Satellite Communications Glossary Analog: A transmission method that uses a variable and continuous voltage to represent the current state. Analog devices monitor movement, temperature, and sound and convert them into a continuous mechanical or electrical output that changes with the condition being monitored. For example, telephones turn voice vibrations into electrical vibrations that represent the human voice. Analog implies a continuous signal, even though it varies in strength; this is contrasted with digital, which converts everything into numbers. ASCII (American Standard Code for Information Interchange): This refers to the encoding of characters of the English alphabet. ASCII codes or numbers represent text to computers and communications equipment. There are 128 characters represented by ASCII codes. ASCII is strictly a 7–bit code, while a byte, consisting of 8 bits, often was the smallest unit of information used by earlier computing systems. The eighth bit can be used for error checking; often, it’s set to zero if not used. ATC: Ancillary terrestrial component; i.e., a ground station. Azimuth: For a satellite system, azimuth is the horizontal angle or rotation that an antenna can be rotated through to point to a specific satellite in a geosynchronous orbit. It is usually referenced to north for convenience. Bandwidth: An expression of the capacity of a communications signal. For digital signals, the bandwidth is the data speed or rate, measured in bits per second (bps). For example, a communications device with a fixed bandwidth of 56kbps can transmit a maximum of about 56,000 bits of digital data in one second. If the system supports more than one user, the bandwidth or capacity is shared, and the data rate is proportionately reduced for each user as needed to maintain the channel bit rate at or below 56kbps. Baud: A measurement of the number of symbols that can be transmitted in one second. A symbol, which could be a character or data in any format, may contain multiple bits. Baud is frequently used interchangeably with bits per second, but this is incorrect. For example, a modem transmitting at 2,400 bits per second is actually operating at 600 baud. Bit: A bit, or binary object, refers to a digit in the binary numeral system. There are only two possible values: 0 or 1. Binary digits are almost always used as the basic unit for digital information storage and communication Bit rate: In communications and computing, bit rate (also written as “bitrate”) or data rate refers to the number of bits transmitted or processed in a specific unit of time. Bit rate is usually measured in seconds and written as bps or bit/s, often with a metric prefix such as kilo (kbps) or mega (Mbps). Byte: A byte is commonly used as a unit of storage measurement in computers, regardless of the type of data being stored. Originally a variable number of bits, th…
Satellite Communications Glossary
Analog: A transmission method that uses a variable and continuous voltage to represent the current state. Analog devices monitor movement, temperature, and sound and convert them into a continuous mechanical or electrical output that changes with the condition being monitored. For example, telephones turn voice vibrations into electrical vibrations that represent the human voice. Analog implies a continuous signal, even though it varies in strength; this is contrasted with digital, which converts everything into numbers.
ASCII (American Standard Code for Information Interchange): This refers to the encoding of characters of the English alphabet. ASCII codes or numbers represent text to computers and communications equipment. There are 128 characters represented by ASCII codes. ASCII is strictly a 7–bit code, while a byte, consisting of 8 bits, often was the smallest unit of information used by earlier computing systems. The eighth bit can be used for error checking; often, it’s set to zero if not used.
ATC: Ancillary terrestrial component; i.e., a ground station.
Azimuth: For a satellite system, azimuth is the horizontal angle or rotation that an antenna can be rotated through to point to a specific satellite in a geosynchronous orbit. It is usually referenced to north for convenience.
Bandwidth: An expression of the capacity of a communications signal. For digital signals, the bandwidth is the data speed or rate, measured in bits per second (bps). For example, a communications device with a fixed bandwidth of 56kbps can transmit a maximum of about 56,000 bits of digital data in one second. If the system supports more than one user, the bandwidth or capacity is shared, and the data rate is proportionately reduced for each user as needed to maintain the channel bit rate at or below 56kbps.
Baud: A measurement of the number of symbols that can be transmitted in one second. A symbol, which could be a character or data in any format, may contain multiple bits. Baud is frequently used interchangeably with bits per second, but this is incorrect. For example, a modem transmitting at 2,400 bits per second is actually operating at 600 baud.
Bit: A bit, or binary object, refers to a digit in the binary numeral system. There are only two possible values: 0 or 1. Binary digits are almost always used as the basic unit for digital information storage and communication
Bit rate: In communications and computing, bit rate (also written as “bitrate”) or data rate refers to the number of bits transmitted or processed in a specific unit of time. Bit rate is usually measured in seconds and written as bps or bit/s, often with a metric prefix such as kilo (kbps) or mega (Mbps).
Byte: A byte is commonly used as a unit of storage measurement in computers, regardless of the type of data being stored. Originally a variable number of bits, the popularity of computers based on 8–bit microprocessors in the 1980s redefined the term to describe a unit containing 8 bits.
