Satnav goes off piste

[la14oldie]

Hi everyone!

The Garmin satnav in my SE Navi generally works well but from new it has been prone to "getting lost" on start up. About once a week it will show the general area where I am but doesn't follow the roads. There can be up to about 5 minutes when I appear to be driving through fields, crossing rivers etc until eventually it sorts itself out and finds the correct road. During this period, the orientation of the image is randomly changing. Once running properly it's ok - the problem only arises sometimes on startup.

I recently updated to 2017 maps from the website and the same thing still happens.

Has anyone had a similar experience?

[JohnAlways]

I'm used to a Tom Tom (Nuvi with the lifetime maps is the way I am going) but often the Tom Tom will turn me to turn left (or right) coming out of a hotel car park and then change its mind and tell me to do a U turn. I wonder if both need to traverse a little bit of distance to orientate themselves. As Tom Tom says I can no longer update my maps the Nuvi is becoming more used but I switch it on first thing to find the satellites while I'm loading the car. I suppose it's like booting a computer but 5 minutes does seem a long time. I also use a 16Gb micro SD in the slot (a fast camera version one) so that possibly has some bearing.

[John Ratsey]

Any slowness in getting an accurate position (as opposed to slow starting of the satnav itself) is because satnavs need to find the satellites to be able to work out the position. Finding them is easier if it knows which satellites to look for (and it needs a reasonable view of the sky). To assist in the process, the most recent satellite orbit data is stored in memory and, once the GPS is locked onto a satellite, it receives updated orbit data. Some smartphones speed up this process by using A-GPS https://en.wikipedia.org/wiki/Assisted_GPS.

However, the doesn't explain the problem of showing the wrong position for the first few minutes unless it's caused by the calculation of an extremely inaccurate position from the minimum number of satellites (3 are necessary to get any position). Does the satnav have an info screeen showing the number of satellites received? If the signals are weak then the GPS takes longer to lock onto the satellites so I wonder if the GPS antenna connection is OK. If the calculated positions are inaccurate then so is the direction of travel between each fix, which would explain the changes in orientation of the map. Is the position error in terms of tens of metres (less than 100 yards in old money), or more?

[Skyrider]

My Tomtom uses the QuickGPSfix system to find its position, this data is updated whenever you log your device onto the TT website.The sat nav finds its position much faster with up to date info on where the satellites are. A Google of Quickgpsfix will reveal all.

[ColinS]

It sounds to me that it is not picking up the satellite positions (it needs 3 to get any kind of position but will be inaccurate just like you are experiencing).  In the early days I had to stop the car and wave the unit out the sunroof to get it to work and 5 minutes was not uncommon.  Modern units store the constellation so will know where they are based on time of day and will startup almost immediately and finely adjust as it picks up the true position (which you won't even notice).

In conclusion, I think your unit is faulty.  Take it back to the dealer.

[culzean]

My Nuvi seems to have trouble picking up satellites quickly after the last couple of updates, although I have lifetime UK and Europe maps available I usually only choose UK+Rep of Ireland and these fit onto the internal storage.  Notices it wanders around a bit just like yous initially,  I don't think your unit is faulty - just the last updates have been a bit flaky,  seems to have lost some features too, when you used to press the 'satellite bars' (top LH corner of screen) it used to bring up another screen that showed which satellites were available and which you were receiving,  does not seem to do that any more.

It just shows that updates can be a mixed blessing and can cause problems.

What satnavs normally do is remember the last position when they are turned off and which satellites it was using,  if you turn it off and move positions it takes longer to find its position when you turn it back on,  don't know why because I expect that anywhere in the UK it would be using same geostationary sputniks.

[Graham Dicker]

The GPS satellites are not geo-stationary

[ColinB]

... I expect that anywhere in the UK it would be using same geostationary sputniks.

