Re: Elevation of observations

Alan Pickup (alan@wingar.demon.co.uk)
Thu, 1 Jan 1998 18:48:44 +0000

Alphonse POUPLIER  writes
>
>Do they also take into acount the atmospheric refraction?
>

This is really a question for those who undertake the analyses but I
would imagine that the answer is "yes". Refraction is clearly an
important factor if the position is reported as an altitude and azimuth
since it acts directly on the altitude. I suspect, however, that the
majority of experienced observers report positions in celestial
coordinates (RA and Dec) by reference to the star background. Light rays
from the stars and the satellite are refracted by the same amount, but
(I think) refraction effectively elevates the observing location above
ground level, to the point the unrefracted rays would have reached. I
note also that refraction is an uncertain quantity, depending on
variable atmospheric conditions (temperature, pressure) and the height
of the observer above sea level.

>It had been given by my GPS.
>But I agree that such an accuracy is not needed at all and you'll see
>that I rounded it to the nearest minute.
>
I did not argue, and I do not agree, that such accuracy is not needed.
You may be happy with an (in)accuracy of 0.5 arcminute (~1 km) in your
observing coordinates, but this translates directly into the same error
in the implied position of the satellite. Why devalue your observations
in this manner?

However, I fully agree that the "ultimate" accuracy in the observing
location coordinates is not needed when it comes to *predictions*.

Alan
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