RESOLVED: Heading jitter

Forum for questions and problems regarding PathAway GPS 4 SE for Palm OS

Moderator: Scott

Joe42
Posts: 4
Joined: Tue Aug 21, 2007 10:35 pm
Location: Toronto, Canada

RESOLVED: Heading jitter

Postby Joe42 » Sun Jul 06, 2008 1:30 am

When stationary my GPS receiver has a small but constant jitter, sort of a "Brownian motion" within a region of a meter or so across. This makes the direction arrow on the PathAway screen flap around like a flag in a hurricane. The relative bearing arrows on the point list keep changing and the "Track up" mode is pretty much useless because PA keeps rotating and re-rotating the map. While moving there's no problem, but as soon as I stop (say to look at the map) the jitter sets in.

Is there a way to suppress updating the indicated heading, and the map rotation and nearby-point list, until some minimum distance or time has elapsed in the new direction? Under Preferences > Tracks there's a "Min. Movement" setting that prevents recording extra track points while jittering, but that doesn't affect the heading update. Is there some other setting that might help?

My GPSr is an IOgear Bluetooth mouse model GBGPS201 and I'm using PA 4.00.80 on a Palm T|X.

Thanx /Joe

Paulo
Posts: 19
Joined: Wed Jun 18, 2008 3:55 am
Location: Porto Alegre, RS, Brazil

jitter

Postby Paulo » Thu Jul 10, 2008 8:19 am

I think this behaviour is due to what is called "Selective Availability". As this technology was originally developed by the military, they were concerned about its use by the enemy, so they include this jitter, as you named it, as a safeguard against unintended use.

Today you find it to be about a meter or so, but I can remember when it was some 20, even 50 meters wide! It used to be switched on and off at the military will, but it seems to be rather small most of the time today.

If you wanto to get an exact fix the best way is to store a lot of points and average the coordinates.

Joe42
Posts: 4
Joined: Tue Aug 21, 2007 10:35 pm
Location: Toronto, Canada

Jitter: SA?

Postby Joe42 » Fri Jul 11, 2008 2:26 am

Hi Paulo,
I assumed it was air turbulence affecting signal propagation or something like that. Isn't Selective Availability supposed to be permanently turned off now?

Well, whatever causes the jitter, it's annoying. I also assume that, since I can't find any mention of the effect in the forums, either most receivers filter it out of the coordinates before they get to PathAway, or there's a way to enable such a filter in PathAway that I'm just not seeing.

It's not so much an exact fix that I want--a fix within a meter is pretty good. Besides, Pathaway's PinPoint feature can average samples automatically to fix a point accurately. It's the constantly spinning heading indication as the map point moves a tad north, then a skitch west, then a smidgin south, and so on that I'd like to get rid of.

/Joe

Paulo
Posts: 19
Joined: Wed Jun 18, 2008 3:55 am
Location: Porto Alegre, RS, Brazil

Postby Paulo » Sun Jul 13, 2008 8:40 am

Hi Joe42

You are right, SA is not the culprit anymore, it is indeed atmospheric influence the responsible for the majority of the observed errors today. I found it at Wikipedia's article on GPS errors:

http://en.wikipedia.org/wiki/Global_Pos ... or_sources

Unfortunately, unless you have a P(Y) enabled pro unit and corresponding daily decoding passkey, you'll not be able to get your fix within 1 meter as I thought. Combining all those errors together results in roughly 15 times more error than that. I left my receiver standing still, recording, and found more than 5 meter longitude error and almost 20 meter latitude error during a 10 minutes period.

There is a filter within the software to deal with this error, but it affects Track recording only: you can state how much should be minimum displacement to account for a real move to be recorded. It is technichally possible to add an option to freeze last heading before the stop (within minimum movement threshold) and releasing it again when movement resumes. Who knows in a future revision it could be done?


Return to “PathAway GPS 4 SE for Palm OS - Support”

Who is online

Users browsing this forum: No registered users and 1 guest