So, finally: data (previous posts here and here).
I may not have said it outright, but I certainly implied that the primary form of orbital data available to operators for collision prediction purposes is Two-Line Elements (TLEs). Specifically the TLEs published by the US Air Force. There are of course other types of data, although they are either not publicly available or not comprehensive.
Here’s a rundown…
Operator data
By “operator data” I mean whatever orbital data a satellite owner/operator uses for orbit control and/or prediction for its own spacecraft. This data is typically based on radiometric or GPS measurements and is much more accurate than TLE. The obvious shortcoming of operator data is that no such data exists for inactive spacecraft and debris orbiting Earth. Furthermore, satellite operators normally only have access to this information for their own spacecraft. Accurate knowledge of your own spacecraft’s orbit doesn’t really help much when you don’t have accurate information about all the other objects it might collide with.
In the geostationary realm, where the ratio of active spacecraft to debris is still relatively favorable, operators have recently started an effort to share data between themselves for purposes of collision avoidance. This is a unique cooperation among the otherwise very competitive telecommunication satellite owners (of which my employer, Intelsat, is one) and an important first step to safer satellite operations in the geostationary belt.
In lower orbits, such as those of Iridium 33 and Cosmos 2251, the ratio of operated spacecraft to debris is so small that sharing operator data would only marginally improve safety.
Two-Line Elements
The US Air Force publishes a catalog of orbital data in the form of TLEs for the majority of Earth-orbiting objects through its space-track.org web site.
TLEs are based on measurements made by the US Space Surveillance Network (SSN). The measurements are fitted to a relatively simple analytical model (SGP4) by an undisclosed process. The accuracy of any given TLE is generally unknown, perhaps even to the Air Force. Factors affecting the orbit quality are:
- The amount and age of measurements used to generate the TLE,
- The applicability of the SGP4 model to the particular orbit and object properties,
- The presence of artificial disturbances (i.e. maneuvers) during or after the time when measurements were collected.
In general the accuracy of TLEs is thought to be on the order of kilometers. However, the TLE catalog also has other problems, notably that classified US spacecraft (and related debris) are purposefully omitted from the catalog and that certain classes of non-classified objects are omitted for technical reasons (e.g. inability to track them).
Special Perturbations (SP) data
SP data is the moniker for orbital data from a classified catalog maintained by the US Air Force. It is essentially equivalent to the TLE catalog but with higher accuracy data based on a more accurate orbital model. It also includes the classified US spacecraft. The accuracy of SP data isn’t publicly known but it is thought to be significantly better than that of the TLEs.
The SP data catalog is used routinely for collision prediction for the Air Force’s (and other DoD organizations’) satellites as well as NASA missions (notably human spaceflight). However, by virtue of being classified the SP data isn’t generally available to other satellite operators. There is a process through which satellite operators can request support from the Air Force to confirm isolated close approaches. This process currently requires the operator to first identify the risk of a close approach by another means (e.g. using TLEs) and then go through a few layers of bureaucracy before the Air Force will investigate the close approach. It works somewhat in the not-too-densely-populated geostationary realm but I cannot see that it would scale to the level of support e.g. Iridium would need.
One can only hope that the collision between Iridium 33 and Cosmos 2251 prompts the Air Force to increase the level of support it provides to satellite operators and streamline the processes involved.
DISCOS
The European Space Agency maintains the DISCOS database (Database and Information System Characterising Objects in Space) which is based primarily on data from the US Space Surveillance Network. I don’t know any details about that data, perhaps it is simply the TLEs that the US Air Force publishes?
Late last year ESA initiated a program to develop its own space tracking capabilities.
MIT Lincoln Laboratory
MIT’s Lincoln Laboratory offers a close approach prediction service to geostationary satellite operators. To my knowledge the data used in the predictions is collected by the laboratory’s own sensors and is comparable to SP data. It’s a good service but the cost for operators is pretty steep (though certainly nowhere near the cost of losing a satellite in a collision) which prevents its use from being ubiquitous.
International Scientific Optical Network
A relatively new player in this field is the International Scientific Optical Network (ISON) coordinated by the Keldysh Institute of Applied Mathematics in Moscow. ISON is a collaboration between 18 scientific institutions in 9 states with the objective of establishing and maintaining a comprehensive database of objects in high altitude orbits (including the geostationary belt). ISON has so far identified several hundred objects not included in the US Air Force’s TLE catalog (roughly half of which are classified US objects).
The quality of ISON’s orbital data seems to be significantly better than TLEs, but at present the networks focus is on tracking object not in the TLE catalog rather than providing comprehensive orbital data for collision prediction purposes. However, this is a player worth keeping an eye on if you are operating geostationary spacecraft.
There’s more to it…
As is obvious to anyone working with the problem of satellite close approach prediction and avoidance I done a lot of hand waving and sweeping generalizations in these posts. My intent was never to go into pain-staking detail or to expose every facet of the problems satellite operators face. I am certainly not aware of them all myself despite the problem being part of my everyday work (just today we decided to reschedule a maneuver on one of our satellites in order to maintain safe separation distance during an upcoming close approach with a spent rocket body).
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