2.1

OGS Site Diversity

A typical ground segment network with optical ground stations is depicted in Figure 1. As discussed in [2], the overall traffic can be divided between (N) active OGS, using smart gateway diversity techniques. A number of redundant OGS (P) is needed in order to achieve the required aggregate feeder link availability. The ground segment therefore consist of N (active) + P (redundant) optical ground stations. The feeder link is considered fully operational when at least N out of N+P sites are under Cloud Free Line of Sight (CFLOS) conditions. To handle atmospheric blockage events, it is assumed that the network has the capability to switch traffic dynamically to redundant OGS when active OGS are no longer in CFLOS conditions, without any loss of data. The particular case of N=1 active OGS means that the full traffic of the feeder link goes through a single OGS, while other OGS are in standby.

Figure 1.

A VHTS system with a network of N+P OGS, connected to the core network with dedicated optical fiber, together with a central computing facility.

When sizing the network of N+P OGS, it is expected that:

2.2

OGS Sizing

Table 1 presents different optical terminal sizes ranging from 100 cm down to 20 cm, that would be required for different values of N.

Table 1.

Scaling OGS for different values of N.

For simplicity, it is assumed that the throughput per terminal (calculated based on the user requirements described in [4]) is proportional to its effective area (so to the square of the diameter), with a transmit power of a few hundred Watts. In practice noise floors may limit the efficiency of the telescope and lead to different values. Uplink disturbances such as index of refraction fading and beam wander must be pre-compensated in the OGS transmitter with adaptive optics techniques, which should be easier to achieve when the terminal is smaller. The cost of the analogue part of the terminal, including the telescope, has been estimated for different OGS sizes and calculated relative to the throughput.

2.3

Selection of OGS Sites

Next, the selection of the OGS sites is carried out to form the feeder network. In doing so, it is important to keep OGS locations sufficiently separated in order to avoid correlated cloud blockage events, as clouds usually extend over tens of kilometres. In practice, the separation should be typically larger than 300 to 1000 km, which effectively limits the number of OGS that can be instantiated on a given geographical area. Another important aspect is the distance between the OGS site and the IXP, which heavily impacts the cost of interconnection by optical fiber cable.

Considering a VHTS satellite positioned between 5°W and 30°E of longitude on the GEO arc, two scenarios are envisaged for the OGS sites: Europe only, and EMEA (Europe, Middle East and Africa).

An example network with 18 OGS sites in the EMEA region is illustrated by Figure 2. Most OGS sites are located close to optical fiber cables or to undersea cables landing sites to lower the cost of connectivity. The average annual CFLOS for these OGS sites is shown in Table 2.

Figure 2.

Example network of OGS locations in Europe, Middle East and Africa.

Table 2.

Average Annual CFLOS for OGS locations from Figure 2.

In the study, an initial list of OGS sites in the European scenario has been chosen from previous publications and ESA studies. When extending to Africa and Middle East for the second scenario, the candidate sites were chosen considering favourable atmospheric conditions, proximity to fiber points, and existing ground stations facilities (e.g. for earth observation satellites).

A sophisticated cloud blockage time series generator developed by ONERA, that makes use of meteorological databases of past years [5], has been used as a reference to select the OGS sites giving the best CFLOS performance. Feeder link CFLOS availability was calculated not only per OGS site (the time period considered was 2005-2006), but most importantly the aggregated availability was computed for a combination of OGS sites at different geographical locations, taking into account the actual correlation of their respective CFLOS in time. In other words the time series have been used to assess the complementarity between N+P OGS to satisfy the required availability over time for at least N OGS. It should be noted that the feeder network availability has been estimated assuming ideal and instantaneous switching between OGS, assuming perfect knowledge of the channel conditions. Hence, results do not account for operational constraints of a large optical ground network.

The problem to be solved for selecting the best sites from the initial list is the following: amongst the M candidate OGS sites of the initial list, how many redundant sites (P) are required in order to achieve CFLOS conditions for at least N active sites within the target availability (for instance 99.9% of the time)?

The algorithm to select OGS consists of the following steps:

This incremental algorithm gives a list of OGS sites sorted by CFLOS performance. The notion of aggregated CFLOS actually depends on the number of active (N) and redundant (P) OGS for the required availability. For this purpose the incremental algorithm uses weighting factors on the cloud blockage time series when calculating the performance gain brought by an additional OGS. The advantage of this simple algorithm is to provide a quick answer to the problem for all values of N, without requiring exhaustive calculations involving large combinatorial, which are computer intensive. In Table 2, the 18 OGS sites listed are actually ordered according the incremental algorithm: from an initial list of M=40 OGS, site 1 (Dubai) corresponds to the highest CFLOS, and other sites have been added 1 by 1 with the objective to maximise the combined CFLOS. Interestingly, even OGS sites with relatively low individual availability like site 7 (Jos, Nigeria, with 50.21% average CFLOS) appear to provide a high aggregated CFLOS when combined with previously selected OGS.

