[time-nuts] DGPS at home

Hal Murray hmurray at megapathdsl.net
Sat Nov 26 17:35:38 EST 2011


> Maybe you can figure out for use how long one must average the data to get
> down to a given position accuracy.   The fact that you have a poor location
> is good.  You are generating real-world numbers.

I'll be glad to provide lots of crappy data if anybody wants to play with it.

----------

The refclock (nmea, PPS, TBolt, ...) support in ntpd has code to discard 
outliers on a clump of timestamps.  I think something like that would be very 
helpful when processing position data.

The code is pretty simple in one dimension: sort, compute average, compare 
distance to left and right ends, discard one, adjust average...  After the 
sort, the processing time is linear in the number of samples to be discarded.

I haven't figured out how to do something like that in 2 dimensions: there is 
no left or right end.

The basic idea you want to implement is to start with a large circle centered 
on the center of mass and shrink that circle until it hits a point.  That's 
the point you want to discard.

Pure brute force would compute the center of mass and then scan all the data 
points computing the distance...  That's an N-squared process which might 
take too long with a large clump of data.  For offline research like this, it 
might be OK.

There is a slightly better approach that I'll call semi-brute force.  The 
idea would be to make two lists: one for NS and one for EW, sort them, then 
use the longest end as a trial point.  Then you scan in from the 4 ends.  The 
semi- part is that you can stop when you get to the trial / sqrt(2).  At 
first glance, discarding isn't cheap since you have to scan the other 
list/array.  Actually, you don't have to scan the other list.  Just mark that 
slot as dead.  In either case, you can fixup the center location rather than 
recomputing it.  If you notice a dead slot on the end of a list you can 
delete it.

[I'm pretty sure that will get the right answer.  I'll try again if that 
description isn't clear.]


-- 
These are my opinions, not necessarily my employer's.  I hate spam.







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