Stereo Install Questions

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Not sure how a speed sensor could increase accuracy of the GPS system. Especially since most people with calilbratable speedometers will calibrate from a GPS unit.

As to why it takes a while to obtain an accurate fix....

When you first turn on a GPS, the unit must download an almanac from the satellites. The almanac is transmitted every 12 minutes and tells the GPS unit where it should expect to see the GPS satellites. The satellites also transmit what is called ephemeris data which is sent in blocks, each about six second in length, starting every tenth second. Ephemeris data is precise orbit information of the satellites.
If you are traveling cross country with the GPS on, the almanac and ephemeris data is kept up to date. However, if you have the GPS off on a cross-country trip, and you turn it on 3000 miles from where it was last turned on, it is a bit confused, since it retained the last alamanc informaton, so until it downloads the almanac and the ephemeris data, accuracy will be affected.
Almanac data is sent by all satellites and includes information about all the satellites. Ephemeris data in regard to a satellite is only sent by that particular satellite.
If the GPS system is WAAS capable and WAAS is enabled, the GPS needs to download the WAAS almanacs from all Satellites. The WAAS almanacs are transmitted every five minutes. This almanac information is used by the GPS to correct the information received from the other satellites.
Right now, on some GPS units, depending on your location in the USA, having the WAAS disabled may give you better accuracy. The US government last year launched a few more WAAS satellites and they are currently moving the old ones and new ones around to new locations, and testing them.
On Units with the SiRF III chip, the chip will not recognize a test signal from the WAAS satellite. Older non-SiRF III chip models will recognize, but the signal may not be correct.
WAAS satellite 33 is over the Atlantic somewhere near Europe. Number 35 is being moved to the west, and I believe 47 is over the Pacific. Right now, the best areas for reception are the southwest, or so I have been told.