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Ground station networks vs. GEO relay satellites for polar orbiting satellites data download

Baard Eilertsen.1 Kongsberg Satellite Services AS, Prestvannveien 38, 9291 Tromsø, Norway

and

Petrus Hyvönen2 SSC, Solna Strandväg 86, 171 04 Solna, Sweden

Abstract

The European Data Relay Satellite system (EDRS) is regularly attributed with a number of advantages over traditional ground stations for the purposes of telemetry, command and data download to and from scientific and commercial satellites in low, polar orbits, using optical link technologies to obtain a major advance in overall utility and performance. These advantages seem intuitively credible and are seldom questioned, probably because they have never been publicly expressed in other than very general terms. The program is furthermore hailed by ESA as a successful example of the application of the Public Private Partnership concept and as such is clearly positioned as a competitor to the existing European ground station network operators. This paper presents an objective comparison between the two systems in terms of the major, relevant performance parameters: real time coverage and data download capacity as well as download of stored data and the expected latency of such. A financial comparison and value for money evaluation is also undertaken.

I. Introduction The EDRS Data Relay Satellite system, currently being developed as a Public Private Partnership (PPP) venture

under the European Space Agency’s Artes 7 program, has a stated goal of providing Europe with an independent access to a data relay satellite system for Low Earth Orbit (LEO) observation and scientific satellites. EDRS is initially aimed at serving future, public programs such as the ESA/EU Sentinel series, nominally providing these satellites with a Real Time telemetry and control (T&C) and high capacity data download facility whilst overflying the European continent.

Europe already disposes of an independent means of performing TT&C and data download services through a large number of European satellite ground stations, both under private and institutional ownership. In particular, the privately owned ground station networks operated by the Swedish Space Corporation (SSC) in Sweden and by the Norwegian company Kongsberg Satellite Services AS (KSAT) have both been successively extended to provide what can today be characterized as a global Near Real Time service for LEO satellites. The networks comprise near-polar and high/mid-latitude stations to that end, disposing of a large number of full-motion S- and/or X-band capable antennas and currently handle in excess of 20 000 satellite passes per month.

EDRS represents a massive undertaking, initially investing 275 MEUR1 of public funds to establish a system which will be in competition with an existing, fully commercial and successful European space industry sector. The investment is being justified by the premise that the system will deliver real-time access and high capacity for publicly funded and commercial satellites within the EDRS coverage area, claiming that it largely outperforms the ground station networks all these areas2. In the following, the authors will compare the two systems and examine the justification for that claim.

1 Director Business Development, KSAT, Prestvannveien 38, 9291 Tromsø, Norway 2 Systems Engineer, SSC, Satellite Management Services, Solna Strandväg 86,Solna, Sweden

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II. System Descriptions The two communication systems are fundamentally different both in their architecture as well as in the

requirements on the LEO satellites’ on-board TT&C equipment and systems design. In the following, the main characteristics of the two systems are highlighted in terms of architecture, overall performance and technical consequences for the LEO satellites.

A. Ground Stations 1. Architecture and Coverage

An RF link (typically S- and/or X-band) is established between the satellite and a full motion ground antenna during the part of the orbit when the satellite is more than 5 degrees above the local horizon at the antenna site. Given the overwhelming dominance of polar orbits for LEO satellites an antenna site as close as possible to the polar regions maximises the number of possible passes per 24 hours. Each pass can have a duration of up to approximately 13 minutes; typical duration is on the order of 10 minutes and anything under 6 minutes is normally not considered as commercially acceptable. With stations close to each Pole, access to the satellite can be assured twice per orbit on most orbits, thus permitting data download at intervals of 45 minutes. Increasingly, customer insistence on ever decreasing time from image acquisition to data delivery (latency) has resulted in the construction of mid-latitude stations, further reducing the average latency.

For European real time coverage, there are multiple ground network stations available, providing X-band, real time communications with a coverage corresponding to the proposed EDRS coverage. Figure 1 gives examples of European stations available for inclusion. A continuous downlink capacity over the entire European area is therefore possible using a combination of two (or more) of the listed stations.

