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Satellite beam coverage patterns

Downlink beam:

The satellite downlink beam coverage maps show contour lines where each line refers to a particular power level from the satellite. The lines are marked with EIRP values like 45 dBW, 44 dBW, 43 dBW, 42dBW etc in descending order from the maximum, normally about in the middle of the beam.

The highest number, towards the middle of the coverage map shows where the downlink beam is strongest and most easy to receive. In the centre of the beam a smaller receive dish on the ground is required. As you move further away from the beam peak the beam becomes less powerful and a larger dish is required. A 3 dB reduction in power level (like going down from 45 dBW to 42 dBW) means you need a receive dish of 2 times the area. A 6 dB reduction requires a receive dish with 4 times the area.

The expression dBW refers to the power radiated from the satellite in the direction towards the contour line.

The term EIRP means "Equivalent Isotropic Radiated Power". 45 dBW is the same as 10^(45/10) = 31,622 watt transmitter feeding an omni-directional antenna.

In practice, an EIRP contour of 43 dBW could be produced by a satellite using a 200 watt transmitter (200W = 10log(200) dBW = 23 dBW) plus a satellite transmit down antenna with a gain of 22 dBi (maximum) but on the -2 dB beam contour.

Both downlink and uplink beams are often similar, with the most significant differences towards the periphery where the contour levels change quickly with distance from the centre. In some cases the beams are quite different, such as an uplink beam with USA coverage cross strapped to a downlink beam pointed at South Africa and shaped like south Africa. Modern High Throughput Satellites (HTS) jabe very many small circular spot beams. In these cases the customer user beams are cross connected in the satellite to spot beams aimed at gateway earth stations. 8 customer beams may interconnect to one gateway beam.

Uplink beam:

Sometimes you may see satellite coverage maps marked with G/T and PFDsat contours. These are uplink maps and refer to the sensitivity of the satellite receive system to signals sent up from the ground. This is of particular importance to satellite internet access services since it is desirable to keep the personal satellite earth station as low size and with as low power transmitter as possible, yet powerful enough to get a good signal into the satellite. If the satellite uplink quality is very good (i.e. with high G/T) then it can pick up weaker signals from your little transmit dish.

G/T contours are typically +4 dBK, +3 dBK, +2 dBK etc. The beam peak number varies a good deal according to the size of the coverage beam. Small spot beams receiving from just the area of one country or state have much higher G/T than larger regional zone or hemi or global beams.

G/T means gain to noise temperature ratio. Gain is simply the gain (in dBi) of the satellite receive (uplink) antenna with consideration of what contour you are on. The temperature is the system noise temperature of the satellite receive system. This will primarily be the satellite LNA noise temperature plus the earth surface temperature (approx) which, of course, is fully visible to the beam from the satellite.

The shape of the PFDsat contours follow the G/T contours exactly. PFDsat is the uplink power flux density required at the satellite to saturate a transponder. So if you put up sufficient power to achieve that PFD you will just saturate the transponder. Note the PFDsat is altered to suit the use of the transponder by selecting on board gain step settings using telecommand.

For the satellite internet outlink DVB-S carrier, a high PFDsat is better since this forces the large hub dish to transmit a more powerful uplink signal, thus maximising the uplink C/N and reducing the potential for uplink interference.

For the satellite internet return links you need a low PFDsat to keep the customer dishes small and transmitter costs down but you can't go too far otherwise you pick up too much noise and interference from the ground and uplinks to nearby satellites.