Risk Areas for Satellite Communications

Experienced VSAT technicians are often loath to quote satellite reliability above 99 percent availability. Key risk factors for service interruption include the following:

• Rain fade. Normal or even reasonably heavy rain will not necessarily disrupt communications. However, a heavy downpour can weaken the signal so much that transmission stops. The hub operator can adjust power on the hub to a certain extent but at some level of rain, nothing more can be done. As expected, some areas of the United States are far more susceptible to rain fade than others (e.g., some parts of Florida).

• Satellite malfunction — fuel shortage. While any number of destructive elements, such as meteorites, can disable satellites, they are most commonly rendered useless because of fuel shortages. A geosynchronous satellite must necessarily stay within tight limits of position in the sky because all VSAT dishes must be fine-tuned for direction to ensure a strong signal. And because satellites naturally tend to wander in an elliptical path, they must be constantly homed to the correct position in space via small thrusters mounted at appropriate locations around the outside surface. These thrusters require fuel; when there is no more fuel, ground control is unable to keep the satellite on target and it drifts away, thus becoming useless for communications. There is a story about the early days of commercial satellites in which technicians new to satellite management used a mouse hooked to a control unit to position the "bird." Apparently, it was so much fun that they moved it around too much, depleting its fuel and rendering a multimillion-dollar satellite useless.

• Satellite malfunction— transponders. Transponders receive signals on the uplink, translate them to the downlink frequency, and amplify them for retransmission to Earth. Transponders can and do fail. Because there are multiple transponders in a satellite, the failure of a single transponder does not necessarily mean the end of the satellite's life. However, if an organization's communications are going through the failed transponder, the result is the same as if the satellite had been knocked out — that is, no service for that organization.

• Ice and snow. If VSAT dishes are not properly heated or enclosed in a radome (special purpose plastic cover), they may not receive and transmit a sufficiently strong signal to function.

• Lightning and power surges. Engineers working day-to-day on VSATs generally agree that the most frequent reason for breakdown of the dishes is electrical.

• Relatively short mean time to failure. VSAT equipment stays hot and wears out relatively quickly.

• Frequency conflicts. Although the FCC controls the frequencies used, occasionally a technician will set up a VSAT incorrectly, resulting in interference. The solution is for all parties to return to their assigned frequencies to avoid interference.

• Sun transit errors. When the main beam of an Earth station receiving antenna is in a straight line with the sun, significantly larger noise will occur, sometimes temporarily stopping communications. These errors are more likely to occur during the fall and spring equinox. Sun transit problems are far more likely to occur with the older, type I VSAT dishes. Type II and III dishes are smaller in diameter and less sensitive to concentration of noise.

• Temporary "commandeering" of frequencies by government authorities. For purposes of safety and security, government authorities or military personnel may temporarily take over certain frequencies in a geographical area. These are lawful actions but in some cases organizations using those frequencies are not timely notified and spend considerable time researching the cause of the downtime.