[LG Optimus 2x] echo_21:
No, battery replacement does not necessarily mean GPS cold start.

citation from http://en.wikipedia.org/wiki/Time_to_first_fix: Warm or Normal (start): The receiver has estimates of the current time within 20 seconds, the current position within 100 kilometers, and its velocity within 25 m/s, and it has valid almanac data.

Sure, when you just plug out the battery our P990 loses the current time. But after restart it uses to get the current time either via wifi or from the mobile network. Exact knowledge of the current time is one important step for GPS fix.Only if getting the current time fails a GPS cold start is required.

Let's move on to the other GPS warm start requirements: Does the P990 knows its coarse position after you plugged in the battery? Yes it does. The data has been stored somewhere on the \data partition before you plugged out the battery. The same should be true for the GPS almanac data. I guess that you don't drive fast or you don't drive at all if you plugin the battery and boot your P990, right? Thus the current speed of the P990 is less than 25m/s.

Summary: the first GPS fix after one removes the battery, re-plugs it in, and reboots the P990 will normally be a GPS warm start.

A GPS cold start will happen if e.g. (i) the P900's clock goes wrong, (ii) the current position estimated data is incorrect, (iii) the current speed is more than 25m/s, or if (iv) the almanac data is missing, corrupt or outdated.

(ii) and (iv) are true after one wipes the /data partition (e.g. during ROM update). (ii) is also true if you travel more than 100km with GPS switched off before you switch it on again.

(iii) is true if you switch on GPS while you are driving. Therefore it is a good idea to start GPS before you drive.

As you read in the article a GPS cold start can last up to 15 minutes. GPS cold start means that the GPS receiver downloads a digital data stream directly from the satellites. The data stream contains the current time. The receiver synchronizes its clock to the current UTC satellite time. The data stream also contains the required GPS alcmanac. It takes such a long time as the satellite transponder bandwidth is very low. The signals are very noise and can only carry a few 100 bits per second (remember the 300 Baud telephone modems in the 1970s and 80s ? ).

No one likes to wait 15 minutes for a first fix, right? Fortunately there is aGPS. The "a" means assisted GPS. With aGPS the time and position estimated as well as the almanac are downloaded from the internet. This should only last seconds.

Knowing all this uncovers many many possible reasons for long time to first fix. Some may seem hypothetical, but who really knows why the P990 fixes GPS slow? Let me list some scenarios:

1. By a software error every P990 GPS fix is a fix that requires a GPS cold start whereas all conditions for a GPS warm start are met. aGPS might work afterwards and you get a delayed GPS fix (e.g. 30 seconds instead of 7). Or aGPS doesnt't work and you will get a GPS fix after 15 minutes if you are patient enough.

2. P990 wants to make a GPS warm start but aGPS works incorrecty and e.g. spoils the GPS almanac data

3. aGPS sometimes works and sometimes it doesn't (server busy or offline or simply no internet connection)

4. By a software bug the P990 shows the correct local time but has an incorrect UTC time. This could be the in case of wrong time zones.

Telling all this GPS stuff does not mean any contradiction to your other statements. I just want to help the community to understand.