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KA-SAT technical system: My guess as to how it works.

Earth stations:

8 Unmanned 9.1m diameter Ka-band gateways. Key features of these ViaSAT antennas: Large hub mounted radio equipment box with 2 high power amplifiers connected using short waveguide runs to the feed system. Dual circular polarisation with monopulse tracking for high accuracy and stability.  Surface accuracy better than λ/16 at 30 GHz. Warm air heating with fans to assure even reflector surface temperature. Uplink power control to maintain steady signal level into the satellite, compensating for uplink rain-fading.

The gateway hubs are GW2 France-Rambouillet; GW4 Spain-Arganda,Madrid, Finland-Helsinki; GW1 Cyprus-Makarios; GW6 Sicilia-Lago Scanzano,Palermo; Nemea, Corinth, Athens, Greece; GW3 Ireland-Elfordstown, Cork; GW5 Berlin, Germany. I'm not sure but there may also be gateways in Italy-Turin; Trieste-Italy;

Customer terminals:

Customer antennas with normal dish size of 72 x 77cm with BUC rated at 3W, but normally operated below 2W. Latest outdoor TRIA uses single IFL cable and has audible receive signal quality level indication. Polarisation (vertical/horizontal) is remote switched from the modem via the cable. A fixed polariser is used to convert linear to circular polarisation. Use of circular polarisation eliminates a major installation skill requirement as it requires no precise adjustment to minimise interference.  A 1.2m dish with 4W BUC is available for for professional applications and for operation beyond normal beam edge coverage patterns.

One IFL cable is used

Surfbeam modem: customer indoor unit
Indoor Surfbeam modem from ViaSat,

Outdoor equipment: front fed offset antenna with circular polarisation, switchable by automatic control from indoors. Assumed HPA rating 3W, normally operated at 2W or below, possibly as low as 0.5W under clear sky conditions. Normal dish size 72 cm.  1.2m dish and 4W BUC for professional applications and operation beyond normal beam edge.

Customer beam frequency plan:

19.7-20.2 GHz for the downlinks to the customer VSAT terminals.
29.5-30 GHz for the uplinks from the customer VSAT terminals.

Each customer terminal sees a single transponder of which approx 230 MHz is useable. The spot beam coverages come in four colours and are arranged so that no two same colour sports are adjacent, thus minimising interference.  Orange and Blue are one circular polarisation, while purple and green are the other. The 500 MHz is divided in two haves, only a 250 MHz half being used by a beam. Thus four non-interfering permutations.

On the downlink to the customer I think there are 4 carriers per beam, comprising 3 carriers of 52 Msps plus 1 carrier of 40Msps, making a total used bandwidth of approx 206 MHz.  Total downlink bit rate per spot is 470 Mbit/s clear sky.

This gives the satellite a maximum download bit rate of 82 beams x 470 Mbit/s = 38.54 Gbit/s.  The claimed capacity is 90 Gbit/s, so I guess this 90 Gbit/s, figure refers to the customer uplink plus the customer download capacity. My calculations seems credible, although the actual uplink capacity for the remote sites will be reduced signifantly due the the TDMA nature of the traffic.

Gateway hub frequency plan

27.5-29.5 GHz for the uplinks from the hub gateway earth stations.
17.7-19.7 GHz for the downlinks to the hub gateway earth stations.

Each gateway deals with around 10 spot beams.  As shown in the frequency plan above the gateway transmits 4 carriers per 236 MHz, total 20 carriers on each polarisation, using one HPA per polarisation, approx 350 watts per polarisation, into a 9.1m diameter dish. See  Xicom Model number XTD-500KaL for the probable HPA used.

The satellite

Ka-SAT satellite was launched Dec 2010 and has 83 spot beams covering Europe, North Africa and parts of the Middle East.  Ka-SAT is located at 9 deg east orbital longitude position.

Each spot beam provides 236 MHz bandwidth.

Beams 80 (Libya) and 83 (Azerbaijan) are mutually exclusive.

Beams 59, 69, 70 and 74 are half bandwidth (125 MHz).

Spot beams have -3 dB beamwidth of 0.5 deg.  Gain on axis is 50.6dBi  and 47.6dBi at the -3 dB contour beam edge. I expect the satellite antenna (approx 2.6m dia) for the downlink (18 GHz) is slightly larger than for the uplink (29 GHz) so that the diameters and gains of the beams are all the same.

I estimate the uplink system noise temperature to the 400K, which combined with the antenna gain of 50.5 dB gives a satellite uplink receive G/T of 50.6 - 10 log(400) = 24.6 dB/K at beam centre or 21.6 dB/K at beam edge.

