Iridium satellite technology, theory and frequency bands
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. 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:
- Four telemetry tracking and command and control stations (TTAC).
- Satellite operation network centre (SNOC).
- 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:
|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
|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
|Approximate satellite view time||9 – 10 minutes|
|Access scheme||FDMA / TDMA|
|Frequency re-use factor||12|
|Total system capacity||172 000 users|
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.
Poole, I. (n.d.). Iridium satellite technology, theory and frequency bands. Retrieved June 01, 2016, from https://www.radio- electronics.com/info/satellite/communications-systems/iridium-theory-history-technology-frequency.php