On Fri, Sep 13, 2013 at 10:13 AM, Darren Addy <[email protected]> wrote:
> Elevation of location. (ASIDE: This seems to be where "all GPS units > exhibit a weakness" but I'm not sure why that is. According to this > PDF, > http://www.nasa.gov/centers/johnson/pdf/584738main_Wings-ch5c-pgs360-369.pdf > the space shuttle created topographical data sets that can give the > elevation for virtually any location. Perhaps it is just not built > into the GPS system yet, or there is no method for the GPS to "look it > up" and enter it into the calculations.) GPS receivers, in general, calculate elevation the same way they calculate position, from the difference in arrival times of the GPS signals. While it is conceivable to use SRTM (the Shuttle data) or another elevation dataset, I would be surprised of the O-GPS1 were doing so. (If you hoist the O-GPS1 up a flagpole or something, you should be able to tell.) Also, SRTM is itself somewhat coarse in terms of the post spacing on the ground (30 meters or worse), so I'm not sure if it would be an improvement or not. Why do GPS receivers do worse with elevation than with lat/lon? The reason is geometric. When measuring position via arrival times, you get the best results from having satellites all around you. If you're measuring your latitude, you'd like a satellite to the north, and another to the south. That ways if you move a meter north, you've decreased the distance to the north satellite by a meter, and increased the distance to the south satellite by a meter, and thus changed the difference between them by 2 meters, which is reflected in your difference of arrival time measurement. With elevation, the stupid earth is annoying in the way, preventing you from seeing a satellite beneath you. They're all above you, and thus somewhat correlated in the up-down direction. When you move up, you move closer to all of them. They're not all directly above you, so there is some variation to measure, but it's a harder measurement. There are various "dilution of precision" metrics that the GPS receiver calculates to estimate these uncertainties based on the geometry of the satellites at any particular time. I don't know that the O-GPS1 makes those visible to the user, though. -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
