active application
An application that is currently running and is in the foreground. The active application is the one in which you are currently working.
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AC
Alternating Current. The electricity provided by wall-sockets. Many devices, like laptop/notebook computers, use AC adapters.
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AC adapter
A power cord designed to allow appliances designed to utilize DC current to be powered from a standard wall outlet by receive incoming power from that source. Common examples of AC adapters include the power cable for a laptop computer that transfers current from a standard wall outlet through a conversion box and into a smaller cable and then plugs into the rear of the laptop computer
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active hydrogen maser
Active Hydrogen Maser technology provides the best known frequency stability for any frequency generator commercially available today. Active masers are used when the best stability is needed in a time domain of 1 sec. to a day. At a 1-hour averaging time, the Active Maser exceeds the stability of the best-known cesium oscillators by a factor of at least 100.
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ACTS
Automated Computer Time System, a NIST service that provides announced time via telephone.
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address
A hexadecimal number designating the location of a device or of a piece of data. The bus address is determined by a setting on a peripheral device that allows the computer to distinguish between two devices connected to the same interface. A bus address is also referred to as a device address.
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address lines
The lines in a bus or cable that carry address information rather than data.
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AIX
A version of UNIX produced by IBM for their RS/6000 computer systems.
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alphanumeric display
A display that gives the information in the form of characters (numbers or letters).
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alphanumeric LED display
A display that gives the information in the form of characters (numbers or letters) using light-emitting diodes.
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AM
Amplitude modulation (AM) is a transmission technique that blends a data signal into a carrier by varying (modulating) the amplitude of the carrier waveform.
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ANSI
ANSI (American National Standards Institute, www.ansi.org) A membership organization founded in 1918 that coordinates the development of U.S. voluntary national standards in both the private and public sectors. It is the U.S. member body to ISO and IEC. The acronym is used to identify standards created by the institute, such as the ANSI 8-bit character set, or “ANSI_C”.
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anywhere fix
The ability of a receiver to start position calculations without being given an approximate location and approximate time.
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API
Application Program Interface. This interface allows software developers to write their software so that it can communicate with the computer’s operating system or other programs.
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application
A set of computer instructions or programs that accomplish tasks for the end user, rather than control the computer (that is what the operating system does). Examples of applications include spreadsheets, word processing programs, graphics, database management, and communications.
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architecture
The structure and organization of a computer’s hardware and/or system software, including items such as the CPU type, bus type, etc.
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ASCII
(American Standard Code for Information Interchange) Pronounced “ask-ee.” The American Standard Code for Information Interchange 7-bit character set. It consists of the first 128 (0-127) characters of the ANSI character set (and most other 8-bit character sets). The ASCII character set is the most universal character-coding set. Historically, ASCII developed from telegraphic codes and first entered commercial use as a 7-bit teleprinter code promoted by [AT&T | Bell]] data services.
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ASCII text file
A File that contains nothing but ASCII characters without special formatting. ASCII text files are sometimes used in file transfers.
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AT
Any IBM or compatible computer that contains an 80286 processor and 16-bit bus.
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ATbus
See ISA
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ATM
Asynchronous Transfer Mode. A network technology for both local and wide area networks (LANs and WANs) that supports realtime voice and video as well as data. The topology uses switches that establish a logical circuit from end to end, which guarantees quality of service (QoS). However, unlike telephone switches that dedicate circuits end to end, unused bandwidth in ATM’s logical circuits can be appropriated when needed. For example, idle bandwidth in a videoconference circuit can be used to transfer data. ATM is a cell relay network protocol which encodes data traffic into small fixed sized (53 byte; 48 bytes of data and 5 bytes of header information) cells instead of variable sized packets as in packet-switched networks (such as the Internet Protocol or Ethernet).
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atomic clock
An atomic clock is an extremely accurate timekeeping device regulated by the natural regular oscillations of an atom or molecule. An atomic clock powered by a hydrogen atom (maser) is accurate to 1 part in 2 quadrillion. A cesium atom powered clock has an error of about one second in one million years. GPS satellites carry multiple atomic clocks, regulated by these precise atomic vibrations, to ensure accuracy. The GPS Master Control Station uses cesium clocks and a hydrogen maser clock.
