inReach Explorer: Battery Life (Updated)

Disclaimer: I haven't tried this lately.

I'm not sure the back fence test is a valid test of battery life in real world conditions, extended tracking mode or not. As you correctly point out, the device reverts to 4 hour send intervals when it senses that it is stationary. I rather suspect that this is going to interact with extended tracking mode in ways that you might not expect. You will still observe some benefit from extended tracking mode. If nothing else, this disables the Bluetooth radio and automated message checking. But I would not trust the result to predict real world experience when moving.

As you conjecture, the device initially detects the fact that it is stationary by comparing recent position readings. When successive readings indicate a position change below a particular threshold, the device reverts to 4 hour send and sleeps. It wakes periodically to acquire a new position. When it detects a large enough change in position, it reverts to normal operation. Nobody but Garmin knows the details (position change threshold, wake frequency, etc.). I don't know whether or not extended tracking affects any of those parameters.

I would expect that positional accuracy to suffer in extended tracking mode. The point of extended tracking is to maximize battery life. Like Bluetooth and Iridium, the GPS radio consumes energy. So the device attempts to minimize the time the radio is on. This may well mean that it sacrifices fix accuracy in favor of battery life. This is one of the places where extended tracking may interact with stationary position detection in your back fence test. I believe you had the unit set for 1 hour intervals in extended mode in your most recent test. If you saw any points at one hour intervals, it's likely that the unit incorrectly detected motion on the basis of an approximate fix.

With regard to the cliff, you don't say whether you were actually on top or at the bottom. If you were at the bottom, you were experiencing predictable inaccuracies based on the environment (in addition to any effects occasioned by extended tracking mode). When you are near a steep obstruction (cliff, steep hill, slot canyon, urban canyon, etc.), you have to deal with two things. First, a portion of the sky is obstructed. This means fewer usable GPS satellites, as well as issues because they are clustered to one side creating bad geometry. Result is a less accurate fix. Second, you may be dealing with multi-path effects. GPS signals reflect off the obstructions, which means that the unit may well see multiple signals from the same satellite. It does its best to pick out the real (direct path) signal, but does not always succeed. The multi-path effect is mostly responsible for any large excursions you may see in your track. If you check the "legs" of the excursions, you will see very high apparent speeds. This is what happens when the unit changes its mind about which signal is the "real" one. It makes a potentially sizable adjustment in position between successive samples (or averaged samples, or whatever).

These two effects are common to all recreational GPS units. Extended tracking mode may aggravate the issues because the unit is not doing its very best to establish an accurate fix each time.

Temperature does have a measurable effect on battery life. Low temperatures tend to negatively affect most battery chemistries, including the lithium ion batteries in the inReach devices. Although it's unlikely to be a serious problem in Florida, most of us at least consider carrying the unit in an inside pocket in very cold weather where battery life is important. Do note, though, that this may have its own negative effect on battery life. Antenna position and orientation matters, both for the GPS receiver and for the Iridium radio. And your body is mostly water, which does a fine job of blocking signals.

When temperature is not an issue, the ideal place for the unit is on a shoulder strap, face up at approximately a 45 degree angle. This is a compromise between GPS and Iridium antennas. The GPS antenna is a patch antenna which points up from the face of the unit. It would prefer to point straight up, meaning the unit is "on its back pointing to the sky". The Iridium antenna is the coil antenna on top of the unit. It would prefer to point straight up. Both of them would prefer not to have body parts between them and the sky.

Well I'm accused of moving really slow, but at even my pace, the device would detect some motion! You're right that there are some non-real-world aspects to the fence test. But knowing how the device performed on my last long journey as compared to the fence test, I feel pretty good about this one. For sure I'll take it out for a spin on a shakedown hike for some actual point sending.

You made a note about disabling bluetooth and message checking. Whether I had XT ON or XT OFF, I had disabled both bluetooth and message checking, these features should have not been a factor in any one of the three tests. I'm still a little puzzled what is deactivated in XT mode, but it works, and I'm OK with that, even if locates are a little wonky. You're probably spot on regarding how much energy it consumes obtaining the best fix--it may compromise the best fix for battery preservation.

I did have many cases where the locates were different enough to lead to successive hourly point sending. The device itself was completely stationary with a clear view of the sky, so it somehow compares the previous locate to the current fix. Again, not a big deal when actually moving, but interesting as part of the story.

Thanks for the notes on the antennas. Didn't know the difference between antenna types. Something to be aware of. I always assumed that any shielding of the antenna (in a pants pocket, jacket, etc.) would lead to excessive attempts to obtain a fix and drain the battery faster. I'm not expecting to be out in crazy cold weather, so the shoulder strap is my spot.

The only things that are documented as being disabled in extended tracking mode are Bluetooth, automatic message checking, and detailed track logging. All of these things are aimed at minimizing radio on-time. Detailed track logging would require more frequent fixes, which consumes both CPU and GPS radio time.

Most types of dry fabric are mostly transparent at the frequencies used. Your body is the biggest shield about which you need to worry. You don't want to bury the unit in your pack, but it'll typically work OK riding on top of the contents. You do have to be somewhat concerned about wet fabrics. It only takes a thin coating of water to interfere substantially.