[time-nuts] GPS position survey

[Mark Sims]

The 48-hour precision survey in Lady Heather uses a statistical weighted median filter to arrive at its final location instead of a simple average of fixes.   It processes data of one minute, hour, and overlapping 24 hour intervals to calculate the final position.   It can produce a location that is 3-10 times better than simple averaging of the fixes.
A further complication is storing the precise location into the receiver.  The Tbolt firmware only lets one write the location using single precision floating point numbers (24 bit mantissa).  This is well below the desired resolution of the position.  Lady Heather gets around this by limitation by doing a royal kludge.  Once the precise position has been calculated,  it issues single point survey commands to the receiver until one just happens to lie within one foot of the calculated position.