Method and apparatus for detecting spread spectrum signals using a signal from a secondary source

DETAILED DESCRIPTION OF THE DRAWINGS The method and apparatus of the present invention enables efficient detection and acquisition of spread spectrum signals, including spread spectrum signals which may be too weak to be detected by prior-art methods.
In particular, the method and apparatus of the present invention enable rapid detection and acquisition of GPS spread spectrum signals for determination of a GPS Unit's location, especially in environments where incoming GPS signals are attenuated.
The method and apparatus of the present invention enhance the detectability and trackability of spread spectrum signals by first obtaining approximate timing and frequency information from a relatively high-power, alternate signal, and then using that approximate information to perform narrow-band detection of the spread spectrum signal.
As used herein, the wireless unit which acquires and tracks the spread spectrum signal is referred to generally as the "Receiver Unit".
For example, the Receiver Unit could be a handset which is adapted to receive GPS signals.
The spread spectrum signal to be detected and acquired is referred to generally as the "Spread Spectrum Signal",and the source of the Spread Spectrum Signal is referred to as the "Primary Source".
For example, the Primary Source could be one or more GPS satellites, and the Spread Spectrum Signal could be the GPS signals emanating from those satellites.
In addition, the alternate signal which is used to determine preliminary timing and frequency information is referred to generally as the "Secondary-Source Signal" or "Secondary Signal", and the source of the Secondary-Source Signal is referred to as the "Secondary Source".
For example, the Secondary Source could be a non-GPS satellite, and the Secondary-Source Signal could be a signal transmitted by that satellite from which a Receiver Unit could obtain timing and frequency information.
The Secondary-Source Signal could be any signal which includes relevant timing and/or frequency information and which has a good probability of reception by the Receiver Unit