[pct-l] smartphone as GPS

[David Ellzey]

Dennis, et al,

As I understand it, it is possible to overcome the GPS chip's lack of sufficient integrated correlators with software. Most likely this is what happened for the people here who got it to work. They had software loaded that contained not only map data but correlator data that the phone's CPU could then calculate.

According to the Apple development team, they do not recommend this approach as it has several downsides, (1) it causes the processor to use excess power to operate, severely diminishing battery life, (2) the solution for the GPS signals cannot be processed in parallel which can cause a much higher rate of error and (3) because the CPU is not designed specifically to process GPS correlators it takes much longer than a standard GPS chip or even AGPS to record a solution, with fewer solutions the mean average can be further displaced by errant position records.

We did some field testing in Florida last month at a National Park with no cell coverage and moderate tree cover and found that we could not reliably use an iPhone or iPad without a functioning AGPS to accurately track where a visitor was at in the park. On the other hand an ultra-mobile PC with a full GPS chip in it performed flawlessly.

The best solution would be a smartphone with an AGPS chip that also had sufficient integrated correlators but for some reason (probably a good one) no one offers that.

BigToe

PS - FYI, this is a long running hot topic among iOS developers and still difficult to get a comprehensive straight answer from Apple even now.

-----Original Message-----
From: Denis Stanton [mailto:denisstanton at mac.com] 
Sent: Monday, October 25, 2010 4:28 PM
To: David Ellzey
Cc: pct-l at backcountry.net
Subject: Re: [pct-l] smartphone as GPS

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.


No virus found in this incoming message.
Checked by AVG - www.avg.com 
Version: 9.0.862 / Virus Database: 271.1.1/3217 - Release Date: 10/25/10 01:34:00