In addition, downlink of on-board stored data is provided by a large number of stations that can be combined into networks with low latency, as shown in Figure 2 below.

Figure 1. European coverage provided by existing ground stations (all owners). SvalSat (Svalbard), Tromsø (Norway), Esrange (Sweden), Neustrelitz (Germany), Matera (Italy) and Maspalomas (Spain), all on European territory. Coverage for 700 km altitude satellite shown.

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2. Ground Station Downlink Rates Most current Earth Observation satellites utilize an X-band link for data download with the highest data rates

situated around 800 Mbps. DigitalGlobe’s WorldView-3 satellite, to be launched in 2014, will operate at a downlink rate of 1.2 Gbps3.

There is a lot of activity within the LEO community looking at the option of using Ka-band for the data

downlink. Work performed by L-3 Communications on the SCRAM (Space Communication Rates at Multi-Gbps) Ka-band link, as reported in a presentation 20114, indicate that rates up to 3.75 Gbps should be achievable between a LEO satellite and a ground station in the foreseeable future. Considering that the adoption of Ka-band technology is likely to occur well within the lifespan of even the initial EDRS payloads, it would have been reasonable to account

Figure 2. Example of stations that can be used as a high capacity network, or combined to a a low-latency network for download of on-board stored data, coverage circles for 700 km altitude satellite. This full network is also referred to as Net-C in simulations shown in the paper.

Figure 3. Artist’s impression of WorldView-3

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for this in the comparison, but the authors have elected not to do so given the prevailing lack of detailed information on actual equipment characteristics and system performance.

3. System Flexibility

An inherent quality of the ground network solution is the capability of handling multiple satellites simultaneously through the use of either time share on the antenna or using two or more antennas when multiple satellite passes occur at the same time. The larger European sites, such as SvalSat and Esrange, each operate a number of antenna systems in the 7-13 meter diameter range. Multiple antennas furthermore assure a significant level of overall system redundancy. If more capacity is necessary, an antenna typically represents an investment on the order of 1 MEUR, dependent on frequency capability and size and assuming that it will be placed on an existing station with the proper infrastructure already available. The deployment time to an operational antenna is less than a year.

Taking full advantage of the geographical distribution of the entire European ground station network as illustrated in Figure 1 above, a LEO satellite can be in continuous contact with a ground station for the entire duration of its passes over Europe.

B. EDRS 1. Architecture and Coverage

When fully deployed by 2015, the EDRS system in its current definition will consist of two Geostationary Orbit (GEO) satellite payloads, one as a hosted payload on a Eutelsat communications satellite (EDRS-A) and the second one on a dedicated platform (EDRS-C), situated at 9°E and 22.5°E respectively. The primary means of assuring the downlink will be through an optical Inter Satellite Link (ISL) from the LEO satellite to EDRS effectuated through a pointing laser terminal on both satellites, and then a “normal” RF link between EDRS and the ground. This allows for continuous communications with a LEO satellite at all times when it is within line of sight view of the EDRS terminal. Each EDRS payload will carry a single optical terminal. EDRS-A is additionally equipped with a Ka-band payload which can be used for an RF ISL.

ESA and its private partners have stated that the EDRS services will be offered on the commercial market. Indeed, this would seem to be a prerequisite for the whole concept of a PPP; without the commercial component EDRS would just be a purely publicly funded program operated on revenues from other public programs. The commercial operators do not limit themselves to a European coverage. Taken together with latency issues as well as total download capacity limitations, this would then dictate that the coverage should eventually be global. The recently launched EDRS SpaceDataHighway5 web site makes reference to plans for two additional EDRS payloads with launches scheduled for 2017 and 2019 for that purpose.

Figure 4. EDRS Architecture6

2. Downlink Rates The maximum downlink rate using the optical ISL is 1.8 Gbps6. The Ka-band ISL on EDRS-A is capable of

providing a low, 300 Mbps rate and will therefore be disregarded in the present comparison.

