Re: how visible is a pass?

Chris Peat (Chris.Peat@dlr.de)
Fri, 2 Oct 1998 18:24:00 +0200

Walter Nissen wrote...

>> Thanks for the note. Why does www2.gsoc.dlr.de only list the 9:42 pm ED=
>> T pass
>> while www.hq.nasa.gov/osf/mir/a.html only lists the 8:20 pm EDT pass for
>> tonight.
>> I would think that both passes should be visible naked eye and both sit=
>> es
>> should list
>> both passes. Just curious. thanks.
>
>The problem of screening passes for visibility is a central problem.
>Generating a correct rating is very, very difficult.  Even with such a
>rating, a cutoff must be imposed.  Implemented programs are far from
>ideal, often very far from ideal.  Most do not actually rate, but merely
>impose various criteria.  Unless these criteria are very elaborate, they
>will be quite imperfect.


The GSOC site selects only those Mir passes whcih reach 10 deg. elevation or
higher, and also require that the sun be at least 6 deg. below the horizon.
These criteria seem perfectly reasonable for the average observer, and you
must remember that we target mainly the interested general public and not
experienced observers. For other satellites (on the All visible satellites
pages) we have a 20 deg. elevation cutoff, and also filter on the selected
magnitude.

Walter is right to say that selection of screening criteria is very
difficult, and we plan to circumvent this problem by allowing registered
users to choose their own selection criteria. This will be possible after a
major upgrade to our site which we are planning to implement before the end
of this year.

>My ancient and reliable version of QuickSat generates:
>
>  41.374  81.864  840.    Nissen, OH            1950 17.5  2 F F F F F
>***  1998 Sept 29  *** Times are UT ***   0 2 1031
>16609 Mir Complex                       .1  -3
>
> H  M  S  TIM AL AZI C   U  MAG   REVS  HGT SHD  RNG  EW PHS  R A   DEC
> 0 26  8   .1 42 331 C  62  -.1   40.8  371 224  541 2.2 106 1410  68.7
> 2  0  1   .1  8 307    71  4.9   41.8  370  28 1507  .7 140 1341  32.6
>
>At an unfavorable phase angle, the second pass of Mir was most likely even
>more difficult than suggested by the predicted altitude and magnitude.
>
>Because the GSOC site generated an incorrect time, it must have been given
>incorrect data, probably observer location data; or have had a bad or old
>elset for Mir; or (not strongly indicated by this example) use very rough
>criteria.


I entered the given coordinates and got a pass predicted by our site for
0:26:18 (20:26:18 EDT) at 41 deg. elevation, so I presume the false
prediction came from false coordinates. I doubt the Mir elset was in error,
because we download the latest every day from OIG, and have had no other
reports of false Mir predictions.

Chris

Chris Peat, Anite Systems
Currently working at the German Space Operations Centre near Munich