No subject

Mon Apr 16 15:45:47 UTC 2012

they came from you can back track and work out where you are.<br>
<br>
- Andrew -<br>
<br>
<br>
----- Original Message ----- From: &quot;Stephen H. Smith&quot; &lt;<a href=
=3D"mailto:wa8lmf2 at aol.com" target=3D"_blank">wa8lmf2 at aol.com</a>&gt;<br>
To: &quot;TAPR APRS Mailing List&quot; &lt;<a href=3D"mailto:aprssig at tapr.o=
rg" target=3D"_blank">aprssig at tapr.org</a>&gt;<br>
Sent: Sunday, May 06, 2012 2:28 PM<br>
Subject: Re: [aprssig] How gps works<br>
<br>
<br>
<blockquote class=3D"gmail_quote" style=3D"border-left:1px solid rgb(204,20=
4,204);margin:0pt 0pt 0pt 0.8ex;padding-left:1ex">
On 5/5/2012 11:21 PM, Andrew Rich wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"border-left:1px solid rgb(204,20=
4,204);margin:0pt 0pt 0pt 0.8ex;padding-left:1ex">
Hello<br>
<br>
I am learning how gps works<br>
<br>
I understand that gps satellites contain accurate clocks<br>
<br>
How does the receiver know the distance to the bird ?<br>
</blockquote>
<br>
Basically, the very simplified concept:<br>
<br>
1) All the super-accurate atomic clocks in all the satellites &quot;tick&qu=
ot; in unison. =A0 =A0These &quot;ticks&quot; are &quot;broadcast on spread=
-spectrum signals at 1575 MHz (for the civilian GPS service). =A0The signal=
s for all satellites are &quot;stacked&quot; on top of each other on the sa=
me center frequency. =A0Each satellite uses a different spreading code, whi=
ch allows it&#39;s signal to be separated from others by the receiver on th=
e ground.<br>

2) The &quot;ticks&quot; for all the satellites travel outward at the speed=
 of light (300,000,000 meters/sec --or-- about 300 meters/uSecond --or-- =
=A0about 0.3 meters/nanosecond. =A0 I.e. about 1 foot / nanosecond. 
 
3) Each satellite is also constantly broadcasting it it&#39;s OWN location.=
 (The US Air Force measures the precise location of each satellite several =
times a day with ground-based radar, and uploads updated orbital data into =
each satellite several times a day, which the satellites broadcast continuo=
usly until the next update.)

4) The receiver compares the DIFFERENCE in time-of-arrival of &quot;ticks&q=
uot; from several satellites. =A0 The receiver, based on knowing where each=
 satellite was at the time of the tick, computes the one location in 3D spa=
ce where this particular combination of delays, due to differing-distances-=
traveled, =A0 =A0must be. =A0 =A0 Differences of 5 or 10 nano seconds in ti=
me of arrival =A0(corresponding to delta distance of =A01.5-3 meters) are q=
uite easily measured with basic logic circuits.

5) =A0Comparing the DIFFERENCE is easy; having a reference clock in a cheap=
 device, not endowed with a rubidium time standard, that can provide the AB=
SOLUTE time to compare to, is not. =A0 Instead GPS receivers &quot;cheat&qu=
ot; by using the ticks from one satellite as the triggering absolute time r=
eference, and then compare three or more others against the first one.

Compared to a reference time, the delay for a SINGLE satellite says you mus=
t be somewhere on the surface of a sphere of approx 10,000 miles radius fro=
m that satellite. =A0 =A0(GPS satellites orbit about 10,000 miles above the=
 earth&#39;s surface.)

Measuring the delay for TWO satellites will define two intersecting spheres=
. You must now be somewhere on the circle where the two spheres intersect.<=
br>
 
Measuring the delay for THREE satellites will define three intersecting sph=
eres where only two unique points will match the time delays observed for a=
ll three. One of these will be an obviously impossible solution because it =
is located deep inside the earth, which leaves a single point that is your =
location.

<blockquote class=3D"gmail_quote" style=3D"border-left:1px solid rgb(204,20=
4,204);margin:0pt 0pt 0pt 0.8ex;padding-left:1ex">
 
Does it look at the pattern coming from the gps sat ? 
 
Can it work out when comparing the code from others sats to know the differ=
ence in time between the different birds, much like trying to line up a set=
 of rulers ? 
 
What does sending empheris data do to help ? Does that help the receiver pi=
cture the constellation ? 
</blockquote>
 
More or less. =A0This is how the receiver &quot;knows&quot; where each sate=
llite is at a given instant. 
 
 
 
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