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3. System Flexibility At any given time, each of the EDRS optical ISL terminals can only service a single customer. Hence, with

more than two satellites simultaneously in view, a resource sharing will be required, and continuous communications within the full coverage area cannot be maintained for each customer. The time allocated to each customer satellite will be inversely proportional to the total number of users. Capacity augmentation, unlike the ground network situation, is costly, time consuming and complex, involving the procurement and launch of an additional EDRS payload, hosted or dedicated. The cost of an additional EDRS GEO payload is estimated to be on the order of 175 MEUR given the publicly released number quoted for the initial system.

III. Satellite Systems and Risk

A. Ground Stations From the very beginning of the space age, the contact between satellites and the ground was assured by an RF

link between the satellite and a ground station with the obvious limitation that the required line of sight condition could not be assured worldwide on a continuous basis given orbit characteristics and a limited number of actual stations (except for geostationary applications). Practically every satellite ever flown has carried an RF TT&C subsystem. RF based satellite TT&C subsystems are therefore extremely well proven, robust and widely available off the shelf.

The pointing and stability requirements for an RF antenna will likely be less demanding than the optical ISL.

B. EDRS The LEO satellite will need a laser communication terminal (LCT) for the ISL. The EDRS LCT is an

adaptation of the LCT which has been flown on the German TerraSAR-X satellite. According to information contained in an ESA presentation6 in 2011, the mass and power characteristics of the EDRS operational LCTs will be 60 kg and 150 W, respectively, significantly higher than for an RF equivalent. Given the higher degree of complexity of the optical terminal, the cost can be expected to be higher than for the simpler RF baseline.

Additionally, the LCT onboard the EDRS satellites, constituting an obligatory link in the overall LEO to ground chain, increases the overall system failure risk. This is somewhat mitigated by the EDRS-A/C redundancy, but losing one satellite will halve the available system communications capacity, significantly reducing the data transmission volume and time slots allocated to each customer.

C. Ground Stations vs. EDRS EDRS will require costlier, bulkier and heavier equipment on board the customer satellites. The LCT on EDRS

represents an extra risk factor in the LEO to ground communications reliability figure. The mass penalty is a significant factor for commercial satellites given the trend towards lower overall satellite mass. In a commercial environment, customer acceptance of these disadvantages would have to be compensated through a significantly improved service and coverage offering. This will be analysed in the next two chapters.

IV. Service The service provision from a satellite communication system for EO satellites can be specified in two main

capabilities, direct reception and data transfer. The direct reception provides a real-time link to the satellite and enables acquisition of imagery and sensor data without intermediate storage in the on-board memory. The geographical coverage of where this direct link can be established is an important parameter.

For the data transfer, payload data such as imagery has been acquired and stored on-board. The network needs to transfer this large amount of data, with a low latency to the users.

A. Direct Reception Coverage 1. Ground Networks

Given the locations of the ground stations in Europe, real time coverage can be provided to a LEO satellite for the entire time it is over European territory. The figure below gives an example of such network, providing the full European coverage using two existing ground stations. For all intents and purposes, this coverage can be provided to an unlimited number of satellites given the inherent flexibility of adding an (low cost) antenna whenever total capacity demand so warrants.

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Figure 4. Example of real-time ground network consisting of two ground stations, providing full coverage over Europe

2. EDRS

The initial, operational EDRS system provides a European coverage, with an ISL possible as soon as a satellite is in view of an EDRS LCT. The system can provide full, uninterrupted coverage for up to approximately 45 minutes each orbit6, but will be limited with an increasing number of simultaneous users.

B. Data Volume in Direct Reception 1. Ground Networks

We have seen that it is possible to assure real-time coverage over the entire European area for all satellites. The longest ground track over the European continent is a little less than 4500 km in distance, corresponding to around 10 minute’s flight time for a LEO satellite. The corresponding data volume during a direct reception pass is 90 GByte in X-band (and could be several times higher in a future Ka-band system). 2. EDRS

Assuming that the satellite has access to a link for the full duration of the10 minutes over the European territory, and given the same assumptions as in the previous paragraph, the real-time downloadable data volume would be 135 GByte, not a quantum increase compared to the ground network. As soon as the number of satellites served by each LCT is larger than two there is an increasing probability of a conflict during the pass over Europe, eventually to the point of not assuring real time coverage during the whole passage over the European mainland and thus reduced download volume.

