Iridium Satellite System.

Iridium satellite system is used to provide voice and data coverage to satellite phones, pagers and other equipment with full global coverage (including the poles). The Iridium satellite system is owned by Iridium LLC based in the USA. This corporation not only operates the satellite constellation, but also provides equipment and services.

The system was originally aimed at providing a global satellite phone system, in the days when terrestrial cellular based systems were not as widely used. However the speed at which cellular telecommunications systems were deployed meant that Iridium is now used for more specialised applications.

Iridium satellite system basics

The Iridium satellite system consists of a constellation of 66 satellites which are arranged in six planes. There are eleven satellites in each orbit which is a near circular orbit inclined, each one being inclined at 86.4°. These orbits provide full global coverage, including the polar areas which are not covered by satellites in geostationary orbits.

The Iridium satellite system uses a frequency band in L band 1616 - 1626.5 MHz for communication with the users. The phone systems communicate directly with the satellite which then routes the data accordingly. The data may then be routed to other satellites or to the ground.

The data may be routed directly to the ground network from the satellite. The ground network comprises two major elements:

• System control segment (which supplies the operational support and control of the constellation as well as providing tracking data to the gateways and messaging control).
• Telephony gateways (used to provide connectivity with the terrestrial PSTN system. Gateways also provide local management functionality).

The system control segment has three main constituents:

1. Four telemetry tracking and command and control stations (TTAC).
3. Satellite operation network centre (SNOC).
5. Operational support network.

In addition to the direct links to the ground, each Iridium satellite is linked to four other satellites in the constellation. It links to two other satellites in the same orbit - these will be the ones either side of it, and it also links to two other satellites in adjacent orbits. These links provide a network in space which allows data to be routed between satellites without having to return to the ground from each satellite. Messages may even be routed across several satellites before reaching the ground. This provides a number of advantages:

• It provides a robust network that can still operate even if a ground station becomes non-operational.
• It enables a network to be run with stations placed in available regions - a particular advantage as it is not possible to set up stations in all areas for geographic reasons (polar, oceanic, etc) or for political reasons.
• It allows the network to be run with fewer ground stations - this allows costs to be reduced as manned ground stations can be expensive.

Iridium frequency bands and channels

The Iridium satellite system uses L band transponders to communicate with the ground based users with frequencies in the band 1616 - 1626.5 MHz while the backhaul to the terrestrial gateways is in the band between 29.1 and 29.3 GHz for the up-link and in the band between 19.1 and 19.6 GHz for the downlink. Links for the inter-satellite links to are in the frequency band between 22.55 and 23.55 GHz.

The user-satellite link uses a total of 10.5 MHz starting at 1616 MHz. This provides 240 channels, each separated by 41.67 kHz. This allows for a channel bandwidth of 31.5 kHz together with a suitable guard-band to accommodate inter-modulation and Doppler shifts caused by the satellite velocity relative to the ground stations.

Iridium access schemes

In order to support many users, it is necessary for the Iridium satellite system to operate a scheme where the different users can be managed so that they may gain access to the satellite system without interfering with each other. Iridium satellite technology uses both FDMA (frequency division multiple access - where users are allocated different frequencies) and TDMA (time division multiple access - where users are allocated different time slots in a transmission).

Iridium satellite summary

A tabular summary of the Iridium satellite technology is given below:

PARAMETER	SPECIFICATION
Iridium satellite orbit	LEO
Orbit altitude	780 km
Iridium applications	Voice and data
Satellites in constellation	66
User satellite link band	1616 - 1626.5 MHz L band
Gateway -> Satellite up-link	29.1 - 29.3 GHz
Satellite -> Gateway downlink	19.1 - 19.6 GHz
Inter-satellite link	22.55 - 23.55 GHz
Satellite relative velocity	26 804 km/hr
Minimum angle of elevation for acceptable operation	8.2°
Approximate satellite view time	9 - 10 minutes
Access scheme	FDMA / TDMA
Frequency re-use factor	12
Total system capacity	172 000 users

History of Iridium satellite system

The concept of the Iridium satellite phone system arose before cellular telecommunications systems had been globally deployed in the way they are today. It was therefore anticipated that a phone system offering global coverage would be of great commercial value.

To achieve the goal, Iridium SSC was set up, with Motorola providing the technology and much of the finance. The system gained its name from the fact that originally 77 satellites were planned to orbit the Earth. This had a strong similarity to a nucleus with 77 orbiting electrons - a description that fits the element Iridium.

The satellite system was completed in may 1998, but some trails and commissioning were then required. The Iridium communications service itself was launched on 1st November 1998 with a call made by the then Vice President of the USA, AL Gore. Services were then offered to all forms of personal users and business.

Despite the successful technical launch of the system, the uptake was very much slower than anticipated. This was attributed to the fact that the deployment of cellular telecommunications systems, especially using GSM was much faster than had originally been anticipated. With roaming features, users could take their phones to many countries and make calls cheaper than they could over the Iridium system. Also there was a massive initial cost for building and launching all the satellites. Accordingly Iridium SSC entered Chapter 11 bankruptcy on 13 August 1999.

Initially no buyers were found for the company and it was anticipated that all the satellites would need to be de-orbited so that they would not be a danger when they re-entered the atmosphere. During the period that discussions about this were ongoing a set of private investors re-launched the company as Iridium Satellite LLC and they restarted the service in 2001.

The service was obviously more expensive than cellular systems and it was therefore used for specialist applications, especially in areas where no cellular coverage was available. For example rescue services, missions to remote areas of the globe, military, etc found the service useful.

With satellites becoming older, some have failed. These have been replaced with in-orbit spares. It takes some days for these replacement satellites to be moved into service, but with the number of satellites in orbit, service is only marginally degraded. Examples of replacements have occurred at various times, e.g. Iridium satellite 28 failed in July 2008 and was replaced by Iridium satellite 95.

Another incident took place in 2009. At 16.56 UTC on 10 February 2009, an Iridium satellite (satellite 33) and Kosmos-2251, an out of service Russian satellite collided. Although a major collision creating large amounts of space debris, it was be replaced by a spare satellite already in orbit. This was completed during March 2009

Iridium Next

With the original constellation of satellites nearing the end of their life, Iridium announced its vision in February 2007 for the next generation of Iridium satellites. Known as Iridium Next, the new Iridium satellite system promises not only to replace the existing constellation of satellites, but Iridium Next also incorporates many new features. The basic functions for the system include:

• Backward compatibility with existing Iridium systems
• Full global coverage as before, including the polar areas not accessible to Geostationary satellites
• Low earth orbits enabling low power phones to be used.

Although it would be possible to provide a simple replacement programme for the existing Iridium satellites, it has been decided that Iridium Next will incorporate new technology to enable the system to move forward and keep up wit the enhanced requirements expected by many users today. Accordingly the new system will incorporate the following features:

• End to end IP technology
• Additional bandwidth capability to allow users more flexibility
• Incorporation of Earth imaging and other secondary payloads - thereby providing additional functionality to further utilise the satellites.
• Ability to provide data links to other space based payloads.

It is planned to replace the existing Iridium satellites so that a seamless upgrade can be effected.

Summary

The Iridium satellite system offers many advantages and is currently used by many people. This satellite phone system offers full global coverage for voice and data (albeit at a low rate). It is also certified for airborne use and along with many other roles, the Iridium satellite phone system is able provide a useful service to many.