How does ‘rain fade’ limit satellite Internet?

Satellite communications are affected by moisture and different kinds of precipitation like rain or snow. All of it happens in the signal path between end users or ground station and the satellite being utilized. This interference with the signal is known as “rain fade.”

Rain fades are less pronounced on the lower frequency “L” and “C” bands, but can become quite severe in the higher frequency “Ku” and “Ka” band. For satellite Internet services in tropical areas with heavy rain, use by of the C band (4/6 GHz) with a circular polarization satellite is popular. Satellite communications on the Ka band (19/29 GHz) can use special techniques such as large “rain margins,” “adaptive uplink power control” and “reduced bit rates” during precipitation.

“Rain margins” are the extra communication link requirements needed to account for signal degradations because of moisture and precipitation. They are of acute importance on all systems operating at frequencies over 10 GHz.

The amount of time during which serve is lost can be reduced by increasing the size of the satellite communication dish, a dish-shaped type of parabolic antenna designed to receive microwaves from communications satellites, which transmit data transmission or broadcasts, such as satellite television, so as to gather more of the satellite signal on the downlink and also to provide a stronger signal in the uplink. In other words, increasing antenna gain through the use of a larger parabolic reflector is one way of increasing the overall channel gain and, consequently, the signal-to-noise (S/N) ratio, which allows for greater signal loss due to rain fade without S/N ratio dropping below its minimum threshold for successful communication.

Modern consumer-grade dish antennas tend to be fairly small, which reduces the rain margin or increases the required satellite downlink power and cost. However, it is often more economical to build a more expensive satellite and smaller, less expensive consumer antennas than to increase the consumer antenna size to reduce the satellite cost, as the antenna cost reduction is magnified through economies of scale, which in microeconomics, are the cost advantages that an enterprise obtains due to expansion; whereas any reduction of the satellite  cost is not.

Large commercial dishes of 3.7 m top 13 m diameter are used to achieve large rain margins and also to reduce the cost per bit by requiring far less power from the satellite. Satellites typically use photovoltaic (PV) solar power, a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect, so there is no expense for the energy itself, but a more powerful satellite will require larger, more powerful solar panels and electronics, often including a larger transmitting antenna. The larger satellite components not only increase materials costs but also increase the weight of the satellite, and in general, the cost to launch a satellite into an orbit is directly proportional to its weight. In addition, since satellite launch vehicles (i.e. rockets) have specific payload size limits, making parts of the satellite larger may require either more complex folding mechanisms for parts of the satellite like solar panels and high-gain antennas, or upgrading to a more expensive launch vehicle that can handle a larger payload.

Modern download DVB-S2 carriers (“Digital Video Broadcasting – Satellite – Second Generation), a digital television broadcast standard that has been designed as a success for the popular DVB-S system, with RCS feedback, are intended to allow the modulation method to be dynamically altered, in response to rain problems at a receive site. This allows the bit rates to be increased substantially during normal clear sky conditions, thus reducing overall costs per bit.