Re: Staying alive in the snow
Posted: Fri Jan 25, 2019 11:59 am
Some good points and questions here. Take home message is there are errors inherent in both dedicated GPS (e.g. garmin etc.) and in cell phones using GPS. Our SAR research group started looking into this a few years back when we got reports of people reporting their coordinates read from a cell phone. Errors would be 1 mile (and 4,000 vertical!) to 6 miles.Obviously significant and a danger to responders. So,what's going on?
For those of you yearning for a long, very long, explanation and introduction to cell phone gps capability, I wrote this a couple of years ago:
https://drive.google.com/open?id=0B5Snn ... HFFYW1xN1E
A shorter version meant as a workflow to get coordinates from someone reporting an emergency via a cell phone is here:
https://drive.google.com/open?id=0B5Snn ... kVHWndOWWc
This latter document, though meant for 911 centers and SAR managers, is useful because it provides a few apps you can put on your phone to provide responders with more accurate location coordinates.
I'll try to give a brief overview (uproarious laughter from anyone who's read my assorted eye-drooping explanations of anything...).
First, gps and satellites. The GPS network is pretty old and, when designed, had/has really limited bandwidth to send data from satellite to gps. It only works one way. Your gps, whether dedicated or cell phone, is a passive receiver. It calculates location based on the time signal from several satellites, ideally at least three. To do that, the receiver needs a list of where each satellite is going to be and the distance so it can calculate when the signal will have been sent and the time it gets to the device. When you have at least 3 satellites, it can triangulate your position and show the coordinates. As bobby49 suggests, multi-path can be a serious problem. I've seen individual pings as much as a mile off. This is especially a problem in deep canyons, like the Grand Canyon. However, averaging over time usually shows a semi-accurate fix. A gpx track can be a mess with the occasional ping that's way off but, overall, the route can be seen.
To get a fix, the receiver needs two files. An almanac and an ephemeris. Both contain the information on satellite location. The problem is, the almanac, especially, takes about 20 minutes to download because of the narrow bandwidth available. This is fine when you use your gps often and in the same general area. If, though, you've not turned it on for a couple of weeks and/or moved over 50 miles (and I'm not sure of exact times or distance...), then the receiver has to download a new almanac. Until it does, the results displayed could be very inaccurate. Which is to say that for an accurate reading, especially where you're using it for the first time in awhile or you've traveled a distance, then turned it on, your readings may be way off.
This applies to dedicated gps and, under some conditions, to cell phones.
(and here I take a short break to pound my head on my keyboard. I'd finished the post in the quick editor, went to Preview, where I signed in again, which was blank. I went back and, auuuughhhhh!, it was all gone though I'd previously saved the above elsewhere.)
Still, I'll try to reconstruct my brilliant prose.
Cell phones use several methods to acquire location. The goal of cell manufacturers is to get as fast a Time To First Fix (TTFF) as possible. To do that, they take some shortcuts and don't rely entirely -- or sometimes at all -- on the phone's gps. The location of a cell tower is known, so the phone can calculate the distance from the tower using the round trip time of a ping. Direction is more difficult and, if it's an omni-directional tower (360 deg antenna vs. individual antenna with a cone of coverage), impossible. With a cone you get only a very rough direction but, combined with help from the gps, you can get a location. Finally, best case is triangulating -- or more -- using multiple towers. But, in building canyons, you also get multi-path so there can be error there. I'm not clear on it, but I think known wifi can also be used. There's references indicating that the Googlemobile and other companies suck up all the mac addresses and locations of wifi, allowing the phone to use that data set. I'd read an article that Europe was going to ban that practice but not clear of current status.
Still, phones seem to be accurate most of the time -- except when they're not. The errors we saw in the cases where location was way off appeared to show the phone's last good fix but not it's current location. Also, and not 100% sure, these cases involved reception off only one cell tower. So, in areas with spotty connection, the odds seem to increase of an inaccurate location shown by the phone. Setting the phone to gps only or using a gps-only app would probably reduce the chance of error.
With Android, you can actually set the phone to the level of accuracy you want though perhaps sacrificing speed. I think the setting "high accuracy" is gps only and the other two use different combinations of the above to solve for location. With iPhone, you don't have that choice though I'm not clear on it's effect on accuracy except to say that several of the errors we looked at were from iPhones.
There's workarounds with apps I link to in the Workflow paper. One will send a text to a person who doesn't know their location. In the reply, permission is given to use the phone's gps to send location. Another app by one of our crew will take you to a site which, again with permission, reads your phone's gps and posts it, including a map location. Other apps, like Backcountry Navigator, I believe use only the phone's gps (but not sure...).
While I'm here, I'll also mention that PLB's also can have serious errors as a result of the tech they use. Those are the gizmos used by ships and aircraft (ELT) and individuals (PLB) that work with a dedicated satellite system and the Air Force Rescue Coordination Center to derive the location. Here's another short paper on those:
https://drive.google.com/open?id=0B5Snn ... XVYb3U1NkE
The good news is, as far as I know, SPOT and InReach are pretty spot-on accurate. They use only GPS though multi-path could effect their results. That said, I've never seen it so they might average the results or use some other post-processing when the route is displayed.
