[time-nuts] OT - GPS and North

[Neville Michie]

I once went through the exercise of finding North from a station at  
home.
A gps fix would give me +- - 1metre northing and  + - 1 metre  
easting, another
fix 1000m away would be the same, a lousy base for an accurate azimuth.
Using an ephemeris for constants, a theodolite fix on a circumpolar star
taken at 6 hour intervals starts to get a value with the limits being  
the
usual limits of observing stars but less the refraction errors.
I recorded the data with respect to a Referred Object, ( a street  
light 4 km away)
Several times I repeated this exercise ( on other nights) and I was  
disssapointed by the precision.
Now my theodolite is a Wild T1, and the method does not lend itself  
to getting the maximum
precision from the instrument, but I was deeply impressed by the  
difficulty of getting
an accurate measure of true North.
cheers, Neville Michie

On 22/11/2009, at 1:13 PM, J. Forster wrote:

> Not so. I'm very familiar with laying in accurate North lines for gyro
> testing. To get anything close to accurate (1 arc second or better)  
> takes
> many hours of stellar observation with a Wild T-3 class instrument.
>
> -John
>
> ===============
>
>
>
>> Neville Michie wrote:
>>>
>>> When you think of time specifications from GPS, the GPS system is a
>>> poor way to find north.
>>> Even with a base line of 1000 metres you only have a fraction of a
>>> degree.
>>> The GPS system may be useful to get accurate time to simplify a star
>>> observation, from a known (GPS)
>>> position on this planet, but finding north is still a problem  
>>> because
>>> of the accuracy of a small
>>> number of observations from a star fix.
>>> Gyrocompasses take some time to get a measurement
>>> ( one hour) but even their estimate of North  cannot match the
>>> precision that the GPS system can get us with time.
>>> cheers, Neville Michie
>> If you are taking star shots a stellar compass can easily provide a
>> boresight pointing accuracy of a few arcsec.
>>
>> Bruce
>>
>>
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