[time-nuts] Is possible precise 1pps?
azelio.boriani at screen.it
Wed Mar 13 13:15:33 EDT 2013
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 <jimlux at earthlink.net> wrote:
> On 3/13/13 7:06 AM, Bob Camp wrote:
>> A GPS that uses position hold gets it's coordinates from one of two
>> 1) You measure the actual antenna location with a precision survey grade
>> and enter them.
>> 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
>> 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
> 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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