The Iridium satellite constellation is a system of 66 active communication satellites with spares in orbit and on the ground. It allows worldwide voice and data communications using handheld satellite phones. The Iridium network is unique in that it covers the whole earth, including poles, oceans and airways.
The company, based in Bethesda, Maryland, US, derives its name from the chemical element iridium; the number of satellites projected in the early stages of planning was 77, the atomic number of iridium.
The satellites are frequently visible in the night sky as satellite flares – a phenomenon typically observed as short-lived bright flashes of light.
Iridium communications service was launched on November 1, 1998. The first Iridium call was made by then-Vice President of the United States Al Gore. Motorola provided the technology and major financial backing.
The founding company went into Chapter 11 bankruptcy nine months later on August 13, 1999. The handsets could not operate as promoted until the entire constellation of satellites was in place, causing a massive initial capital cost running into the billions of dollars. The increased coverage of terrestrial cellular networks (e.g. GSM) and the rise of roaming agreements between cellular providers proved to be fierce competition. The cost of service was prohibitive for many users, and the bulkiness and expense of the handheld devices when compared to terrestrial cellular mobile phones discouraged adoption among potential users.
Mismanagement has also been cited as a major factor in the original 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 "difficulty gaining subscribers".
The initial commercial failure of Iridium 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 other proposed schemes were never constructed.
At one stage there was a threat that the Iridium satellites would have to be de-orbited; however, they remained in orbit and operational. Their service was restarted in 2001 by the newly founded Iridium Satellite LLC, which was owned by a group of private investors. Although the satellites and other assets and technology behind Iridium were estimated to have cost on the order of US$6 billion, the investors bought the firm for about US$25 million.
The system is being used extensively by the U.S. Department of Defense 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, the petroleum industry, scientists, and frequent world travelers.
Iridium satellites are now an essential component of communications with remote science camps, especially the Amundsen-Scott South Pole Station. As of December 2006, an array of twelve Iridium modems was put online, providing 24/7 data services to the station for the first time. Total bandwidth is 28.8 kbit/s, making real time e-mail conversations finally possible.
Kyocera phone models SS-66K and SD-66K are no longer in production but still available in the second-hand and surplus market. The SD-66K phone was a small 900 MHz GSM phone that fitted in a cradle (KI-G100) that included a large antenna and facilitated connection to the Iridium network. The SS-66K was a self contained phone, but featured a rather unusual ball antenna.
All handsets can receive SMS, but only the 9505, 9505A and those based on the 9522 can send them.
These can be used for data-logging applications in remote areas, now a common practical use for Iridium's services. Some types of buoys such as those used for the tsunami warning system use Iridium satellites to communicate with their base. The remote device is programmed to call or send SBD messages to the base at specified intervals, or it can be set to accept calls in order for it to offload its collected data.
The 9522A is the most current version of the OEM L-Band Transceiver module designed for integration into specific applications. It is based upon the original 9522 transceiver made by Motorola. Several variants of this modem exist, some with built in GPS receivers and autonomous position reporting capabilities. The 9522 provides audio, RS-232 and power supply connectors through a DB-25 connector and supports voice calls, SMS and low-speed data calls. Recent versions of the 9522 are able to send and receive SBD messages.
The 9601 modem supports only SBD and several tracking devices and other products have been built around this modem. It is the only mass produced Iridium transceiver that does not use a SIM card, instead it only uses its IMEI number for identification.
Since Iridium will not sell prepaid cards or even its subscription call service directly, it is hard to obtain the exact price of making a call. There are numerous distributors that will activate Iridium phones and sell pre-paid vouchers and SIM cards.
Iridium and other satellite phones may be identifiable to the listener by the "clipping" effect of the data compression and the latency (time delay) due to the electronic equipment used and the distances the signal must travel. The voice codec used is called Advanced Multi-Band Excitation.
Iridium operates at only 2.2 to 3.8 kilobaud, which requires very aggressive voice compression and decompression algorithms. Latency for data connections is around 1800 ms round-trip using small packets.
Despite the bandwidth limitations, transparent TCP/IP is supported. Iridium claims data rates up to 10 kilobits per second for their "direct internet" service. Phones can be connected to computers using a RS-232 connection, as can the 9522A transceiver module.
The most common card is the 500 minute one, which remains valid for one year and can usually be bought for US$600–750, while the 75 minute card can cost up to US$200 and the 5,000 minute card costs around US$4,000.
Since spring 2007, postpaid Iridium subscribers have an option to associate their Iridium numbers with a direct U.S.-based number (the so-called +1 Access service).
The Iridium system requires 66 active satellites in orbit to complete its constellation, with spare satellites in orbit to serve in case of failure. Satellites are in low Earth orbit at a height of approximately and inclination of 86.4°. Satellites communicate with neighbouring satellites via Ka band intersatellite links. Each satellite can have four intersatellite links: two to neighbors fore and aft in the same orbital plane, and two to satellites in neighboring planes to either side. The satellites orbit from pole to pole with an orbit of roughly 100 minutes. This design means that there is excellent satellite visibility and service coverage at the North and South poles, where there are few customers. The over-the-pole orbital design produces a "seam" where satellites in counter-rotating planes next to one another are travelling in opposite directions. Cross-seam intersatellite-link handoffs would have to happen very rapidly and cope with large Doppler shifts; therefore, Iridium supports intersatellite links only between satellites orbiting in the same direction.
