Inmarsat Global Limited SB-SAT Sustained Data - SpaceOps 2012

Inmarsat Global Limited

SB-SAT

Sustained Data Relay for Low Earth Orbit Satellites

14 June 2012

Johnston, B (1); Haslam, M (1); Trachtman, E (1); Goldsmith, R (2); Walden, H (3); McGaugh, P (3)

(1) INMARSAT (United Kingdom), 99 City Road, London, EC1Y 1AX, U.K. Johnston, B [email protected] +44 (0) 20 7728 1146 Trachtman, E,

[email protected], Haslam, M, [email protected] (2) COM DEV Europe (United Kingdom),Triangle Business Park, Stoke Mandeville, Aylesbury,

Bucks, HP22 5SX, U.K. Goldsmith, R [email protected] +44 (0)1296 616491, (3) Broad Reach Engineering (United Stated of America), 1113 Washington Ave., # 200 Golden,

CO 80401. Walden, H, [email protected], (303) 216-9777 x217, McGaugh, P, [email protected], (303) 216-9777 x215

SB-SAT SpaceOps Symposium Paper

June 2012

© Inmarsat – 15 May 2012


1. Introduction Real-time data relay to and from LEOs is difficult to obtain with few existing options. Ground station access to LEO spacecraft has a limited window of a few contacts a day for short periods of time. Alternatively, a dedicated ground infrastructure is expensive to install and operate. Existing GEO data relay systems offer limited or restricted access with scheduling or booking requirements. Additionally, time delays between downlinks can be an operational issue, especially during anomalies. Inmarsat recognises that a low latency communication link between the spacecraft and mission control has many benefits for LEO operators and has developed the SB-SAT product by extending the capabilities of the existing Broadband Global Area Network (BGAN) satellite network. The SB-SAT terminal provides the LEO subscriber with the benefits of real time connectivity to their orbiting assets delivering an always-on link to the LEO operator’s mission control ground facility. The network connection is via three geostationary, Inmarsat-4 satellites maintained by Inmarsat. This presents a unique opportunity to develop new LEO satellite applications based on real-time delivery of relayed satellite DATA via the SB-SAT service. This paper describes the general operating concepts, terminal specifications and service capabilities of the SB-SAT product and will propose applications for LEO principle investigators and mission designers.

2. The SB-SAT Concept

An SB-SAT terminal is simply a communication device that can be installed on low Earth orbiting spacecraft to maintain a data link between the spacecraft and mission control. The terminal provides all of the required communication protocols and the radio link to the existing Inmarsat network. Each SB-SAT subscriber becomes another user on Inmarsat’s network in the same way that existing users of services such as FleetBroadband or SwiftBroadband utilise the network, as shown in Figure 1. The SB-SAT terminal provides persistent, real time, global broadband data IP connectivity to low-Earth orbiting spacecraft. Data services are provided through Inmarsat’s Broadband Global Area Network service (BGAN) which can deliver always-on IP connectivity to the LEO operator’s mission control ground facility. The SB-SAT terminal connects to the network via the Inmarsat I4 constellation consisting of three geostationary satellites maintained by Inmarsat.


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Figure 1: System Overview

3. Inmarsat and the BGAN Network Inmarsat owns and operates the Inmarsat-4 constellation of geostationary satellites over which the BGAN network functions. Launched between March 2005 and August 2008, each satellite has the following characteristics: Spacecraft Power: 12 kW L-band EIRP: 67dBW Launch Mass: 6 Tons Solar Array Span: 48 m The SB-SAT terminal communicates with the BGAN Radio Network Controller (RNC) nodes. The I4 satellite design provides transparent amplification and frequency translation of the BGAN communications and signalling traffic. Transmission between the I4 satellite and the SB-SAT terminal (host spacecraft) is via the L-band ‘service link’ which is re-used by dividing the coverage into a large number of beams (an I4 satellite can generate up to 256 beams simultaneously). The beam overlays are operator configurable but a typical arrangement consists of a global beam over which is overlaid 19 regional beams and upon which is arrayed approximately a further 200 narrow beams (also called spot beams). The SB-SAT terminal automatically transitions between beams as required by the movement of the LEO spacecraft. The beam hand over process is autonomous, seamless and transparent to the user. The BGAN network therefore consists of a mature, established architecture and, as a BGAN service variant, the SB-SAT service will benefit from this stable platform as shown in Figure 2.


