At the end of the last century, satellite operators from the U.S.A. and Europe began to develop mobile satellite communication (MSC) systems for voice, data and tracking solutions. Ground cellular communications were already at an early stage of their development, but were not able to fully cover all areas on Earth. With the development of MSC all remaining regions in the world will be covered with great importance for science, mobile industry, telecommunications and defense.
MSC HISTORICAL BACKGROUND
Big low-Earth-orbit (LEO) systems are large low-earth-orbit satellites that provide comprehensive communications services in addition to real-time voice. They are used to provide global mobile telephone services via small personal handsets.
In September 1991, Inmarsat announced its strategy for the future development of Project-21 using an intermediate circular orbit (ICO). The culmination of this project was the introduction of a handheld phone prototype to the whole world under the name of the Inmarsat-P service. The Inmarsat-P phone was envisaged as a dual-mode terminal to work with a cellular system, from a satellite when out of home region cellular coverage, in a region with a different cellular standard or in a region without roaming arrangements. To implement this service, a new space segment architecture would be required, such as the proposed medium Earth orbit (MEO).
U.S. satellite operators Globalstar, Iridium, Ellipso, Odyssey, Aries and AMSC proposed the exploitation of the big LEO satellite constellation, so that on January 31, 1995, the Federal Communications Commission (FCC) granted licenses in the U.S. to Globalstar, Iridium and Odyssey.
TRW proposed to exploit the MEO satellite solution using a satellite configuration called Odyssey. The Odyssey constellation was to comprise 12 satellites, equally divided into three orbital planes, inclined at 55 degrees to the Equator. The satellites were to be placed 10,600 km above the Earth. The FCC granted TRW a license to set up its satellite MEO system in 1995, noting that construction of the first two spacecraft should begin by November 1997. The Odyssey was scheduled to begin operations in 1999 at an estimated cost of $3.2 billion. Unable to find another major investor willing to support the project, Odysseus was abandoned in December 1997.
The investment of $ 1.2 billion in ICO by Teledesic was announced in November 1999, but the system was abandoned after many problems. Meanwhile, Iridium and Globalstar received bankruptcy protection in the U.S.A. after difficulties with the establishment of space segments and problems with market penetration. Finally, both systems received sufficient funding to proceed to the next stages of development for the upgrade of terrestrial networks and personal satellite communications.
Loral Space and Communications, with Qualcomm, developed the Globalstar system concept at a similar time as Iridium. Globalstar received an operating license from the FCC in November 1996. The first launch of four Globalstar satellites occurred in May 1998 with a Delta rocket from Cape Canaveral. The deployment of 48 satellites plus four spares was finally accomplished, using Delta and Soyuz-Ikar rockets.
The first six second generation Globalstar satellites were launched on 19 October 2010, and additional six were launched in July 2011 followed by another six satellites in December 2011; so, the launch of 24 satellites of the second generation constellation was completed on 6 February 2013. In February 2022, it was announced that Globalstar purchased 17 new satellites to continue its constellation built by MDA Ltd. and Rocket Lab for $327 million. The satellites are expected to be launched by 2025.
The Globalstar satellite constellation supports the company's current line of products and services for voice, duplex and simplex data, including its SPOT branded consumer products that have launched over 2200 rescues in over 70 countries and at sea since its initial launch in 2007. Globalstar is known for its crystal clear voice service “landline quality,” ease of use and value for customers.
The Globalstar system (see Figure 1) uses code division multiple access (CDMA) and frequency division multiple access with an efficient power control technique, multiple beam active phased array antennas for multiple access, frequency reuse, variable rate voice encoding, multiple path diversity and soft-handoff beams to provide high-quality satellite service.1-4
Figure 1 Overview of the Globalstar MSC network. (Source: Ilcev)
GLOBALSTAR ARCHITECTURE
Handheld and semi-fixed satellite phones are communications tools available to businesses, professionals and military troops in mobile and fixed environments for satellite telephone access via the Globalstar big LEO MSC system at sea, on land and in the air. Compared to little LEO systems (comprising small satellites measuring around one meter cubed, to provide mobile data and messaging services), big LEO systems are bigger have more power and bandwidth and offering comprehensive services to their subscribers. The bigger size of these satellites enables more complex data processing in the transponders than the simple store-and-forward feature of the little LEO systems. These systems provide a wide variety of services, such as voice, data and fax, SMS and paging, Search and rescue (SAR), environmental monitoring and position, velocity and time data and determination.
Globalstar is a LEO satellite-based digital telecommunications system that offers wireless telephone, messaging, tracking and other telecommunications services worldwide, starting from the end of the last century. The communications system is designed to provide worldwide digitally crisp voice, data and facsimile services to portable, mobile and fixed user terminals (UTs). To the user, operation of a Globalstar phone is like that of a cellular phone but with one main advantage: while a cellular phone works only with its compatible system within its coverage areas, the Globalstar system offers worldwide coverage and interoperability with current and future publicly switched telephone and land mobile networks.
The Globalstar system comprises three major segments: Space, Ground and User, including a terrestrial telecommunication network, as shown in Figure 1. The Globalstar satellites receive signals from mobile devices in S-Band (forward link) and send signals to mobile devices in L-Band (return link). Links between satellites and ground earth stations (GESs) are in C-Band and the system is controlled by Operations Control Centers (OCCs). GES terminals are connected via satellite links with different users, such as personal earth stations, personal trackers, vehicle earth stations, ship earth stations, aeronautical earth stations and global vehicle trackers.
The Globalstar system does not have an inter-satellite connection like the Iridium network, so it cannot cover both pole regions. Thus, it provides near-global coverage to users anywhere in the world between the two poles, even when affected by propagation interference and non-ideal environmental conditions. Globalstar CDMA is a modified version of Interim Standard 95 (IS-95), originally developed by Qualcomm.