The further two satellites planned to be launched in 2017 and 2019 would increase the overall coverage area of the EDRS system and the total daily stored data download capacity as well as improving the latency, but would not change the real-time limitations as outlined above.

C. Download of On-board Stored Data There are two main parameters for the download of on-board stored data, the capacity of the network (such as

how many TByte that can be downloaded per day), and with what latency it is delivered (i.e. how frequent is the access to the satellite, determining latency or age of the collected data).

1. Ground Networks

Given the global distribution of European-owned ground stations, the maximum latency would be on the order of 50 minutes and the total data volume download capacity would be on the order of 3.7 TByte/day in X-band. The number of satellites served is of no particular consequence for the capacity since additional antenna capacity is inexpensive to install. The maximum latency for a ground network will be determined by the number and locations of sites in combination with the selected orbit.

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2. EDRS The latency for EDRS is determined by the GEO visibility in combination with the LEO terminal

characteristics, resulting in 45 minutes6 – about half the orbit. However, when the system serves multiple satellites, the actual allocated time slot could very well be at the beginning of the coverage area during pass n and at the end during pass n+1, thus resulting in a latency of up to 105 minutes (example using 15 min timeslot). The total data downlink capacity for a satellite that has priority access to an EDRS satellite and no interruptions is 8.5 TByte / day.

3. Data Volume Comparison

The capacity of a relay system and a ground network scales differently with increasing number of satellites, as the ground network can be more easily adapted to the forecasted load. The EDRS system provides a very high initial capacity for a few satellites, but the geostationary resources do not scale without launching a completely new satellite.

Figure 5. Total daily volume comparison (Tbyte) for a simulated scenario using EDRS and three different ground networks, composed of currently established ground stations. The differences in ground network volumes with increasing number of satellites is due to orbit differences in the added satellites.

A simulation was performed, with a set of orbits taken from the following set of satellites: Sentinel, Spot-4, WorldView-1, GeoEye, QuickBird-2, Pleiades-1, TerraSar-X, Cosmo-Skymed-2, Spot-5, Ikonos-2, Aqua and Formosat-2. Losses due to switching between satellites for EDRS were not taken into account for the volume simulation. The ground networks were:

Net A = Esrange, Inuvik, Troll, Punta Arenas, Hartebeesthoek, Matera Net B = Esrange, Svalsat, Inuvik, Troll, Punta Arenas, Hartebeesthoek, Matera, WASC, Net C = Esrange, SvalSat, Inuvik, Troll, Punta Arenas, Hartebeesthoek, Matera, WASC , South Point,

Santiago, Mauritius, Bangalore, Dubai, Singapore

From the simulation, as illustrated in Figure 5 above, the EDRS data volume is nearly 50% higher than the large ground system for up to two satellites. For supporting a number of satellites, the systems are equal in volume at around 7 supported satellites for the large ground network, and 10 supported satellites for a smaller ground network. 4. Latency Comparison

The latency determines the duration from the moment the satellite passes over a location on the Earth until it reaches a communications node. Figures 6 and 7 show the maximum latency as a function of geographical position for EDRS and a 6 station ground network, respectively. Areas in dark blue are in direct contact with a communication node and can be used in direct reception mode.

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In Figure 6, a histogram comparing the two

systems, shows that EDRS has a slightly higher fraction in the 0-10 minutes latency, compared to the 6-station ground network. As the satellites are out of view for a long time with the EDRS system, a large fraction is around 50 minutes. For the ground system, the majority fraction outside the 0-10 min range, is in the 20-30 minutes, with a few outliers going up to 90 minutes.

The EDRS system has been assumed supporting only one LEO satellite. For a real case, the EDRS will support a number of LEO satellites and perform some kind of switching of the link capacity between the users. The details of this switching have not been published and will depend on the operational scenario combined with the performance of the optical intersatellite link.

For example, a pass duration of 4 minutes and a switching time of 1 minute means that satellites are supported in a 5 minutes cycle. For 10 supported satellites, an additional maximum latency of 20 minutes should thus be added both within the coverage region and outside. This would affect the experienced maximum latency significantly for the EDRS system.

