[time-nuts] Strange GPS behaviour
saidjack at aol.com
Sun Dec 30 03:20:16 UTC 2012
Our Position-Hold enabled units (Fury, Mini-JLT, various CSAC units, and GPS-405 support accurate position reporting in ECEF, NMEA, and various other standards.
Users can select how many fixes are averaged for the position report.
They can also be instantly switched back and forth between position hold, and mobile mode using simple English SCPI commands. Some of them have built in 3 axis accelerometers that allow instant detection of when the vehicle starts moving so it can be switched into mobile mode at that time.
We recently did a test by putting an antenna in a side window in the office that had maybe 10% view of the sky, then let the unit run in mobile mode while doing the auto survey. After two days, we switched it into position hold mode. Most of the time it sees only one sat direct, all others are only seen through multipath. The results are astonishing, SD went down from about 30ns+ to about 8ns. Spikes to +/-150ns went away completely.
I can post some GPSCon graphs of this significant improvement during the test next week if there is interest.
Sent From iPhone
On Dec 29, 2012, at 18:56, Bob Camp <lists at rtty.us> wrote:
> The interesting thing is that you sometimes can get a position hold receiver to report it's estimated location…. Not so much on current product, but on some of the old stuff.
> On Dec 29, 2012, at 9:51 PM, Jim Lux <jimlux at earthlink.net> wrote:
>> On 12/29/12 6:34 PM, Bob Camp wrote:
>>> The gotcha is that often the "navigation" and "timing" receivers are identical in terms of hardware. There is no upgraded hardware in the timing device.
>>> When you put a receiver into position hold, you are telling it "I don't care about the location solution". It reduces the weight of that part of the filter. Yes, that's only one way to look at it and there are other ways to look at it.
>> or, another conceptual model is: I'm fixed, I know where the transmitting satellite is, and can calculate range and doppler a priori, so anything else must be local clock variation and propagation. And you can average out the propagation among multiple satellites (or use one satellite as your timing reference)..
>> Particularly if you are post processing and have precise ephemeris data for the satellites....
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