[amsat-bb] Keps accuracy

Ray Hoad ray.hoad at mypbmail.com
Fri Jan 26 04:00:42 UTC 2018

Thank you Dale for your comments. Good questions. I will try to answer them.

I think part of the confusion described may be the result of just the process of launching a new amateur CubeSat with other CubeSats.  CubeSats in the 1U format (4 x 4 inch) are launched in groups of typically 3 objects. Unavoidably, the placing in orbit of a new CubeSat is followed by the process of determining which object in a "flock" of CubeSats is the satellite in which you are interested. This is a usually relatively short (5 to 10 day) period of tracking a group of CubeSats until they separate enough to isolate the one of interest. A spectrum analyzer program is used to identify which object is our satellite. 

Also a set of "PRE-Launch" TLEs (that is calculation based on the EXPECTED orbit at launch) are usually produced by the launching entity.  The "PRE-Launch" TLEs are used until POST-Launch TLEs (for the group of objects that your satellite is in) are released from the Space-Track website. The POST-Launch TLEs released from Space-Track are determined by tracking sensors of the U.S. Air Force.  

Finally, your satellite and its associated TLEs are determined and we settle down to an single, accurate, reliable set of TLEs ... and all the other "best guesses" go away. (Although they may be still floating around on the internet.)  But, there are so many variables (did you launch on time, did you get released on time, has the group your CubeSat is in separated enough to identify your satellite, etc.) that the process that can be both confusing and annoying at the same time. Remember, we did not pay for this ride and it took a lot of work to get our University sponsor and us on that rocket. In spite of all the hassle, it is still very much worth the effort.

The source for AMSAT-NA TLEs (or KEPS) is from the Space-Track website (www.space-track.org). This is the most reliable and the most accurate primary data (not filtered through another website) available on a continuing basis. This data is used to keep track of the cloud of objects that circle our Earth, so accuracy is a major concern for Space-Track. Anyone who has watched the International Space Station (ISS) suddenly appear sunlit in a dark sky (right on time and in the predicted place) knows how accurate these TLEs are.

TLE data is updated on a continual basis and that depends on when the object is available to the Air Force tracking sensors. Most objects cannot change their orbits and have orbits that do not change radically over a seven day period. Thus weekly updates are completely adequate with one exception. The ISS is an example of an object that has the ability to change its orbit. Thus the TLEs for the ISS are updated more frequently. 

The weekly AMSAT-NA TLE distribution is updated every Thursday evening (CST) and is available from the AMSAT website www.amsat.org under the dropdown menu <Satellite Info>, <Keplerian Elements Resources>. Look under "Current Bulletins" and click the blue HyperText "NASA (2-line) format elements".  You can have an email with these TLE's send to you every Thursday evening by signing up for the KEPS mailing list. Under the same heading "Current Bulletins", click on the blue HyperText "Keps mailing list". Go down to the heading "Subscribing to Keps" and follow the instructions to sign up.

You can also get a TLE element set with the ISS TLEs updated at least twice a day by going to the "Keplerian Elements Resources" website page as discussed above and under the heading "Current Bulletins" click on the blue HyperText "NASA (2-line) stripped of headers".  This TLE data on the ISS is updated based data from NASA's Johnson Space Flight Center.
Ray Hoad
Orbital Elements Manager

I see all these (hard to follow) AMSAT-BB postings about Keps implying there is NO ONE site that has accurate Keps. That we need to get this from here or that from there and combine to make a KEP.TLE for our programs. 
Is there no HAM SAT Keps that are complete & accurate? 
I was under the impression the AMSAT Keps were the most accurate for HAM SATs? Is that true or not, and if not, WHY?   

AMSAT Keps - when are they updated? If daily - what time? 
How often should Users update their program Keps? 


Dale Kubichek, MS-EET, N6JSX

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