XT acting up near large buildings

[Yuthe]

Morning all
Had a bit of a weird one with my Zumo XT yesterday. Pulled up for a coffee near one of those big gaming complexes and the unit just went mental. The bloody thing was showing I was doing 70mph while I was sat stone cold sober in the car park. It just froze up and wouldn't recalculate the route at all. As soon as I got a mile or two down the road, it sorted itself out and worked fine. Don't think it's a dodgy satellite lock because I had clear sky. Seems like all the WiFi or interference from those big buildings just mucks up the chipset or something. Or maybe the POI database is struggling to keep up when there’s too much going on in one spot—like when sites with lots of dynamic content, say coolzino, are nearby. Bit annoying for a kit that costs this much. Anyone else had theirs play up like this near those big sites?

[lkraus]

I would not blame the XT. GPS signals can encounter interference, spoofing, and jamming anywhere. Tall buildings not only block signals but they can cause random reflections due to metallic skins or even metallic UV-blocking window coatings. The reflections alter the satellite-to-receiver timing and change the position calculations. Even mounting a GPS near a TFT instrument panel emitting stray radio frequencies can cause unreliable reception.

[jfheath]

That occasionally happens when surrounded by large buildings. Also hills, trees, motorway traffic etc.

It is worth bearing in mind if you use the satnav to show your actual speed. It is only accurate if you have been travelling at a steady speed and it is showing the same reading for 5 seconds or more, and you are not surrounded by any of the above.

The city issue can be a problem if you are just setting off - before it has found a good collection of satellites. The later models - the XTs seem to be much better at getting an accurate fix - but at times they ‘assume’ - notice what they do if you go into a tunnel and change your speed !

[jfheath]

The bloody thing was showing I was doing 70mph while I was sat stone cold sober in the car park.

I should have added that the satnavs do not show your actual speed - ever. They show the speed that you have been traveling in the last 5 seconds or so - and that is calculated based on the distance between two ‘fixes’ as you move. It calculates the distance between those points and divides by the time to get the speed.

It gets a ‘fix’ on your location by calculating your distance from a number of different satellites - using the time that it takes for the Zumo to receives a signal from each satellite. From those timings it can work out distance from each satellite and hence your position (and surprisingly, elevation).

The margin for error is very tiny. It is using ‘trigonometry’ to calculate a small change in distance using a time signal from a satellite 12,000 miles away to work out a change in position of a few metres. So if the signal has been bounced off a building on its way to your GPS, that is a significant detour - which places you further way from the fix it uses from 5 seconds ago. So it calculates your speed based on that, which is wrong.

You can observe a similar effect on the displayed speed when negotiating tight hairpins on mountain passes, your displayed speed is much less than it should be, because after each bend the road you have just ridden is more or less below you. That’s if the satnav can see enough satellites.

[danham]

Two possibilities in addition to the good info so far...

1) you could have a corrupt almanac. This is data stored in your XT which tells it where to look for the sats based on their last location. This used to happen on my old 660. At the moment I cant recall how to force it to acquire a new almanac, but if I find the answer I'll edit this post.

2) when you started your ride, did you wait for the XT to acquire sats before putting your bike in motion? Sometimes if you drive off prior to sat lock, especially in an area with obstacles (buildings, hills), it never locks properly.

-dan

[proofresistant]

Users on other forums have reported that their vehicle's dashboards can also have a negative impact on the GPS receivers in their navigation systems.
(In this case, the navigation device was mounted directly above the motorcycle's dashboard).
Edit: If GPS signals are now disrupted by interference from the surrounding environment and additionally by the motorcycle's dashboard, it is entirely possible that the experiences described above may occur with the GPS device.

However, I have not observed this myself.
You can try it out for yourself by checking the signal strength in the satellite info menu with and without the ignition (power on) of your own bike.

If you encounter problems, shielding the dashboard should help.

PS
If you're interested, I can search for the discussion again and share the source link here.

[jfheath]

I read that somewhere as well. If I remember correctly (which is a pretty tall order these days), people were blaming the TFT units for the interference. Whether this as the actual TFT screen (Thin Film Transistor technology used for displaying brighter, sharper, faster refreshing pixels - according to Google AI), or the electronics interference from whatever is powering the display, I don't know.

I suspect the latter. When I first got my XT2, I had a wired headset, so I bought a BT device that would plug into the 4 pole 3.5 mm socket on my autocom device to transmit stereos sound to my headset and mic output to my phone.

That didn't work out very well - the XT2 kept dropping the connection because I put the BT device in the rear cowl of my bike, and that is where the ECM - The engine management system is located. I moved it under the dash and all was well again.

But I'm glad you brought that up again, because I had forgotten, and I am about to mount my XT2 on me new-to-me bike and that has all sorts of stuff going on behind the multi-coloured screen. Maybe I'll check it out before I drill holes !