Architecture of ORBCOMM Little LEO Global Satellite System for Mobile and Personal Communications

The communication subsystem is the principal payload flown on the satellite, consisting of five major parts:

1. The Subscriber Communications Section is the main payload part consisting of one subscriber Tx, seven identical Rxs and associated Rx and Tx filters and antennas. Six of the Rxs are used as subscriber receivers and the seventh is used as the Data Center as a Service (DCAAS) Rx. The subscriber Tx is designed to transmit an operational output power of up to about 40 W, although the output is less during normal operation. The power of each Tx can vary over a 5 dB range, in 1 dB steps, to compensate for aging and other lifetime degradations. Symmetrical Differential Phase Shift Keying (SDPSK) modulation is used on the subscriber downlink at a data rate of 4800 b/s. (It is capable of transmitting at 9600 b/s.). The satellite uplink modulation is SDPSK with a data rate of 2400 b/s. Raised cosine filtering is used to limit spectral occupancy.

2. The ORBCOMM Gateway Communication Section contains both the gateway satellite’s Tx and Rx. Separate right-hand circular polarization antennas are used for Tx and Rx functions. The gateway Tx is designed to transmit 5 W of RF power. The 57.6 Kb/s downlink signal to the GES is transmitted using an offset quadrature phase shift keying (OQPSK) modulation in a TDMA format. The gateway Rx is designed to demodulate a 57.6 Kb/s TDMA signal with OQPSK modulation. The received packets are routed to the onboard satellite network computer.

3.The Satellite Network Computer receives the unlinked data packets from the subscriber and the ORBCOMM gateway Rxs and distributes them to the appropriate Tx. The computer also identifies clear uplink channels via the DCAAS Rx and algorithm and interfaces with the GPS Rx to extract information pertinent to the communications system. Several microprocessors in a distributed computer system aboard the satellite perform the satellite network computer functions.

4.The UHF Tx is a specially constructed 1 W Tx that emits a highly stable signal at 400.1 MHz. The Tx is coupled to a UHF antenna designed to have a peak gain of about 2 dB.

5.The Satellite Subscriber Antenna Subsystem comprises a deployable boom containing three separate circularly polarized quadrifilar antenna elements.

The attitude control system is designed to maintain both nadir and solar pointing. The satellite must maintain nadir pointing to keep the antenna subsystem oriented toward the Earth. Solar pointing maximizes the amount of power collected by the solar cells. The satellite employs a three-axis magnetic control system that operates with a combination of sensors, which also obtains its position through its onboard GPS receiver.

Satellite planes A/B/C are designed to maintain a separation of 45 ± 5 degrees between satellites in the same orbital plane. Planes D/E provide 51.4 degrees spacing between satellites, while highly inclined satellite planes (F/G) are spaced for 180 ± 5 degrees apart.

The springs used to release the satellites from the launch vehicle give them their initial separation velocity. A pressurized gas system is used to perform braking maneuvers when the required relative in-orbit satellite spacing is achieved. An Orbital Sciences Corporation formation-keeping technique maintains the specified satellite intra-plane spacing. One of the benefits is that, unlike GEO satellites, it does not affect the satellite’s life expectancy in fuel usage.^2,4,5

Second Generation of ORBCOMM OG2 Satellites

ORBCOMM in May 2008 signed a next generation satellite constellation contract with Sierra Nevada Corporation (SNC) to build 18 modern ORBCOMM Generation 2 (OG2) satellites with an option to purchase up to 30 additional OG2 satellites to augment its existing satellite constellation (see Figure 4).

Figure 4 ORBCOMM OG2 satellite coverage.⁶

As prime contractor, SNC is an experienced integrated space team with unique and established space heritage, resources and performance record, including Boeing Intelligence and Security Systems (I&SS), ITT Space Systems and MicroSat Systems. The integrated space team also includes several other key subcontractors and industry leaders with unparalleled experience in both the design and construction of complex communications systems and satellites.

SNC, Boeing and ITT provide oversight, systems engineering, technical management, integration and mission assurance functions to assure the successful performance of the OG2 program. MicroSat Systems (MSI), a wholly owned subsidiary of SNC, leveraged its experience on the TacSat-2 mission to design the spacecraft and perform integration and test activities for the OG2 satellites.

In June 2008, SNC selected Argon ST to develop and deliver the satellite payloads for the OG2 satellite constellation. Each OG2 satellite is equipped with an enhanced communications payload designed to increase subscriber capacity by up to 12 times over the OG1 satellites. ORBCOMM customers can transmit data over the OG2 satellites at greater speeds and send larger data packets using future modems.

From 8 October 2012 to 22 December 2015, 18 next generation OG2 satellites were launched, but only 17 became operational. Figure 5 shows the ORBCOMM OG1 spacecraft (a), ORBCOMM OG2 spacecraft (b) and ORBCOMM OG2 orbital satellite constellation (c).

The OG2 satellites are backward compatible with OG1, so that existing subscriber communicators function seamlessly with the OG2 satellites. In addition, all OG2 satellites are designed with S-AIS payloads to receive and report transmissions from AIS-equipped oceangoing vessels. In fact, ORBCOMM markets this S-AIS data to the USA and international coast guards and government agencies, as well as maritime companies engaged in security or logistics businesses for tracking shipping activities or for other navigational purposes.

