GPS Certainly Not a Perfect Science

wa8yxm wrote:
Actually you are not describing a failure of your GPS. it was working perfectly.
You are describing a failure of the Cartographers who wrote the map for the GPS.

The gps has basically one duty. To properly report your geographical location in 4 dimensions (Altitude, Latitude. Longitude and Time) It may also carry "optional information" to your receiver (Weather. Alerts, Traffic) but that is just "Radio" not GPS.

Translating 2 (or even all 3) of the GEO varibles (Lat, Long, Alt) to a spot on a map or represention of a building.. Well that is the job of the map maker. and the map software.. Locating a nearby "Attraction" (Target destination) is yet other optional software... And part of the map package. NOT THE GPS. it just runs the map program.

Your smart phone has another advantage. IF a new store opens. it will knwo about it almost before the ribbon is cut.

Actually it isn't all the cartographers fault. The GPS solution can be very precise but isn't always because of the implementation. The US array adds fuzz to the signal that makes getting a precise solution impossible unless the receiver has the ability to remove the fuzz. That ability is contained in a classified piece of HW not available to the public. Some of this can be circumvented to an extent if the WAAS option is enabled in the receiver and it detects WAAS signals. I suspect the Russian GPS uses a similar method (without WAAS) in their system, but not sure about the EU or the Japanese systems.

It generally takes a solid lock on four signals or satellites to obtain a solution. But satellite signals get masked by terrain, buildings and such. Also the GPS software will mask any satellite signal coming from a bird that is too close to the horizon, or in some cases it it's more or less in-line with the sun's position. And for the US array one needs to remember that it was designed for the military and the birds are positioned to give better signals over Europe, not the US. I suspect the Russian array is designed to offer better solutions over the US so nowdays when I'm looking for a new GPS I look for one that can handle both US and Russian arrays.

Another issue with GPS systems is they are designed to continue to offer solutions even when they haven't got a solid lock on enough signals. This is called "coast mode" and it projects your position based on your last known good position, your direction of travel and your speed. The amount of time the GPS remains in "coast" telling you where you are (even when it has no good idea) is somewhat indeterminate and depends on the GPS unit. I have some units that tell me pretty quickly, others that might tell me when I'm in the next State.