The Globalstar satellite comprises communication systems in S and L-Band, a trapezoidal body and two solar arrays. Each satellite operates at an altitude of 1414 km (approximately 876 mi). On any given call, several satellites transmit a caller's signal via CDMA technology to a satellite dish at the appropriate gateway where the call is then routed locally through the terrestrial telecommunications system. Globalstar phones look and act like mobile or fixed phones. Like "bent-pipes," or mirrors in the sky, the Globalstar constellation of LEO satellites picks up signals from over 80 percent of the Earth’s surface, everywhere outside the extreme polar regions and some mid-ocean regions. Several satellites can pick up a call; this “path diversity” helps assure that the call is not dropped even if a phone moves out of sight of one of the satellites.
The system's software resides on the ground, not on the satellites, which means fast and easier system maintenance and upgrades. The Satellite OCC manages the Globalstar satellite constellation. As soon as a second satellite picks up the signal and can contact the same terrestrial gateway, it begins to simultaneously transmit. If buildings or terrain blocks a phone signal, the “soft-handoff” prevents call interruption. The second satellite now maintains transmission of the original signal to the terrestrial GES or gateway. Additional advantages of using LEO satellites within the Globalstar system include no perceptible voice delay (latency) and lighter/smaller all-in-one phones.
Gateways process calls, then distribute them to existing fixed and cellular local telephone networks or the Internet. Terrestrial gateways are an important part of Globalstar's strategy to keep key technology and equipment easily accessible and to integrate its services as closely as possible with existing local telephone networks. This helps make the Globalstar system and its service easier to manage, expand and improve coverage.
Actual Globalstar coverage may vary because of gateway deployment, local licensing and other factors. Globalstar service is a satellite radio technology subject to transmission limitations caused by the type of terrain, service area limits, customer equipment use and other variable conditions including the functionality and orbital locations of the satellites themselves.
To provide full voice and duplex data coverage, Globalstar expanded its terrestrial network in 2012 with an additional 6 GESs for the Pacific, 3 GESs for the Atlantic and 5 GESs for the Indian Ocean region. Today, the total number of GES terminals is 24, however, for polar coverage, Globalstar must establish GES terminals in each polar region.4-7
SPACE SEGMENT
The Globalstar satellite transponder is transparent; unlike the Iridium system, without cross or intersatellite-links and onboard traffic processing, all traffic switching service happens on the ground and traffic routing is through the existing fixed public switched telephone network (PSTN) and associated networks. A satellite phased array antenna produces 16 elliptical spot beams that enable continuous multiple satellite global coverage, path diversity and position location. Lowering the angle to the satellite will increase the overlapping coverage. Thus, small changes can dramatically increase the coverage area, which is particularly apparent in the polar regions. If operated at low elevation angles, polar areas that otherwise could not be covered can receive service.
The Globalstar space segment has a constellation of 48 satellites in eight planes with six satellites per plane inclined at 52 degrees to the Equator at an altitude of 1414 km LEO. Four in-orbit spares are parked at a lower altitude. The low orbits permit low-power user phones, like cellular. The constellation is a 48/8/1 Walker Delta pattern with a 52 degree inclination, designed to provide global Earth coverage between 70 degrees N and S latitudes, see Figure 2.
Figure 2 Globalstar satellite coverage. (Source: Lloyd Wood – L.Wood@society.surrey.ac.uk)
Globalstar provides coverage from any point on the Earth’s surface to any other point worldwide with multiple overlapping satellite beams for simplex and voice/duplex data, exclusive of both polar regions. The simplex data coverage map in Figure 2 shows the coverage with the current 24 GES terminals indicated as ground satellite antenna units.
This service is for satellite asset tracking (SAT) and SAT and fleet management (SATFM) of all mobile assets including aircraft, known as global aircraft tracking (GAT). Globalstar also provides fixed data service for fixed assets known as satellite supervisory control and data acquisition (SCADA) of machine-to-machine communications (M2M). These units are designed to transmit just a single packet message 3-times (the original transmission plus 2-repeats) per day in the frequency appropriate for the given regions in the coverage area.
The coverage area may vary based on terminal location, terrain features, signal strength and other factors affecting satellite communications. The Globalstar simplex data service is provided via units containing GPS Rx and Satellite Tx only. Thus, to provide complete coverage for simplex data, like the Inmarsat coverage map, Globalstar must provide an additional 4 GES terminals for the Pacific area, 2 GES terminals for the Atlantic area and 3 GES terminals for cities in the Indian Ocean Region.
The voice and duplex data coverage map (see Figure 3) is available for satellite personal, mobile and fixed voice (Tel) and duplex data transmission and data service for SATFM of all mobile assets including aircraft. The map indicates coverage for voice and dial-up data calls only. Direct Internet calls (Dialing #777 send) can be made from all regions except China and the Central American countries of Belize, Panama, Guatemala, Honduras, Nicaragua, El Salvador and Costa Rica, along with their surrounding coastal waters.
Figure 3 Globalstar spot and simplex data coverage map.
In both polar areas, the need for overlapping coverage is increased and power demands may be increased because the look angle to the satellite is limited. High gain directional antennas become practical for fixed and even portable installations. The payback is that Globalstar can serve areas that otherwise might be unserviceable. The same considerations for polar areas apply to equatorial areas, where the overlapping coverage is less than 100 percent.
The Globalstar communication satellite is a simple, low-cost satellite designed to minimize both satellite and launch costs. The first-generation satellite, spacecraft orbital planes and second-generation of Globalstar satellite are illustrated in Figure 4.3-6
Figure 4 Globalstar satellites and space constellation; first generation satellite (a), spacecraft orbital planes (b) and second generation satellite (c).