For the large ground network, the ground system outperforms EDRS on maximum latency, already with only one supported LEO satellite, as shown in Figure 9.

Figure 7. EDRS Maximum latency, single LEO satellite, orbit altitude 786 km

Figure 8. Latency for ground network, 6 stations, orbit altitude 786 km

Figure 6. Maximum Latency histogram comparing 6-station ground segment (blue, left) with EDRS (red, right).

Figure 9. Maximum latency for a 14-station ground network (blue, left) compared to EDRS (red, right).

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V. Financials As a PPP, and therefore by definition a (at least partly) commercial endeavour, EDRS has to be seen as a direct

competitor to the existing and successful European ground station networks. In that context, the PPP will benefit from a very generous 275 MEUR start-up present courtesy of the European taxpayer. There will nevertheless be further investments and costs associated with the venture. According to ESA, one of the PPP partners, EADS Astrium, is expected to inject approximately 100 MEUR7 in order to round out the initial investment requirements estimated at around 380 MEUR. It is expected that the investment for the additional two spacecraft in 2017 and 2019, presumably primarily aimed at meeting the commercial operators’ requirements, will have to be financed through PPP revenues and not by yet another subsidy, otherwise the whole concept of a PPP makes no sense. Based on the investment required for the two initial payloads it would appear realistic to assume that the two follow-on spacecraft will demand at least an additional 250 MEUR outlay.

The PPP will also have to sustain costs for its commercial activity, as well as to maintain and operate the space and ground infrastructure, probably a few millions of Euro per year.

Unless the foreseen service price levels are an order of magnitude higher than what is current business practice on the ground station side, revenues from the “captive” launch customer programs, the Sentinels, will not be sufficient in order for the PPP to be able to finance these investments and costs, much less make a profit. Hence, the PPP must base its business plan on capturing a significant part of the current ground station market, coincidentally weakening one of the very few healthy, and purely commercial, global space markets that Europe has managed to develop.

Naturally, the authors have not had access to the PPP business plan, but even a very crude modeling of a hypothetical plan based on the cost assumptions above, and a number of customers within the limits of where EDRS provides at least an equal service level to that provided by a ground network, illustrates that EDRS will not be profitable unless the per customer price level is least on a par with current practice and in all probability higher. The alternative is to sign on more customers, but this would reduce the service level to well below ground station network standards thereby rendering it less interesting to a commercial customer.

VI. Conclusions The EDRS system has been championed by ESA as a next generation, high tech successor to the ground

networks for the purpose of LEO satellite T&C and data download services. The 380 MEUR program is claimed to eliminate the need for “expensive and complex”8 polar ground stations which are also supposedly squeezed for capacity (an assertion particularly difficult to understand for the network operators since a capacity problem is very easily resolved by the addition of an extra antenna at a fraction of the cost of an EDRS payload). EDRS is furthermore reportedly superior in terms of both real-time data download volume and latency. So how do the two systems stack up in reality based on the analysis above?

Let us look at two scenarios: 1) EDRS as a wholly publicly funded system dedicated to servicing publicly funded (ESA/EU/national) satellites; and 2) EDRS as a commercial system serving the larger number of satellites required in order to finance sustained operations on commercial terms.

A. EDRS as a public infrastructure vs Ground Stations This case is primarily geared towards a need for services for publicly funded satellites within the initial, Europe-

centered EDRS coverage area. As long as EDRS can serve each satellite for the full duration of the passage, the total data volume which can be downloaded in real time over Europe will be equal to 135 GByte, as compared to the 90 GByte achievable by two, overlapping GS passes (always achievable).

As for total capacity, a maximum of 500 GByte per orbit can be downloaded in a full coverage, non-interrupted pass on EDRS (achievable only if users are very few) whereas three passes on the ground network will always yield approximately 270 GByte, regardless of number of users.

In terms of latency, EDRS will, again depending on satellites served, exhibit a latency of initially around 50 minutes, increasing with the number of supported satellites. The ground network, using the example shown in Figure 6, will provide almost all latencies below 50 min, with a few exceptions running up to 80 minutes.

