[AZ-Observing] Re: Another Shuttle and ISS pass tonight (06/20)

Neville,

I didn't see the pass, so when Tom "Did the Math" I got to thinking as to
why the discrepancy. The first thing I though of was the age of the keps. I
downloaded afresh set right before I did my calculations. The elements I
used are below.  For those unfamiliar with the format of TLE's, I put the
Epoch variable in quotes. This shows the elements for the ISS were valid at
0258 20 July UT, Which would've been pretty old by the time of the pass.
Given that the Shuttle would be maneuvering a bit to gain separation from
the ISS and phase it's orbit for reentry. I'm not surprised that the
discrepancy was as large as it turned out to be. The ISS elements would more
than likely been pretty good. But STS-117 would have been way out of whack
with reality. 

ISS (ZARYA)             
1 25544U 98067A   "07171.12363426"  .00005766  00000-0  36804-4 0  4289
2 25544  51.6360  88.7334 0007500 341.6186 231.2152 15.78168180491237

STS 117                 
1 31600U 07024A   "07171.04015913"  .00011996  00000-0  72294-4 0   234
2 31600  51.6352  89.1604 0006674 341.5415 116.7271 15.78174410  1765


What I am surprised at is how old they were when I downloaded them. SkyMap
has a download feature that automatically downloads the elements from
Celestrack (you can set the URL for whatever other service you use). I'll
compare the elements SkyMap downloads vs. downloading directly from
Celestrack. Not sure where H-A gets it's elements (my guess would be the
DLR, German Space Agency) but I know they update every three hours, at least
for iridium flares. Maybe the timing for other satellites is less frequent. 

FWIW, I did also check the numbers I came up with using the same method you
did (RT=D) and came up with, using a 2 second lag as predicted in H-A,
numbers that corresponded to my Calculations from SkyMap.

BTW, I also noticed that there was a Progress supply vehicle leading the
parade by a good bit. Did anyone see that?

At least I have an explanation :)

Clear Skies
Rick Tejera
President, Editor SACnews
Saguaro Astronomy Club
Phoenix, Arizona
www.saguaroastro.org
saguaroastro@xxxxxxx 
-----Original Message-----
From: az-observing-bounce@xxxxxxxxxxxxx
[mailto:az-observing-bounce@xxxxxxxxxxxxx] On Behalf Of Neville Cole
Sent: Thursday, June 21, 2007 13:34
To: az-observing@xxxxxxxxxxxxx
Subject: [AZ-Observing] Re: Another Shuttle and ISS pass tonight (06/20)

Rick,

Wikipedia says the ISS' average speed is 17,240 mph, and I counted approx 14

seconds for the ISS to catch up to the orbiter.

If you plug those numbers in the "distance = velocity x time" formula, like 
this:

17240 m/h
----------------      X   14 s
3600 s/h

You get 67 miles, and say +/- 10 miles.

True, I was counting, and not using a stopwatch, but we're still out by an 
order of magnitude on this one.

Do you know if SkyMap used predicted locations, or did it obtain get actual 
locations some how?

Heavens Above seemed to predict a separation of only one second, which would

calculate out to around 5 miles.  Maybe SkyMap used similar TLE's as Heavens

Above?

Neville


----Original Message Follows----
From: "Rick Tejera" <saguaroastro@xxxxxxx>
Reply-To: az-observing@xxxxxxxxxxxxx
To: <az-observing@xxxxxxxxxxxxx>
Subject: [AZ-Observing] Re: Another Shuttle and ISS pass tonight (06/20)
Date: Wed, 20 Jun 2007 22:18:12 -0700

Greg,

At 2200 they were 5.52 miles apart. OK, how'd I come up with that, you ask
(and I know someone will).

First I looked up their positions in SkyMap at 2200.

ISS: S 50 37 04, W 7 23 02
STS-117: S50 38 07, W 7 15 41

I then ran these coordinates in a utility I have call Forward/Inverse to
come up with the answer.

Now that being said this is a topographic solution, it would not take into
account the greater separation due to the higher altitude. I don't imagine
it would add up to much, but if anyone would like to do the trig, feel free.
:)


Clear Skies
Rick Tejera
President, Editor SACnews
Saguaro Astronomy Club
Phoenix, Arizona
www.saguaroastro.org
saguaroastro@xxxxxxx

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