Alan Young | SatellitePro ME | January 2018 | p. 40
While geostationary satellites are no longer the only economically viable way of broadcasting television programming across large geographies, they are still arguably capable of being the most reliable. This is due to the fact that geostationary satellites are built to extremely high standards with sufficient on-board redundancy to last in orbit for 15 or so years. In addition, there are few touch points in the signal path – basically the signal is uplinked from an antenna to the satellite and then amplified and frequency shifted before being beamed back to earth to receive antennas. This makes for a fundamentally reliable network because there are so few places something can go wrong.
Indeed, geostationary satellites have already been proven to deliver very high reliability over decades. However, we cannot get complacent, especially given the huge amount of competition from other ways of broadcasting television programming, and the consumer’s expectation for perfection. Therefore, continual monitoring and control is vital to maintaining uplink reliability to as close to 100% as is feasible. Automating actions based on that monitoring can make the whole process much more efficient and drastically reduce, if not eliminate, any potential downtime.
One of the biggest challenges with satellite communications is one which cannot be controlled – the weather! Weather conditions, in particular precipitation, can adversely affect the uplink by attenuating the RF signal received by the satellite. If the RF signal power level drops enough at the satellite, the transmission will be lost. Precipitation affects higher frequencies more than lower frequencies, so Ku-band is affected more than C-band and Ka-band is affected more than Ku-band.
This is where Automatic Up Path Power Control (AUPPC) comes in. Not only is it important to continuously monitor the amount of signal attenuation, it needs to be done from multiple sources. This is necessary in order to verify that it is in fact signal attenuation causing the issue – measuring the satellite beacon level from just the antenna controller is not sufficient as the antenna’s pointing relative to the satellite may have changed (an entirely separate issue). If attenuation is detected in the uplink path, the transmit power should be automatically adjusted, i.e. increased so that the received signal level at the satellite remains at its nominal level. It is critical that any system is smart enough to reduce the power again when the weather-induced attenuation subsides. Otherwise the satellite may be overloaded which will also result in loss of signal).
There are times, such as during a heavy rainstorm, when the attenuation becomes so severe that it is simply not possible to compensate sufficiently. It could be that the antenna’s amplifiers do not have enough headroom or the uplink antenna’s size means that off-axis emissions exceed regulatory limits. In these cases, switching to another site is necessary to ensure continuity of the broadcast. Diversity switching logic continually monitors the rate of increase of signal attenuation. When the attenuation is about to reach the disruptive limit, and the diverse site has less attenuation, a sequence of events is triggered to automatically switch the transmission to an alternate uplink site, even if it is thousands of miles away.
In these cases, there are a few important things that need to be achieved. One is ensuring that the power level is matched so the satellite receives the same power from the alternate uplink. It is also important to make the switch as fast as possible. This involves being able to take into account the total propagation delay to the satellite from the alternate site and pre-sending the command to illuminate the satellite from the alternate site before it mutes the main uplink. This ensures that receivers in the field are only impacted transiently, thereby minimizing, or eliminating, any service disruption.
Ultimately, without Diversity switching and AUPPC, video transmissions are subject to weather at the uplink site and as such have a significantly lower availability.