GPS location "wanders" while car is stationary.

**sjmanikt** · 03-10-2008, 04:57 PM

Hi all,

I'm running SD and have a USB GPS mouse (BU 353). I've had this thing working for quite a while, but there's not a lot to configure.

When my car is stopped, frequently the reported position will hop or wander around. Sometimes it's pretty minor, and sometimes it's kind of ridiculous (like 3 blocks off!), which throws the auto re-routing into a tizzy from time to time.

I'm assuming this is caused by small timing discrepancies in the timing of satellite signals, but then again, I really have no idea. Is there a way to fix this?

**brodgar** · 03-10-2008, 05:55 PM

I don't have the answer but I also have the same problem. I'm using the same GPS as you but different maps (PC Navigator 7), so it must be to do with the gps

**sjmanikt** · 03-10-2008, 06:09 PM

After doing some research, it seems like this is caused by cumulative signal position error. When the device is stationary, the error in position tends to "accumulate," so the position reported tends to jump around more than if the device is in motion.

When the device is in motion, the errors are only along a "track" of positional points, so the software is better about filtering them out / the errors are less "visible."

Still, you'd think that it'd be relatively easy to filter this kind of info out at the NMEA sentence level somehow?

**KryptonianJorel** · 03-10-2008, 11:06 PM

Are you sure this has nothing to do with the receiver being a serial device? A serial mouse or what have you may be interferring

**Woofnstuff** · 03-11-2008, 12:33 AM

its a common issue with gps.
i have a usb device and it does it to a small degree too.

the sats are pretty far away dont forgert

**skuller** · 03-11-2008, 01:45 AM

is there any tall building on the area?
gps are not very accurate when there are tall building, the signal bounces off like crazy.

**Marvin Hlavac** · 03-11-2008, 07:17 PM

Test it while the GPS receiver is outside the car. Place your BU-353 on the roof of the car (for testing purposes). Make sure you are in an open area, not obstructed by tall buildings. If you experience GPS location jumping large distance (hundreds of feet), then the GPS receiver may be faulty. It could also be affected by interference from your car/computer/something. If you experience the same problems during this test, you could still test it on another computer at home or at work. If you see the same symptoms, my guess would be your GPS unit is faulty.

**Nerd2** · 03-11-2008, 07:28 PM

I have this a bit too.

I'm sure SirfStar software should be able to fix this, by reducing the resolution to get rid of the cumulative errors. Havent got time to mess with it now.

**BrianC72bgt** · 03-12-2008, 12:40 PM

I suspect this is due to a lack of vehicle speed and direction info from the car. What I mean is, in a factory installed GPS system, the unit also gets vehicle speed and direction (Reverse or not) info from the electronic speedo and gear selector. This helps when the signal is spotty.

Not to thread hijsack, but does anyone know of an aftermarket GPS card/device, what have you, that will accept this information? I'm guessing no because different auto mfs use different methods and signals.

**Laidback** · 03-12-2008, 06:10 PM

Originally Posted by BrianC72bgt

Not to thread hijsack, but does anyone know of an aftermarket GPS card/device, what have you, that will accept this information? I'm guessing no because different auto mfs use different methods and signals.

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