[time-nuts] OT - GPS and North

[Bruce Griffiths]

When a star tracker is used as a stellar compass in effect takes 
simultaneous fixes on several stars and the better ones are capable of 
an rms error of a few arc seconds, largely limited by atmospheric 
instability.
These are usually used for determining space vehicle attitude, in which 
case the instability due to seeing is much smaller than when immersed in 
the atmosphere.
They have been used as relatively inexpensive position encoders for 
pointing a telescope to within a few arc seconds.
Pattern recognition techniques are used to identify stars in a 
relatively wide field (a few degrees) slightly defocused image (improves 
centroiding accuracy).

Star tracker for Clementine mission:
http://www.osti.gov/bridge/servlets/purl/86977-ULkJcR/webviewable/86977.pdf

A more accurate version:
http://www.newworldt.com/download/DTU/microASC%20Summary.pdf

telescope pointing application:
http://www.ifa.hawaii.edu/faculty/pickles/AJP/spie3351.07.pdf
http://www.ifa.hawaii.edu/users/pickles/AJP/AMOS2003_v4.pdf

Measuring the position (altitude + azimuth) of one star at a time using 
a theodolite is not the most efficient method of determining the 
direction of the meridian.
Its far better to measure the position of several stars at once as in 
the stellar compass, however a longer focal length camera lens than 
usually used in a star tracker is desrable for increased accuracy.

Bruce

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
>>
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>>
>>
>>      
>
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>
>