Repurposing old smartphones to assist your primary phone with navigation (increasing the range)

The problem I have is on long trips (via bicycle or on foot) my phone’s battery hits 15% remaining and screen dims mid-trip, which is essentially blank in daylight when navigating. I’m in airplane mode with wifi also disabled. So the only power consumers are the screen and the GPS receiver. Yet I’m still forced to power down, swap batteries, lose the clock time (which GPS strangely fails to correct), and wait to reacquire a GPS signal. Then OSMand remembers the route parameters but forgets the route (a bug). And because the phone’s time is 1am, I have to either update the time or force OSMand into daytime mode.

Big hassle and unwelcome interruption. I see 3 fixes:

  1. Repurpose an old phone to receive the GPS signal and feed the lat/long over bluetooth to your navigation phone. Since a bluetooth radio in receive mode consumes around ⅒ the energy of a GPS receiver, the main phone battery will last much longer. The GPS phone need not power a screen, so it can obviously run quite long if it’s only powering GPS chips and bluetooth in tx mode. (refs: GPS uses 13-38%, bluetooth uses ~1.8% / 17.9mA on one chip; math-intensive research I didn’t read because it would make my brain explode)

  2. Attach an external USB battery. I reject this because I don’t want to strap another box to my arm and run a cable into my water resistant phone strap.

  3. Get an Android-compatible phone with a dual mode LCD, so a low-power e-Ink mode can be used in daylight. I reject this because I boycott Russia and IIRC only Russia has phones with dual mode displays. I would perhaps be open to buying just a raw dual mode screen (not from Russia or Israel) and then use it to replace a cracked screen on a 2nd hand phone.

I guess it’s debatable relevance to solorpunk travel. Two phones in case 1 consumes a little more power overall but it keeps a phone out of the landfill and makes it useful.

update

Found an f-droid app that looks good for this. It will even run on AOS 2 which means quite old phones can be used to feed GPS coords over BT. This app could be useful as well.

Question: I always disagree to “Google’s location service” nag -- (using towers and/or wifi APs) to supplement navigation (no idea what gets shared with Google and also don’t want wifi or GSM eating battery).. but if a separate phone is feeding the fix, then the power problem goes away. But there’s still the sharing problem. Is there a way to harvest the tower info before a trip anonymously and use it without feeding Google?

update 2

I tried using an external bluetooth GPS device -- one that is dedicated to that purpose from the palm pilot days. I was able to pair to it over bluetooth but after pairing it would not connect to it for any kind of session. It’s as if the android does not know what to do with a GPS server.

Some instructions out in the wild say: “In the Android playstore fetch ‘bluetooth GPS’ or ‘bluetooth GNSS’ App.” Well, I don’t do Playstore.

One step is to go into settings → “Developer options” → Debugging → Allow Mock location → enable. That makes no difference for me.

The instructions also say: “Before you launch your GPS software, launch ‘bluetooth GPS/GNSS’, click “connect” and check “Enable Mock GPS Provider” -- which is a non-starter for those not inside Google’s walled garden. Guess I need a free-world variation of this app which apparently uses the external GPS device to feed a mock location. I found these two apps:

  • GPSTest - this is an apparently useful test app but seems unable to use external devices
  • RtkGps (abandoned¹) - claims to make a connection over bluetooth to an external GPS, but does not work for me. Mentions SiRF IV but not SiRF III, which may be my problem. IIRC, RTK was a SiRF III competitor.

¹ This repository has been archived by the owner on Mar 28, 2023. It is now read-only.

plactagonic ,

I use Sigma ROX 4.0 on bike and supplement it with old mobile with maps. This setup lasts about 3 days.

If the phone dies it just doesn't matter that much because I use different one for calls...

activistPnk OP Mod ,

That looks interesting. I might have to keep my eye out for these at the 2nd hand street markets. When you say supplement, do you mean the ROX feeds coordinates to the phone?

Apparently Sigma has a proprietary app for the phone. If you don’t use that app, are open standards supported? In the pre-smartphone days, it was common to get a dedicated device that merely ran a GPS receiver and the sent to coords to any bluetooth device (e.g. palm pilot) that paired to it. I think the standard is called NMEA. The ROX 4.0 manual makes no mention of NMEA so I’m not sure if that could be used to feed OSMand.

In any case, your finding seems to suggest using an external GPS has a substantial power savings on the phone that hosts the maps.

plactagonic ,

It is pretty limited device. You basically have to use the app but I found some 10 yo github repo that I will try (I have it only for few months).

I get the planned route to the device through the app (which is horrible) and have mobile on standby with same route. When the route inside is bad I crosscheck it with phone.

I figured out how to get .fit file dump of the recorded route out through USB (they don't always sync to mobile). But don't know how to get routes in without the app.

The device itself doesn't have maps it only draws line and have some pre-programmed instructions (turns and intersections). I never tried using it connected to phone on ride (because the app is horrible), it has apparently some extra functionality that I don't need.

Other option for me would be some Garmin but that is about 3-4 times more expensive.

Cuttlefish1111 ,

Maybe you could learn your way around and leave the phone off. Perhaps only power on the phone for emergency/when necessary

activistPnk OP Mod , (edited )

I only use it when I don’t know the route. Usually it’s when I’m on foot all day long in an unfamiliar foreign city.

Sometimes my memory is almost sufficient for the trip, in which case I turn off the screen and go purely off the audible instructions, which greatly increases the range by using less battery. But the timing and accuracy of the audibles is not accurate enough for completely unknown routes.

AchtungDrempels ,

So do you have your screen turned off most of the time? I usually get through a day fine with a charge. And i use some internet, photos snd videos, record my track and navigate with osmand. I do bring a powerbank though.

If you bring a second phone, that is also a second device you're carrying around, might as well be a small powerbank.

activistPnk OP Mod ,

So do you have your screen turned off most of the time?

Yes, because I’m usually not using it. I never use it as a phone and keep it permanently in airplane mode. Daytime navigation is its most common use, in which case I have the backlight on full power and the GPS on.

I usually get through a day fine with a charge.

I could probably get through a week if navigation were not involved. But when I do a day trip in a foreign city I have to carry a spare battery and still take every opportunity at bars and restaurants to recharge (which just gives ~5—10%). I also turn off the GPS when stopped to save battery, but this brings the inconvenience of reacquiring a fix.

If you bring a second phone, that is also a second device you’re carrying around, might as well be a small powerbank.

A powerbank needs to be wired to the phone and thus strapped to my arm. I’ll first test what an external GPS does and if that’s insufficient then I might consider an external battery.

The phone gets quite warm when navigating. I believe that’s because the GPS is computationally intensive. The heat is not only waste energy but it also heats the battery which then possibly impacts the battery performance and charging. So by using a separate device for the GPS, the impact from the heat should be reduced.

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