Last! I'm a poor but mostly honest forestry major and semi-retired ranger. I don't have a background in gizmos and took this up because of some serious errors in location on self-reported SARs, and even including 911 transfers. We figured someone somewhere would have a good summary document on how cell phones acquire location. We never found one so I ended up writing this stuff from research our team compiled and contributed. If anyone here has additions or corrections, they'd be much appreciated! Send them to me directly.
Thanks.
For those of you yearning for a long, very long, explanation and introduction to cell phone gps capability, I wrote this a couple of years ago:
https://drive.google.com/open?id=0B5Snn ... HFFYW1xN1E
A shorter version meant as a workflow to get coordinates from someone reporting an emergency via a cell phone is here:
https://drive.google.com/open?id=0B5Snn ... kVHWndOWWc
This latter document, though meant for 911 centers and SAR managers, is useful because it provides a few apps you can put on your phone to provide responders with more accurate location coordinates.
I'll try to give a brief overview (uproarious laughter from anyone who's read my assorted eye-drooping explanations of anything...).
First, gps and satellites. The GPS network is pretty old and, when designed, had/has really limited bandwidth to send data from satellite to gps. It only works one way. Your gps, whether dedicated or cell phone, is a passive receiver. It calculates location based on the time signal from several satellites, ideally at least three. To do that, the receiver needs a list of where each satellite is going to be and the distance so it can calculate when the signal will have been sent and the time it gets to the device. When you have at least 3 satellites, it can triangulate your position and show the coordinates. As bobby49 suggests, multi-path can be a serious problem. I've seen individual pings as much as a mile off. This is especially a problem in deep canyons, like the Grand Canyon. However, averaging over time usually shows a semi-accurate fix. A gpx track can be a mess with the occasional ping that's way off but, overall, the route can be seen.
To get a fix, the receiver needs two files. An almanac and an ephemeris. Both contain the information on satellite location. The problem is, the almanac, especially, takes about 20 minutes to download because of the narrow bandwidth available. This is fine when you use your gps often and in the same general area. If, though, you've not turned it on for a couple of weeks and/or moved over 50 miles (and I'm not sure of exact times or distance...), then the receiver has to download a new almanac. Until it does, the results displayed could be very inaccurate. Which is to say that for an accurate reading, especially where you're using it for the first time in awhile or you've traveled a distance, then turned it on, your readings may be way off.
This applies to dedicated gps and, under some conditions, to cell phones.
(and here I take a short break to pound my head on my keyboard. I'd finished the post in the quick editor, went to Preview, where I signed in again, which was blank. I went back and, auuuughhhhh!, it was all gone though I'd previously saved the above elsewhere.)
Still, I'll try to reconstruct my brilliant prose.
Cell phones use several methods to acquire location. The goal of cell manufacturers is to get as fast a Time To First Fix (TTFF) as possible. To do that, they take some shortcuts and don't rely entirely -- or sometimes at all -- on the phone's gps. The location of a cell tower is known, so the phone can calculate the distance from the tower using the round trip time of a ping. Direction is more difficult and, if it's an omni-directional tower (360 deg antenna vs. individual antenna with a cone of coverage), impossible. With a cone you get only a very rough direction but, combined with help from the gps, you can get a location. Finally, best case is triangulating -- or more -- using multiple towers. But, in building canyons, you also get multi-path so there can be error there. I'm not clear on it, but I think known wifi can also be used. There's references indicating that the Googlemobile and other companies suck up all the mac addresses and locations of wifi, allowing the phone to use that data set. I'd read an article that Europe was going to ban that practice but not clear of current status.
Still, phones seem to be accurate most of the time -- except when they're not. The errors we saw in the cases where location was way off appeared to show the phone's last good fix but not it's current location. Also, and not 100% sure, these cases involved reception off only one cell tower. So, in areas with spotty connection, the odds seem to increase of an inaccurate location shown by the phone. Setting the phone to gps only or using a gps-only app would probably reduce the chance of error.
With Android, you can actually set the phone to the level of accuracy you want though perhaps sacrificing speed. I think the setting "high accuracy" is gps only and the other two use different combinations of the above to solve for location. With iPhone, you don't have that choice though I'm not clear on it's effect on accuracy except to say that several of the errors we looked at were from iPhones.
There's workarounds with apps I link to in the Workflow paper. One will send a text to a person who doesn't know their location. In the reply, permission is given to use the phone's gps to send location. Another app by one of our crew will take you to a site which, again with permission, reads your phone's gps and posts it, including a map location. Other apps, like Backcountry Navigator, I believe use only the phone's gps (but not sure...).
While I'm here, I'll also mention that PLB's also can have serious errors as a result of the tech they use. Those are the gizmos used by ships and aircraft (ELT) and individuals (PLB) that work with a dedicated satellite system and the Air Force Rescue Coordination Center to derive the location. Here's another short paper on those:
https://drive.google.com/open?id=0B5Snn ... XVYb3U1NkE
The good news is, as far as I know, SPOT and InReach are pretty spot-on accurate. They use only GPS though multi-path could effect their results. That said, I've never seen it so they might average the results or use some other post-processing when the route is displayed.
Last! I'm a poor but mostly honest forestry major and semi-retired ranger. I don't have a background in gizmos and took this up because of some serious errors in location on self-reported SARs, and even including 911 transfers. We figured someone somewhere would have a good summary document on how cell phones acquire location. We never found one so I ended up writing this stuff from research our team compiled and contributed. If anyone here has additions or corrections, they'd be much appreciated! Send them to me directly.
Thanks.