satellites, downto only one. Timing grade receivers have better internal
oscillators and give better PPS always, either in position hold or not.
OCXO disciplining, a navigation receiver will do. Then you will start to
correction. I say this because I have travelled this way over the years.
Post by Jim LuxPost by Bob CampHi
A GPS that uses position hold gets it's coordinates from one of two possible
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.
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