Globalstar’s constellation is made up of 40 (LEO) satellites with an additional four satellites in orbit as spares. The satellite design is very simple. Each satellite consists of an antenna, two solar arrays, a magnetometer, and a trapezoidal body. The Globalstar constellation orbits at an altitude of approximately 1,500 kilometers of (876 miles). First generation Globalstar satellite weighed 550 kg. Their second generation satellites will weigh substantially more. Globalstar satellites orbit with an inclination of 52 degrees. Because of this and the fact the satellites lack inter satellite linking the constellation picks up signals from over 80% of the planet excluding the extreme polar regions and some ocean regions.
The satellites are simple repeaters or mirrors in the sky. Globalstar calls this “bent pipe” technology. Voice and data transmissions come up from the satellite phone or data device to one of the 40 satellites, that signal is then bounced back down to one of the ground gateways where it is transferred to the public switched telephone network and/or the Internet.
The Globalstar gateways are a very important port of the ground segment which includes Ground Operations Control Center (GOCC), Globalstar Data Network (GDN) and Satellite Operations Control Center. Each gateway or ground station is owned and operated by a service provider which receives transmissions from orbiting satellites and switches the calls to the appropriate ground network.
Gateways have three to four dish antennas, remote operating controls and a switch station. With the switches and complex hardware on the ground it is easier to maintain and upgrade the network than it is for Iridium, their software and switches are located on the satellites.
Globalstar utilizes a standard T1/E1 interface to the existing PSTN/PLMN systems giving seamless integration with local landline and wireless networks. Encryption is provided giving a peace of mind that voice and signal security is safe.
Ground Operations Control center (GOCC) is responsible for controlling the use of satellites by gateway terminals and coordinating with Satellite Operations Control Center (SOCC). The GOCC controls the allocation of satellite resources for each gateway.
SOCC manages the constellation by tracking satellites, providing telemetry, controlling their orbits and command services for the constellation. Each Globalstar satellite constantly transmits telemetry data providing on board health reports for each satellite. The SOCC oversees satellite launch and deployment. The GOCC and SOCC remain in constant contact using the Globalstar Data Network (GDN).
The GDN is the connective network providing wide area intercommunications for the Gateways, GOCC and SOCC.Network Performance
Globalstar’s Low Earth Orbit (LEO) constellation offer significant advantages over Geosynchronous (GEO) networks for mobile satellite services (MSSs). Advantages include having multiple satellites to receive service from instead of one. Low power handheld and vehicle mounted equipment can be utilized. GEO satellite networks use a signal satellite servicing a region of the earth with an approximate altitude of 35,800 kilometers. GEO networks are best suited for high speed data, video transmission and other wideband applications.
Mobile users want hand held and vehicle mounted devices with omnidirectional antennas that provide instant access to voice and data services. GEO network normally require larger satellite terminals with directional antennas that must be placed stationary to acquire line of site to the satellite. Because with a LEO network the distance the transmission travels is much shorter than GEO networks, they provide higher quality voice transmissions. The user does not normally experience time delays or echo associated with GEO networks.
With Globalstar’s LEO satellites, even when a satellite fails service will be provided by the other 40 LEO satellites in orbit. A GEO system with a satellite failure would suffer an outage in an entire region. Also GEO systems require larger more powerful terminals which does not normally accommodate a low powered handheld device.
Globalstar’s patented path diversity method of signal reception permits the combining of multiple signals of varying power into a single coherent signal. The Globalstar user can operate with one or four satellites at one time if they are in line of site of the satellite device.
Using a rake receiver satellite signals from three satellites can be combined into a single static free signal. The satellite device will also alter power levels to compensate for shadowing and interference if needed.
The Globalstar satellites are constantly moving in and out of view. This allows the Globalstar system to provide service to a wider variety of locations with less dropped calls from buildings, trees and other tall structures.
Globalstar uses a version of Code Division Multiple Access (CDMA) technology based upon the IS-95 CDMA standard. This standard provides high quality, crisp voice, data and fax services. CDMA utilizes digital transmission which users share time and frequency allocations assigned by unique codes. The CDMA technology allows a large number of users to simultaneously access a single frequency channel orthogonally reducing interference. This creates a many fold increase in capacity compared to analog systems like the Frequency Division Multiple Access (FDMA).
Globalstar uses a combination of CDMA with FDMA and spread spectrum modulation enabling it to support multiple users simultaneously. Globalstar’s CDMA technology with their path diversity with multiple satellites results in higher quality calls and fewer dropped calls.
The Globalstar simplex data and asset tracking services uses the L-Band uplink of the satellites. The simplex data service is operating at a successful transmission rate of greater than 99 percent. Globalstar SPOT satellite messenger and simplex transmitters utilize this service and has been a strong part of Globalstar’s business.
The SPOT Satellite Messenger product is marketed through Globalstar’s subsidiary SPOT Inc. The SPOT is manufactured by Axonn using the Nemerix GPS chipset.