The uplink PFD required, per uplink beam to meet the required downlink EIRP per 256 MHz is estimate to be about -94 dBW/m^2.  It is likely that this figure is adjustable to. More information would be appreciated..

Downlink operating EIRP per 1180 MHz towards the hub is estimated also. I have assumed 130 watts satellite HPA, with two of these per gateway beam. EIRP = 50.6 + 10log(130) = 71.7 dBW beam centre or 68.7 at -3dB beam edge.

Power analysis:

Comms payload 11kW

TWT amps assumed. See https://www.thalesgroup.com/sites/default/files/asset/document/thales_space_k_ka_band.pdf
Customer beams (82 TWTAs) 18 GHz with 50W = 4100 watts
Gateway beams (10 beams.  20 TWTAs) 18 GHz with 130W = 2600 watts

6700 W at 60% eff = 11.1 kW.  This seem credible.

Additional bus power approx 3kW.  Solar array 14-15 kW.


Traffic forecast:

2010: "cost 350 million euros ($490 million), will generate 100 million euros in fresh revenue per year within three years. Paris-based Eutelsat, which has been planning Ka-Sat since 2007, expects Ka-Sat will reach profitability by the three-year mark with around 300,000 subscribers, although depending on the mix of professional and consumer users, that milestone may come earlier or later. Counting just consumers, Ka-Sat can accommodate up to 1 million subscribers, according to Eutelsat estimates."

Traffic analysis:

Given each beam has download capacity of 470 Mbit/s clear sky and there are 83 beams, the total download capacity is around 39 Gbit/s. Each beam has a similar capacity on the return link (uplink from the customer).

2000 sites share a 64 Mbit/s download carrier. In clear sky all get their "up to 20 Mbit/s" standard service. Note that many sites share the capacity in a time shared fashion. Three sites could be active simultaneously, each at 20 Mbit/s. 100 sites could be active simultaneously with bit rate of 640 kbit/s each. If a 100% fill was possible and all sites active, they could each get 32 kbit/s.  32kbit/s allowance per customers sounds low but is an improvement the typical satellite internet access services bit rate allowance of 10 kbit/s around year 2000. It is important that users are sparing in their use of satellite resources and minimise use of 'streaming' applications like video and audio which involve massive transfers of data and cause congestion..

During mild propagation events, a small basic link margin, say 3 dB, means that there is no adverse effect. This assumes site dishes are accurately aligned and not casually set up so they only just work OK.

In the download direction during more degraded propagation conditions at a single site the bit rate to that site is reduced to say 21 Mbit/s, but there is the opportunity, if the carrier is not congested, for the data block to that particular site to be tripled in length, thus maintaining the customer bit rate per whole second. Because the full bit rate down (64 Mbit/s) to all the other sites is maintained (in clear sky), the total carrier capacity is hardly affected.

Obviously if many or all sites in a beam are suffering from rain then there will be congestion as the total capacity available is reduced to 21 Mbit/s (10.5 kbit/s per site).

A typical DVB-S2 carrier has a structure made up a blocks of different bit rates and modulation/coding types, say 64, 50 and 21 Mbit/s, and the proportion of each type, say 0.95 8PSK, 3/4 8PSK, 1/2 QPSK, will depend on the distribution of types of weather across the beam coverage.

Uplink attenuation is similarly mitigated. Good customer dish pointing should give a basic small link margin. If the customer transmit amplifier is rated at say 3 dB above the clear sky operating power then there is scope for uplink power control to deal with some uplink attenuation. Uplink burst bit rate can be reduced by half and burst length doubled. As with the downlink, the effect on everyone's service is minimal if only a few sites are affected. Obviously if all sites in a beam are affected by rain the uplink capacity is reduced, since there will not be enough time for them all to transmit with double length bursts (at half speed) to achieve the normal per-second throughput.

The above attenuation mitigation methods can reliably deal with attenuations of around 5 - 6 dB, thus dealing with all but the most severe rain events. Given that satellite connection to the internet is the "last hope" option for many customers I think they are generally grateful for a service that works for at least 99.5% of the time, since otherwise they might have nothing at all. A very few customers have special high reliability or high bit rate needs and sales people should be careful not to mislead such people. If a customer needs an outside broadcast feeder link, for example, there is no reason why a special inbound carrier cannot be set up, perhaps using a larger customer dish and transmit amplifier.

Regarding dry snow, this is not a big deal, the problem comes when the snow melts or turns to solid ice on the dish. Carefully brush it off or use heaters.

Please if anyone knows better, tell me the facts ! eric@satsig.net


Page started 10 March 2016.   Page amended 11 March 2016, 18 April 2017