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autoconfig
Automatic configuration. The ability of computer hardware to detect the presence of expansion devices, and, using information resident in those devices, configure the system to run the device properly.
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AUTOEXEC.BAT
A batch program (set of commands) that is automatically carried out by MS-DOS whenever the computer is started or restarted. The file contains basic startup commands that help configure the system. Often, when a computer stops doing what it normally does when it boots (enters dos shell automatically or starts Windows, etc.), it is because the autoexec.bat file has been deleted. It is a good idea to have a copy of the autoexec.bat file (printed or on a separate disk) in case this happens.
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backplane
A circuit board (usually a printed circuit board) that connects several connectors in parallel, forming a computer bus. It is used as a backbone to connect several printed circuit board cards together to make up a complete computer system. Backplanes are normally used in preference to cables because of their greater reliability. A backplane’s service life is limited only by the longevity of its connectors. For example, the DIN 41612 connectors used in the VME bus system can withstand 50 to 500 insertions and removals (called mating cycles), depending on their quality. A backplane is generally differentiated from a motherboard by the lack of on-board processing power where the CPU is on a plug-in card.
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bandwidth
Bandwidth is the range of frequencies that a communication cable or channel can carry. In the computer world, it generally refers to the amount of data that can be carried by a specific cable or bus.
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BCD (Binary Coded Decimal)
Also called packed decimal, this is the representation of each digit of a decimal number by four-bit binary numbers. So the number 29 would be encoded as 0010 1001.
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Berkeley Software Distribution
see BSD.
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beta
A conventional designation of pre-release software. A beta version of software is usually in secondary testing (alpha versions are earlier releases), and is sometimes publicly available, but sometimes only available to particular users (beta testers). Beta software is usually somewhat unstable, and may have more bugs than a regular release.
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binary file
A file containing information that is in machine-readable form; it can be read only by an application. “Binary_file” usually refers to a file that uses all 8 bits of each byte for information. Text files usually use only 7 bits, leaving the 8th bit as 0.
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BIOS (Basic Input / Output System)
Hard-wired instructions the CPU uses to boot up and to control I/O devices. The BIOS controls most computers basic input and output functions such as communications with the drives or the system memory
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BIPM
One of the three organizations established to maintain the International System of Units (SI) under the terms of the Convention du Mètre (Metre Convention). The averaging and steering of TAI [International Atomic Time, (Temps Atomique International)] is done by the Bureau International des Poids et Mesures (BIPM).
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bit
Binary digit. Smallest unit of digital information, either on or off (1 or 0).
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boot
The loading of the operating system and starting of initial processes into a computer. From the saying, “pulling_oneself_up_by one’s_bootstraps.”
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BootP
Bootstrap Protocol
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board
A piece of fiberglass that holds integrated circuits (ICs or chips) and contains the connections between chips. A board is also called a card or circuit board.
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boundary clock
Generally a switch with more than a single IEEE 1588 equipped port, which is a slave on one port and a master on all others.
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bps
Bits Per Second. Speed at which data is transferred. Often used in discussing the speed of modems and serial transmissions.
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bridge
Traffic filter within a single network. They operate like a traffic cop to direct messages toward a destination and to exclude messages that are intended for other destinations. Like repeaters, they can join network segments. Unlike repeaters, they can extend the network by resending the signal. You may find you need a bridge as traffic on your network increases.Bus: Part of a chip or circuit board designed to send and receive data. The bus on a computer’s motherboard contains slots for expansion cards.
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BSD
Berkeley Software Distribution is the UNIX derivative distributed by the University of California, Berkeley starting in the 1970s. The name is also used collectively for the modern descendants of these distributions.
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BSP
Board Support Package
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BUS
In computer architecture, a bus is a subsystem that transfers data or power between computer components inside a computer or between computers. A bus logically connects several peripherals over the same set of wires. (Also, part of a chip or circuit board designed to send and receive data. The bus on a computer’s motherboard contains slots for expansion cards.)
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bus level products
Plug-in timing boards for various computing platforms.
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bus level timing
Board level products may be used to precisely time synchronize one or more computers and/or provide very precise time to the host or a application program.
