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Discussion: GPS tracking

in: Orienteering; Gear & Toys

May 27, 2019 3:52 PM # 
I'm having major problems recording accurate GPS tracks, wondering if anyone can offer help or advise? For the last 3 years I was using a Suunto Spartan GPS watch, it usually worked fine. I used the Movescount app to download GPS data, and then uploaded it into QuickRoute to overlay GPS tracks on the O-map. Mostly worked great.

But over the past year I noticed that the GPS data deteriorated in quality, sometimes not recording at all, sometimes missing portions of the course altogether. I became convinced my watch was faulty, so bought a new Suunto Spartan Baro 9, top of the line watch. It seems to perform even worse. For example in the two races this past weekend, in Connecticut and Massachussetts, it failed to record any map data at all. I have been in contact with Suunto and they say there may be an issue with the watch.

Before I try to replace the Suunto watch, does anyone have these kind of isues with other GPS watches (e.g. Garmin) that they fail to record GPS data correctly?
May 27, 2019 4:27 PM # 
First thing I'd check is what settings your watch is using. I don't have Suunto, but on my Garmin FR235, you can have one second recording or smart recording. The latter provides really lousy tracks for orienteering purposes, one second is pretty good, particularly using both GPS & GLONASS.

If the original worked well, then 'deteriorated', it could be a software update reset the mode back to default - which is usually the 'smart' mode.

No idea why it would not record at all, unless something prevented it from getting a GPS lock.

Just a few ideas to check..
May 27, 2019 4:51 PM # 
Thanks. The Suunto support people thinks it is because the watch wasn't getting a GPS lock. They just told me I need to make sure the watch is synched to the Suunto App on my phone before using it each time, as the app provides a GPS optimization service. Without it the watch can take a long time to get a lock. Who would know...
May 27, 2019 7:02 PM # 
That's pretty lame if you need to run a phone app in order to get your watch to get a lock. Older Garmin's used to take a long time to get satellites, but newer ones seem pretty solid.
May 27, 2019 9:12 PM # 
I think all GPS watches download GPS almanac data every time they download/sync, you shouldn't need your phone every time. My Suunto is fine unless I don't synch it for ages or straight after a software update when it is best to leave it a while to find satellites. It also gets confused if I fly a long way so the almanac data may be specific to where it thinks you are. If it doesn't find satellites fast it will usually get there if left stationary...
May 28, 2019 6:01 AM # 
I've found the traces from my Garmin Forerunner 310XT have gotten worse over the past few years. Used to get accurate tracking but now not so much and yet it's the same device on one second recording. Maybe some satellites have fallen out of orbit and they didn't tell us.

On the flying thing, anytime I go to a new timezone it takes ages to get satellites again and in the process it inevitably condenses all the historical data stored on the watch so I can't view separate activities anymore. Sometimes it'll still let me upload these but sometimes not.
May 28, 2019 7:57 AM # 
Well thats how GPS works, cold boot of GPS device without up to date almanacs& satellite info takes at least 12 minutes to download this information using just GPS signal (50 bytes/ sec as far as I remember). Thats where mobile devices/syncing apps kick in, they push latest info into the watch so that device acquires the fix faster.
May 28, 2019 12:07 PM # 
Most modern GPS receivers preload satellite data via the computer or phone app. This shortens the time to first fix since it knows which satellites to look for without waiting 12 minutes to download a new ephemeris. You don't need to have the phone during the run, just for a few minutes at the start line.
May 28, 2019 7:15 PM # 
I’m not a Suunto user but when I arrive at an Orienteering event I put my watch into orienteering running mode and wait for it the GPS. Then it’s ready to go.
May 28, 2019 11:34 PM # 
mndgs, does that mean it erases the data from my previous location every time I go to a different timezone? It does it every time I travel, even to locations I've been to before and again when I return home. Seems counterproductive.
May 29, 2019 7:42 AM # 
Here's what I remember about GPS:

There are two kinds of satellite data: the almanac contains rough positions for all satellites. Is is valid for quite some time (months?), and broadcast by all satellites.
The ephemerides contain precise data. They are valid for a short time (a day or so) and are broadcast by each satellite for itself only.

If you don't have the almanac, you have to perform a "factory start": first, listen on all channels (the satellites all use the same frequency, but they're at different Doppler shifts) for all satellite signatures. Then, download the almanac. This takes at least a quarter of an hour. Then, proceed as below.

If/once you have the almanac (and a rough idea of your position), you know what satellites should be visible, and listen only for their signatures. Of course, if you have travelled far since the device has been on the last time, your position is no longer valid, and satellite visibility is different; and because you do not yet receive sat data, the device does not know where it is, and so has to look for all satellites instead of just the 10 or so that it knows would be visible at its last known location.

Whenever you "catch" a satellite, you can start downloading its ephemeris (20 seconds or so), and once you have that, you can use it to calculate your position.

About ephemeris data: it is possible to extrapolate the data to generate something that is valid for a week or so. This can be done on the device (client generated anhanced ephemeris, CGEE) or on a server (SGEE). The latter is what Suunto does, and that's why they get a quick fix if you've updated SGEE data recently.

About satellite signatures and "listening": Even under ideal circumstances, GPS signals are quite weak -- 50 dB or so below noise. That's why it would be impossible to receive with old-fashioned analog technology.
The solution is to look for known satellite signatures ("correlating") and averaging over time so the noise cancels out. Of course, the receiver doesn't know exactly when a given satellite signature starts, so it has to try different time shifts. That's why it takes some time until you can even start receiving satellite data.
May 29, 2019 7:54 PM # 
tRicky, Ansgar explained more or less how it works.
As why it takes a long time after travel: All GPS satellites transmit on the same frequency, but because of Doppler effect (some satellites are moving away from you, some getting closer) the frequency shifts a little bit. To know which channels/frequencies to look for, watch uses its last known location + current time (wristwatch's clock is not very precise for these kind of calculations and it increases complexity). But if the gps receiver moved far away since it was last on, all the data is wrong and it needs to start scanning all frequencies to lock onto the satellite signals (it needs a minimum of 4 to calculate location - we have 4 unknowns - x,y,z and time)

This discussion thread is closed.