[time-nuts] GPS message jitter (preliminary results)

Chris Albertson albertson.chris at gmail.com
Thu Jul 21 00:00:35 EDT 2016

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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