I am writing my Master thesis atm and analyzed about 400 different GPS-Trajectories (paths consisting of a list of longitude/latitude points and an according timestamp) recorded with the MediaQ-App (probably not important) for Android (about 310 trajectories) and iOS (about 90).
When analyzing the time intervals between the timestamps, I was expecting the inaccuracies around the 1 second frequency, in which the timestamps were supposed to be recorded, since neither Android or iOs are real-Time-OS from apps point of view. Still, why are there, what appears to be local peaks in about 200ms steps? Be nice, I do not know a lot about mobile operating systems :-)
(x-axis: seconds, y-axis: the relative probability that a timestamp will be recorded at this point in time. Ignore the exact values of the y-axis...)
The usual behavior is that an App takes the timestamp direct from the location delivered by the system. Most probably this is the case in that App, otheriwse this would be a heavy programming bug. (The delay between nominal GPS time and receiving system time of about 200ms is plausible, I saw that in my app, and in ios Map app, which makes a correction in their map api to extrapolate the location into the future, to try to compensate that delay)
Under android (and ios) it is import to know whether the location are determined exclusively by GPS. On both plattforms (ios, android) an App can set whether GPS only or combined GPS + WLAN + Cell Locationing is used.
For high quality recording is is highly recomended to use GPS only.
A GPS chip once having a location usually outputs a location with a timestamp exactly at 000. Exception can happen before the location is valid.
Under ios if the location was obtained by GPS the millisecond are always 0.
I have no experience with android, but usually the GPS chip itself sets the value to millis = 0. (However, some android phones are known to use a crippled cheap GPS chips, that are not able to get an intial position without the help of an internet conection)
It is possible to determine whether the location was obtained by GPS by looking at the course (or called heading) and speed value, which are invalid for non GPS locations (cell Tower locationing)
To know whether this time variations come from the Application (not using the GPS only mode) or if it is caused by Android, you can test that with another application on the same phone. (E.g Motion-X GPS) (Or by comparing both apps on another android phone)
Related
There are any documentation for this? I've checked many formulas and algorithms but didn't find an explicit definition how they get the speed provided in Geolocation object.
The GPS chipset provides the current velocity to the system along with the current location.
The chipset may compute the velocity by comparing location over time and correcting for the curvature of the Earth at the current location, or from the Doppler shift of the received satellite signals.
Whichever method is used, the operating system doesn’t need to do any processing to get the speed. The velocity data is provided along with location and altitude data directly from the GPS chip (actually chips in many newer devices support both GPS and GLONASS, but which system is used is not visible to the user).
Physics.org has a simple explanation of how GPS works
Wherever you are on the planet, at least four GPS satellites are ‘visible’ at any time. Each one transmits information about its position and the current time at regular intervals. These signals, travelling at the speed of light, are intercepted by your GPS receiver, which calculates how far away each satellite is based on how long it took for the messages to arrive.
Once it has information on how far away at least three satellites are, your GPS receiver can pinpoint your location using a process called trilateration.
But you can search for more detailed explanations of the mathematics and physics involved.
It basically scans your location (i.e. your latitude and longitude) and compares with the last check. Using euclidian distance, it can get the distance in the period of time. With the time (for example, if it scans your location every 2 seconds), it can easily estimate your speed by doing Speed = Distance/2.
I'm using intel xdk and cordova for an android app that uses geolocation this is the code that i use:
if(navigator.geolocation) {
navigator.geolocation.getCurrentPosition(geolocalizacion,error,{enableHighAccuracy:true, maximumAge:Infinity, timeout:100000});
} else {
handleNoGeolocation(false);
}
I don't want to use the 'google location service' because, if that option is disabled, some people can get lost trying to find it in settings, so i'm using enableHighAccuracy:true in order to use GPS.
However with that it takes so much time, almost a minute to call the success function 'geolocalizacion' when the app is used indoors. I know that GPS is better in outdoors (in that case it takes 5 or 7 seconds) but i think 1 minute is too much for indoors.
Am i doing something wrong?
Is there a way to make faster the geolocation with GPS?
or a way to activate the 'google location service' without the user doing anything?.
The GPS signals transmitted by GPS satellites are very weak if you are not located outside. Even if you are outside, weather conditions, tall buildings and other large metal structures can hide or degrade the GPS signal. In addition, the GPS receivers inside of your typical phone or tablet are very low grade receivers and antennas, they are not as capable as the more expensive dedicated GPS devices, due to compromises associated price and physical size.
You should really take a two prong approach: get an initial reading using the "coarse" geo reading, with the geo cache enabled. The coarse reading will use the wifi, network, GPS and cached readings and return it's best estimate based on the parameters you specify, usually within a very reasonable time frame.
Then, after you've established an approximate position (which can be very accurate, as #SeahawksRdaBest points out, if based on the current wifi), switch to the "fine" location setting to get an accurate position. This part of your app can run in the background so it doesn't interrupt the responsiveness of your app, and can then update the position information in your app as more accurate position data becomes available.
One thing to keep in mind, many users have the GPS turned off because it severely degrades battery life. If all your app needs is to "find a restaurant nearby" then the coarse location should be good enough, since you're probably more interested in placing the destination on the map. This will be much more battery friendly, and is one of the reasons that the geo settings on the more recent Android devices have changed to make it harder for an app to enable GPS programmatically.