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Figure 2: BGAN network architecture

4. SB-SAT Terminal Characteristics and Operation The SB-SAT terminal is intended to be deployed on stable, nadir following platforms. Other mission types can be considered on a case by case basis. The fundamental components of the SB-SAT terminal are shown below: Terminal Box 7.8 kg Gimbal 1.9 kg BGAN Antenna 1.1 kg GPS Antenna 0.1 kg Total 10.9 kg The standard configuration of the terminal box includes the following major hardware functions and the physical layout can be seen in Figure 3: • Host spacecraft interface (Spacewire and UART) • Processor complex for BGAN protocol support and user interface • Digital modulation/de-modulation processing complex • BGAN radio transmit chain • BGAN radio receive chain • BGAN transmit/receive diplexer • GPS RF receive chain • GPS processor • Motor driver for antenna gimbal • Additional test ports

CN

CN

BGAN DCN Backbone

RF/IF

Satellite

RF/IF

RF/IF

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PLMNsPDN PSTN ISDN

PoP Sites

UT Community UT Community

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TT&CStations

Service Providers

BOC

SAS4

NOC

SCC

SAS1

SAS 3SAS2

C-Band FeederLinks

L-Band UserLinks

C-Band FeederLinks

C-Band Feeder Links

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Satellite

I4 Satellite I4 Satellite

OSA BSSHLR/AuC/EIR

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PS CSCN

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OtherSAS Sites


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Figure 3: SB-SAT Physical Envelope

Antennae The BGAN antenna requires clear visibility to the geostationary satellite belt which can be accomplished by satellite slews, or by utilising an antenna gimbal system. The distance between the gimbal assembly and the terminal box is assumed to be less than 1 meter but can be considered on a case by case basis. Alternative antenna solutions include multi switched antennas and electronically steered phased array antennas which can be evaluated on a case by case basis. The GPS antenna is a small, fixed, patch style antenna that requires visibility in the zenith pointing direction and must be mounted such that the antenna is not blocked by other obstructions on the spacecraft. The antenna should be located to minimize interference from the BGAN service. In order to meet the tracking threshold for the baseline GPS processor of 40 dBHz for an L1 signal, an L1 signal level of at least -132.6 dBm must be incident at the GPS receiver chain input port (on the side of the Terminal Box). Based on a -135 dBW/m2 minimum power flux density of a GPS L1 signal at LEO altitude, the GPS antenna must provide a minimum net gain of -2 dB.


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Spacecraft Interfaces Spacewire The primary spacecraft interface consists of a point to point Spacewire link over which the IP data packets flow for transmission over the BGAN network. Spacewire is a full-duplex, bidirectional, serial, point-to-point data link. It encodes data using two LVDS signal pairs in each direction. That is a total of eight signal wires, four in each direction. The default data rate for the SB-SAT Spacewire link is 10Mbps. A nine-pin micro-miniature D-type is specified as the Spacewire connector. Command and Telemetry UART The command and telemetry interface is provided as a UART over RS422 that separates the user data flow from the local terminal command and telemetry path. Discrete Fail-Safe Controls The terminal provides two discrete control lines for safety and assured control of the BGAN transmitter and the motor driver. The two control lines are inputs to the terminal box and are both RS422 interfaces. The controls are used to disable/enable the BGAN transmitter and to disable/enable the motor driver. When the BGAN transmitter is disabled, all radiation is prohibited from the BGAN antenna. When the motor driver is disabled, all movement controls to the gimbal motors are prohibited. These control inputs default to a disabled state and must be actively driven by the spacecraft to permit normal operation. Other Communication Interfaces The terminal provides an Ethernet interface for test and debug facilities. The terminal also provides an optional GPS output for providing position knowledge to the spacecraft. The GPS output is provided as a UART over differential RS422. Also included is a 1PPS output over RS422. DC Power Supply Interface The power interface to the host spacecraft has the following characteristics:

+ 28V +/- 6V (unregulated) Return Chassis – chassis ground to spacecraft structure 9-pin, D-Sub, normal density, plug style connector (SND9M1000G)


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Figure 4: Host Spacecraft Interfaces

Power up sequence The spacecraft powers the terminal by powering the main 28V input to the terminal. For a stable spacecraft, feasible initialization times are depicted in Figure 5.