At the risk of being exposed as a nerd, a minor correction: the US GPS constellation is not in geostationary orbit, the orbits are arranged at approximately 55 degrees inclination (ie angle to the equator) with an altitude such that the orbital period is half a sidereal day (ie roughly 12600 miles altitude). Therefore the same satellites are not always in view, and any GPS receiver when booted from cold will take some time to acquire a usable set of signals (it needs four to get precise position, including height) and sort out where it is. That period can be shortened or even imperceptible if the receiver has an initial rough fix, eg if it hasn't moved greatly from it's last known position or if it uses some other technology (eg A-GPS) to get an approximate fix first. In short: an initial variability in position is not a fault and is characteristic of the system.

[culzean]

Don't know why I didn't remember that (oldtimers disease), the majority of satellites are orbiting but 3 special ones which control WAAS and EGNOS are geostationary (these broadcast to ground stations and the ground stations send out signal to correct the differences in travel time and distortion cause by atmospheric conditions,  these were installed so that aircraft could use the much better and more reliable accuracy to land and take off,   modern satnavs (including the Nuvi) use these correction signals to get better accuracy - even if they do show you in the middle of a field sometimes).

[Skyrider]

My Tomtom has a GPS data display of how many satellites are being received and how many of them are being used. It has a ten channel receiver and can use all of them if it can see ten satellites. It needs a minimum of four satellites for a fix.

[culzean]

My Nuvi has not lost the screen showing satellites,  you just have to press the satellite bars at top left of main screen for about 5 or 6 seconds. If you press the battery symbol at top RH for 5 or 6 seconds you get masses of diagnostic screens to play with.

[guest1372]

Can be quite noticeable with phones, but as said A-GPS helps by getting up to a months worth of almanac & ephemerides plus the errors and offsets into memory.  If you record the raw fix data at say 10Hz on a serial console and plot the results, then it's quite scattered initially over maybe a 30 - 40m radius, but the centre of the plot is quite obvious.

The magnetometer (compass) is quite important, I've had to hard reset my phone and without calibrating it (rotating in 3 dimensions) the car was displayed sideways on the road with the map repeatedly flipping 180°.  I'm guessing driving in a circle should calibrate an OEM satnav.

Quadcopter flyers using a ground control station might run GPS for 20 minutes before flying for certainty.

--
TG

[la14oldie]

Thanks to everybody who replied to my original post.

I have now found a way of avoiding the "off piste" problem. On setting off from rest, if I wait until all the satellite bars turn green then the problem does not occur. But if I set off with the bars greyed out then the unit seems to have difficulty in discovering its position while the car is in motion.

Don't understand this, but it does work.

Thanks again!

[John Ratsey]

This discussion in the HR-V forum might be relevant since it is basically (exactly?) the same unit in both vehicles.

One bad signal reception problem was fixed by replacing the touchscreen unit http://www.hrvforums.co.uk/viewtopic.php?f=11&t=527&start=20#p3357.

[guest6570]

Thanks to everybody who replied to my original post.

I have now found a way of avoiding the "off piste" problem. On setting off from rest, if I wait until all the satellite bars turn green then the problem does not occur. But if I set off with the bars greyed out then the unit seems to have difficulty in discovering its position while the car is in motion.

Don't understand this, but it does work.

Thanks again!

I have a EX Navi that has exactly the same problem as you describe. It doesn't happen regularly but at random and takes like yours about 3 to 5 minutes to correct itself, even a reboot doesn't speed things up.

I will give your suggestion a try and wait for the greyed out satellite signal bars to go green and see if that helps. (not that I noticed and bars when I was last using the SatNav).

You also mentioned that you updated the maps to the 2017 version. I'm assuming mine has the 2016 version being it was registered in June that year. Now the instructions for updating the maps suggests that you need to allow up to 4 hours of time to do this and keep the engine running throughout the update process. I don't fancy sitting in the car for 4 hours with the engine running, I know it says you can incorporate it into your journey but even so that's a hell of a long process! How long did your updates take if you don't mind me asking?