Iridium
– Iridium Air Interface
– Start of Iridium and Bankruptcy
Globalstar
– Globalstar Air Interface
– Start of Globalstar and Bankruptcy
Inmarsat und Thuraya

Satellite based telephony

A natural consideration in the 1980s and 1990s was to station a base station virtually in orbit and use it for telephone calls. This would result in a huge radio cell and could cover areas where it is either not worth setting up a base station or where this is impossible. However, a large investment is required to build such a system and capacity will always be limited.

Technically speaking, satellite phones were based on digital 2G technology and were created at the same time or shortly afterwards in the 1990s. They were also mainly created by two players in the mobile space: Motorola and Qualcomm.

Iridium

Motorola has always been the leader in mobile communications and Motorola was ambitious. In 1987 they had a bold plan. A satellite-based mobile communications system that should be accessible across the entire planet using handheld devices. A first draft was published in 1988. The system would consist of 77 satellites, which would orbit from pole to pole in six low orbits (270 km). The satellites could communicate with each other (with neighboring satellites) and of course with ground stations.

Because there were (originally) 77 satellites, the system was called Iridium, because the element Iridium has the atomic number 77 and therefore 77 electrons spinning around an iridium atom. Later the system only had 66 satellites but the name was not changed.

In 1991, a company called Iridium Inc was founded to later operate the system.

Iridium Air Interface

A frequency band of 10.5 MHz from 1616 MHz to 1626.5 MHz was reserved worldwide for Iridium communication. The first 10 MHz are divided into 30 channels with a width of 333 kHz. These channels must be divided among the satellites to avoid interference. These bands are only used to communicate with mobile devices. The system uses frequencies between 19 GHz and 29 GHz for the satellites to communicate with the ground stations and with each other. Each band is in turn divided into 8 sub-channels with a bandwidth of 31.5 kHz and a spacing of ample 41.66 kHz.

There is no Uplink and Downlink band like in GSM. The sub-frequencies are divided into frames of 90 ms length. This includes 4 slots for the uplink and 4 slots for the downlink. In total, a satellite can process 30 x 8 x 4 or around 1000 data channels. This means that fewer than 1000 connections can be made in a region the size of Europe.

In addition to the channels for voice and data transmission, there are also 12 channels above 1626 MHz. These are used for so-called simplex channels, which are used for control information. The data transfer rate of the traffic channels is just 9.6 kbit/s. Only 2.4 kbit/s are available for voice transmission. That’s by far not enough for a good speech encoder. The quality does therefore not correspond to a normal mobile network, but rather to a simple walkie talkie.

The cell size of an Iridium satellite has a radius of around 2000 km. For example, one satellite covers the whole of Western Europe area. The satellites are in low orbits. Therefore the satellites move relatively quickly over the earth. For this reason also for the Iridium System a handover must be done, when a satellite moves over the horizon and a new satellite comes up.

Start of Iridium and bancruptcy

Starting in 1997, the Iridium satellites were placed into orbit using 20 rockets. The official launch of the network began in November 1998. Iridium’s business plan called for 500,000 subscribers by the end of 1999. However, this did not happen. Instead, there were only 50,000 users. The were many reasons for this failure and this was somehow predictable. With $6,000, the devices were too expensive. On top, they were too big and too heavy. Furthermore a minute of talk time cost up to $6. And all this came with poor voice quality and not very stable connections. Connections could only be established at “line of sight” with the satellites. This is often not the case in deep valleys. An advantage of the system was that it could be used in extremely remote regions. But in those regions where not sufficient users to „pay the bill“.

Iridium therefore had to declare bankruptcy at the end of 1999. As a result, a $5 billion of investment was lost. There was considerable damage, especially for Motorola. The Iridium disaster was one of the reasons for the decline of Motorola starting in the late nineties. Motorola could hardly recover financially.

After the bankruptcy it was considered that all satellites were to be deliberately crashed because their operations could no longer be maintained. But due to public pressure this was prevented. There were many adventurers and expeditions that meanwhile depended on communication with Iridium.

Eventually a new operating company was founded, Iridium Satellite LLC, which bought the entire system for just $25 million. The American military also saw value in the system and placed a $72 million contract. The system gradually gained more customers and became finally profitable in 2004.

Globalstar

Parallel to Iridium, another system for a satellite phone was developed in the USA called Globalstar. The origin came from Qualcomm. In effect, they wanted to expand the use of their CDMA system (IS-95) to satellites. Globalstar was a joint venture between Qualcomm and Loral Cooperation, an American defense company that specialized in satellite technology. The basic idea of Globalstar was that the satellites should only work as repeaters. In practice, they should forward the signals from the end device to a “base station”. Other than Iridium, there was no connection between the satellites and there always had to be a base station within the satellite’s reception range.

Globalstar used 48 satellites for its system, which were distributed over 8 orbits. However, these did not fly over the poles as with Iridium, but rather between the longitudes 70° and -70°. They flew on a higher orbit at an altitude of 1400 km.

GlobalStar Air Interface

Globalstar uses the same band as Iridium for its uplink, the so-called L-band: 1616 MHz to 1626.5 MHz. They also got the license of another band in the S-band: 2483.5 MHz – 2500 MHz for the downlink. For the connection to the base station higher frequency ranges were used around 5000 MHz and 7000 MHz. There the bands could be used for 13 sub-bands of 1.23 MHz width.

The IS-95 CDMA system was used to run these bands with only minor changes. Furthermore a satellite did not only transmit a broad signal to Earth but so-called radio beams. 16 radio beams transmitting in different directions where used which could be operated independently like sectors of a cellular network. So theoretically there are 16 x 13 x 55 = 11440 channels that can be routed via a satellite. In fact, the Globalstar base stations were the most powerful base stations at that time and can handle 10,000 calls simultaneously. However, the “area” of this base station was also as large as half of North America.

Start of GlobalStar and bancruptcy

With $1.8 billion, Global star was far less expensive than Iridium. The first satellites were launched in 1998 and Qualcomm’s CEO Irvin Jacobs made the first call with this satellite system in the same year. Full expansion with 48 satellites and full operation began in 2001. Despite the better performance, Globalstar was also a failure and had to file for bankruptcy in 2002. Unlike Iridium, Globalstar could not be received everywhere in the world. Mainly in the USA, Europe and Australia where there were already well-developed cellular networks. Nevertheless, like Iridium, the system was saved and continued to be operated by a successor organization.

Inmarsat and Thuraya

There are two other satellite systems which are based on geostationary satellites. This means that the satellites are always over the same place on Earth. However, it is impossible to reach the polar regions.

Thuraya is a satellite system consisting of only 2 satellites covering Europe, Asia and Australia. Founded 1997 in Abu Dhabi, it began operations in 2001. Like a GlobalStar satellite, a Thuraya satellite has so-called spot beams. This creates 512 independent sectors that can be served independently. Thuraya has two bands around 1500 MHz and 1600 MHz with a bandwidth of 34 MHz each. This is divided into many narrowband sub-bands with a spacing of 31.25 kHz. These are QPSK modulated and contain TDMA slots. A satellite is capable of making up to 13,750 calls simultaneously.

The oldest satellite system is Inmarsatsat. This goes back to an initiative of an international organization of the United Nations, the International Maritime Organization (IMO). The aim was to ensure the safety of all ships on the world’s oceans (later also aircrafts). Intelsat was founded in 1979. In the 1990s, several satellites were launched that cover all of the world’s oceans. In 1999, Inmarsat was privatized in order to better finance the system. Since then, the system has also been available to private users.

Inmarsat has 4 satellites that cover almost all seas and land areas except the polar regions.