Figure 5: Typical Initialization Time Line

Network Connection Connection to the BGAN network may be initiated automatically after the terminal has successfully acquired the regional beam frequency, alternatively it may be initiated under control of the Host when it is ready to transfer data. When ready to connect to the BGAN network, the terminal transmits signalling messaging to register on the network. The network also requests the terminal to send its GPS position to determine if it is allowed to access the BGAN network in the area corresponding to its


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position. If this is the case, the RAN accepts the terminal and allows it to proceed with the UMTS attach procedure. Spacecraft Data Transfer To transfer data across the BGAN network the terminal must set up a Packet Data Protocol (PDP) context with the CN (Core Network), after which the terminal will be assigned an IP address, user plane capacity and Quality of Service for the data transfer. The PDP context also establishes a gateway to the associated ground networks. When the host is ready to transfer data and after it has successfully attached to the CN, it will command the terminal to initiate a Packet Data Protocol (PDP) context with the network via the BGAN regional beam. If the request is successful the terminal will be handed over to the narrow spot beam where all data transfer takes place. The terminal will remain on the narrow spot beam while data is being transferred - if no data is transferred after an idle period of 90 seconds the narrow beam resources allocated to the terminal will be handed back to the regional beam until there is more data to transfer at which time the terminal will be allocated resources on the narrow beam again. The PDP context will remain active even when the terminal is tuned to the regional beam frequency. After the terminal has established connection to the BGAN system, an IP network connection will be established. Typically an IP address is dynamically assigned by the system and the spacecraft initiates communication with the ground. A static IP feature providing a permanent IP address to the spacecraft is supported and offered by SB-SAT on a per-subscription basis. Narrow Beam Handover The terminal will automatically perform a seamless handover between narrow beams. The narrow beam handover procedure may only be used for transitions between narrow beams on each satellite. In some cases the spacecraft may transition into a narrow beam that is not activated. There is some time delay associated with activating the narrow beam which may delay the user data stream transmission. In these cases the terminal will continue to use the previous beam to avoid delays in the user data stream. The narrow beam regions are defined by a table of vertices in a narrow beam map. The spot beam map rarely changes and is pre-loaded on the SB-SAT terminal. If the SB-SAT detects an update to any map, the new version is downloaded and replaces the older version automatically. Ground Segment Data Delivery There are two options for the ground segment connectivity to the BGAN network for data delivery from the spacecraft to the end-user control environment, and forward link command and telemetry. A leased line linked into an Inmarsat ground network access point can deliver the data over a secure, dedicated connection. It is the responsibility of the user to install and maintain the direct link into the Inmarsat network. Alternatively, as a cheaper and simpler option, traffic could be passed via the public domain internet allowing connectivity through a virtual private network.


