Providing time sychronization to a GPS locator
A method and system for knowing positioning system standard time at a mobile unit with respect to a system, such as the Global Positioning System, when normal, direct measurement may be impracticable owing to low signal-to noise ratio, by calibrating the timing signal of an available communication network, such as a cellular telephone transmission network ( 610, 615 ). A time reference is set for the communication network with respect to the positioning system at a time when an adequate signal-to-noise ratio prevails and the offset of a timing event in the communication network control signal measured by means of the mobile unit's internal clock may be determined ( 620, 625 ). Subsequent times are measured with respect to this time reference by using the internal clock to measure time intervals therefrom.
1. A method for calibrating a network time of a communications network that has at least one cell and that transmits a control channel signal including a plurality of known bit sequences according to the network time from at least one base transceiver station into the at least one cell of the communications network, to provide an absolute time reference for a local clock in a mobile unit located in one of the at least one cell, comprising the steps of:
a) providing:
i) a source, of a standard time, that transmits at least one synchronization signal; and
ii) a transceiver operative to communicate with the communications network and with the mobile unit;
b) synchronizing the local clock with said standard time;
c) synchronizing the network time of the communications network with the local clock;
d) calculating a time offset, according to the local clock, of the network time of the communications network from said standard time; and
e) calculating an absolute time reference as a sum of said time offset and said standard time.
2. The method of claim 1 , further comprising the step of:
f) providing at least one location server communicating with at least one said base transceiver station for storing said absolute time reference.
3. The method of claim 2 , further comprising the step of:
g) providing the absolute time reference to said location server.
4. The method of claim 3 wherein providing the absolute time reference to said location server is effected by steps including:
i) transmitting the absolute time reference by said associated transceiver to said base transceiver station; and
ii) communicating said absolute time reference by said base transceiver station to said location server.
5. The method of claim 1 , wherein said source of the standard time is an Earth satellite navigation system.
6. The method of claim 1 , wherein said synchronizing of the local clock with said standard time is effected by steps including:
i) receiving said at least one said synchronization signal by the mobile unit; and
ii) noting a time according to said local clock when a known bit sequence of said at least one synchronization signal is received.
7. The method of claim 1 , wherein said synchronizing of the network time of the communications network is effected by steps including:
i) receiving the control channel signal of the communications network from the at least one base transceiver station by said transceiver; and
ii) noting a time according to the local clock when one of the known bit sequences is received.
8. The method of claim 7 , wherein said synchronizing of the network time of the communications network further includes the step of:
iii) updating said absolute time reference to provide an updated time reference to the mobile unit.
9. The method of claim 8 , wherein said updating of said absolute time reference is effected by steps including:
A) receiving the control channel signal of the communications network from the at least one base transceiver station by said transceiver;
B) demodulating a frame number of a subsequent known bit sequence in the control channel signal;
C) noting a time according to the local clock when said subsequent known bit sequence is received;
D) calculating from known network signal characteristics a time difference, according to the communications network, between said known bit sequence of claim 7 and said subsequent known bit sequence; and
E) correcting the local clock time according to said time difference.
10. The method of claim 9 , wherein said updating is performed at least twice per day.
11. The method of claim 1 , wherein said time offset is a time period, according to the local clock, between said reception of a known bit sequence within said at least one synchronization signal and reception of a known bit sequence within the control channel signal.
12. The method of claim 1 , further comprising the steps of
f) determining, according to the local clock, a time offset of a subsequent event from a most recent time reference; and
g) adding said time offset to said most recent time reference.
13. The method of claim 12 , wherein said most recent time reference is received from said at least one location server via the at least one base transceiver station serving the at least one cell wherein the mobile unit is located.
14. The method of claim 1 , wherein said synchronizing is effected independently of the communications network.