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Iridium (satellite)

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.

History

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.

Present status

Iridium Satellite LLC claims to have 285,000 subscribers as of early August 2008 (compared to 203,000 in July 2007). Revenue for the second quarter of 2008 was US$81.7 million with EBITDA of US$25.8 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.

Subscriber equipment

Handsets

The former Iridium provided phones from two vendors, Kyocera and Motorola. Neither still manufacture these handsets. The Motorola phone 9500 is a design from the first commercial phase of Iridium, whereas the current 9505A model is the most current version of the handset and was released in 2001. The 9505A phone is functionally identical to the 9505 but is no longer manufactured by Motorola and contains a slightly different set of components.

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.

Pagers

Two pagers were made for the Iridium network - the Motorola 9501 and Kyocera SP-66K these are one-way devices that could receive messages delivered in the form of SMS. Both of these pagers have been discontinued.

Other satellite phones

Several other Iridium based telephones exist such as payphones, bag phones and car phones and equipment intended for installation on ships and aircraft. A handset made by NAL research combined with a 9522 transceiver is used for some of these products. The DPL handset provides a user interface nearly identical to that of the 9505 series phones.

Standalone transceiver units

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.

Services

Calls to Iridium phones are notoriously expensive, ranging from US$3 to US$14 per minute. It is possible to call with charges reversed by first dialing a number in Arizona; the call is charged to the receiver at the standard rate for satellite to landline calls, but the caller only pays for the call to Arizona.

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.

Voice and data calls

The Iridium system deals with 'minutes' which are subdivided into several much smaller 'units'. These minutes are the "basic rate" to landlines and ordinary mobile phones around the world. For a 500 minute annual plan the cost of the "basic rates" fluctuates around US$1.50/min depending on a distributor. There are also regional plans that offer slightly cheaper rates than the normal, but these minutes can only be used in a specified geographic location (such as Africa, North America, Canada or Alaska).

  • Calls to landlines worldwide: 1.00× basic rate
  • Calls to other Iridium phones or voice mail: 0.50×
  • Sending SMS messages: 0.33×
  • Calls to other satellite phones: 5–13.50×
  • Data calls: 1.00×

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.

Prepaid service

Prepaid SIM cards are available from a variety of different outlets and sometimes appear on auction sites such as eBay. Their values range from 50 to 5,000 minutes; the 50 minute cards have no validity and the 75 minute vouchers are valid for only a month, but the 5,000 minute cards stay valid for two years. Since Iridium charges quite a bit for merely accessing their network without making calls it is possible to extend the validity of such an account by a month for around US$45. It is also possible to refill such an account without purchasing a new SIM card.

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.

Post-paid service

There is a basic "Emergency" plan for around US$30 to US$40 a month that offers no minutes at all with calls charged at around US$1.39 per minute, and also numerous plans with included minutes. For the more expensive plans (around US$250 per month) the price per minute dips slightly below US$1.

Phone numbers

Iridium controls the virtual country codes +8816 and +8817, part of the 881 range designated by the ITU for the Global Mobile Satellite System. Each subscriber is given an 8-digit number prefixed by one of these country codes. However many regional telephone service operators such as Meteor have no interconnect agreement with Iridium or other satellite networks and users on these networks need to call reversed charge to a U.S.-based number.

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).

Paging service

The one-way paging service is still operational despite the pagers no longer being in production for many years now. Messages are delivered to pre-selected "MDAs" which cover a certain geographic area. Three of these MDAs may be selected on a web-based portal or updated automatically if the paging service is bound to an Iridium phone. Each country has its own MDA based on its country code, some of the larger countries are divided into several MDAs while separate MDAs exist for sections of ocean and common aeronautic routes. This service costs around US$70 per month with a limited number of messages allowed or US$140 for an unlimited number of inbound messages.

Short burst data

Special modems such as the 9522A and Quake Q9612 can be used for sending and receiving short bursts of data <2 kilobytes at a time. This service is often used for asset tracking and remote monitoring. Messages are converted to be delivered in email format or over [] to a pre-configured address - the mobile unit does not include a destination address when sending a SBD message. A crude positioning report is also included in each message sent. SBD messages take between 6 to 22 seconds to send or receive

Technical details

The constellation

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 satellites

The satellites each contain seven Motorola/Freescale PowerPC 603E processors running at roughly 200 MHz. Processors are connected by a 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 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.

Air interface

Communication between satellites and handsets is done using a TDMA and FDMA based system using L-band spectrum between 1616 and 1626.5 MHz. Iridium exclusively controls 7.775 MHz of this and shares a further 0.95 MHz. In 1999 Iridium agreed to timeshare a portion of spectrum allowing radio astronomers to observe hydroxyl emissions; the amount of shared spectrum was recently reduced from 2.625 MHz.

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.

Handoff

The Iridium system uses 3 different types of handoff. As a satellite travels over the horizon calls are handed to adjacent spot-beams; this occurs approximately every fifty seconds. A satellite only stays in view for seven minutes at the equator. When the satellite disappears from view an attempt is made to hand the call to another satellite. If no other satellite is in view then the connection is dropped. This may occur when the signal from either satellite is blocked by an obstacle. When successful the inter-satellite handoff may be noticeable by a quarter-second gap.

The satellites are also able to transfer mobile units to different channels and timeslots within the same spot beam.

Earth base-stations

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

Other technical information

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.

Tracking transceiver units

Without an extra GNSS receiver tracking is difficult, but not impossible as the position of a mobile unit can be determined using a Doppler shift calculation from the satellite. These readings however can be inaccurate with errors in the tens of kilometers. Even without using Doppler shifts a rough indication of a unit's position can be found by checking the location of the spot-beam being used.

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.

Patents, Manufacturing, and Launch Campaign

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).

Bugs

In the early days the system experienced many bugs. Satellites in orbit are subject to high levels of ionizing radiation that cause single event upsets (SEUs) in the memory chips - random bit-changes. The caches in the PPC603 processors were so susceptible to SEU's that they had to be disabled, canceling most of the performance benefits of a RISC processor. The memory system had automatic scrubbing hardware that would read and write memory words to correct SEUs before they could affect the CPU, by stealing CPU cycles. However, this hardware cut bus bandwidth in half again, so that each 200 MHz PPC603 could not perform better than a mid 1980's Macintosh II computer.

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.

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