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AGPS working without GPS

 
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learner71
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Joined: Jul 29, 2013
Posts: 8

PostPosted: Fri Sep 06, 2013 8:25 am    Post subject: AGPS working without GPS Reply with quote

Hi,

I read that there are two types of AGPS.
1. Mobile Station Based : It will take almanac and ephemeris data from server and provide information to GPS module.
2. Mobile Station Assisted: where GPS module will send signal information (almanac and ephemeris data) to base station, where it will calculate the position and send back to module.

Question?
1. Can AGPS work, even when i am not receiving any signal or weak signal to GPS module?
2. which is best method of getting AGPS coordinates for every attempt and accurate one?

Regards
Learner
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M8TJT
The Other Tired Old Man
The Other Tired Old Man


Joined: Apr 04, 2006
Posts: 10118
Location: Bexhill, South Sussex, UK

PostPosted: Fri Sep 06, 2013 8:37 am    Post subject: Reply with quote

Q1. As AGPS is to assist getting a shorter time to first fix from the satellite signals especially if you have moved position greatly since the last fix or it is a considerable time since last fix, if you have no sat signal, you will not get a fix regardless of the AGPS input.
Q2. Don't know. One might be a bit better than another, but as it only affects time to first fix by more accurately predicting the position of the sats etc., does it really matter? No sat signals=no fix.
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learner71
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PostPosted: Fri Sep 06, 2013 9:26 am    Post subject: Reply with quote

Thanks,

But this is case, that when we are sitting in home, where no satellite is visible to us, but we are able to get our position. as we saw in mobile, when we are at home, even then we are able to get location.

Does it means that even when no satellite is visible to us, we can get location?

I am not sure, please can you tell, then why are we able to get location using mobile?
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M8TJT
The Other Tired Old Man
The Other Tired Old Man


Joined: Apr 04, 2006
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Location: Bexhill, South Sussex, UK

PostPosted: Fri Sep 06, 2013 11:03 am    Post subject: Reply with quote

Quote:
Does it means that even when no satellite is visible to us, we can get location?
As I said before, AGPS only shortens the time to first fix. I think that when you are 'sitting at home' you are still getting satellite signals: I know I do and get a fix indoors. But go into a big building with steel all around and you won't and AGPS won't help either.

Some phones will give you an approximate location based on (triangulation?) from mobile phone masts, but I have not found it very accurate (200yard+ out) and in fact if it is enabled in the CA app on my android phone, it screws up my position and average speed big time in CA
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MaFt
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Joined: Aug 31, 2005
Posts: 14687
Location: Bradford, West Yorkshire

PostPosted: Fri Sep 06, 2013 11:32 am    Post subject: Reply with quote

learner71 wrote:
Thanks,

But this is case, that when we are sitting in home, where no satellite is visible to us, but we are able to get our position. as we saw in mobile, when we are at home, even then we are able to get location.

Does it means that even when no satellite is visible to us, we can get location?

I am not sure, please can you tell, then why are we able to get location using mobile?


Do you mean using your mobile phone? If so then the phone will be picking up local WiFi hotspots and using their known location. You can test this by turning off wifi on your phone - see if it shows a good location then. Phones can also use cell-triangulation - it knows how far you are from your 3 nearest mobile base-stations and can get a rough idea of your location.

Another fun test is to move your wifi router to a different house, plug it in etc. Then see if your location is showing as your normal house Smile We saw this a few times at MWC in Barcelona last year - companies displaying new products had brought their router over from, say, Chicago and because we were indoors the mobile phones put people's location as chicago as the router was 'known' to be in chicago Smile

MaFt
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learner71
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Joined: Jul 29, 2013
Posts: 8

PostPosted: Tue Sep 10, 2013 8:11 am    Post subject: Reply with quote

Quote:

Another fun test is to move your wifi router to a different house, plug it in etc. Then see if your location is showing as your normal house Smile We saw this a few times at MWC in Barcelona last year - companies displaying new products had brought their router over from, say, Chicago and because we were indoors the mobile phones put people's location as chicago as the router was 'known' to be in chicago


Question:
1. Does it mean, if wifi is enabled,then it will take wifi router position, irrespective of where are we? will it always shows position of wifi router?
2. During wifi enable, will it use AGPS or WIFI router to fix location?

Regards
Learner
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M8TJT
The Other Tired Old Man
The Other Tired Old Man


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PostPosted: Tue Sep 10, 2013 9:06 am    Post subject: Reply with quote

You seem to be miss-understanding AGPS. AGPS does not provide a fix. It enables the device to more quickly get a fix from the satellites on your GPS device. And AGPS is used for GPS location only. Not WiFi or cell location.
MaFt wrote:
Phones can also use cell-triangulation - it knows how far you are from your 3 nearest mobile base-stations and can get a rough idea of your location.
But it is not very accurate. My cell location is always given as about 200 yards from my house whereas the GPS fix puts me in my lounge (through the roof etc. of a two story house).

MaFt's example of the router is anecdotal, and should not be taken as gospel. That's the first time I've heard of a phone using your router (or anyone else's router) as a location data source. Surely the router does not transmit its location? If so, from where does it get it? Or how does that work? Or does the phone (etc) think it knows where the router is by comparing the GPS or cell location with the signal strength of the router and remember the location?

Other than for interests sake, you should forget about AGPS unless you move a considerable distance with your device switched off or have it off for a long period of time when AGPS will enable your device to get a fix quicker than it would normally without AGPS data.
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MaFt
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Joined: Aug 31, 2005
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PostPosted: Tue Sep 10, 2013 9:32 am    Post subject: Reply with quote

M8TJT wrote:
MaFt's example of the router is anecdotal, and should not be taken as gospel. That's the first time I've heard of a phone using your router (or anyone else's router) as a location data source. Surely the router does not transmit its location? If so, from where does it get it? Or how does that work? Or does the phone (etc) think it knows where the router is by comparing the GPS or cell location with the signal strength of the router and remember the location?


When Google did Streetview they grabbed every MAC address of every open WiFi hotspot / router and matched that to a geolocation. If using Google's services then this is how 'wifi location' works. If that router is moved then the service doesn't know this but 'matches' you to where it has a record of it.

Skyhook were one of the pioneers in this (info here on how it works: http://www.skyhookwireless.com/howitworks/ ) which Apple used to use but now have their own system: http://gps.about.com/od/glossary/g/wifi_position.htm

However, it is NOT GPS positioning. BUT it can be used to speed up the GPS fix. As M8TJT says, if you have moved a LONG way between GPS fixes then it can take a long time to pick up the satellites again. So the device will get a rough location based on cell signal and any local wifi signals. It can then use this rough position to give a fairly accurate location while the full GPS signal is being found.

AGPS (Assisted GPS) is a separate technology. AGPS involves having a local database on the device's GPS chips or as a file on the device that stores the ephemeris details - i.e. the position of all the GPS satellites for the next few days. So if the device knows what time it is it knows roughly where it needs to 'look' for the satellites in order to get a quicker fix. TomTom Home can download one of these files when you connect so if you move a long way between fixes it can use its latest data to speed up the fix.

MaFt
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M8TJT
The Other Tired Old Man
The Other Tired Old Man


Joined: Apr 04, 2006
Posts: 10118
Location: Bexhill, South Sussex, UK

PostPosted: Tue Sep 10, 2013 9:42 am    Post subject: Reply with quote

MaFt wrote:
When Google did Streetview they grabbed every MAC address of every open WiFi hotspot / router and matched that to a geolocation. If using Google's services then this is how 'wifi location' works.
Ah yes, thanks for the reminder. I remember the furore about them gathering other private data as well.
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