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5. Service Coverage and Capabilities Services BGAN services include packet switched, symmetric ‘always on’ data connections and the network supports Standard IP data and Streaming IP data services. The SB-SAT terminal supports both of these services. The IP-based packet-switched data rates supported by the SB-SAT terminal consist of up to 475 kbps in the return link (downlink from LEO), and up to 464kbps in the forward link (uplink to LEO). Standard IP contexts offer a service which is shared among users of the system in the same narrow spot beam, providing a 'best effort' service. This type of connection is ideal for file transfer. The user pays for the amount of data sent and received. This type of connection is also called Background IP. The BGAN service can also be used at a pre-determined quality of service, i.e. with streaming data at a rate of 8, 16, 32, 64, or 128 kbps. This service is called Streaming IP and has time-based charges, while the standard IP services (packet switched) are charged by the volume of data transferred. The SB-SAT terminal has the ability to allocate up to 11 public IP addresses per terminal, and run up to 11 parallel IP sessions to/from the terminal. The maximum attainable bit-rate is shared across the IP sessions. As an example and assuming an orbital duration of 100 minutes, the maximum volume of traffic passed asymmetrically through the SB-SAT system in one orbit would be in the region of 220MB. This volume would depend on the service rate chosen by the user, the beam loading, as well as the service coverage (see below). Service Coverage The SB-SAT terminal is designed to provide service for low Earth orbiting spacecraft in circular orbits with altitudes from 300km to 900km. As the LEO satellite progresses around its orbit it will pass sequentially through the coverage of the three I4 satellites equally spaced around the geostationary arc. The composite coverage of the I4 constellation of satellites provides maximum coverage for the Earth and for satellites in low Earth orbits as shown in Figure 6.


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Figure 6: Polar View of Inmarsat I4 Constellation and Composite Coverage Coverage at the poles is more limited as the altitude is increased. In other words, coverage may be reduced with increasing altitude mainly near the poles.

Figure 7: Example of the SB-SAT coverage area for 400km orbit at 50° inclination


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Figure 8: Example of the SB-SAT coverage area for 720 km Sun Synchronous orbit

Table 1: Service Coverage for a SB-SAT Terminal with a high gain antenna A high gain antenna SB-SAT terminal benefits from extended coverage. Coverage may be reduced by 1% to 1.5% due to satellite to satellite handover. Service Capabilities The attainable throughput over the SB-SAT system will be commensurate with the BGAN network capabilities, which automatically adapts to the prevailing transmission conditions to ensure maximum possible data throughput at all times. Terminal and antenna characteristics dictate the service performance and high gain antenna SB-SAT terminals benefits from extended coverage as shown in Table 2.


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Table 2: Service data rates Throughput values above assume the highest priority level of IP service. 6. SB-SAT Service Provision and Applications

The SB-SAT product eliminates the need for global ground infrastructures: mission data and T&C can be delivered end-to-end as an IP service over a reliable and mature air-interface system combined with a global ground backbone. This on-demand and real-time telemetry and command service could significantly reduce risks to satellite assets as it allows near instantaneous detection of LEO anomalies which could be followed with immediate resolution measures. This in turn leads to longer life spans of LEO missions. Real-time access to mission data would reduce response times for LEO satellite missions with time-critical requirements. The ability to deliver satellite tasking information in real-time means that mission data key-holing would be available. By sending real-time low resolution data samples, tasking in response could be implemented immediately in certain missions. It is acknowledged that a complete overhaul of the conceptual approach to LEO operational activity is required in order to make use of SB-SAT’s real-time functionality. Design modifications will be required to satellite interfaces, operational systems, on-board memory, and software. Additionally, the ground infrastructure software and routing of traffic will need to be adapted. However the benefits of SB-SAT adoption as a telecommunications system on LEO satellites are wide and varied and the applications are numerous:

Earth observation key-holing of images Scientific - rapid relay of space weather Hosted satellites – servicing multiple payloads Aircraft Tracking via ADS-B Fractionated systems – data aggregation of wirelessly connected modules In orbit testing - monitoring and controlling experimental and testing flights Launch vehicles and shuttles - short term connections at assured rates Manned space stations - sustained and on demand telephony, email, video

streaming, web-browsing and file transfer Control during retirement of orbit - short term connections at assured rates prior to re-

entry 7. Conclusion

This paper provides a relatively high level overview of the new SB-SAT technology which is being developed. The terminal and service capabilities deliver a “disruptive” technology which is a paradigm shift in the way data is relayed to and from LEO spacecraft. It will allow the deployment of smaller and cheaper LEO missions that require frequent communication contacts in the near-earth environment. Future variants of the system will allow deeper


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penetration of the product onto the market for LEO data transfer and once deployed, this system should revolutionise many aspects of LEO satellite operations and management.

Inmarsat Global Limited SB-SAT Sustained Data - SpaceOps 2012

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