[pct-l] smartphone as GPS

[Denis Stanton]

Hi David

It sounds like you know more about developing GPS systems than I do, but I have to say part of your advice below doesn't tie in with my experience.  

I have used an iPhone on short PCT sections.  Around Glen Pass in 2009 and Selden Pass in 2010 I used the Topo Maps application extensively.  There was no cell coverage available, and in 2009 my iPhone could not have used one even if it was available as I had no US phone subscription.

My iPhone was able to get a location point via GSP without any access to cell towers.

iPhone use AGPS when they can get it, but if no cell signal is found they will still pickup the GPS signal from the satellite just as any GPS unit would do.  It takes noticeably longer to get a fix without the AGPS, and this does tend to use the battery more, but it works and in my case saved me a lot of wasted effort by getting me back on the trail after a long snow field.

The biggest problem with using an iPhone on the PCT is battery life.  If you like to check your position every 10 minutes (as I do) you will exhaust the battery.  I'm have a solar charger system, but I have not proved its effectiveness of a long hike.

Denis Stanton


On 26/10/2010, at 10:02 AM, David Ellzey wrote:

> A few people have indicated that smartphones use cellular tower triangulation and not satellites. This is not entirely accurate, every smartphone I know of uses what's called Assisted GPS, or AGPS for short. Whereas a normal GPS chip takes the signals from satellites and calculates position, an AGPS chip only records the signals and sends that data to an online server for calculation. This means that you MUST have a good wireless connection for the AGPS to work. This is the disadvantage, especially when in the backcountry.
> 
> An advantage of AGPS is that it speeds up the satellite acquisition considerably by getting a accurate time and general starting location through tower triangulation. Of course, this is dependent on having a good signal as well.