Comment on “Analysis of GEO spacecraft anomalies: Space weather relationships” by Ho‐Sung Choi et al.

Abstract

We raise several issues and concerns with the study by Choi et al. [2011] regarding GEO satellite anomalies and their possible causes. We start by saying that accurate attribution of the space environment as a contributing factor in any in-flight anomaly is usually fraught with peril for many reasons, including the lack of relevant environmental data at the satellite and engineering unknowns about the anomaly itself. We acknowledge the difficulties faced by Choi et al. [2011] in their study of anomalies in GEO vehicles; they are the same difficulties that we encountered and enumerated in an earlier study of space environment-induced anomalies in multiple orbit regimes [Koons et al., 2000]. However, there are aspects of the anomaly database that the authors used for their study that must cast significant doubt on the database's usefulness for anomaly attribution. Specifically, the Satellite News Digest (SND) event list (http://www.sat-index.co.uk) provided the universal time (and by inference local time for GEO) for only 33% of the anomaly events. It is virtually impossible to support the claim that a space environment hazard contributed to an anomaly if all that is known is the calendar day on which the event occurred. We must qualify this statement with the following: If the authors were presupposing that internal charging at GEO was the environmental cause for most of the database events, then the particular local time would not be an organizing parameter and possibly the calendar date would be sufficient. Internal charging might then explain why the authors found a better correlation between event frequency and the maximum Kp measured up to 3 days prior to the anomaly. However, internal charging is clearly not the only organizing hazard as shown by the local time dependence that correlates with hot plasma sheet electrons at GEO [Choi et al., 2011, Figure 3], as has been seen in many GEO anomaly surveys. Another specific problem with the SND database is the unclear and unsupported selection of events. For example, it is highly unlikely that the space environment contributed to events such as a failure to deploy a solar array (event #8 of Choi et al. [2011]), a safehold triggered by faulty ground-control software (event #25), or a pressure drop in a fuel tank (event #52). Here we (and Choi et al.) are taking the SND descriptions at their word, which adds an unknown level of uncertainty to the analysis. We would claim that the authors should have culled the database for events like the ones that we highlighted along with others for which the root cause was “unknown” and then should have put those events into a separate category. As it stands, their analysis appears to be polluted with nonenvironment-related anomalies. In sympathy with the authors, we (and others) have argued for an agency that would maintain adequate and open anomaly and abnormality lists. Such lists would contain, at a minimum, the subject vehicle, the date and time of the event, the 3-D location of the vehicle at the time of the event, the 3-D velocity or complete orbital elements for the vehicle at the time of the event, the affected subsystem, the suspected type of anomaly, and the level of confidence in that assessment. This information would enable robust statistical analyses of anomaly occurrence in order to develop statistical models [e.g., O'Brien, 2009] and first-principles models [e.g., Davis et al., 2008] of anomaly phenomena for improved satellite design and operations. Last, we would like to point out that the surface charging hazard depends on many parameters, such as the detailed shape of the electron spectrum [e.g., Fennell et al., 2001], and material properties. However, it is understood that the coupling mechanism to internal electronics is not the incident electrons themselves, as the authors questioned in their summary (“…it seems to be hard for such low-energy electrons to penetrate satellite shielding.” [Choi et al., 2011, paragraph 26]), but rather the electrostatic discharge caused by charging somewhere on the surface of the satellite bus.