2.4

Simulation Results

Tables 3 & 4 present the results of simulation for sizing the number of redundant OGS (P) for a certain number of active OGS (N values from Table 1), for availability targets of 99%, 99.9% and 99.99%, based on the time series from 2005 and 2006. The initial number of OGS sites was M=27 in the European scenario, and M=40 in the EMEA scenario.

Table 3.

N+P results for OGS sites in Europe.

Table 4.

N+P results for OGS sites in EMEA.

The results can be analysed as follows:

The relative overhead of redundant sites (P) compared to active sites (N) can be expressed as P/N, to reflect the additional resources needed to be deployed to accommodate for redundancy. P/N ratio is plotted in Figure 3 for values of N ranging from 1 to 12, for the 3 availability targets, and considering 3 different cases: the European and EMEA scenario (using the time series), and a binomial model. The binomial model corresponds to the case where all OGS sites are assumed to be independent from each other, and where the probability of CFLOS per site is 80% at any time, reflecting typical CFLOS performance from Table 2. The aggregated CFLOS availability follows a binomial distribution as a function of N and P. This model was analysed in [2] and the results have been extended in Figure 3 for higher availabilities and for higher values of N, where the probability distribution tends to become Gaussian.

Figure 3.

Relative overhead for OGS redundancy P/N.

The expected statistical gain, corresponding to a decrease of P/N as N increases, can be seen especially for the first values of N and for the binomial distribution. However there appears to be some asymptotic effect for the two scenarios based on the time series when N is larger than 4. This behaviour could indicate some correlation of time series between adjacent sites. Another reason could be the limited number of candidate sites as N increases, reducing the benefits of site diversity. For instance, in the European scenario targeting 99.9% availability, N+P=12+28 case makes use of all the M=40 candidate sites from the initial list. If such results were encountered also for higher values of M, considering more candidate sites, it would mean that the geographical de-correlation hits a limit, and it would be more efficient in that case to deploy a limited number of active sites and instantiate multiple OGS per site.

3.1

Ground Segment

Technologies for optical reception and transmission on ground continue to progress, notably in Europe. The main challenges concern the transmit part (forward uplink), which has to achieve a higher throughput than the return link. One advantage of splitting the feeder link traffic between multiple smaller optical terminals is to reduce the requirements in terms of aperture and throughput, as shown in Table 1, without having to wait for the most demanding technology (1 m, 492 Gbit/s transmit) to be available. Another benefit is to relax the requirements for adaptive optics on the uplink transmission, which are less stringent for smaller size terminals. Other technical challenges in the optical domain could be the output power levels that can be achieved with High Power Optical Amplifiers (HPOA) and bulk multiplexers following the HPOA bank to collect all sub-channels into a single optical beam.

For the digital part of the OGS, most functions are already available off-the-shelf for RF VHTS systems: modulators & demodulators, digital processing (network control, traffic classification, data compression…). The amount of redundancy needed, both at system level (N+P dimensioning of OGS network) and inside the OGS gateway, reduces in a distributed approach (N>1). Further savings can be envisaged in the digital part, for instance by moving digital processing functions (N+P elements) to a central place where only N active elements are needed, but requiring dedicated connectivity between all OGS, as discussed in [2]. Section 4 presents the economical aspects of these different options.

Connection between the OGS and the core network requires optical fiber cables. Two cases are distinguished:

Different fiber solutions can be envisaged: owning a fiber network, long term rental of dark fibers (IRU scheme), rental of wavelengths (shared fiber). Whatever the scheme, the fiber needs to be available at all times because switching between OGS occurs dynamically. Fiber costs very much depends on two factors:

3.2

Space Segment

While the space segment characteristics for a VHTS system are discussed in detail in companion paper [4], the present section addresses the specific needs of the satellite optical terminal in the case of multiple OGS.

The approach of having as many dedicated on-board telescopes as OGS, which can be several tens, appears to be too demanding to be accommodated on a single satellite platform, whether N active OGS are tracked (requiring at least N telescopes), or N+P fixed OGS sites. An alternative approach is a single multi-feed optical terminal on-board the satellite, as illustrated in Figure 4. Such concept was presented by RUAG in [6]. Assuming the OGS locations are known and fixed during the lifetime of the satellite, a single coarse pointing assembly can be designed, integrating both receiving and transmitting functions. Provided that the satellite position is kept in a small box on the GEO arc, the multifeed telescope can track all OGS at the same time, in particular for pointing ahead angle (PAA) and to compensate for residual satellite rotation.

Figure 4.

Multi-feed optical terminal. Each colour corresponds to a different OGS.