The ORBCOMM LEO satellites are “orbiting packet routers” ideally suited to “grab” small data packets from mobile or fixed sensors and relay them through a tracking Earth station and then to a GCC.

The current satellite constellation, OG1 and OG2, provides one central location for the two-way delivery of data over multiple carriers to customer back-office applications via the Internet, cellular wireless, satellite and dual-mode cellular and satellite services; however, more than 42 percent of the market share of ORBCOMM's current satellite system provides M2M service to users for fixed and mobile applications.^6-8

Figure 5 ORBCOMM OG1/OG2 satellites: OG1 spacecraft (a), OG2 spacecraft (b) and ORCOMM OG2 orbital satellite OG2 coverage (c).⁸

GROUND SEGMENT

The ORBCOMM ground segment, which contains most of the system intelligence, comprises gateways or GESs, control centers and both mobile and fixed SCU customer terminals. The space segment of satellite constellations and orbits are controlled by one satellite control center (SCC).

Gateways, which include the GESs, GCCs and the NCC, are located at ORBCOMM headquarters in Dulles. Within the USA, there are four other GESs located in Arizona, Georgia, New York State and Washington State. The NCC also serves as North America’s GCC and manages the overall system worldwide. ORBCOMM gateways are connected to dial-up circuits, private dedicated lines or the Internet. The SCU handheld devices for personal messaging are fixed and mobile units for remote monitoring, control and tracking applications.

GES

ORBCOMM continues the deployment of additional regional GESs to provide near-real-time service for all major areas of the world, as well as developing and launching a new generation of satellites that will enhance and expand the current system’s capabilities. All ORBCOMM’s GES terminals link the ground segment with the space segment and are in multiple locations worldwide.

The GES acquires and tracks satellites based on orbital information from the GCC, links ground and space segments from multiple worldwide locations, transmits and receives transmissions from the satellites, transmits and receives transmissions from the GCC or NCC, monitors the status of local GES hardware and software and monitors satellite system level performance “connected” to the GCC or NCC (see Figure 6).

The GES terminal is redundant and has two steerable high-gain VHF antennas that track the satellites as they cross the sky. It transmits to a satellite at a frequency centered at 149.61 MHz at 56.7 Kb/s with a nominal power of 200 W. It receives 3 W transmissions from the satellite in the 137 to 138 MHz range. These up-and-downlink channels have 50 KHz bandwidths.

The mission of the GES is to provide an RF communications link between the ground and the satellite constellation. It comprises medium gain tracking antennas, RF and modem equipment and communications hardware and software for sending and receiving data packets. An ORBCOMM licensee requires a gateway to connect to satellites in view of its service area. Namely, the gateway consists of a GCC and one or more GESs, as well as the network components that provide inter-facility communications.^2,9,10

Figure 6 Diagram of ORBCOMM GES.¹⁰

GCC

The GCC is the operations center for ORBCOMM gateway activities. Auxiliary systems such as subscriber management and business support systems are typically located in the GCC. GCC terminals are in territories licensed to use the ORBCOMM system. They locate wherever ORBCOMM is licensed, link remote SCUs with terrestrial-based systems, communicate via X.400, X.25, leased line, dial-up modem, public and private data networks and E-mail networks including the Internet and efficiently integrate the ORBCOMM infrastructure with new or existing customer management information system solutions.

The GES transmits messages over a dedicated line to the GCC that places them on the public switched network for delivery to the receiver subscriber’s PC Internet provider. The GCC Local Area Network (LAN) and GES LAN are considered part of the global OCCNet Wide Area Network (WAN).

The OCCNet is the overall, worldwide WAN hardware network that ties together the ORBCOMM LAN hardware in GCC stations of ORBCOMM gateways established throughout the world. Thus, it consists of the routers, bridges, modems and interconnecting circuits over which command and control telemetry passes.

Within the context of an ORBCOMM gateway, the OCCNet also consists of the routers, bridges and modems by which the ORBCOMM gateway elements (in particular, the NMS, GMSS, and GES) are interconnected. The OCCNet functions include: 1) providing message transport between an ORBCOMM gateway, GMSS and NMS, 2) providing message transport between an ORBCOMM gateway, GCC and its GES terminals and 3) providing message transport with the GCC.^1,2

NCC

The NCC is responsible for managing the ORBCOMM communications network elements and the USA gateways through telemetry monitoring, commanding and mission system analysis. It monitors real-time and back-orbit telemetry from the ORBCOMM satellites, sends real-time and stored commands to the satellites, provides the tools and information to assist engineering with resolution of satellite structure and ground anomalies, archives all satellite and ground telemetry data for analysis and monitors performance.

