EUMETCast Europe Link Margins Explained
This bulletin gives background information about observed variations and long term trends in the link margins for the different EUMETCast Services.
For each EUMETCast service (Basic, High Volume, transponder 1, transponder 2), the link margin measured in dB is the essential parameter describing the link quality, i.e. the link margin must be positive for error-free reception. The clear sky link margin (clear sky at uplink and downlink sites) gives information about the yearly availability that can be achieved under typical weather conditions.
The link margin depends mainly on the following factors: The uplink equipment, satellite transponder settings, the satellite position, the reception equipment, and weather in the uplink and downlink path.
The uplink system is feeding the telecommunication satellite with the modulated carriers for the different transponders. The output (downlink) power of each transponder has a given set point and is automatically compensating (within certain limits) for variations in the uplink signal. Therefore, weather related attenuation in the uplink path will be fully or partially compensated up to a certain limit. The maximum transponder power setting is called saturation. A saturation test has to be performed to find the optimal configuration of uplink and transponder settings.
On the reception side the optimisation includes pointing of the antenna and find the best focus and skew angle of the LNB. Figure 1 shows a typical Basic service link margin plot.
In this plot the following typical variations in link margin can be observed:
- For fixed pointing reception antennas: Daily variations due to satellite orbital movement by inclination. With increasing reception antenna size this daily oscillation gets stronger due to the narrowing beam width.
- Monthly variation due to satellite station keeping. The East-West orbital position is corrected by maneuvers in the order of a month causing a similar but less pronounced monthly effect to the daily oscillations.
- Weather effects, mostly due to Thunderstorms, causing deep dips.
- Start of operation of a neighboring transponder. This effect is shown in the plot in week 25. The impact is caused by non-optimal antenna and LNB alignment and/or by LNB with insufficient cross polar isolation. This event is described in more detail below.
- Change of transponder output back-off in week 27. (see also below)
- Optimisation of transponder output back-off end week 31. (see also below)
- Switchover between prime and backup uplink station. This is mostly visible in the High Volume Service link margin and can have the opposite effect on transponder 1 and 2.
On 21 June there was a perceived degradation in link margin for the EUMETCast Europe (transponder 1) service.
On that day a service was started on the neighbouring transponder with the opposite (vertical) polarisation on E10A. The degradation occurred only on antennas which were not optimally aligned to suppress the signal from this transponder or with LNBs having insufficient cross polar isolation.
If you experienced a degradation in link margin on 21 June, please optimise the antenna alignment following the antenna pointing guide.
In particular it is important to optimise the skew angle of the LNB for maximum link margin. In a few cases, where the LNB itself does not have sufficient cross-polar isolation, an improvement of the link margin is not possible and the LNB must be replaced. Please contact our User Service Helpdesk to confirm that this is the case.
On 3 July the second transponder was upgraded from half to full capacity and on 6 July output back off of both transponders reset to nominal values for the MODCODs used.
The output back off of transponders determines how far the transponder output power is away from saturation. Close to saturation the transponder is no longer working in the linear range and signal distortion occurs. This has less impact on the 8PSK MODCOD, but high impact on the 16APSK MODCOD, since 16APSK is also using amplitude modulation. The effect is that, although the signal level is increased, the actual link margin will decrease. The increase of output back off resulted in a decrease of link margin by more than 1 dB for 8PSK (Basic Service), no change for HVS-1, and increase by more than 1 dB for HVS-2.
From 2–4 August the output back off settings of both transponders were optimised.
The output back off of both transponders were changed step-by-step in order to reach the optimal setting. On transponder 1 only a best compromise was possible due to the different optimal points for Basic Service (8PSK) and High Volume Service (16APSK), and due to different optimal points for different receivers. As a result the observed link margins are now very close to the long-term maxima. The difference between transponder 1 (HVS-1) and transponder 2 (HVS-2) is due to different transponder characteristics.
It should be noted that the achieved link margins are above the guaranteed values. The antenna availability plots used for antenna size requirements are based on the guaranteed values.
Keeping link margins optimal
We recommend that users repeat a fine tuning of the antenna alignment on a yearly basis, to correct misalignment from environmental sources (weather, corrosion, etc.). A yearly check of the LNB condition is also suggested and a replacement of it if needed. It is important for the new LNBs to support focusing and tilting.