This section reviews the current systems already designed or in orbit that aid towards enabling distributed operations using distributed computing and intersatellite links.
The IRIDIUM satellite constellation [154, 155] is a wireless personal communications network designed to permit a wide range of telephone services-voice, data, fax, paging to connect to destinations virtually anywhere on earth. Sixty six operational satellites at 662 kg are configured in six near-polar orbital planes, in which 11 satellites circle in one plane to track the location of the telephone handset (this allows IRIDIUM to cover all areas of the Earth). Each satellite operates 7 PowerPC CPUs to control routing and voice compression drawing 620W. This system is the only example of a DSS where on-board computing and intersatellite links are utilised to complete a mission. Despite being a technological success, the project was an economic failure [156].
Emerald [157, 158, 159] is a two satellite mission to explore “Robust Distributed Space Systems” between the Space Systems Development Lab (SSDL) at Stanford University and Santa Clara Remote Extreme EnvironMent (SCREEM) lab at Santa Clara University. Here, a distributed bus architecture without ISLs is used for on-board distributed computing. The “smart” subsystems/peripherals contain their own computational power on an f C bus and data bus, allowing the main MCU to take on a co-ordinating role, free from the burden of menial tasks. Comparable to object-oriented computer programming, this ‘object-oriented hardware’ uses these
Chapter 3.Distributed Satellite Systems
subsystems to perform data processing at the source (e.g. at sensors/payloads). Emerald was never launched.
The Cluster mission is 4 x 1200 kg spacecraft to gather detailed data for a three-dimensional map o f the magnetosphere [160]. The satellites are each in different orbits and collaboratively collect data when all satellites pass the same area without intersatellite link capabilities. This configuration has been providing scientific data and breakthroughs. All computing is done on ground, offloading any online computations ftom the satellites.
TechSat-21 [161] [162]: The Autonomous Sciencecraft Constellation flight demonstration (ASC) was to fly onboard the Air Force’s TechSat-21 constellation of 3 x 150 kg satellites. The onboard flight software includes several interesting software components, notably the use o f Object Agent and TeamAgent cluster management software that enables Techsat-21 spacecraft to autonomously perform manoeuvres and high precision formation flying to form a single virtual instrument. This mission was cancelled in 2001 due to the problem complexity and project overruns.
Milstar [163] is the most advanced militaiy communications satellite system to date that provides secure, jam resistant, worldwide communications to meet essential wartime requirements for high priority military users (such as ships, submarines, aircraft and ground stations). The 4,500 kg satellites operate intersatellite link capability. The operational Milstar satellite constellation consists o f six satellites positioned around the Earth in geosynchronous orbits with one non- operational. Launched over 9 years from 1994 to 2003, each satellite serves as a smart switchboard in space by directing traffic from terminal to terminal anywhere on the Earth, like a LAN network. Since the satellite actually processes the communications signal and can link with other satellites through intersatellite links, the requirement for ground controlled switching is significantly reduced. It has the ability to interface many communications protocols via point-to- point and point-to-multi-point communications.
NASA’s ST5 [164] was launched on 22nd March 2006 to act as buoys for monitoring the weather o f space and the enormous storms spawned by the sun. The three 55 lb ST5 microsatellites were hauled into a highly elliptical orbit aboard an air-launched Pegasus rocket (inclined 105.6 degrees). Designers have packed 10 advanced technologies into the satellite, including an X-Band transponder communication system which is smaller than current systems for command, telemetry, and tracking communications between the microsatellites and ground stations.
The transponder is low-power (5.5 W @ 7.2 V DC) and low-weight (862 g). It also uses less power than previous flight-proven systems using a high power amplifier, diplexer, band pass filter, and two X-band antennas [165]. This mission aims to give the X-band transponder some space qualification and testing for future missions. It is not used for intersatellite links.
Chapter 3.Distributed Satellite Systems
The FORMOSAT-3/COSMIC [166] constellation of six (70 kg) microsatellites was launched successfully from Vandenberg Air Force Base in California on Friday, 14th April 2006. COSMIC’s (also “Constellation Observing System for Meteorology, Ionosphere and Climate”) mission is to collect atmospheric sounding data for scientific research and operational testing for constellation design and analysis, development of the spacecraft bus and payload instrument development. There are no ISLs on this mission either.
All these discussed missions are summarised in Table 3-2.
Table 3-2. Comparison of Current Distributed Satellite Systems
Mission Distributed Satellite System
IRIDIUM Uses ISLs, distributed telecoms routing, stable constellation, high cost
Emerald No ISLs, on-board distributed computing, never launched
Cluster No ISLs, no on-board distributed computing, varied constellation periods
TechSat-21 Uses ISLs, distributed formation control, never launched, high cost
Milstar Uses ISLs, distributed routing, stable constellation, high cost (military)
NASA ST-5 No ISLs, stable constellation, no on-board distributed computing
FORMOSAT-3/ No ISLs, stable constellation, no on-board distributed computing
COSMIC
Table 3-2 summarises the discussed DSSs focusing on key features for a distributed satellite system: the use of ISLs, the use o f on-board distributed computing, the constellation design, and the cost. Each constellation has some progress towards a complete distributed satellite system but no current mission meets all the requirements towards intersatellite communications with multiple satellites, on-board processing, and the utilisation o f smaller cost effective platforms.