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bus speed
The speed (measured in megahertz, MHz) at which information or data can move across the bus on the motherboard.
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bus timing instrumentation
Bus Level Timing boards provide time stamping and synchronization requirements. Synchronizes one or more computers to an external time standard, to provide very accurate time stamps to application programs or storing the time occurrence of external events.
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byte
The amount of memory needed to specify one ASCII character: eight bits. Kilobytes (~1000 bytes) and Megabytes (~1,000,000 bytes) are usually used in discussing the amount of memory a computer uses.
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C
A popular programming language.
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C++
An extension of the C programming language.
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C/A code
The standard (Course/Acquisition) GPS codeconsisting of a sequence of 1023 pseudo-random, binary, bi-phase modulations on the GPS carrier at a chip rate of 1.023 MHz. Also known as the “civilian code.”
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card
A hardware expansion unit that is installed inside the case of a computer, into an expansion slot on the motherboard.
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carrier
A signal that can be varied from a known reference by modulation.
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carrier-aided tracking
A signal processing strategy that uses the GPS carrier signal to achieve an exact lock on the pseudo random code.
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carrier frequency
The frequency of the unmodulated fundamental output of a radio transmitter.
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carrier phase GPS
GPS measurements based on the L1 or L2 carrier signal.
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CBO (Cesium Beam Oscillator)
A highly-precise frequency source. Also known as a cesium clock or cesium atomic standard.
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CBT (Cesium Beam Tube)
The key component of a cesium-based atomic clock.
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CD (Carrier Detect)
A signal that indicates whether an interface is active. Also, a signal generated by a modem indicating that a call has been connected.
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CDMA (Code Division Multiple Access)
A method for transmitting simultaneous signals over a shared portion of the spectrum.
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Central Processing Unit
The master chip in a computer that controls the execution of program instructions, arithmetic functions, and access to memory and peripherals. Abbreviated CPU, and also called the microprocessor.
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cesium
Also cae·si·um (Symbol Cs) Atomic number 55; atomic weight 132.905; melting point 28.5°C; boiling point 690°C; specific gravity 1.87; valence 1. [From Latin caesius, bluish gray (from its blue spectral lines).]
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cesium beam clock
Device that uses as a reference the exact frequency of the microwave spectral line emitted by atoms of the metallic element cesium, in particular its isotope of atomic weight 133 (“Cs-133”). The integral of frequency is time, so this frequency, 9,192,631,770 hertz (Hz = cycles/second), provides the fundamental unit of time.
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cesium clock
A clock containing a cesium standard as a frequency-determining element.
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cesium drcbt
Cesium Direct Replacement Cesium Beam Tubes
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cesium frequency standard
Cesium frequency standards work by squirting Cesium atoms from a reservoir that’s heated to boil off some atoms from a small piece of cesium liquid metal. These atoms are processed is such a way that they become an atomic frequency standard.
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cesium oscillator
Often used as a primary frequency standard since the SI second is defined from the resonance frequency of the cesium atom (133Cs), which is 9,192,631,770 Hz. A properly working cesium oscillator should be close to its nominal frequency without adjustment, and there should be no change in frequency due to aging. Environmental conditions (motion, vibration, magnetic fields, and so on) do cause small frequency shifts.
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channel
A channel of a GPS receiver consists of the circuitry necessary to receive the signal from a single GPS satellite.
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chip
The transition time for individual bits in the pseudo-random sequence. Also, an integrated circuit. Also a snack food. Also a betting marker.
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CLE
Customer Located Equipment
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CLI
See Command Line Interface.
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clock
1. An instrument other than a watch for measuring or indicating time, especially a mechanical or electronic device having a numbered dial and moving hands or a digital display. 2. A time clock. 3. A source of regularly occurring pulses used to measure the passage of time, as in a computer. 4. Any of various devices that indicate measurement, such as a speedometer or a taximeter.
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clock bias
The difference between the clock’s indicated time and true universal time.
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coaxial cable
A type of cable that is commonly used due to its insensitivity to noise interference. Coaxial cable is often used in Ethernet networks; both thick (10Base5) and thin (10Base2) ethernet cables are coaxial.
