant

• Current Assignee: Universita' Degli Studi Di Roma Tor Vergata (Repubblica Italiana)
• Original Assignee: Universita' Degli Studi Di Roma Tor Vergata (Repubblica Italiana)
• Inventors: Galati, Gaspare
• Primary Examiner(s): Sotomayor; John B
• Application Number:
  US11/596,592
• Publication Number:
  US 20080231494A1
• Time in Patent Office: 1,547 Days
• Field of Search: 342 29- 40
• US Class Current: 342/30
• CPC Class Codes: G01S 13/781 Secondary Surveillance Radar [SSR] in general
• CPC Class Codes: G01S 13/878 Combination of several spaced transmitters or receivers of known location for determining the position of a transponder or a reflector G01S13/874 takes precedence
• CPC Class Codes: G01S 7/285 Receivers
• CPC Class Codes: G01S 7/4091 during normal radar operation

Abstract

A system able to locate and identify aircraft and vehicles based on the reception and processing, with novel means and methods, of signals emitted by the transponder of the secondary surveillance radar, shortly SSR. The system has a number of fixed stations distributed in the area of interest, e.g. in the airport area; any signal (the well known SSR reply/squitter) transmitted by the on-board transponder is received by four or more stations and the measurement of three or more differences of times of arrival (TOA) permits the reconstruction of the position of the transponder in spite of the fact that the transmission time is unknown. Suitable algorithms based on optimal estimation enhance both the accuracy of TOA measurements and the accuracy of the reconstructed position. The effects of possible overlapping of signal in time are avoided or mitigated by multiple source separation techniques based on least squares algebraic processing.

Claims

1. A system for locating and identifying cooperating mobiles carrying a transponder of a secondary surveillance radar (SSR) apt to emit an SSR signal, the system comprising:

a plurality of measuring stations;

one or more reference transponders, installed in known positions, for performing time synchronization of the measuring stations with respect to an assigned measuring station used as a temporal reference station, so producing a single temporal reference for the system, on the basis of a difference between a time of arrival of a signal from the transponder to the assigned measurement station and a time of arrival to the station to be synchronized; and

a processing subsystem,

wherein each measuring station comprises a processing section for processing an SSR signal emitted by a mobile, said SSR signal comprising a preamble including a sequence of pulses, so to obtain a measure of the time of arrival referred to a clock of said station, said processing section being configured to;

detect an SSR signal emitted by a mobile by analysing the output of a matched filter to a preamble of said signal;

filter said SSR signal by application of a single-pulse matched filter;

differentiate the output of the single-pulse matched filter;

determine the time of arrival of each pulse by determining the zero-crossing of the differentiated matched filtered signal;

reduce error variance of the time of arrival by computing an arithmetic mean of the values relative to the several pulses that constitute the signal, and

wherein said processing subsystem collects data related to said measured time of arrival and reconstructs position and identity of the mobiles.

2. The system according to claim 1, wherein the processing is implemented through an approximation of the cascade of the matched filter and the differentiator and, in the approximation, the impulse response of approximated filtering has a first group of consecutive coefficients equal to one (+1), a second group of consecutive coefficients equal to zero and finally a third group of consecutive coefficients equal to minus one (−

1).

3. The system according to claim 1, wherein, in order to realize high precision and resolution in the measurement of the time of arrival, an interpolation between two points of the amplitude-time plane is carried out, where the amplitude is the one of the signal after the processing used for the measurement, the two points being related to the two consecutive amplitude samples having opposite signs, evidencing a zero-crossing.

4. The system according to claim 1, wherein alignment of the clock of each measuring station with respect to the one of the reference stations is executed by a filtering and prediction process through a Kalman filter.

5. The system according to claim 1, wherein, in the measuring stations, the in-phase and quadrature components (I &

Q) of the received signals are obtained, thereby making possible super resolution algorithms for the discrimination of signals.

6. The system according to claim 1, wherein the measuring station includes an array antenna and a multichannel receiver, with as many channels as array elements, and the output of the channels is organized as a data matrix that is processed according to least square techniques in order to estimate the mixing matrix for the various channels by exploiting time intervals where only one signal is present.

7. The system according to claim 6, wherein the estimated mixing matrix is used to un-mix, by projections in linear vector space, the superimposed signals.

8. The system according to claim 6, wherein, when a signal is completely superimposed by an interfering one, the interfering signal is separated, by projection in linear vector space, and eliminated in order to exploit the contribution of the former signal to the estimate of the mixing matrix.

9. The system according to claim 1, wherein in the central processing subsystem the precise position of the aircraft or vehicle is calculated by processing the measurements of the times of arrival, with a single temporal reference, through a multilateration processing that includes a first non-recursive part followed by a recursive part.