[time-nuts] OT - GPS and North

Brooke Clarke brooke at pacific.net
Mon Nov 23 00:44:40 UTC 2009


Hi Bruce:

Do you know where I might find connection information on the MD-1 star tracker 
used in the B-52?  See:
http://www.prc68.com/I/MD1.shtml

The system was good to 1 arc minute (including temperature, pressure, etc.) and 
I've heard that the sensor was far better than that (into the arc second range).

Have Fun,

Brooke Clarke
http://www.prc68.com

Bruce Griffiths wrote:
> 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
>>>
>>>
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>>
>>
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>
>
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