The cellular lookdown antenna has 48 spot beams arranged as 16 beams in three sectors. The four intersatellite cross links on each satellite operate at 10 Mbit/s. The inventors of the system had previously worked on a government study in the late 1980s that showed that microwave cross links were simpler and had fewer risks than optical crosslinks. Although optical links could have supported a much greater bandwidth and a more aggressive growth path, microwave crosslinks were favored because the bandwidth was more than sufficient for the desired system. Nevertheless, a parallel optical crosslink option was carried through a critical design review, and ended when the microwave crosslinks were shown to support the size, weight and power requirements allocated within the individual satellite's budget. In recent press releases, Iridium Satellite LLC has stated that their second generation satellites would also use microwave, not optical, intersatellite communications links. Such cross-links are unique in the satellite telephone industry, as other providers do not relay data between satellites.
The existing constellation of 66 satellites is expected to remain operational until at least 2014, with many satellites expected to remain in service until the 2020s. Iridium is planning a new generation of satellites with improved bandwidth to be operational by 2016. This system will be backward compatible with the current system. In August 2008, Iridium selected two companies - Lockheed Martin and Thales Alenia Space - to participate in the final phase of the procurement of the next generation satellite constellation, with the winner to be announced in mid-2009.
The original design envisioned a completely static 1960s "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. It was found that this design did not have enough bandwidth in the space-based backhaul to upload each satellite quickly and reliably over the poles. Therefore, the design was scrapped in favor of a design that performed dynamic control of routing and channel selection late in the project, resulting in a one year delay in system delivery.
Each satellite can support up to 1100 concurrent phone calls and weighs about 700 kg. The vast majority of patents filed by Motorola during the Iridium project concern ways to manufacture and launch satellites affordably. The satellites were designed to mount sideways on a gimbal for easy access during manufacture (most satellites up until that time had been assembled vertically.) Motorola hired the chief manufacturing engineer from Apple Computer, who had set up the first Macintosh manufacturing line, to help design and automate satellite production.
The type of modulation used is normally DE-QPSK although DE-BPSK is used on the uplink (mobile to satellite) for acquisition and synchronisation. Each timeslot is 8.28 ms long and sits in a 90 ms frame. Within each FDMA channel there are four TDMA timeslots in each direction. The TDMA frame starts off with a 20.32 ms period used for simplex messaging to devices such as pagers and to alert Iridium phones of an incoming call, followed by the four upstream slots and four downstream slots. This technique is known as time division multiplexing. Small guard periods are used between timeslots. Regardless of the modulation method being used communication between mobile units and satellites is done at 25kBd.
Channels are spaced at 41.666 kHz and each channel occupies a bandwidth of 31.5 kHz—this allows space for doppler shifts.
The satellites are also able to transfer mobile units to different channels and timeslots within the same spot beam.
Iridium routes phone calls through space. There are four earth stations, and the space-based backhaul will route 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. Communication between satellites and earth stations is done at 20 and 30 GHz
Gateways are located in
In previous years there were eleven gateways in service, many of which have since been closed. Gateways have also been built in Pune, India; Beijing, China; Moscow, Russia; Nagano, Japan; Seoul, South Korea; Taipei, Taiwan; Jiddah, Saudia Arabia; and Rio de Janeiro, Brazil
Like other satellite networks, Iridium terminals need open line-of-sight to open sky in order to function. For instance, units will not work consistently indoors, or under forest cover. Iridium does have a very powerful paging channel that can ring the phone indoors, but the customer may have to walk outdoors to take the call.
There is a Web/e-mail to SMS gateway which enables messages to be sent from the internet or an e-mail account to Iridium handsets for free. There is also a voice mail service.
Iridium generally does not have roaming agreements with terrestrial/cellular operators. Telstra in Australia allows postpay GSM subscribers to use their SIM card. However, global roaming has to be activated and both incoming and outgoing calls are charged to this account, and the call rate is around US$4 per minute; the incoming calls are via the GSM phone number of the account, with country code etc. prefixed. In order to use the network, it is necessary to have not only appropriate equipment, such as a handset or the optional cellular cassette for the Motorola 9505 phone, but also a pay-as-you-go or contract Iridium SIM card.
The position readings can be extracted from some transceiver units and the 9505A handset using the
-MSGEO AT command. It returns values as the number of kilometers from the prime meridian and the equator
In the past Iridium has used this method of tracking to block service to US Embargoed countries such as North Korea and other unpopular regions such as Northern Sri Lanka. It is also used to stop geographically bound plans from being used outside the designated area.
The main patents on the Iridium system, U.S. Patents 5,410,728 and 5,604,920, are in the field of satellite communications, and the manufacturer generated several hundred patents protecting the technology in the system. Satellite manufacturing initiatives were also instrumental in the technical success of the system. Motorola made a key hire of the engineer who set up the automated factory for Apple's Macintosh. He created the technology necessary to mass-produce satellites on a gimbal, taking weeks instead of months or years and at a record low construction cost of only US$5 million per satellite. At its peak during the launch campaign in 1997 and 1998, Motorola produced a new satellite every 4.3 days, with the throughput time of a single satellite being 21 days.
Motorola used launch vehicles from three companies from three different countries: the Delta II from McDonnell-Douglas; the Proton K from Krunuchev Industries in Russia; and the Long March IIC from China Great Wall Manufacturing Company. The original constellation of 66 satellites plus six spares was launched in 12 months and 12 days, between May 5, 1997, and May 17, 1998, with an astounding success rate of 15 out of 15 successful launches and all 72 satellites put into the intended orbits. In one 13-day period they successfully put 14 satellites into orbit (late-March to early-April 1998).
The memory chips in early satellites were not military grade, and several of them failed due to temperature extremes in space, requiring some satellites to map out the failed memory regions. One satellite was launched with 2 wires mixed up, and it had to have a special load of software or configuration that was always different from the other 65 satellites.