| Orbcomm | ||
Orbcomm was a commercial venture to provide global messaging services using a constellation of 26 low-Earth orbiting satellites. The planned system was designed to handle up to 5 million messages from users utilizing small, portable terminals to transmit and receive messages directly to the satellites. T The first two satellites of the constellation experienced communications problems after launch, but were recovered and placed into operational status. The nominal 26 satellite constellation was to be deployed by 1997, with the potential for an additional 8 satellite plane and two more polar orbiters depending on demands for increased coverage. The vehicles were controlled from a single control centre located in Dulles, Virginia. The cost per satellite was estimated at $1.2 million. A small forerunner vehicle, Orbcomm-X, was launched in 1991 as a feasibility demonstration. This vehicle had a different design than the operational vehicles and was not included in the operational system. ORBCOMM provided global data services (similar to 2-way paging or email) via low-Earth orbit (LEO) Satellites and ground infrastructure. Subscriber Communicators (SC) transmitted data messages to the LEO Satellite, which relayed the message to the U.S. Gateway Earth Station (GES), or International Gateway Control Centers (GCC). The GES/GCC sent the message via satellite or dedicated terrestrial line to the ORBCOMM Network Control Centre (NCC). The network was packet data only. ORBCOMM satellite coverage was available world-wide. The message delivery time (or "latency") depended on the specific hardware set-up, message size and location of the remote unit. In the continental US, ORBCOMM was committed to providing 90% in 6 minutes or less, and 98% in 15 minutes or less for all message traffic. ORBCOMM was ideal for short, daily messages and only required minimum amounts of power to operate. The ORBCOMM satellites constantly moved, so large obstructions did not prohibit communications and coverage was available in remote rural areas. In comparison, cellular telephone coverage depended on tower location, usually centered around major highways and cities, and could not reach remote areas. Geostationary satellites required large, costly and power intensive hardware. Large data files (such as graphics) or emergency response latencies would however not be appropriate applications for ORBCOMM. The spacecraft was circular, disk shaped, with circular panels hinged from each side after launch to expose solar cells. These panels articulated in 1-axis to track the sun and provided 160 W. The deployed spacecraft measured 3.6 m from end to end with a 2.3 m span across the circular disks. VHF telemetry was at 57.6 kbps. The satellite had an on-board GPS navigation and timing system, and a 14 volt power system. Gravity gradient stabilization provided 5 degrees control with magnetic torquers for damping. Cold gas (nitrogen) propulsion system. Each spacecraft carried 17 data processors and seven antennas. These were designed to handle 50,000 messages per hour. On a long boom was a 2.6 meter VHF/UHF gateway antenna. Receive: 2400 bps at 148 - 149.9 MHz. Transmit: 4800 bps at 137 - 138 MHz and 400.05 - 400.15 MHz. The system used X.400 (CCITT 1988) addressing. Message size was 6 to 250 bytes typical (no maximum). The $ 330 million program consisted of a constellation of 3 planes x 8 satellites. The first satellite was lost but put back into operation on 22 May 1995. Satellite costs were $ 135 million for 26 units. Design Life: 4 years. Typical orbit: 785 km circular orbit. Length: 0.17 m (0.55 ft). Maximum Diameter: 1.05 m (3.44 ft). Mass: 22 kg (48 lb). Associated Launch Vehicle: Ariane 4, Pegasus, Pegasus XL. Orbcomm Chronology
Bibliography and Further Reading
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