[time-nuts] Lady Heather survey accuracy

[John Nelson]

I hesitate to ask this question on here since it's perhaps more one for
position-nuts rather than time-nuts. My excuse is that it involves a
Thunderbolt ;-) 

I've been a happy user of a Thunderbolt for a while as an accurate time and
frequency source but until recently I hadn't got round to mounting it in a
proper enclosure with its PSU and finding a permanent home for the result in
the office. Having done so I asked Lady Heather to carry out a 48h
'precision survey' and got a result which although close to that done when I
first acquired the unit was about 10m different. I then carried out two
further surveys which gave similar disparate results.  

LH reports latitude and longitude results to eight decimal places, which in
principle suggests centimetre accuracy. I don't know enough about the finer
points of GPS to know whether this is actually achievable over a 48h period
but the results I'm seeing suggest that it's not.  

So the question amounts to this -- how reliable is the position produced by
LH after a 48h 'precision survey' and what level of accuracy is achievable
in practice? I'm guessing it's more like four places of decimals than eight.


John