GPS and you

[matthew]

People are confusing things by calling every position locating system GPS.

You don't actually need satellites to find your location electronically these days.

There is the US 'GPS' satelite system. It's basically for the use of the US defence forces and it's paid for and operated out their defence spending. When originally implemented the only way to use the system was via large, expensive and strictly controlled by the US DOD receivers.
The Russians also launched a similar system GLONASS, although they let the maintenance laps in turmoil following the fall of communism and while they are now rebuilding it again, I don't know if it's actually operational enough now to be useful. Countries like China and India have contributed to the rebuilding of the Russian system so they have use of a non US controlled system. China and India have asperations of launching their own systems too as not to be dependant on any foreign power for 'location services'.

The Europeans are building a system too called 'Galileo' , but the expense of the system is slowing them down some what and the system is no where near operational.

So there could be potentially 3 satellite based location systems in the near future and in the longer term even more.

But your mobile phone has other options - the mobile phone network itself. Your phone is in constant contact with the network, evaluating the signal from multiple towers and selecting the best one and 'handing over'. Each of those towers has a unique ID. Their locations are also known. Your distance from them is also known (The phone can calculate the distance by measuring protocol delays in the GSM/3G communications protocol).

Enter a program like Google Maps. On a phone with out GPS or with the GPS disabled, the Google Apps application can access the phones 'tower and signal strength' table. It then uses the data connection to send all the towers (base stations) that your phone can see to Google. The 'googleplex' looks up it's monster database of mobile phone base stations and sends back to the application on your phone, the physical co-ordinates of all those mobile base stations/towers.

Knowing the location of all the base stations and their relative signal strengths or distance to the base station you can make a pretty good 'guess' as to where the phone handset actually is. No GPS satellite signal needed.


A-GPS is just a merge of these methods. GPS can be 'difficult' to start up - the receiver has to listen on all channels, trying to find satellites and then wait to get a complete set of data from at least 3 different satellites. Then calculate the position. If you are moving the calculations might never complete.

A-GPS uses the trick google-maps mobile uses when there is no GPS signal. The GPS application grabs the base station list from the phone and sends it via a Internet to a 'positioning server' some where. It looks up it's location database, makes a pretty good guess at where you are and also what GPS satellites should be in your 'field of view', it may also send 'ephemeris' data for your location as well.

The GPS receiver is then preloaded with this as starting information. It now knows which satellites to listen for, so the radio channel scan can be skipped, it knows the specifics of your location (ephemeris) and it has starting co-ordinates to load into the engine that calculates the position from the satellite signals.
All this reduces the 'time to fix' from several minutes to several seconds.


You could also of course just look out the window

 

[CyberTron88]

If not wrong, the iPhone 3G and onwards uses 3 ways to give gps location. One is through know wifi hotspot location, another thru cell network, finally through satellite. The accuracy is also in that order as well. Depending on the developer requirement when they write their location aware apps, the iphone might not use satellite location to determine the location if accuracy was not important. That's what I think developers were told when they attended one of the developers conference held at SMU (2 years back I think).

If ya use the google map on iPhone, if u try to find your location, you will find the circle goes from big to the dot. The big one is use wifi or cell, and it gets smaller, until the dot where they got satellite lock on.

 

[Limsgp]

It is necessary to define what is real.
In this case means read directly off the satellites above you in open ground, without first going through any assist. And relying solely on your iPhone exposed to the sky, pick up the GPS signal

Best way to check is to remove the sim card. After removing the sim card, do not put it back yet.

If the iPhone can give position within 10m accuracy, is it considered "real" then?

I feel that removing SIM card is not a good test, because Google Maps requires data to work. So, after blocking data, and it doesn't work, how to tell if it's google map not working or GPS not working? Unless there's a 3rd party App that uses offline maps then it's possible to test.

btw, why I mentioned 10m, is bcoz I believe that's the accuracy that a real satellite GPS gives. I have a HD2, and everytime Google Map runs, it'll show "no. of Satellites active", so definitely it's using satellite. And by far the best accuracy is within 10m.

If the so-called AGPS w/o satellite can gives the same accuracy, then I guess real satellite signal is not required. In fact I would prefer this type of GPS probably, bcoz it doesn't require LOS to the sky like the satlellite based one.

But from experience, the iPhone only give Good accuracy within 10m when there's LOS to the sky or in a car. So I'll suspect that it does lock on to satellites.

If not wrong, the iPhone 3G and onwards uses 3 ways to give gps location. One is through know wifi hotspot location, another thru cell network, finally through satellite.

If ya use the google map on iPhone, if u try to find your location, you will find the circle goes from big to the dot. The big one is use wifi or cell, and it gets smaller, until the dot where they got satellite lock on.

that what I believes too.. it just A-GPS to give a indicate position Fast, then try to lock on to satellites.

btw, the positioning is accurate only when the indicator is a pulsating blue circle. If it is a static blue circle, probably it has not lock-on to satellites yet. that's from observation, fyi.

 

[ricohflex]

Real as in reading off from satellites directly from the phone without any assist.
It's not about the accuracy of placement.
Do let me know if you hear of anyone who has taken out his sim card and can still navigate via GPS signal using his iPhone.

Nokia has given away the maps and navigation free. That is the software part. They can be loaded into the phone.
This is Nokia's big advantage over others that require Google maps to be constantly downloaded updated via sim card data link.

Let's hope Nokia knows how to leverage on this big advantage.
They should put a high quality GPS chipset like SIRF III into every Nokia phone that is priced above $X. This is the hardware part.

Nokia does not have a Steven Jobs who knows how to market these triumphs and turn it into sales of millions of iPhones.

 

[CyberTron88]

Ok...I'm here at my campus apartment in US, where I practically get no phone signal (both at&t or t-mobile, no signal, no iPhone problem... every people here have this problem).... and inside the room....so cannot get gps signal.. but with wifi signal...

When you do google map and try to locate, the blue dot will give u a rough location (which is through my wifi), and then u will see a circle around the dot, which is telling ya the accuracy (or diameter where it think you might be).

Only when you get satellite triangulation, the circle around the blue dot will disappear and it will start blinking.

The iPhone (from 3G) onwards, does have real satellite gps tracking, as this wat what is told in the developers conference.

 

[Limsgp]

Actually, A-GPS is better than GPS only (At least for phones). A-GPS systems operate just like GPS where they read signal off satellites, but they do it faster.

Maybe dedicated GPS units can response quickly as compared to a phone, but for a phone with not so high end GPS chips, the A-GPS implementation actually improves performance compared with GPS only.

 

[matthew]

Actually, A-GPS is better than GPS only (At least for phones). A-GPS systems operate just like GPS where they read signal off satellites, but they do it faster.

Maybe dedicated GPS units can response quickly as compared to a phone, but for a phone with not so high end GPS chips, the A-GPS implementation actually improves performance compared with GPS only.

All 'A' GPS is is 'Assisted GPS'. By making use of the phone data network and a 'location server' the embedded GPS software can get 'starting data' which speeds up (considerably) the initialization of the satellite tracking part.

A phone (with a data plan) and A-GPS should be able to beat the 'initial time to fix' of just about any 'stand alone' GPS unit, as the standalone GPS unit has to initialize it radio and position algorithm from scratch, the A-GPS phone can 'cheat' by getting approximate starting position from the network, a standalone GPS receiver has to work that out for itself.