The remote terminal receive figure of merit (G/T in dB/K) plays a role in the link budget, particularly if it varies as a function of steering angle.
The SFD that arrives upon the receive terminal aperture is proportional to the satellite transponder downlink power, which is a result of the inherent gain, amplification, and back-off settings in the satellite itself in combination with the signal received from the teleport.
The SFD is strongest at the "beam peak" and weakest at the "beam edge".
Generally a network is designed so that beam edge is 4-7 dB down from beam peak.
In combination, this means the PSD presented to the modem varies about 4-7 dB across a given transponder beam. This is true in both wide beam and spot beam.
Spot beams are smaller than wide beams, hence a moving remote terminal would fly across the contours more quickly in a spot beam than in a wide beam. This also leads to frequent spot-spot handoffs.
The limit in PSD or SFD is established in the beam peak, all other areas would be lesser.
I model Ku wide beam transponders varying from +50 dBW at beam peak down to +45 dBW at beam edge.
For Ku or Ka spot beams, I assume beam peak could range between +56 and +61 dBW, and between +51 and +56 dBW at beam edge.
Average performance in either a spot beam or a wide beam is a function of which contour you are operating.
It is pointless to argue whether on average spot beams or wide beams yield closest to beam peak, so I always look at any given scenario across multiple contours from beam peak to beam edge.
Peter Lemme
peter@satcom.guru
Copyright 2015
All rights reserved
Check out these related posts
Link Budget Background, Definitions, and Assumptions
Rain Fade
Skew Angle and Effective Aperture of Airborne Antennas
Spot beams Vs. Wide beams Ku band
Forward Channel Considerations
Forward Channel Downlink Regulations
Ku-band Airborne Antenna Figure of Merit (G/T)
Ka-band Figure of Merit (G/T)
Return Channel Link Budget
Ku-band Return Channel Maximum PSD
Ka-band Return Channel Maximum PSD