GPS and policeman question

**imdying** · 11th April 2006, 19:38

Originally Posted by sAsLEX

A bigger headache is signal drop out completly, they have done urban location using cell phone tech but obviously that wont work in hostile terrirtory, best bet is an Inertial NAv unit that updates when GPS is in range

Apparently the European system Galileo will work indoors etc, as well as being more accurate... bet the americans love that

**Patrick** · 11th April 2006, 19:52

Originally Posted by GR81

how would you know? arent you a school janitor? *cough* hahaha

And seller of flowers...but sshhhhh.... thats secret...

**sAsLEX** · 11th April 2006, 20:20

Originally Posted by imdying

Apparently the European system Galileo will work indoors etc, as well as being more accurate... bet the americans love that

How got any info on it? They are geosynchrounous aint they those ones?

edit, No sAs they are MEO orbiting constellation of 30 sats, 3 spare, designed for civilain use

**imdying** · 11th April 2006, 20:34

Heaps of white papers on the net about them, pretty sure they EU have a big site on it somewhere.

**lb99** · 11th April 2006, 21:12

Let me foigure this out....
so your GPS snds out a signal saying "where am I?", and the satellites figure it out and reply "you are (Here)", ... is that right?

If so then isn't it possible that the sattelite is recording your position each time you query it? thus tracking your movements!!!

Is this possible?

Should I start final preperations?

**imdying** · 11th April 2006, 21:25

Nope, nothing like that son

**ajturbo** · 11th April 2006, 21:46

Originally Posted by lb99

Let me foigure this out....
so your GPS snds out a signal saying "where am I?", and the satellites figure it out and reply "you are (Here)", ... is that right?

If so then isn't it possible that the sattelite is recording your position each time you query it? thus tracking your movements!!!

Is this possible?

Should I start final preperations?

you think tooo much... i hope

**sAsLEX** · 11th April 2006, 22:59

Originally Posted by lb99

Let me foigure this out....
so your GPS snds out a signal saying "where am I?", and the satellites figure it out and reply "you are (Here)", ... is that right?
Should I start final preperations?

The sats send out an L1 and a L2 sig which are encoded radio signals, in this it sends GPS time, the exact location the satallite is in , the vauge area the satilites will be in for the next week or so and some other stuff. It has a rather complex code that is sent in the signal to tell all that stuff and sync your reciever.

L1 => s(t)=SQRT(2*Pi)d(t)c(t)cos(wt + phi) + SQRT(2*Pq)d(t)p(t)sin(wt+phi)

Pi and Pq are respective carrier powers
d(t) is the 50 bps data modulation
c(t) and p(t) are a C/A and P pseudorandom code wavforms
w is the L1 carrier freq
Pq is 3dB less than Pi

L2=> s(t) SQRT(2*Pq) d(t)p(t)sin(wt+phi)

fun eh?
When you turn on your reciever it remebers the vauge info of the last sats it seen and searches for those that should be visible, its kinda neat that all the sats transmit on the same freq, and once recieving sat signals the reciever works out the delay from each one, and with some of the atmospheric data transmitted in the signal it works out the distance to each sat in view.

Once it has that it can calculate your position to 10 m accuracy or so.

**Lou Girardin** · 12th April 2006, 08:24

A guy in ChCh used GPS data to defend a speeding ticket. He succeeded.

**MisterD** · 12th April 2006, 08:47

Originally Posted by sAsLEX

The sats send out an L1 and a L2 sig which are encoded radio signals, in this it sends GPS time, the exact location the satallite is in , the vauge area the satilites will be in for the next week or so and some other stuff. It has a rather complex code that is sent in the signal to tell all that stuff and sync your reciever.

If you don't mind me adding some to that sAsLEX....

The signal the satellite sends is unique and and repeats over a pretty long time frame (think wallpaper with a really long repeat on the pattern). The receiver knows what the pattern is (ie has a wallpaper sample) and basically slides it up and down past the received signal to match it up (a process called "correlation") by this means the receiver can work out the time delay and therefore the distance to the satellite.

Distance from 3 Sats will give you 2D and 4 will give you 3D position...

**MisterD** · 12th April 2006, 08:51

Originally Posted by imdying

Apparently the European system Galileo will work indoors etc, as well as being more accurate... bet the americans love that

It'll work indoors becuase they're trying to bump the power of the transmitted signals up, as for more accurate that's not particularly relevant as the two systems are mandated to be compatible so you'll have more accuracy because receivers will be using both systems at once and you'll always have more satellites in view.

GPS will work indoors if you've got a good enough timing reference in your receiver and a chipset that'll make use of it. SiRF Star III is the current standard setter if you want to not get lost on the way back from the lav...

The important thing is, can you store "Points of interest" ie speed cameras?

**sAsLEX** · 12th April 2006, 13:41

Originally Posted by MisterD

If you don't mind me adding some to that sAsLEX....

The signal the satellite sends is unique and and repeats over a pretty long time frame (think wallpaper with a really long repeat on the pattern). The receiver knows what the pattern is (ie has a wallpaper sample) and basically slides it up and down past the received signal to match it up (a process called "correlation") by this means the receiver can work out the time delay and therefore the distance to the satellite.

Distance from 3 Sats will give you 2D and 4 will give you 3D position...

yeah the C/A code with a 1023 chip sequence and a period of 1ms and a chipping rate of 1.023MHz

the P code is similar and has a period of a week and is used for mainly military options