'Official' TiPS elsets are very accurate

Tue, 12 Aug 97 12:19:17 PDT

A week or so ago there was a brief mention of the TiPS elsets as published
by the Naval Research Lab (NRL) at their website:


The NRL says that the accuracy of their elsets has improved greatly, and I
have seen no evidence to the contrary.

A claim was made here that the NRL elsets shouldn't be considered reliable.
Perhaps that was true in the past, but I offer another opinion based on
my recent observations, bolstered by some statistics published on the
following page also hosted by the NRL:


An excerpt from that page reads:

   "With increased SLR tracking during April '97 and the
    addition of tracking from the ALTAIR radar on Kwajalein
    Atoll, we were able to refine our drag parameters to the
    point where we can now issue predictions of the orbital
    motion which are accurate to a few tens of milliseconds
    over a week.  This is equivalent to being able to predict
    the position of the TiPS center-of-mass to within E50
    meters.  With this accuracy, the NASA MOBLAS lasers in
    Monument Peak and Greenbelt have been able to use a
    "blind" tracking mode for the TiPS end-bodies from
    4/25/97 through 5/3/97."

I've been using the NRL elsets for quite some time, and have found them
very accurate.  I track the tether at 40 power, with a 1 deg. field of
view, and the predictions have been within a second of USNO time.  As a
test, I used a 12 day old NRL elset last night and observed two passes,
right on time and track.  Of course, a much fresher elset was available
but I decided to see if the NRL elset could withstand aging well.  In
this case there was no problem, but if you have an elset with a negative
drag term, as happens from time to time, it might be prudent to check their
website for the most recent one.

One minor problem is that the elsets as published lack checksums, but
the software I use ignores checksums anyway.  If your prediction
software balks, just try adding any number to the end of each line to
pad them to the right length, or use one of the elset utilities available
to do the same, though I've found adding any arbitrary number to each
line works for me.

While on the subject of TiPS, I thought I should mention that a new chart
was recently added to the TiPS website:


It's an interesting graph of viewing opportunity windows for the US, Europe,
Australia, and South America.

 Craig Cholar    3432P@VM1.CC.NPS.NAVY.MIL
 Marina, California
 36 41 10.3N,  121 48 17.9W    (36.6862, -121.8050)      UTC -7