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code phase GPS
GPS measurements based on the pseudo random code (C/A or P) as opposed to the carrier of that code.
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command
A word or phrase, usually found in a menu, choosen in order to carry out an action. Choose a command from a menu, or type a command at the MS-DOS prompt. Some commands are Format, Edit, Open, etc.
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command line
Commands to run an application. Type commands at an MS-DOS prompt or in the Run dialog box in the Program Manager of Windows. Interfaces where commands are typed rather than choosen from a menu, are often called command line interfaces. MS-DOS has a command line interface while the Macintosh does not.
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Command Line Interface (CLI)
A way of accessing a computer, based on typing commands rather than selecting icons and menu items (compare with GUI).
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command prompt
Usually refers to the MS-DOS prompt. The character or characters that appear at the beginning of the command line (usually “C:”), indicating that the computer is ready to receive input.
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control segment
A world-wide network of GPS monitor and control stations that ensure the accuracy of satellite positions and their clocks.
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compactPCI
PCI bus technology implemented on a Eurocard. Eurocards provide more rugged packaging and a more secure plug and socket for embedded systems than the standard PCI card used in desktop computers. CompactPCI may support hot swap and provides higher performance (32-bit, 33MHz).
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compiler
A program which translates source code written in a particular programming language into computer-readable machine code that can be executed on its own. Compare with Interpreter.
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confidentiality
Keeping secret data from unauthorized eyes.
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content filtering
A filter that screens out data by checking, for example, URLs or key words.
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crystal oscillator
Oscillator that produces electrical oscillations at a frequency determined by the physical characteristics of a piezoelectric quartz crystals.
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crystal oscillator low distortion
Designed to operate with fundamental crystals with less than mW dissipated in the crystal.
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Coordinated Universal Time (UTC)
See UTC
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COTS
Commercial Off-The-Shelf. Products or services that are generally available and not built to customized specifications.
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cPCI
Compact Peripheral Component Interconnect - a bus standard
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CPU
See central processing unit
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crystal oscillator PLL (Phase-Locked Loop)
A circuit in which a crystal oscillator is used as a time base for a voltage controlled oscillator maintaining frequency and phase lock between the two oscillators. This is a useful technique for achieving a frequency programmable crystal oscillator and output frequencies that are out of the range of a crystal manufactured to achieve certain performance specifications. However, short-term frequency stability is one or two orders of magnitude worse than for a true crystal oscillator.
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Cs
Atomic representation for cesium
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cycle slip
A discontinuity in the measured carrier beat phase resulting from a temporary loss of lock in the carrier tracking loop of a GPS receiver.
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daisy chain
A method of connecting devices on a bus. On a daisy chained bus, devices not requesting a signal respond to it by passing it on. The daisy chain scheme permits assignment of device priorities based on the electrical position of the device on the bus.
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data message
A message included in the GPS signal which reports the satellite’s location, clock corrections and health. Included is rough information on the other satellites in the constellation.
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dB
see decibel
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DCLS
Direct Current Level Shift, or digital IRIG.
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decibel (dB)
A measure of the ratio between two quantities, and can be used to express a wide variety of measurements in acoustics and electronics. The decibel is a “dimensionless unit” like percent. Widely used as a measure of the loudness of sound.
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DEC
Digital Equipment Corporation
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DEC Alpha
A 64-bit RISC microprocessor originally developed and fabricated by Digital Equipment Corp. (DEC), which used it in its own line of workstations and servers. Designed as a successor to the VAX line of computers, it supported the VMS operating system, as well as Digital UNIX. Later open source operating systems also ran on the Alpha, notably Linux and BSD UNIX flavours.
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Denial of Service
When a network is flooded with traffic, the systems cannot respond normally, so service is curtailed or denied. This is a favorite technique of network saboteurs.
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DGPS
Differential Global Positioning System
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DHCP
DHCP is used to assign dynamic IP addresses to devices on a network. With dynamic addressing, a device can have a different IP address every time it connects to the network. In some systems, the device’s IP address can even change while it is still connected. DHCP also supports a mix of static and dynamic IP addresses.
