How are Spot Beam Systems Modelled?
The algorithm in Recommendation ITU-R S.1503-2 was designed to handle a wide variety of non-GSO FSS system designs, including both those with fixed beam pointing and also those with steerable spot beams. This section gives more information about the modelling of steerable spot beam systems for the EPFD(down) direction
The PFD mask is the envelope of the maximum PFD in any direction, defined either by (azimuth, elevation) or (α, Δlong). This PFD will be greatest for locations where there is a traffic hot spot, as this will require the maximum possible number of spot beams to be pointed in that direction.
However this hot spot, and hence maximum PFD, could occur at in any direction and hence the mask could contain the maximum PFD in all pointing directions simultaneously. This is not physically feasible but is acceptable – indeed likely – for masks provided as input to the algorithm in Rec. S1503-2.
The reason this is acceptable is how the mask is used within S.1503-2. In particular, it is noted that there is a single reference GSO system, comprising a satellite and ES, located at positions fixed for the duration of the run. These locations are will have been calculated using the worst case geometry (WCG) algorithm.
Therefore, only one part of the PFD mask is used at each time step for each satellite. There is no over-representation of the actual emissions of the satellite as the other parts of the PFD mask are not used.
In addition, the non-GSO FSS system will have constraints that limit its ability to provide service from more than a specified number of satellites simultaneously due to intra-system interference constraints. This is modelling using the following field:
A.4.b.7.a the maximum number of non-geostationary satellites receiving simultaneously with overlapping frequencies from the associated earth stations within a given cell
This Nco[Latitude] field is used to ensure that only a set number of non-GSO satellites are considered active so that the aggregate EPFD is not the sum of all visible satellites. Rather it is a sum of:
- Of those satellites outside the exclusion zone and above the minimum elevation angle, the Nco[Latitude] satellites which have the largest single entry EPFD values
- Those satellites inside the exclusion zone