How the Iridium Network Works

The Iridium satellite network includes three principal components which include the satellite network, the ground stations and the satellite phones and data units. Voice and data messages can be routed anywhere in the world by the Iridium network. Calls are relayed from the satellite phone or data unit on the ground to one of the Iridium satellites. It is then relayed from one satellite to another then down to an appropriate ground station. The call is then transferred to the public voice network or Internet when it reaches the recipient.

The Iridium constellation includes 66 satellites flying in six orbital planes with 11 satellites equally spaced apart from each other in that orbital plane.  The satellites have polar orbits at an altitude of 485 miles. 

The satellites communicate with each other using Ka-band intersatellite links. Each satellite has four intersatellite links, two to the fore and aft satellites in the same orbital plane and two to the satellites in orbital plane to either side. These intersatellite links allow calls to be routed among the Iridium satellites before being transferred to a ground station. This intersatellite links makes Iridium impervious to natural disasters like hurricanes or earthquakes which can damage a ground station. If a ground station is damaged the Iridium satellites can rout the call to another ground station.  

Each satellite completely orbits the earth in 100 minutes traveling at 16,832 miles per hour.  From horizon to horizon it takes 10 minutes. As satellites move out of view from the satellite phone user the call is handed over to the next satellite coming into view.  Each Iridium satellite contains seven Motorola Freescale PowerPC 603E processors running at 200MHz.  One processor is dedicated to each cross link antenna and two processors are dedicated to satellite control with one as a spare.  Each Iridium satellite can support up to 1100 phone calls.  Communication with the Iridium network is done using a TDMA and FDMA based system using L band spectrum between 1616 and 1626.5MHz.  Iridium controls 7.775MHz of this. 

The Iridium network uses three different types of handoff. When a satellite travels over the horizon the call is handed to adjacent spot beams. At the equator where the satellites are spaced the furthest apart, a satellite stays in view for seven minutes. This handoff may be noticeable by the satellite phone user by a quarter second gap in the call. The constellation is able to transfer the call to different channels and timeslots within the same spot beam. 

The Iridium gateways include the system control segment and telephony gateways used to connect into the public telephone system. As a satellite leaves the area of a ground station and looses it line-of-site, the routing tables change and frames are forwarded to the next satellite coming into view of the gateway. Gateways are located in Tempe, Arizona and Wahiawa, Hawaii. Like other satellite networks, Iridium satellite phones must have line-of-site to the sky in order to place a call. They will not work consistently indoors or under trees.  There are fixed site units that include an external antenna giving you line-of-site for use on a building. There are also car kits giving you an external antenna. 

Iridium operates the most reliable satellite constellation currently available. One of the reasons why Iridium works so well is because their system has more satellites than any other satellite provider, giving coverage to every part of the planet. With no service gaps, Iridium users can place and receive calls from virtually every part of the planet as long as they have line-of-sight to one of the satellites. With cellular systems only covering 15 percent of the planet Iridium is the only wireless service available to many parts of the world. 

How VSAT Equipment functions

The Very Small Aperture Terminal (VSAT) is a satellite communications device that allows reliable data transmission via satellite using small antennas of 0.9m to 3.8.m. VSAT is a plug and play device. VSAT has terminals arranged in a star configuration into the central hub station that is connected to the host computer. Communication between the terminals has to pass through the network central hub processor. The VSAT technology does not send signal to each other or there is no direct communication between VSAT devices without a hub. The hub consists of three elements, namely Radio Frequency Terminal (RTF), VSAT hub base-band equipment and the user interface. VSAT is the fastest technology compared to point-to-point connection, or dial-up connection due to the fact that VSAT deploys the use of procedure to make connection, which other communication systems either don’t have or are not reliable.

The VSAT entire network has got a satellite, main hub with an antenna measuring 5 to 11metres. VSAT configuration is made up of two segment , where one segment is known as the earth segment  - this segment has equipment both at the central hub and at the remote locations, the other segment is known as the space segment – VSAT pace segment is link to and from the satellite Satellite segments in the sky serves as a radio frequency repeater, when VSAT send information to the satellites in the sky, it is received, amplify the information and retransmit on a higher frequency. As the information is being transmitted, much of the work is done in the ground where the hub controls and regulates the entire operation of the communication network A hub in VSAT network consist of Network Management System(NMS) which accumulates data on the system  giving the system health check, and providing billing information. In VSAT network to operate it has to be in a position of geostationary orbit, which is relatively high from the ground normally around 38,800km from the equator.

There has to be a remote terminal which consists of two units one placed outdoor directly connected to the satellite while one placed indoors and connected to the user devices. When connecting the VSAT network, the central hub is located near the user’s main office or at the central location where the host computer is. The hub normally enables the connection via satellite of all the VSAT terminals within the network and any other communication from host to the VSAT

How the Inmarsat System Works

Orbital Operations and Call Flow

• The Inmarsat satellites are positioned in geostationary orbit. This means they follow a circular orbit in the plane of the Equator at a height of 35,600km, so they appear stationary relative to a point on the Earth's surface.
• The satellites are controlled from the Satellite Control Centre (SCC) at Inmarsat HQ in London, which is responsible for keeping the satellites in position and for ensuring the onboard systems are fully functional at all times.
• Data on the status of the Inmarsat satellites is supplied to the SCC by four tracking, telemetry and control (TT&C) stations located at Fucino, Italy; Beijing in China; Lake Cowichan, western Canada; and Pennant Point, eastern Canada. There are also back-up stations at Eik in Norway and Aukland, New Zealand.
• A call from an Inmarsat mobile terminal goes directly to the satellite overhead, which routes it back down to an LES. From there the call passes into the public phone network for connection to home, office or cell.
• The flow of communications traffic through the Inmarsat network is monitored and managed by the Network Operations Centre (NOC) at Inmarsat HQ.
• The NOC is supported by network co-ordination stations (NCS). Their primary role is to help set up each call by assigning a channel to the mobile terminal and the appropriate LES. There is one NCS for each ocean region and for each Inmarsat system (Inmarsat A, B, C, etc). Each NCS communicates with all the land earth station operators in its ocean region, the other NCSs and the NOC, making it possible to distribute operational information throughout the system
• With the launch of BGAN, two new gateways, called Satellite Access Stations (SASs), are being introduced. Both are owned by Inmarsat. The first, in Burum, The Netherlands, will be operated by Inmarsat partner Xantic, and the other, in Fucino, Italy, by another partner, Telespazio.

The Latest Inmarsat I-4 Satellite System: Gateway to Broadband

• Are among the largest commercial communications satellites ever launched.
• Replacing their highly successful predecessors - the Inmarsat-2 and Inmarsat-3 spacecraft
• Together they have the ability to deliver simultaneous voice and data at speeds of about half a megabit per second.
• Each I-4 is a veritable powerhouse compared with the Inmarsat-3 spacecraft, offering:
  • Greater call capacity than all five I-3s put together
  • 60 times more power than any one of its predecessors
  • 12 times greater efficiency in its use of radio spectrum
  • 16 times the capacity
  • 25 times the receiver sensitivity.
  • A single global beam that covers up to one-third of the Earth's surface, apart from the poles.
  • 19 wide spot beams that provide continuous coverage across the same region for Inmarsat's existing high-end services, including Fleet F77, Fleet F55 and F33, and maritime mini-M.
  • An additional 228 narrow spot beams, designed to form the backbone of Inmarsat's broadband services
• Together the I-4 beams will service about 85 per cent of the world's landmass, covering approximately 98 per cent of the global population.