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Digital Equipment Corporation
a pioneering company in the American computer industry. They are generally referred to within the computing industry as DEC. They were later acquired by Compaq, which subsequently merged with Hewlett-Packard. As of 2004 their product lines were still produced under the HP name. Digital Equipment should not be confused with Digital Research; the two were unrelated, separate entities (despite some similarities between the operating systems they produced);
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differential positioning
Accurate measurement of the relative positions of two receivers tracking the same GPS signals.
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Digital Signature Standard (DSS)
A National Institute of Standards and Technology (NIST) standard for digital signatures, used to authenticate both a message and the signer. DSS has a security level comparable to RSA (Rivest-Shamir-Adleman) cryptography, having 1024-bit keys.
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digital time stamp
See Time Stamp.
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Dilution of Precision (DOP)
The multiplicative factor that modifies ranging error. It is caused solely by the geometry between the user and his set of satellites. Known as DOP or GDOP.
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DIP Switch
Dual Inline Package. Small on-off switches mounted in computer hardware for purposes of user configuration. When adding expansion cards to computers, DIP switches must often be set to make the card compatible with the computer’s configuration.
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directory
The directory is the storage area for network security information such as keys or server names.
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disciplined oscillator
An oscillator with a servo loop that has its phase and frequency locked to an external reference signal.
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disciplined oscillators
Locked outputs to the received time input signal. By disciplining the oscillator to the received time input signal, the oscillator is automatically calibrated, ensuring a precise frequency is available.
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dithering
The introduction of digital noise. This is the process the DoD uses to add inaccuracy to GPS signals to induce Selective Availability.
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DoD or DOD
Abbreviation for The United States Department of Defense, sometimes called the Defense Department. A civilian Cabinet organization of the United States government. The Department of Defense controls the U.S. military and is headquartered at The Pentagon. It is headed by the Secretary of Defense.
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DOP
GPS acronym for Dilution of Precision that describes the geometric quality of a GPS satellite configuration in the sky. The smaller the DOP number, the better the geometry. Factors that affect the DOP are, besides the satellite orbits, the presence of obstructions which make it impossible to use satellites in certain sectors of the local sky. Especially in urban measurements, this may be limiting. GDOP, HDOP, VDOP, PDOP and TDOP respectively, represent Geometrical, Horizontal, Vertical, Position (3-D) and Time Dilution of Precision. These quantities follow mathematically from the positions of the useable satellites on the local sky. GPS receivers allow the display of these positions (“skyplot”) as well as the DOP values.
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doppler-aiding
A signal processing strategy that uses a measured Doppler shift to help the receiver smoothly track the GPS signal. Allows more precise velocity and position measurement.
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doppler shift
The apparent change in the frequency of a signal caused by the relative motion of the transmitter and receiver.
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DOS
Disk Operating System See MS-DOS.
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dynamic host configuration protocol
See DHCP
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DPLL
Digital Phase-Locked Loop
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DPRAM (Dual Ported RAM)
A type of Random Access Memory that allows multiple reads or write to occur at the same time, or nearly the same time, unlike single-ported RAM which only allows one access at a time.
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DSS (Digital Signature Standard)
A National Institute of Standards and Technology (NIST) standard for digital signatures, used to authenticate both a message and the signer. DSS has a security level comparable to RSA (Rivest-Shamir-Adleman) cryptography, having 1024-bit keys. DSA is the Digital Signature Standard.
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E911 (Enhanced 911)
Specification that requires the ability to estabish the physical location of a mobile hand set when transmitting to/from a wireless base station. A North American telephone network (NANP) feature that automatically associates the physical address with the calling party’s telephone number. This is generally done by a form of reverse telephone directory that is supplied by the telephone company to provide emergency responders with the location of the emergency without the person calling for help having to provide it. Enhanced 911 has been deployed in most metropolitan areas in the United States and Canada.
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EEPROM
Electrically Erasable Programmable Read Only Memory is a type of ROM that can be erased electronically and reprogrammed in-circuit (or with a device programmer). . From the programmer’s perspective, EEPROM is very similar to flash memory. The biggest difference is that the bytes (words) of an EEPROM can be erased.