The NCC manages the entire ORBCOMM satellite constellation and its processes and analyzes all satellite telemetry. It is responsible for managing the ORBCOMM system worldwide. Through OrbNet, the NCC monitors message traffic for the entire ORBCOMM system and manages all message traffic that passes through the U.S. gateway. The NCC is staffed 24 hours a day, 365 days a year from Dulles, Va. A backup NCC system was established in 2000, which permits the recovery of critical NCC functions in the event of an NCC site failure.^1,5

SCC

The SCC serves in territories licensed to use the ORBCOMM system and provides control of the ORBCOMM Little LEO satellite constellation. The SCC terminal is owned and operated by ORBCOMM and co-located with the U.S. NCC in Dulles, Virginia.

SCU

The SCU equipment comprises both mobile and fixed terminals used for connection to the ORBCOMM satellite network through gateway stations. The SCU terminal is a wireless VHF modem that transmits messages from a user to the ORBCOMM system for delivery to an addressed recipient and receives messages from the ORBCOMM system intended for a specific user.

Manufacturers have different proprietary designs. Each model must be approved by ORBCOMM and adhere to the ORBCOMM Air Interface Specification, Subscriber Communicator Specifications and ORBCOMM Serial Interface Specification (if an RS-232 port is available). Different versions of SCU terminals are currently available, which include “black-box” industrial units that have RS-232C ports for data uploading and downloading. Current options on several SCUs include internal GPS receivers and/or additional digital and analog input and output ports.

User Segment

The ORBCOMM satellite system is designed to enable short communications between different, often unmanned remote fixed or mobile modems, positions and customer information hubs as a part of the User Segment. The main unit is an SCU. ORBCOMM hardware and software components comprise a global, packet switched two-way data communication service optimized for short messages and small file transfers.

The ORCOMM SCU mobile or fixed satellite terminals are full-feathered compact, light-weight devices with their own power supplies, or are supplied from other sources if installed on some mobile means of transport, such as ships, road or rail vehicles and small craft. Many have RS-232 data ports and some are integrated (black-box versions) with GPS receivers, lap-tops and palm-top computers and other systems.

Data transmissions can be encrypted by application software using standard digital encryption standard computer chips using the same methodology used by STU-111s to encrypt classified conversations over the public telephone network.

The ORBCOMM system provides a less accurate geolocation capability that combines the Doppler frequency shift available from a LEO satellite with the satellite onboard GPS receiver to calculate position within less than 100 meters. This may be sufficient for many U.S. or other country armed forces requirements.^1,5

Second Generation of ORBCOMM SCU Terminals

There are many first and next generation SCU satellite terminals that use the ORBCOMM satellite network, but in this context, two terminals from ORBCOMM’s second generation of SCU terminals will be introduced. Before that, the Magellan GSC 100 satellite modem was developed as the first generation of handheld satellite terminals in the world that allows sending and receiving text and e-mail messages to and from anywhere in the ORBCOMM coverage area (see Figure 4).

1. Stellar DS300 Terminal – The DS300, designed by Stellar and Delphi Electronics, and Safety and manufactured by Delphi, is a two-way satellite communicator for use with the LEO ORBCOMM satellite network. It is a complete hardware solution combining a satellite transceiver and GPS receiver for companies using a wide variety of applications to track, monitor and communicate with their fixed and mobile assets around the globe (see Figure 7a).

The DS300 terminal is a complete hardware solution for companies using a wide variety of applications to track, monitor and communicate with fixed and mobile assets around the globe. It features a satellite modem using a VHF transmit frequency of 148 to 150.05 MHz and a receive frequency of 137 to 138 MHz. Dynamic Range is 40 dB minimum. Its power supply is 12 VDC for both the transceiver and GPS receiver. It transmits at a speed of 120 Kb/s, and has a user-programmable application processor, an integrated 16 channel GPS receiver, adequate software configurable input/output (I/O) options and a battery charger packaged in a rugged, automotive-grade enclosure.

The design and stable performance make the DS300 a reliable satellite device for transportation, heavy equipment, marine, aeronautical and many other markets. The satellite modem is configurable with eight input or output digital channels, four input analog channels and 8 GPS receiving channels.

2. Quake Q4000 Terminal – This satellite three-mode (ORBCOMM, Iridium and GSM) mobile terminal is one of the initial generations of cost effective and fully programmable ORBCOMM transceivers and GSM (cellular) receivers. It operates at a VHF transmit frequency of 148.000 to 150.050 MHz and a receive frequency of 137.000 to 138.000 MHz with a 50-channel GPS receiver for a global tracking capability (see Figure 7b).

It uses the ORBCOMM satellite network and it has almost the same technical characteristics as the Iridium Q4000i for the Iridium Big LEO satellite network. It can be used for SCADA (M2M) and business-to-business Internet links with marine, land (road and rail) or aeronautical based assets and fixed M2M heavy equipment, transportation and oil and gas applications anywhere in the world.

This is a robust industrial modem in a compact form factor that is small enough to hold in one’s hand. It is also designed to meet exacting standards regarding automotive power conditioning requirements and features a low power draw load for battery operated applications of 10.5 VDC.

The Q4000 uses an application programming interface (API) that empowers developers to integrate its functions and build customized onboard mobile applications. Various mobile and fixed clients are given a wide array of fully customizable options, including multiple inputs/outputs, antenna detection, J1939 CAN Bus, memory and network accessibility based on their M2M technical and functional requirements.^{11, 12}