Locator system and related localization method and service with innovative time and frequency sinchronization of localizator transponders

1. Localization method for locating a target that is coupled with a locator transponder ( 24 ) associated with a permanent identification code permanently assigned to said locator transponder ( 24 ); the localization method comprising:

a) upon reception of a user request for locating the target, transmitting, by a radio communications system ( 22 ), a spread spectrum paging signal carrying the permanent identification code and a temporary identification code temporarily assigned to the locator transponder ( 24 ), wherein said temporary identification code is shorter than said permanent identification code;

b) receiving, by the locator transponder ( 24 ), the spread spectrum paging signal and extracting, by said locator transponder ( 24 ), the temporary identification code carried by the received spread spectrum paging signal;

c) transmitting, by a radar-based system ( 23 ), radar signals towards one or more areas of earth's surface or sky and receiving, by said radar-based system ( 23 ), echo signals from said one or more areas of the earth's surface or sky;

d) upon reception by the locator transponder ( 24 ) of one or more radar signals transmitted by the radar-based system ( 23 ), generating and transmitting, by said locator transponder ( 24 ), a sequence of watermarked radar echo signals in which a spread spectrum watermarking signal is embedded, wherein said spread spectrum watermarking signal carries the temporary identification code extracted; and

e) carrying out, by the radar-based system ( 23 ), localization operations that include:

detecting, in the received echo signals, the sequence of watermarked radar echo signals transmitted by the locator transponder ( 24 ),

extracting the temporary identification code carried by the spread spectrum watermarking signal embedded in the detected sequence of watermarked radar echo signals, and

determining a location of the locator transponder on the basis of the detected sequence of watermarked radar echo signals;

characterized by further comprising:

f) transmitting, by the radio communications system ( 22 ), one or more frequency-synchronization-aid signals;

g) receiving, by the locator transponder ( 24 ), the frequency-synchronization-aid signal(s) and estimating, by said locator transponder ( 24 ), based on the received frequency-synchronization-aid signal(s), a frequency drift affecting a reference frequency provided by a local oscillator ( 86 ) of said locator transponder ( 24 );

wherein the locator transponder ( 24 ) transmits the sequence of watermarked radar echo signals by using a transmission carrier frequency obtained based on the reference frequency provided by the local oscillator ( 86 ) and on the estimated frequency drift.

2. The localization method of claim 1 , further comprising:

h) transmitting, by the radio communications system ( 22 ), one or more signaling-related signals carrying signaling data indicating one or more operating parameters of the radar-based system ( 23 );

i) receiving, by the locator transponder ( 24 ), the signaling-related signal(s) and extracting, by said locator transponder ( 24 ), the signaling data carried by the received signaling-related signal(s);

j) estimating, by the locator transponder ( 24 ), based on the radar signals received from the radar-based system ( 23 ) and on the extracted signaling data, timing parameters of the received radar signals;

wherein the locator transponder ( 24 ) transmits the sequence of watermarked radar echo signals by using the estimated timing parameters.

3. The localization method according to claim 1 , wherein estimating the frequency drift includes carrying out:

a coarse frequency drift estimation based on the received spread spectrum paging signal; and

a fine frequency drift estimation based on the coarse frequency drift estimation and on the received frequency-synchronization-aid signal(s).

4. The localization method according to claim 1 , wherein the radio communications system ( 22 ) is a satellite radio communications system ( 221 ); the localization method further comprising:

transmitting, from a ground station ( 25 ) to the satellite radio communications system ( 221 ), the frequency-synchronization-aid signal(s) to be transmitted by said satellite radio communications system ( 221 );

receiving, by the ground station ( 25 ), the frequency-synchronization-aid signal(s) transmitted by the satellite radio communications system ( 221 );

detecting, by the ground station ( 25 ), errors and/or shifts affecting the received frequency-synchronization-aid signal(s);

correcting, by the ground station ( 25 ), the received frequency-synchronization-aid signal(s) to compensate for the errors and/or shifts detected; and

retransmitting, by the ground station ( 25 ), the corrected frequency-synchronization-aid signal(s) to the satellite radio communications system ( 221 ) that retransmits said corrected frequency-synchronization-aid signal(s).

5. The localization method according to claim 1 , wherein the radar-based system ( 23 ) is a synthetic aperture radar system or an inverse synthetic aperture radar system or a radar system.

6. The localization method according to claim 1 , wherein the radio communications system ( 22 ) and the radar-based system ( 23 ) are installed both on board one or more same platforms.

7. Locator system ( 20 ) configured to carry out the localization method as claimed in claim 1 , wherein said locator system ( 20 ) includes:

a radio communications system ( 22 ) configured to carry out the steps a) and f) of said localization method;

a radar-based system ( 23 ) configured to carry out the steps c) and e) of said localization method; and

a locator transponder ( 24 ) configured to carry out the steps b), d) and g) of said localization method.

8. The locator transponder ( 24 ) of claim 1 configured to carry out the steps b), d) and g) of the localization method as claimed in claim 1 .