Hutta, to sum up for you: as you have read, position hold doesn't mean precise PPS, only the ability to determine the PPS (the time) with less than 4 satellites, downto only one. Timing grade receivers have better internal oscillators and give better PPS always, either in position hold or not. Best to use a timing grade receiver but to move the first step towards the OCXO disciplining, a navigation receiver will do. Then you will start to appreciate things that will lead you to a timing receiver with the sawtooth correction. I say this because I have travelled this way over the years.
On Wed, Mar 13, 2013 at 3:30 PM, Jim Lux <jim...@earthlink.net> wrote: > On 3/13/13 7:06 AM, Bob Camp wrote: > >> Hi >> >> A GPS that uses position hold gets it's coordinates from one of two >> possible >> sources: >> >> 1) You measure the actual antenna location with a precision survey grade >> GPS >> and enter them. >> >> --or-- >> >> 2) The GPS does a survey for some amount of time. It averages it's own >> "reasonable" location estimates over this time period. With 48 hour >> averaging and a good sky view the location estimate can be pretty good. >> >> The position hold function allows the GPS to come up with a time estimate >> from a small number of satellites. This is useful when the sky view is not >> very good. >> >> A position error of one meter can translate into a time error of about 3 >> ns. >> Most GPS engines are rated for a 3 meter error, so that would be roughly 9 >> or 10 ns. Since the exact error depends on the stat's location relative to >> the error vector, the actual error will vary a bit (= it looks like >> noise). >> >> > I was just reminded of an interesting observation.. > > The satellites are all moving, so whether your receiver is moving or not > doesn't really change the inherent time accuracy possible, as long as you > can accurately (!) estimate your position. Otherwise, the time uncertainty > is some combination of the position uncertainty of the satellites and your > own position uncertainty. i.e. there's no reason why you can't determine > the position of a LEO satellite to centimeters, even though it's zipping > along at 7km/sec. In fact it's potentially easier than on the earth's > surface: less ionosphere, less multipath, less high frequency variation in > position and velocity vectors. > > What position hold really buys you is a reduction in "own position > uncertainty" and the ability to use fewer satellites to get a "time fix". > > Think of it as solving for 4 unknowns (x,y,z,t) (i.e. your position and > time). And, as a practical matter, you need to solve for their derivatives > as well. Using inputs that are the (multiple) satellites' (x,y,z,t and > derivatives). Position hold essentially says xdot,ydot,zdot =0, so you > have fewer things to solve for (t and tdot). Fewer things to solve for with > the same number of observables means, hopefully, smaller uncertainty on the > resulting solution. > > > There's also a basic issue with some receivers... if they were intended > for an application that didn't need precise timing (e.g. they just time > stamp things to the nearest millisecond or something), then the internal > receiver architecture and software may not bother to actually try to solve > for time to a higher level of precision. Maybe 1 microsecond is "good > enough" to produce position and time outputs with the required accuracy. I > recall seeing a patent (or maybe a paper) for a low precision attitude > determination system (1 or 0.1 degree, as I recall) and it didn't need very > good time or position accuracy at all.. what it needed to know was the > "direction of arrival" of the GPS signal, so they could compare carrier > phase between two antennas. And they didn't need precise measurement of > carrier frequency either. To a first order, to get 1 degree knowledge, you > need to know your position to within 1/57th of the distance to the > satellite, or some hundreds of km. That's pretty crummy in GPS terms, but > it works. I don't recall if that system even solved for own position, or > if it used an estimate from somwhere else, or whether it just acquired and > tracked the carrier and PN code, without doing a nav solution. > > > ______________________________**_________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/** > mailman/listinfo/time-nuts<https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts> > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.