As many of you probably know by now, Iridium 33 (a mobile communications satellite) and Cosmos 2251 collided in low-Earth orbit. You might wonder, is there a way to predict such events? In addition to government organizations monitoring space, there are commercial tools that regular folks can use. SOCRATES is the Satellite Orbital Conjunction Reports Assessing Threatening Encounters in Space (I know, a mouthful right?). It’s a service for the satellite operator community run by the Center for Space Standards & Innovation (CSSI) and can be found at www.celestrak.com/SOCRATES (you guys know how much I love celestrak.com so once again thank you Dr. T.S. Kelso!). What SOCRATES allows you to do is run conjunction analysis reports on satellites over a 7-day period and it uses STK to do this STK/Conjunction Analysis Tool (STK/CAT) enables users to identify close-approach situations and compare against the entire NORAD TLE space catalog on an individual satellite or multiple satellites. Now this doesn’t have to be just for satellites you could run CAT analysis on missiles or launch vehicles as well. You can run your own STK/CAT analysis right inside your scenario, all you need is an STK/CAT license (don’t have one? Send an e-mail to support@agi.com and request one).

To set up this analysis in STK, add an AdvCAT object to your scenario and double click to open up the properties. When you open it up you’ll see the Main properties under the Basic Section. Here you set the Time Period that you want to run CAT over or use the Scenario Interval (remember the longer your time period the longer it’s going to take CAT to run; there are a lot of objects it has to run analysis against so keep it short if you can). Next, set your Threshold, which is distance between the objects you want. For example, if you set it to 10km you will get a warning whenever the distance between the threat volumes of the primary and secondary objects is less than that 10 km threshold distance. Alright now jump down to the Primary Objects section. These are the satellites that you are interested in. STK will give you a list of available objects which will include the satellites in your scenario, ephemeris files (.e), and TLE sets by type, date, and name (the satellite database is in there too). Once you find your satellite in the Available Objects list, highlight it and click the blue arrow to move it over to the Chosen section. Once over in the Chosen section you can set the Class type, which is the dimension definition type (Fixed, Orbit Class, Quadratic). Next set the Tangential, this is the dimension of the threat volume ellipsoid along the X axis (parallel to the object’s velocity vector). Then you have the Cross Track or the dimension of the threat volume ellipsoid along the Y axis (parallel to the orbit normal vector). Finally you have the Normal or the dimension of the threat volume ellipsoid along the Z axis (parallel to the cross product of the X and Y axes).

After that’s set, jump down to the Secondary list. These are the objects that present a potential risk of collision with or have a close approach to your primary satellite. Once again these are all the satellites in your scenario plus the satellite database. Select which ones you want to run the analysis against, and then hit the blue arrow to move it over. You have the same options to set as you did with the Primary Object. Now that all your parameters are set, you can go ahead and hit the Compute button to run the analysis. There are some other settings you can make, but this will get you started. Once it’s computed you can go into the Report for StkCAT object and run a couple of different reports to see things like Close Approach by Min Range or Encounter Warnings. So here are a couple of snaps of the AdvCAT properties panel and Close Approach By Min Range report.

Here is a quick metric in case you are wondering about STK/CAT. For runs of approximately 3000 objects against the full catalog (about 12,000), this can typically take about 1.5 hours on a single STK desktop.  This is using a 5 km threshold for an analysis period of 7 days.

Remember there are thousands of objects orbiting our Earth (satellites, debris) and we have to be careful up there.  Here is an image of what the debris from this event three days later:

STK/CAT is here to help you avoid this and all the rest of the objects out there. 

Till Next Time,

Rocket Girl (aka Amanda)