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EISA
(Extended ISA) A PC bus standard that extends the 16-bit ISA bus (AT bus) to 32 bits and provides bus mastering. ISA cards can plug into an EISA slot. EISA runs at the slow 8MHz speed of the ISA bus in order to accommodate any ISA cards that may be plugged into it. EISA has been superseded by PCI.
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Electromagnetic Compatibility (EMC)
Electrical sciences branch that studies the unintentional generation, propagation and reception of electromagnetical energy with reference to the unwanted effects that such an energy may induce. Due increased clock speeds used in modern digital equipment coupled with the lower signal voltages, EMC is more and more an issue. Many nations have issued directives to the manufacturers of these kind of equipment, which set out the essential requirements to be satisfied before such equipment may be sold. Organizations in each nation where set up to draw up and safeguard these directives. Among the more well known national organizations are: The FCC for the United states. CEN, CENELEC and ETSI for Europe and BSI for Britain.
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embedded system
A combination of computer hardware and software, and perhaps additional mechanical or other parts, designed to perform a dedicated function. In some cases, embedded systems are part of a larger system or product, as in the case of an antilock braking system in a car. Contrast with general-purpose computer.
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EMC (Electromagnetic Compatibility)
See Electromagnetic Compatibility
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EMI (Electromagnetic Interference)
Also known as Radio Frequency Interference (RFI), an electromagnetic radiation emitted by electrical circuits carrying rapidly changing signals, as a by-product of their normal operation. EMI causes unwanted signals (interference or noise) to be induced in other circuits. This interrupts, obstructs, or otherwise degrades or limits the effective performance of those other circuits. It can be induced intentionally, as in some forms of electronic warfare, or unintentionally, as a result of spurious emissions and responses, intermodulation products, and the like.
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ephemeris
The predictions of current satellite position that are transmitted to the user in the GPS data message.
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ethernet
A data transport standard for Local Area Networks (LANs), initially developed by Xerox, and later refined by Digital, Intel and Xerox (DIX). All hosts are connected to a coaxial cable where they contend for network access using a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) paradigm.
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eurocard
Eurocard is a European mechanical system standard format for PCB cards that are plugged together into a standardized subrack and does not define the specific connector to be used or the signals that are assigned to connector contacts.. The subrack consists of a series of slotted card guides on the top and bottom, into which the cards are slid so they stand on end, like books on a shelf. At the “back” of each card is one or more connectors, which plug into mating connectors on a backplane which closes the rear of the subrack. Eurocard subracks have standardized sizes based on the ‘U’, for unit. 1 U is 1.75 inches. A 3U subrack is 5.25 inches high and accepts a 3U Eurocard that is 100 mm high. Another popular size is the 6U high subrack at 10.5 inches high accepting 6U Eurocards which are 233.35mm high. Eurocards come in modular depths that start at 100 mm and then increase in 60 mm increments. The 160 mm depth is the most common today, however standard hardware is available to accommodate depths of 100 mm, 160 mm, 220 mm, 280 mm, 340 mm, and 400 mm. The Eurocard mechanical architecture was defined originally under IEC-60297-3. Today, the most widely recognized standards for this mechanical structure are IEEE 1101.1, IEEE 1101.10 (also known commonly as “dot ten”) and IEEE 1101.11. IEEE 1101.10 covers the additional mechanical and EMI features required for VITA 1.1-1997(R2002) which is the VME64 Extensions standard as well as PICMG 2.0 (R3.0) which is the CompactPCI specification.
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fast switching channel
A single channel that rapidly samples a number of satellite ranges. “Fast” means that the switching time is sufficiently fast (2 to 5 milliseconds) to recover the data message.
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FCC
(Federal Communications Commission) An independent United States government agency, created, directed, and empowered by Congressional statute, established by the Communications Act of 1934 as the successor to the Federal Radio Commission. The FCC is charged with regulating all non-Federal Government use of the radio spectrum (including radio and television broadcasting), and all interstate telecommunications (wire, satellite and cable) as well as all international communications that originate or terminate in the United States.
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FDM
Frequency-division multiplexing (FDM) is a scheme in which numerous signals are combined for transmission on a single communications line or channel. Each signal is assigned a different frequency (subchannel) within the main channel.
