[time-nuts] Thunderbolt precise positions needed

[Mark Sims]

A couple weeks ago (check the archives around 26 July 2009)  I posted some plots of the PPS/OSC error estimates when the receiver's position was intentionally offset from its true position.  The results were rather dramatic...  And the quality of the data scaled with the position error.  It certainly looks like a the more precise your position is known,  the better will be your performance...  it certainly cannot hurt.  And for some strange reason we tend to be a bit obsessive nut cases about such things...

There are several faults in the standard Tbolt survey mechanism.  There are +/- 20 foot changes in the averages of fixes over a 1 hour time interval (a typical self-survey time).   Depending upon the luck of the draw,  you never know if you will get a good survey position.

An ideal survey should occur over 24 hour intervals.  This will mitigate both environmental (atmospheric) and orbital (multipath) effects.  Just specifying a 86,400 point survey has problems.  There will be times when the PDOP is too high for the survey fixes to be usable,  so  it usually takes several more hours to complete.  This will introduce biases into the results.

Also,  simple averages are far from ideal even with perfect 24 hour survey intervals.  I am using weighted medians over 1 minute, 1 hour,  and 24 hour intervals.  I am routinely seeing results under 1 foot lat/lon,  2 foot altitude.  A simple average of the fixes over 24 hours is typically three to five times that.  

The quality of the results match the quality of the antenna.  A geodetic/survey grade antenna gives results to within a few inches.  A conical timing antenna within 8 inches.  A cheap patch antenna around a foot.


----------------------------------------


_________________________________________________________________
Get back to school stuff for them and cashback for you.
http://www.bing.com/cashback?form=MSHYCB&publ=WLHMTAG&crea=TEXT_MSHYCB_BackToSchool_Cashback_BTSCashback_1x1