GPS performance vs Etrex 10

Hi,

I bought the mini a couple of weeks ago and use it in two sunday hikes. Generally speaking I walk in wood, occasionally in deep valley and on top of mountains. I have definitely noticed a poor gps performance compared against my old etrex 10. The etrex gets the fix much quicker and never lose it. The mini takes up to 10-15 minutes to obtain the gps signal, depending on the location natural features (near to a rock wall, under trees, and so on). These terrain features almost never affect the etrex performance.

Moreover, once acquired the mini loses the signal pretty soon so that when I send a preset msg, it takes minutes to take the fix and then a few more minutes to connect to IR and send.

I do not have any track/logging activated, both for battery saving reason and because the etrex does the tracking job.

Now the questions are:

if the logging is activated, the signal is better managed by the unit? Maybe it is forced to keep the fix (just wondering)?
Is this poor performance, in the terms I have described, expected, due to the hardware? I know the etrex use the Glonass satellites too; maybe the mini looks only for GPS?

I am afraid that this kind of problem substantially impair the utility of the device: if something goes bad, it will probably happens in rough terrain.

Thanks in advance for replies.

Best regards.

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0 twolpert over 4 years ago

Mini uses only GPS.

If you log at intervals of 30 seconds or less, the unit will keep the GPS radio on at all times. This should result in a more reliable, consistent fix. But it really drains the battery quickly. In fact, the unit will warn you about the battery drain when you enable logging at 30 seconds or less.

AFAIK, the Mini does not use an EPO file. As a result, time to first fix (on a cold start) is dependent on whether or not the unit has an up to date almanac on board. Other users have reported improved TTFF after making sure the almanac is up to date. To do this, take the unit to a spot with a clear sky view, wait for the unit to acquire, place the unit on a stationary surface and walk away. Allow the unit to "soak" for about 20 minutes. (Downloading a full almanac requires at least 12.5 minutes - best to allow more in case there is any interruption in satellite visibility.)

In theory, the almanac should stay up to date (without an additional soak) as long as you use the unit regularly. You should benefit from improved TTFF as long as the almanac is current. The almanac is good for at least 60 days - and I have seen references to as much a six months.

Even with a valid almanac, you will experience a slow first fix after moving the unit more than 200 miles with power off. The unit uses the almanac, current date/time, and current location to determine what satellites are overhead. After a move like that, the unit will use the last known position until it acquires again.

Anecdotally, it appears that the Mini does not acquire as quickly as other units and is more prone to losing its fix in challenging conditions. Part of this is probably due to aggressive power conservation measures. The conventional wisdom is to be sure you HAVE a fix before setting out. Carry the Mini in a position the minimizes interference from your body - on a pack strap on top of your shoulder is about the best you can do.

0 andtorg over 4 years ago in reply to twolpert

Many thanks for your reply, twolpert

Up to now I have used the device in places not so far apart, less than 50 km, against the 200-miles you quoted.

About the almanac, what if the unit never stay connected for a continous 12.5m-period? I mean, after a few hours of fix-and-loss, does it get the full almanac, in your opinion?

0 twolpert over 4 years ago in reply to andtorg

The technical considerations for acquiring a full almanac are complicated. I no longer remember the gory details. The only reliable way to do it is to soak for at least 12.5 minutes (preferably longer), with a clear view of the sky, and without moving the device. The way the almanac is downloaded, interrupting the signal even momentarily requires the device to begin again on that particular segment of the almanac. This is why carrying the device around while trying to get the initial almanac causes it to take much longer. In fact, it might not complete at all.