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<p class=MsoPlainText>Some thoughts about time tags, satellite time and
receiver time ... I can’t guarantee this is the right explanation, but it is
how I think of it.<o:p></o:p></p>
<p class=MsoPlainText style='margin-left:.5in'><o:p> </o:p></p>
<p class=MsoPlainText style='margin-left:.5in'>The time tag is more a label
than anything else. It indicates the time the signal ideally would have been
transmitted. In practice, things don’t work perfectly, so we have to make
adjustments.<o:p></o:p></p>
<p class=MsoPlainText style='margin-left:.5in'><o:p> </o:p></p>
<p class=MsoPlainText style='margin-left:.5in'>The satellite clock has errors,
but the errors are very predictable. Satellite errors can be extrapolated from
the ephemeris.<o:p></o:p></p>
<p class=MsoPlainText><o:p> </o:p></p>
<p class=MsoPlainText style='margin-left:.5in'>The receiver clock is the real
problem. It is a quartz crystal which keeps good time by human standards, but it
is woefully inadequate for positioning. Think of being a 1/4 mile off for every
microsecond lost and you'll see how important it is to have accurate clocks. A
GPS receiver compensates by tracking an extra satellite and solving for x,y,y,t
instead of just x,y,z.<o:p></o:p></p>
<p class=MsoPlainText style='margin-left:.5in'><o:p> </o:p></p>
<p class=MsoPlainText style='margin-left:.5in'>Including an extra satellite takes
care of most of the clock error. However, part of the clock error is drift. Drift
is like having a measuring tape which stretches; if you know how much it
stretches, you can compensate. The trick is to figure out how much the clock
has drifted and to "unstretch" the pseudo-range and phase
measurements.<o:p></o:p></p>
<p class=MsoPlainText><br>
<br>
<o:p></o:p></p>
<p class=MsoPlainText><o:p> </o:p></p>
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l0 level1 lfo1'><![if !supportLists]><span style='mso-list:Ignore'>-<span
style='font:7.0pt "Times New Roman"'> </span></span><![endif]>John<o:p></o:p></p>
<p class=MsoPlainText style='margin-left:.5in'><o:p> </o:p></p>
<p class=MsoPlainText><o:p> </o:p></p>
<p class=MsoPlainText><o:p> </o:p></p>
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