Question

Why wireless assisted GPS can avoid cold-start occurred in conventional GPS? (Hint, think about how GPS receiver obtain Almanac and Ephemeris)

Answer 1

Answer:- Wireless assisted GPS can avoid cold start because it is the process of Powering up a unit after it has been turned off for an extended period of time and no longer contains current ephemeris data. In this the receiver has knowledge on last position, time of satellite constellation. The receiver starts to search for signals blindly. This is normal behavior, if no backup battery is connected. Cold Start time is the longest startup time for GPS receivers and can be several minutes. On the other hand assisted GPS is a system that uses a RF(Radio frequency) network to improve the performance of GPS receivers by providing real time information about the satellite constellation directly to the GPS receiver. With assised GPS system, a wireless network sends infomation directly to the GPS receiver, which allow the device to quickly locate the sattelite needed and process the data contained in their signals.

A GPS receiver that has just recently switched on, has no a priori knowledge (of the frequency of the signals or of its location) is considered to perform a cold-start. The receiver will have to search a large range of frequency caused by the Doppler effect of satellite motion and receiver oscillator offset, but also due to a small contribution from receiver velocity. The receiver performing cold-start has also perform a search of all possible code phase bins.

A traditional GPS receiver will have two correlators per channel (with a correlator spacing of half a chip), and will thus be able to search one chip at a time. Therefore, the receiver will have to search a two-dimensional space. Once the correlation peak has been found for each satellite, the search is over, but the receiver cannot compute position until it has decoded the TOW and ephemeris data. The total TTFF is therefore at a minimum, provided that the signal is not blocked and it does not fade during this time, or else data bit errors may occur and the receiver will have to wait another 40 s for the data to be retransmitted.

A GPS receiver that has just been recently switched on has no a priori knowledge (of the frequency of the signals or of its location) is considered to perform a “cold-start”.