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bharbour

Member Since: March 8, 2012

Country: United States

  • Your comment on the number of channels beyond 12 being marketing foolishness is not correct. The number of channels availible is a direct contributer to the time to first fix on a receiver. When a GPS receiver is doing a cold start, it does not know what time it is, where it is, how fast it is moving, what satellites to look for, or how accurate it's reference clock is. What this means is that there is a large number of combinations that must be searched. Even though all GPS sats transmit on the same frequencies, the relative motion of the sat as it comes overhead causes a doppler shift. The result of the doppler shift is that a receiver must search over a wide frequency range. Each channel can only search for one satellite over a narrow frequency range. To cover the whole possible range of frequencies for a single satellite can take up to 8 channels per satelite that are being searched for. More if you want extremely high sensitivity. There are 32 possible satellite ID's to search for. You can take time or channels to do this search.

    Once you have a good fix and know what time it is (within a few microseconds) and where you are, 12 channels is good. I am not sure, but I think that WAAS ties up an extra channel or so, so maybe 14 channels. The receivers I have worked on shut down whole blocks of channels (for lower power consumption) once they had found enough sats or they had searched for everything that they could with no additional sats found. A receiver still needs to search for additional sats once it has a good fix because sats rise and set or get blocked by obstructions all the time.

No public wish lists :(