The full SPS document can be found on the National Executive Committee for Space-Based Positioning, Navigation, and Timing website.The National Executive Committee for Space-Based Positioning, Navigation, and Timing (PNT) has released an updated civil GPS Standard Positioning Service Performance Standard, committing the United States Department of Defense (DoD) to an improved level of GPS accuracy for civilian signals.It is the fourth revision of the standard positioning service (SPS) performance standards document, and the first update since October 2001. In addition to specifying GPS minimum performance commitments, the SPS performance standard serves as a technical document designed to complement the GPS Signal in Space (SIS) Interface Specification.The most significant change in the updated SPS standards is a 33 percent improvement in the minimum level of SIS range accuracy, from 6 meters root mean square (rms) accuracy to 4 meters rms (7.8 meters, 95 percent), according to the document, which is drafted by the DoD and released through the PNT committee.
While the stated dedication to improvement is notable, it has a built-in conservative margin for minimum performance; as the documents authors note in the executive summary: "with current (2007) SIS accuracy, well designed GPS receivers have been achieving horizontal accuracy of 3 meters or better and vertical accuracy of 5 meters or better, 95 percent of the time."
Friday, October 17, 2008
Wednesday, October 08, 2008
Saturday, October 04, 2008
This newest Coolpix camera from Nikon has a built-in GPS and Nelso's blog reviews the GPS functionality. Read the Great article on Nikon's Coolpix 6000 GPS qualities - Nelso Blog. He does a pretty good job but I believe he confuses normal GPS start-up lags as being related to the Nikon implementation when the noted lags are essentially present in all single channel GPS devices... The entry of the $500 CoolPix 6000 makes Nikon the alternate offering to Ricoh's geo-cameras. GO NIKON!
An explaining comment for Nelso's article...
Just a comment on what I believe is your disappointment regarding out-of-the-box initial turn-on to first sat lock in the Nikon CoolPix 6000.
All GPS units require two reference tables to be current and if not they must updated by un-disrupted listening to “a” satellite for a period of time. The larger table, the almanac, requires upwards of 10 minutes of uninterrupted lock on a satellite. The second, the ephermic table, needs a minute or three before precise satellite lock to be achieved. A bit of explanation -
The almanac is essentially the bus schedule that provides the essential “coarse” schedule for all satellites. Generally it must be no older than a week or three and if older it must be updated. Absence or presence of a stale almanac requires the update of this table which requires around 10 minutes of “constant lock to one satellite” to be fully loaded at the bit rates in the carrier code. Break the lock and it must reacquire and initiate refreshing of the almanac all over. So if you store your unit for more than a couple of weeks – expect 10 or more minutes minimally to regain a proper "almanac" table.
The next step to getting the "best" positional solution is the updating of the ephermic table. This is the very fine hour-to-hour resolution on the coarse almanac and accounts for things like tidal effects, day/night, and other fine tunings on the satellites’ paths. This information can be updated several times in an hour. The ephermic table is also the reason why cold starts with current almanacs can require upwards of a minute or three to properly lock and for accuracy to stabilize. This is where the a-GPS advantage can be found in certain geo-smartphones to improve time to navigating lock. These phones will hunt the ephermic table first from the carrier gaining this update via the mobile web feature… also allows the carrier to keep you locked to them as well?
Lastly, accuracy can be further improved by a differential correction that is know as WAAS. Its not available everywhere as it is broadcast from fix-position satellites - Europe and the US are covered. If present and if the GPS chip set can utilize this "very-fine" correction, GPS accuracy can be held to under three meters.. better chips-sets can be within a meter 95 percent of the time. This is really the fine tune and accounts for radio delay due to atmospheric density and other delays that vary minute to minute.
So the Almanac gets you to the bus stop. The ephermic tells you if the bus is going to be a bit early or late and the differential suggests delays due to making change or getting a long line properly to their seats.
Lastly, your point on moving a GPS several hundred miles from its last known position when almanac and ephermic are current can also confuse your unit. This lag is a result of GPS unit believing it is in the same area when it was turned off. It looks into the almanac, its clock, and then hunts for the predicted satellites based on its last known position. If it can not find those most likely satellites it falls into a search and find solution… for at least two or more satellites for a hint; this too delays the locked navigation. Giving the unit a hint can speed this up significantly… geo-smartphones use the local area code for the hint.