Google+ Followers

18 February 2009

Pager Satellite Failure and Space Environment

Pager Satellite Failure May Have Been Related to Disturbed Space Environment

A period of particularly bad "space weather" may have played a part in failure of the Galaxy 4 satellite, which silenced about 80% of the United States' pagers last May.

by D. N. Baker, J. H. Allen, S. G. Kanekal, and G. D. Reeves

A very intense flux of electrons, evident in the magnetosphere earlier this year, may have caused a satellite failure (or at least exacerbated the situation) leading to the loss of telephone pager service to 45 million customers, research has shown. The electrons, known as highly relativistic electrons (HREs), were especially numerous in the weeks preceding the failure. Researchers say HREs have triggered spacecraft anomalies in the past when fluxes are elevated. They therefore believe this energetic electron event could have been behind the failure of the attitude control system of the Galaxy 4 spacecraft at 2200 UT on May 19, 1998. A backup system also failed, either at the same time or earlier, so operators were unable to maintain a stable Earth link.

Galaxy 4 is a heavily used communication satellite at geostationary orbit*. Its sudden failure caused not only widespread loss of pager service but also numerous other communication outages. Using a wide array of datasets, our team of scientists analyzed the space environment for the times in question and found evidence of highly disturbed solar, solar wind*, and geomagnetic conditions in late April and early May. The combination of coronal mass ejections*, solar flares*, and high speed solar wind streams led to a powerful sequence of interplanetary disturbances that hit the Earth. These disturbances produced a deep, powerful, and long-lasting enhancement of the HRE population throughout the outer Van Allen radiation zone. The kinds of disturbances witnessed are indicative of the types of events that may commonly occur during the approaching peak in solar activity in the years 2000 and 2001. It will be most important to determine how well space systems can stand up to the multifaceted effects of the space environment over the next several years.

The evidence is strong that HRE fluxes were substantially elevated above average conditions for a period of about 2 weeks before the Galaxy 4 failure. Long-duration HRE enhancements have in the past been convincingly associated with spacecraft operational failure. For example, we know that high fluxes of energetic electrons can lead to a buildup of electric charge deep inside of spacecraft subsystems. In this process, energetic electrons bury themselves in poorly conducting material such as thermal control blankets, electronic boards, coaxial cables, and insulation. Eventually, if the charge builds up more rapidly (because of electrons continuing to hit the spacecraft) than it leaks away (because of low material conductivity), then there can be an electrostatic discharge event. This is much like small, powerful lightning discharge inside of the spacecraft. Such a discharge can damage or destroy a sensitive circuit or subsystem, and the result can be a spacecraft failure.

Scientists involved in the analysis have noted that whether or not the incident on May 19 was caused by "space weather," it nonetheless shows the vulnerability of society to a single spacecraft failure. The vast number of users affected by the loss of just one spacecraft shows how dependent society is on space technology and how fragile communication systems can be. The Galaxy 4 failure had a large impact because the spacecraft was optimally located over the central United States and could best handle digital pager signals. Eighty percent of all pager traffic was being directed through it. Increasingly, phones, televisions, radios, bank transactions, newspapers, credit card systems, and the like depend on satellites for at least part of their links. It seems very inadvisable to have such complex, societally significant systems susceptible to single-spacecraft failures. This seems particularly true as the peak of the 11-year solar activity cycle, in 2000-2001, approaches.

The typical major communication spacecraft has an estimated value of $200-250 million. Over 100 such spacecraft are in operation today and whole new groups of low- to mid-altitude satellites are being placed in orbit. The invested cost of space assets is staggering. Given the recent record of space environmental disturbances, space researchers believe many more highly disruptive spacecraft failures may occur. They suggest that space systems be made immune to the space environment and backup systems be made readily available to cover space system failure whatever the cause.

http://www.agu.org/sci_soc/articles/eisbaker.html

No comments: