The SkyLAN concept proves to be particularly attractive for FSS multispot beam broadband access. A cluster of 4 medium size satellites, each providing 16 beams, allows for the deployment of 64 spot beams over "extended" Europe for a total capacity of 14.4 GHz. Each satellite of the cluster works on a dedicated frequency and provides non-adjacent beam coverage. The satellites of the cluster are interconnected by optical ISL and are arranged in a double ring, for maximum routing flexibility and redundancy.
In the BSS a linguistic beam coverage of Europe was analysed, with a 3 frequency contoured beams Ku-band coverage, for an overall system bandwidth of 4.78 GHz. It is important to notice that such a mission would exceed the performances of the current generations of platform. The preferred solution in this case consists of a cluster of 4 satellites, arranged in a star configuration. Three Rx/Tx satellites handle reception from feeder station and transmission towards user dishes of the Ku-band signals. Each Rx/Tx satellite is in charge of a part of the coverage in one frequency. The fourth satellite provides the onboard processing of 25 channels. Onboard processing is DVB-S demodulation and MPEG-2 TS multiplexing. The fourth satellite is also in charge of the reception of the Ka-band channels, which are dedicated to onboard DVB processing. The processing satellite has an unusual payload: no TWTA, a large digital OBP, a receiving multibeam Ka-band antenna and three ISL terminals In this case an RF ISL is proposed.
MSS missions with the stringent payload requirements that drive up payload mass and power budget and that would require in any case multiple very large satellites for a pan-European coverage offers limited interests for the SkyLAN approach.Impacts on payload
An FSS cluster requires splitting of the switching function and of the antenna coverage. The main impacts at payload level are on the antenna system design, on the regenerative OBP and on the digital ISL. Coverage splitting by frequency allocation simplifies the antenna design. If the coverage splitting is realized following geographical considerations, the resulting antenna subsystem will be much more complex and active antenna techniques will have to be used.
Distribution of switching and ISL are closely related. An optical ISL offers the best compromise between easiness to interface the switching matrix and ability to support high-speed links.
The BSS cluster is characterized by the non-conventional hub satellite. The major change at payload level is the introduction of an ISL to backhaul signal to the hub satellite. RF ISL best fit the overall architecture, but analogue modulated optical ISL are also possible. On the antenna side the most natural solution is to split coverage according to frequency, with virtually no impact on the antenna design.Inter Satellite Link
The ISL is the enabling element of the SkyLAN concept as it provides the flexibility and the scalability that go with the concept. SkyLAN clusters arranged in ring or star topology according to the specific mission requirements, can provide next generation satellite services that are outside the reach of the current platform performances.
RF link can be realized with 25 cm antenna and high power (120 TWTA) in Ka band over short range (10-20 km).
For optical ISL both 860 nm (Silex heritage) and 1066 nm technologies can be used for both co-located and separated clusters. Small terminal apertures (5 cm) can be sufficient.Orbit keeping
Co-location of 4 satellites in the same orbital slot in a ring topology is not feasible due to the tracking requirements that are imposed on the ISL terminal. The best option for a ring connectivity is two place the satellites in two collocation groups separated by 2 deg. Such an arrangement does not pose any safety issues or collisions threats. Ion propulsion will result in a smoother evolution of the ISL pointing parameters and will improve the visibility angles.
For the BSS cluster the best approach consists on co-locating the three TX/RX satellites in a single GEO slot, while keeping the hub in a non-adjacent slot.
Synchronisation of manoeuvres is required to maintain the safety margins inside the co-location slots.Financial assessment
The overall cost of SkyLAN system is generally higher than the one of a corresponding single satellite. The difference sensibly reduces when sparing policy is taken into account. For the FSS mission a sub-regional coverage (as opposed to the frequency splitting) would provide the most interesting solution.
The SkyLAN system provides a more advantageous cash flow (lower upfront investment) that can be modulated on the market demand and, in the worst case, offers a better exit option.