Wednesday, October 19, 2016

GPS and Cell Phones

There seems to be some misunderstanding when it comes to GPS and cell phones.  I have found posts which say "GPS is GPS" and others that think "just ask your provider".

First ... remember this ...

In the frequency allocation filing for GPS signals, the L1 C/A power (that the commoner uses) is listed as 25.6 Watts (14 dbW).  The Antenna gain is listed at 13 dBi.  Thus, based on the frequency allocation filing, the power would be about 500 Watts (27 dBW).

Now, the free space path loss from 21000 km is about 182 dB.  Take the 500 Watts (27 dBW) and subtract the free space path loss (27 - 182) and you get  -155 dBW.  I remember reading that this is analogous to trying to read a book in the dark, but with one candle lit 100 miles away.  Bottom line ... GPS is a weak signal.

The receiver that accepts that GPS signal is influenced by software processing, cost of GPS chips, the noise that is generated in the front end receiver section of the chip, and the noise in the demodulation section, as well as the actual antenna that receives the signal (and it's size and cost).  

So, when testing and measuring a receiver, you spec things like capture performance (what signal level takes how many seconds), MDS (minimum discernible signal), and tracking performance ... and NONE of that is listed on OEM web sites.  Thus, we haven't a clue how good a phone is.  

If I were to GUESS, I'd say that the more you pay for the phone, the better the GPS performance will be.  The phone I used for my WAZE display is a LG Volt LS740, and I can tell you that it's GPS capability is worse then many other cells I've tried ... but it's good enough during dry and clear conditions.  Give it fog or rain, and the signal capture is very poor.

If you want the best GPS signal you can get with a cell, consider adding on the blu-tooth accessory ... like the XGPS150A ... it is a GPS receiver, dedicated, and provides a better GPS signal to your phone.  Or do some googling and find sites that say the iPhone has the best GPS receiver.  Bottom line ... it's a guess.

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