Digital: The word “digital” can have many meanings, but as applied to communications, it often is used to describe a signal that was converted from an analog source—a voice, for example—by sampling the analog signal at a very high rate. A number is created, usually in binary format, to represent each sample taken. Factors such as sampling rate and numerical precision determine the accuracy of the digital representation of the original analog signal.
Earth station: This term describes the combination of antenna and receiver electronics used to receive a signal transmitted by a satellite. (See LES.)
Footprint: A surface map of the satellite’s signal coverage that may include contours of equal signal strengths. Different satellite transponders on the same satellite will often have different signal footprints.
GEO:Geostationary orbit, and orbit directly above the equator that allows a satellite moving at the same speed as the Earth’s rotation to remain virtually fixed in relation to a point on the Earth’s surface. Geostationary and geosynchronous are terms used to describe an object or a satellite that is in a geostationary orbit.
Global beam: A global beam is a signal coverage pattern used by the Inmarsat satellites that effectively covers one–third of the globe. Global beams are aimed at the center of the Atlantic, Pacific, and Indian Oceans by their respective satellites. Because these transmissions cover such a wide area, global beams deliver relatively low–strength signals at the surface of the Earth. Due to the low signal strength, a large antenna is required to receive and process the signals.
GMDSS: The Global Maritime Distress and Safety System is an international safety system that includes requirements for types of equipment and communications protocols used to enhance maritime safety and rescue.
Inmarsat and GMDSS: Satellite systems operated by Inmarsat are an important part of the GMDSS. Four Inmarsat networks participate in the GMDSS systems: Inmarsat-A, -B, -C, and F77 netwrosk provide for communication between ships and shore, plus telephone, telex, and high–speed data services to and from rescue coordination centers. The F77 system must be used with Inmarsat-C to meet GMDSS data requirements. Inmarsat-C equipment provides email, messaging, and a data link between ships and shore, plus the option of sending preformatted distress messages to a rescue coordination center.
ISDN: ISDN (Integrated services digital network) is a standard for the integrated transmission of voice, video, and data. Originally designed for the telephone network system using copper wires, the term is used to describe a communications channel that is capable of transmitting both digital voice and data (including video) simultaneously.
LES: A land Earth station (Inmarsat) is an Earth station that provides a link between the satellites and the ground-based communications network. LESs are sometimes referred to as coast Earth stations, and they are the satellite equivalent of the HF or VHF coast stations. Today, there is no need for an Earth station to be on the coast, so “land Earth station” is probably a more accurate description of these stations.
MPDS: Mobile packet data service (Inmarsat). When you are connected to a network, it appears that information is being continuously sent in both directions. In reality, the information is transmitted and received in short bursts, and small gaps are left between each burst, or packet. This is done so that the receiving station can verify that the data packets were correctly received. This is the same method used to transfer data on the Internet, and it’s aptly named Internet protocol, or “IP” for short. The Inmarsat MPDS service was developed to provide a way of sending packet data over the Inmarsat network. Because each data packet contains the address of the sender and the intended receiver, packet data from several users can be sent through a channel simultaneously. At the network server, the packets are sorted and forwarded to the proper recipient. As more users connect, they must share the channel (in this case, the spot beam) to connect with the satellite. Because the bandwidth, or capacity, of each channel is of a fixed size, the bandwidth available to each user is reduced. When the bandwidth available to a user is reduced, the speed of the data transfer is also reduced.
MSS: Mobile satellite service.
Ocean regions: Satellite coverage is often abbreviated to reflect certain global regions as follows.
AOR: Atlantic Ocean region
AOR-E: Atlantic Ocean region–east
AOR-W: Atlantic Ocean region–west
IOR: Indian Ocean region
POR: Pacific Ocean region
SIM card: A SIM (subscriber information module) card is a programmable card inserted into a phone to store a subscriber’s personal information and settings. Also known is the smart card, the SIM card contains the subscriber’s authorization to use a network and stores the phone number, personal security code, and other data necessary for a phone to operate. The card can be switched from phone to phone, which also transfers the subscriber’s phone number to the new phone.
SMS: SMS (short message service), also known as text messaging, is a service available on most digital cellular phones and other mobile devices that sends short messages between phones. There are many services available on the Internet that allow users to send text messages free of charge. SMS messaging is also provided by some satellite services.
Transponder: A combination receiver, frequently converter, and transmitter installed on board a communications satellite. Communications satellites typically have between 12 and 24 onboard transponders, but double this number or more may be installed if needed.
Uplink: This refers to the Earth station used to transmit signals to a satellite.
