JSAT Satellite Services

Satellite Services Satellite Applications and Uses Why Satellite



Power

For high quality communications, check your satellite’s power levels.
 
The most important technical characteristic of a satellite is the transmit power for the downlink into the coverage area. Why? Power is the primary determinant of the overall quality of the link between the satellite and the earth station and the size (and cost) of receiving dishes.
 
The higher the Effective Isotropic Radiated Power (EIRP) in the direction of the receiving dish, the better. It means a better quality link, or smaller receiving dishes.

More about EIRP.
EIRP is measured in decibels relative to one watt, or dBW. One watt is equivalent to 0 dBW and 50 dBW is equivalent to 105 watts, or 100,000 watts. Because it’s a logarithmic scale, an increase of 3 dB of EIRP corresponds to a doubling of the power in watts. A 10 dB increase reflects a factor of 10.
 
 

As an example, the footprint map above shows that the Horizons-1 satellite will produce a maximum downlink EIRP of approximately 50 dBW. This is visible in the red circle. The radiated power declines from inner contour to next outer contour, reaching a low of approximately 46 dBW at the edge of the Continental United States (CONUS). Coverage still exists to the south and north, but at ever-decreasing power levels.

The EIRP coverage of a satellite determines the size of receiving dishes required. The higher the EIRP, the smaller the antenna needed. For example, for a standard DVB-S digital video broadcast, an EIRP of 50 dBW would be sufficient to allow the use of one meter receiving antennas.

If the EIRP is 3 dB less, or 47 dBW, the antenna diameter will need to increase by about 50%, to 1.5 meters. An actual link budget calculation will give a more precise value.
 
Footprints in space.
The uplink provides the satellite with signals to retransmit into the downlink. Like the downlink, the uplink, typically has a coverage “footprint”.
 
The receive footprint is specified with two parameters - the gain to noise temperature ratio (G/T), which is also called the receive figure of merit, and the Saturation Flux Density (SFD).
 
We won’t try to explain these in detail, but communications engineers need this information to determine the size of the uplink antenna and ground (uplink) transmit power. In a network of two-way VSATs, the SFD can be particularly important because it determines the requirement for VSAT transmit power which determines the type, and therefore the cost, of the required solid state power amplifier on the VSAT antenna.
 
 
Basic elements of a satellite link, showing the transmitting earth station, satellite transponder, and receiving terminal. Satellite EIRP determines receiving dish size and satellite receive G/T and SFD determine transmitting earth station uplink power.



Through our fleet of satellites and exceptional level of customer care, JSAT International makes it easy to communicate and transmit content across North America.