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Hence: for a low number of satellites requiring service, EDRS has a small edge on real-time download volume and can provide double the daily data volume when considering the full coverage area. The ground network provides superior latency. As soon as the number of satellites increases, the EDRS real-time and stored download advantage diminishes and eventually disappears.

Given the above, the question that needs to be asked is: which currently planned ESA, EU or national program drives the requirement of a 500GByte data download volume per orbit over Europe, necessitating a 380 MEUR investment?

B. A Commercial EDRS vs Ground Stations A commercially motivated EDRS system, financed on commercial terms, will need a large number of

customers to be financially viable. However, the analysis shows that with any number of satellites beyond 8 or 9, the existing ground network provides higher performance than EDRS on all main parameters, at a lower complexity and vastly lower system cost. The networks exist, with the capacity to handle the entire current public and commercial markets without any further investment. Upgrading to even higher capacity only requires the investment in additional antennas at around 1 MEUR each, two orders of magnitude less than the EDRS investment! If even lower average latency is required, another six full stations built from scratch, at new geographical locations, would only cost on the order of 25 MEUR.

Commercial customers demand global coverage. Therefore, the EDRS system would have to be extended in order to provide that. Even then, the service level would not be superior to ground networks unless the number of EDRS payloads in each GEO position is augmented at great cost. Unless the investment in the extended system is yet again subsidised by the public partner in the PPP, the service prices would not offer any advantage over the ground networks. Providing subsidies to one private player in a competitive market is hardly a path to be contemplated by ESA without raising grave concerns on the part of the member states.

A further concern in this scenario would be the acceptability within the commercial satellite operator community of a system that requires the adoption of dearer, heavier and more complex communications technology on-board their satellites, yet without offering a significant advantage in performance. The LEO commercial operators are no different from their GEO counterparts: they are looking for technically proven, simple and cost-effective solutions. EDRS currently satisfies none of these criteria.

As the performance of a commercially offered EDRS is at best equal to the ground based systems, it is also questionable if a commercial LEO operator would appreciate being limited to a single service provider. For a ground solution, there are at least two commercially competing providers of ground service, but for the optical relay there would be only a single provider as no purely commercial business case can justify the initial establishment cost.

In summary then, apart from the claimed advantages for the GMES Sentinel and other public programs’ data

access and which we have seen are only valid for certain parameters and under severely limited constraints on number of satellites served, the EDRS public investment is being justified on the merits of the PPP aspect of the program. But: the much higher number of customers that would be required in order to render EDRS profitable as a purely commercial endeavour on equal terms with the existing downlink industry will only exacerbate the capacity problem, enlarging the performance gap relative to the ground stations. Furthermore, the commercial LEO satellite operators, like their GEO counterparts, are infinitely more sensitive to the trade-off between technical performance and cost/price/risk than institutional customers. They can be confidently expected to choose the alternative which provides the best trade-off between price and performance at the lowest risk possible. The PPP partners appear to share the same view: in a Space News article published on 4 October 20119 an ESA official provides information on the creation of a joint ESA-Astrium committee “..that will track EDRS’s development and, if necessary, propose additional ESA investment... to protect Astrium against the possibility that no data relay market materializes...”.

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References References

1 Space News, 4 October 2011 2 European Space Agency EDRS Factsheet 2009 3 WorldView-3 Satellite Sensor, Webpage as of 2012-05-13, http://www.satimagingcorp.com/satellite-sensors/worldview-

3.html 4 Presentation given by L3 at a NASA “Workshop on X-Ray Mission Architectural Concept”, December 15, 2011. 5 EDRS SpaceDataHighway webpage, as of 2012-05-15, http://www.edrs-spacedatahighway.com/ 6 ESA presentation given at the EDA Workshop in Brussels on 7 June 2011 7 SpaceNews, 2011-01-28, ”ESA Secures Full Funding For Data Relay Satellites” 8 Imaging Notes, Spring 2012, Vol. 27, No. 2, pg. 36-38, ”Space Data Highway, Redefining Satellite Data Transfer” 9 SpaceNews, 2011-10-04, “Astrium, ESA Sign Deal for High-speed Data Relay System”