I see you've added the intel-xdk tag, so you might want to checkout this little geolocation test app that I've put together at https://github.com/xmnboy/hello-cordova. It needs a little work but should help you with some experimentation between coarse and fine settings.
I think you can diversify your app a little.
For example if you are indoors and have a WiFi connection why not use that to triangulate your position? WiFi is highly accurate because theoretically it covers a small space so the phone would know exactly where it is and return a position quickly.(if done right in my experience usually <10 secs).
If a Wifi connection is not found(outdoors). You can always drop back to GPS to provide the coordinates. Plus Gps is "fastest" outside anyways.
Checkout these pages: Stackoverflow discussion. Location Manager Android.
If you want to get really fancy you can even use the Geocoder Android library to get Location addresses etc in human readable form.
In most (more likely all) Mediatek mtk6592 phones (and also older models), There is a bug related to GPS UTC time.
the bug causes the GPS to report WRONG UTC time by an offset of exactly 1 hour.
That means that the sky view (which is the number and position of satellites currently visible) is shifted by one hour and that means that the phone is trying to connect to satellites that aren't there and even worst, it IGNORES satellites that are there!
Fixing this small issue will improve GPS in all Mediatek phones.
BTW, I discovered this bug and informed Mediatek but got no reply to date.
The question is whether Android can be fixed to insert a 1 hour shift to the GPS UTC time a thus fixing the problem.
I doubt that your information is correct!
The GPS chip receives the time from the satellites in GPS Time base, which is some seconds differet from UTC (about 17 or 18s), then after some minutes the sattelites also send the GPS time to UTC offset, for better display to the end user.
I am very sure that the chip does his work correct, otherwise nothing would work.
The time has to be correct to about nanos seconds!
If there is realy an UTC time offset, then it is in the phone or software API.
But this has nothing to do with the quality of the location, because again:
inside the GPS chip everything will work!
Probably there is an internal API like in iOS and android that replaces the location timestamps received from the GPS chip, to match the current time zone, and to match the offset from the phone time to the real time (by GPS UTC) (if the user has manualy set the time).
And this internal time replacement might fail.
Event if there would be a bug in the chip, under no circumstances you could externaly correct that time bug! GPS chips allow to set the current time, but this feature is for faster speed up of initial time calculation. In practise this does only work if the time set is extremly precise.
I'm working on a app which requires that a device with this app installed will automatically find other users within a maximum radius of 200ft (worst case scenario 300ft, but that's pushing it) and/or a minimum radius of 40ft.
Ive tried the obvious solution of using GPS and a MYSQL query that query's our location table for other users within the 200ft radius, but as you probably would guess this is not incredibly accurate and if the device uploads coordinates that are off by over 200ft the server will return a list of users that are not within proximity. While I would prefer to just get the app to work the way it was meant to I'd rather the server return no users than false positives.
I know there's probably no simple way to do it accurately, but what other options do I have? And how did Color manage to do it? With all the tech in the avg smartphone and all the location based apps this has to be possible to do.
200ft (60m) is no Problem for GPS. Usually GPS is below 10m.
You even have a location.getAccuracy() method which you should evaluate
Just use GPS as your only location source. do not use cell tower location provider, when you want accuracy < 60m.
Of course inside a building, when you are sitting at your desktop GPS will not work, or is off by 60m.
GPS needs a view to open sky not obstructed (by dense materials).
Take a look at this question:
how to get the most accurate location using GPS and AGPS with given time in android?
Basically it depends on the phone's GPS and the current environment. Besides that, there's probably nothing you can do to further boost the location accuracy other than using GPS.
How would I verify/ track device location within a 5' accuracy? I've heard of people using cell towers/ gps combinations.
As far as I know, the only way to get a 5 feet accuracy figure is to use GPS, then it still isn't always that accurate depending on how good a fix of the satellites (clear view to the sky) you have.
Cell tower / Wifi triangulation methods only serve to speed up positioning and will seldom (if ever) be more accurate than satellite positioning methods.
GPS is the way to go. Cell towers won't cut it. In Android (and I believe iOS) the system will provide you with an accuracy reading in addition to the actual location. You could use this value to determine whether the value you've received should be uploaded to your server. Keep in mind using the GPS is very battery intensive and there's no guarantee of how good the accuracy will be. Depending on conditions you may not be able to achieve 5' precision.
As #CommonsWare points out, 5' is really unrealistic anyway although you can get close.
As CommonsWare says you will not get much better that 10 metters accuracy in a consummer-grade device. Even in open sky, the atmosphere characteristcs change minute by minute and thats enough to change the GPS readings.
However, it's teoreticaly possible to increase accuracy if you could get all of the following:
1-There are some stationary GPS receiver stations with fixed known locations which measure the current GPS signals deviation. You would need to have one of those close to you and have access to the data in real time.
2-You would need to have low level access to your phone GPS receiver to read the unprocessed data received from sattelites. This would be different from device to device, and as far as I know, no supplier is providing this access.
3-Finnaly, you would need to do all the calculations required to determine your location applying the deviations got from point 1 above.
good luck.
The only way you can get this type of accuracy is with WAAS. As far as I know, there are no Android handsets that can receive WAAS corrections. If this is for a specific controlled situation, you could use a bluetooth gps receiver with WAAS, and only in WAAS supported locations. However, if this was for a wider deployment, then I think you are out of luck.