[time-nuts] GPS message jitter (preliminary results)
scott.j.stobbe at gmail.com
Thu Jul 21 11:14:04 EDT 2016
If your not bean counting for a commercial product you can take a look at
particle filtering, its like a real-time monte-carlo simulation.
The location estimate in NEMA is also heavily filtered and referenced to
its utc time estimate (PPS). Conceivably could be filtered to a bandwidth
of Fs/2, 500 milliHz.
On Thu, Jul 21, 2016 at 12:00 AM, Chris Albertson <albertson.chris at gmail.com
> I might have been the one who brought up the problem of the NMEA
> message not being right on the UTC send tick. But now I'm thinking
> of another problem this might create. It is slightly off topic
> because it has to do with location.
> Let's say I have a mobile robot (or a self driving car) that wants to
> know where it is. I have a three axis gyro, three axis accelerometer
> and a three axis magnetometer (acting as a magnetic compass. I also
> have rotational encoders on the wheels to count millimeters of travel.
> In theory I can dead recon from any known point but all of those
> sensors drift or are otherwise imperfect. So I add GPS. GPS's
> problem is it's on order 10 meter level accuracy and I need
> centimeters or better. The combination works. Each sensor makes up
> for the faults in the others.
> But up until now I have forgotten that the NMEA location data is
> likely some tens or hundreds of milliseconds old before it is output
> on the wire. This "noise" was hidden in the large location
> uncertainty inherent in GPS. Accounting for this should make my GPS
> more accurate.
> Now an off topic question that I bet many in this group can answer.
> I'm a total novice when it comes to Kalman Filters. I need to
> expertly combine all this data using one. Anyone got a good
> (hopefully on-line) tutorial or a cheap book on Amazon. (yes I used
> Google, got 1,000+ hits) I'm trying to do this myself and have found
> I forgot most of my Linear Algebra (it's been 30 years) and I doubt I
> ever really understood Kalman Filters completely
> On Tue, Jul 19, 2016 at 10:20 AM, Mark Sims <holrum at hotmail.com> wrote:
> > I ran some tests on the message timing of some V.KEL gps receivers in
> both NMEA and binary mode. These receivers are the cheapest ones I have (3
> for $15 - $20, shipped). They use a SIRF III chip and have an on-board
> ceramic patch antenna. They performed amazingly well. No problems
> tracking sats indoors in a very poor location. Message jitter was less
> than 20 msecs peak-peak, standard deviation less than 2 milliseconds.
> ADEVs at tau 10000 after an overnight run in NMEA mode (hardware serial
> port) were in the 1E-7 range! They also have a 1PPS signal on the
> connector so no need to go digging for a place to bodge on a wire like with
> some of the other cheap GPS modules out there.
> > V.KEL also makes a Ublox based version of the module (around $22 for
> one)... mine reports that it has a Ublox 7 chip, though V.KEL's part number
> implies that it a Ublox 6.
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> Chris Albertson
> Redondo Beach, California
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