Amid the calls for removal of space debris, satellite operators face the tough reality of staying away from these “uncontrolled flying objects.” The awareness has certainly increased over the years, and efforts are being made to avoid leaving debris in highly used areas such as the geostationary (GEO) ring; nevertheless, with more satellites being launched, more debris accumulates from launch vehicle remains and satellites drifting after fatal anomalies.
At SES, we have a proactive approach in three ways. First, through constant monitoring of close approaches between any of our spacecraft and debris that may result in an avoidance action. Second, by relocating satellites with sufficient radial separation to minimize risk of close approach with other GEO satellites. Third, by respecting Inter-Agency Space Debris Coordination Committee guidelines for satellite de-orbiting and thereby minimizing any further accumulation of debris in the geostationary ring.
The Space Data Association’s (space-data.org) Space Data Center (SDC) is our main source of information for monitoring distances between our satellites and space debris. Presently it deals mainly with GEO spacecraft and processes conjunction analysis for more than 65 percent of the operational GEO satellites. This system has the great advantage of working directly with the satellite operators and using their own highly accurate ephemeris, which limits the number of false alarms as the ephemeris includes the planned maneuvers and has long-term reliability (up to seven days). The current limitation of this system is that not all operators are sending their ephemeris, in particular military satellites. Nevertheless, we believe that this approach – fusing “real, authoritative data” for controlled satellites – is the safest and most efficient one.
Our second source of information regarding surveillance of space objects is the Joint Space Operations Center (JSpOC), which tracks more than 1,000 controlled objects and more than 22,000 debris objects and sends us alerts (so-called CSM – Conjunction Summary Messages) so we can investigate potential cases of close approach between an SES spacecraft and any other object. JSpOC uses optical measurements for orbit determination in GEO, which provides the best-available data for space debris but does not include maneuver information and is susceptible to track misassociation and cross-tagging issues. This JSpOC data is typically actionable only when the conjunction event is within the next several days.
On request of the JSpOC, ephemeris of SES spacecraft can be used, but false alarms are frequent if the “external object” is a controlled spacecraft. In cases of controlled and identified spacecraft, we will try to contact the operator of the other spacecraft. Given the large number of warnings that we were getting concerning close approaches involving one of our satellites, SES has partially automated the process and only if a close approach is confirmed using the recent and complete ephemeris (maneuver information included) for our spacecraft will an intervention be necessary. This may simply be to get an updated orbit for the other object if it is controlled, but if not, the required action will be the computation of a collision avoidance maneuver.
With 47 satellites operated by SES, one can say that close approach management has become a daily concern. While most of the activity is monitoring, collision avoidance maneuvers are occasionally required to avoid “uncontrolled flying objects.” Our action toward debris avoidance would not be complete without a mitigation policy.
When relocating spacecraft, a large distance is maintained with the geostationary ring, and at end of life, SES complies with established international best practices to avoid pollution of the geostationary ring, consisting of passivation and de-orbiting of the spacecraft.
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