Science Fair Project Encyclopedia
The Iridium satellite constellation is a system of 66 active communication satellites and spares around the Earth. The system was originally to have 77 active satellites, and was named for the element iridium, which has atomic number 77. Iridium allows worldwide voice and data communications using handheld devices. The service is interdicted for political reasons in North Korea, North Sri Lanka, Hungary and Poland.
The service was launched on November 1, 1998 and went into Chapter 11 bankruptcy on August 13, 1999. Its financial failure was largely due to insufficient demand for the service, the bulkiness and cost of the handheld devices compared to cellular mobile phones, and the rise of cellular GSM roaming agreements during Iridium's decade-long construction period.
Mismanagement has also been cited as a major factor in the program's failure. In 1999 CNN writer David Rohde detailed how he applied for Iridium service and was sent information kits, but was never contacted by a sales representative. He encountered programming problems on Iridium's website and a "run-around" from the company's representatives. After Iridium filed bankruptcy it cited its "difficulty [in] gaining subscribers".
The initial commercial failure of Iridium has had a dampening effect on other proposed commercial satellite constellation projects, including Teledesic. Other schemes (Orbcomm , ICO Global Communications, and Globalstar) followed Iridium into bankruptcy protection, while a number of proposed schemes were never constructed.
The Iridium satellites were, however, retained in orbit, and their services have been re-established in 2001 by the newly founded Iridium Satellite LLC, owned by a group of private investors.
The system is being used extensively by the US Department of Defense for its communication purposes through the DoD Gateway in Hawaii. The commercial Gateway in Tempe, Arizona provides voice, data and paging services for commercial customers on a global basis. Typical customers include maritime, aviation, government, oil & gas, scientists, and frequent world travelers.
Phone rates from land lines to Iridium phones are $2 to $4 per minute, from Iridium to land lines about $1.50 per minute and between Iridium phones less than $1 per minute. Iridium and other satellite phones may be identifiable to the listener because of the particular "clipping" effect of the data compression and the latency (experienced as a noticeable lag or time delay) due to the long traveling path of the signal. Iridium operates at a data rate of 2400 baud, which requires very aggressive voice compression and decompression algorithms.
The former Iridium provided phones from two vendors, Kyocera and Motorola. Kyocera phone models SS-66K and SD-66K are no longer in production, but still available in the second hand and surplus market. The Motorola phone 9500 is a design from the first commercial phase of Iridium, whereas the current 9505 model is a more modern design which is especially popular in military applications.
Currently all equipment is provided by Iridium Satellite through its distribution partners. Motorola no longer manufactures equipment for Iridium. The 9505A is the most current version of the handset and the 9522A is the most current version of the OEM L-Band Transceiver module designed for integration into specific applications.
Iridium phone numbers all start with +8816 or +8817 (which is like the country code for a virtual country) and the 8-digit phone number.
Because of the satellites' peculiar shape with three polished door-sized antennas, 120 degrees apart and at 45 degree angles with the main bus, the Iridium satellites have a highly visible satellite flare. On their orbits, the antennas directly reflect sunlight, creating a predictable and quickly moving illuminated spot of about 10 km diameter when the reflected beam hits the earth. To a spectator this looks like an extremely bright flare in the sky with a duration of only a couple of seconds. Some of the flares are so bright (up to -8 magnitude) that they can be seen at daytime, but they are most impressive after dusk and before dawn. This flashing has been of extreme annoyance to astronomers in that the brightness of the satellites disturbs observations and can damage sensitive equipment.
The iridium satellite has 66 satellites in orbit with about 5 spares parked in orbit in case of failure. Satellites are in low earth orbit at a height of approximately 418 miles. Unlike other LEO communications constellations the satellites fly from pole to pole with an orbit of roughly 58 minutes. This design means that there is excellent telephone coverage at the North and South poles, where there are no customers. Moreover, at each pole there are roughly 21 satellites converging on a single location at any time, and these satellites must be guided (by control signals sent through the ground network) to miss each other at the poles. Because satellites use an over-the-pole orbit there is a "seam" where satellites next to each other are flying in opposite directions and cross-link handoffs must happen very rapidly.
The cellular lookdown antenna has 48 spot beams arranged as 16 beams in 3 sectors. Each satellite has 4 cross links operating at 10 Mbps. The cross links were originally envisioned to be optical but the design engineers could not think of a search algorithm to find neighboring satellites if communications was lost, that would run in less than an hour. It is now possible to achieve optical lock for space communications in under a second due to the work of Vincent Chan while at Lincoln Labs.
The satellite contains 7 motorola PowerPC 602 processors running at roughly 200 Mhz. Processors are connected by an unwieldy custom backplane network. One processor is dedicated to each cross-link antenna ("HVARC"), and two processors ("SVARC"s) are dedicated to satellite control - one being a spare. Late in the project an extra processor ("SAC") was added to perform resource management and phone call processing.
The original design envisioned a completely static 1960's "dumb satellite" with a set of control messages and time-triggers for an entire orbit that would be uploaded as the satellite passed over the poles. Not only was this design ill-conceived and inefficient, it simply would not work because there was not enough bandwidth in the space-based backhaul to upload each satellite quickly and reliably over the poles. This design was scrapped in favor of a design that performs dynamic control of routing and channel selection late in the project, resulting in a 1-year delay in system delivery.
Iridium routes phones calls through space. There are 4 earth stations and the space-based backhaul routes phone call packets through space to one of the downlinks ("feeder links"). Station-to-station calls can be routed directly through space with no downlink. As satellites leave the area of an Earth base station the routing tables change and frames are forwarded to the next satellite just coming into view of the Earth base station.
The main patents on the Iridium system are in the area of mass production of satellites. Iridium made a key hire of the engineer who set up the automated factory for Apple's Macintosh, and he created the technology necessary to mass-produce satellites in weeks (instead of months or years) on a gimbal, at a record low cost of only $5 million per satellite ($40M including launch costs, 1998 dollars.)
- "Iridium will succeed because every time we estimated the growth of cellular phones, we were LOW by a factor of 4x" - Bary Bertiger of Motorola, system inventor.
- In fact, Iridium is a "fill-in system" which depends for its success on these incorrect estimates of terrestrial cell phone growth. So when the inventor of Iridium used this argument to justify the system, he was actually predicting its imminent demise.
- http://www.iridium.com Iridium Satellite LLC's home page
- http://www.heavens-above.com/ Iridium flare predictions based on geographic location.
- http://iridiumflares.sourceforge.net/ IridiumFlares prediction software
- http://homepage.mac.com/kevision/video/iMovieTheater25.html Video of an Iridium flare, from kevision.com
- http://www.ee.surrey.ac.uk/Personal/L.Wood/constellations/iridium.html Iridium information from Lloyd's satellite constellations
- http://www.cnn.com/TECH/computing/9902/24/iridium.idg/ So how do you order Iridium satellite service? (CNN)
- http://ardent.mit.edu/real_options/de%20Weck%20System%20Study/unit1_summary.pdf Technical success and economic failure (MIT)
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