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fiber optic distribution
A technology that uses glass (or plastic) threads (fibers) to transmit data. A fiber optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages modulated onto light waves.
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FIFO
First In First Out This expression describes the principle of a queue or first-come, first-served behavior: what comes in first is handled first, what comes in next waits until the first is finished, etc.
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firmware
In computing, firmware is executable software that is embedded in a hardware device. The programs may written permanently into ROM chips or provided in flash ROMs that may be updated by an end user. Unauthorized modified firmware is sometimes used to provide functionality not intended by the manufacturer, for example to defeat the region encoding of DVD players.
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firewall
Software and hardware systems that protect an internal network from outside data that could be harmful to the network, such as a virus sent via the Internet.
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flash memory
A RAM-ROM hybrid that can be erased and rewritten under software control. Abbreviated as flash. Flash is an in-circuit programmable nonvolatile memory segmented into blocks called sectors. Each sector can be individually erased, then the data within it rewritten.
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FM
Fault Management.
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FN
Fiber Node.
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frequency
Frequency is the measurement of the number of times that a repeated event occurs per unit time. To calculate the frequency, one fixes a time interval, counts the number of occurrences of the event within that interval, and then divides this count by the length of the time interval. In SI units, the result is measured in hertz (Hz) after the German physicist, Heinrich Rudolf Hertz. 1 Hz means that an event repeats once per second. Other units that have been used to measure frequency include: cycles per second, revolutions per minute (rpm). Heart rate is measured in beats per minute.
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frequency and time standards
A radio communication service for scientific, technical and other purposes, providing the transmission of specified frequencies, time signals, or both, of stated high precision, intended for general reception.
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frequency band
A particular range of frequencies. frequency control Circuit that maintains the frequency of an oscillator within the specified limits with respect to a reference frequency.
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frequency spectrum
The distribution of signal amplitudes as a function of frequency.
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frequency standard
A stable oscillator used for frequency calibration or reference. (188) Note 1: A frequency standard generates a fundamental frequency with a high degree of accuracy and precision. Harmonics of this fundamental frequency are used to provide reference points. Note 2: Frequency standards in a network or facility are sometimes administratively designated as “primary” or “secondary.” The terms “primary” and “secondary,” as used in this context, should not be confused with the respective technical meanings of these words in the discipline of precise time and frequency
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FTP (File Transfer Protocol)
Allows users to copy files between the local system and any system reachable over the network. Every UNIX system has ftp and there are version for the Macintosh and Windows. A common way to make software (text files, sounds, pictures, utilities, etc.) available is to put it on an anonymous ftp server. Anonymous ftp servers allow users to login without a password, specifying “guest” as the user, and then copy whatever has been made available. Also, FTP is usually the name of the program the user invokes to execute the protocol.
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Galileo
A satellite-based radio navigation system run by the European Space Administration (ESA), the European Commission and the European Organization for the Safety of Air Navigation. Using 27 satellites and three spares Galileo will be an alternative to the US military-controlled Global Positioning System and the Russian GLONASS. The system should be operational by 2008. This system is intended to provide: Greater precision to all users than is currently available. Improved coverage of satellite signals at higher latitudes, which northern regions such as Scandinavia will benefit from. A global positioning system that can be relied on, even in times of war.
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gateway
Transfers information between physically separate networks that are based on differing protocols (for example, between Internet and AppleTalk networks). It performs high-level information translation, while routers provide low-level.
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Geometric Dilution of Precision (GDOP))
See Dilution of Precision.
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GHz
A GigaHertz is one billion (109) cycles per second.
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gigabyte
1,024 Megabytes. Abbreviated GB.
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Global Positioning System
See GPS (the US military refers to it as NAVSTAR GPS - Navigation Signal Timing and Ranging Global Positioning System.
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GLONASS
The Soviet space-based navigation system comparable to the American GPS system. The operational system contains 21 satellites in 3 orbital planes, with 3 on-orbit spares. GLONASS provides 100 meters accuracy with its C/A (deliberately degraded) signals and 10-20 meter accuracy with its P (military) signals. It is operated for the Russian government by the Russian Space Forces.
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GMT
Greenwich Mean Time, the mean solar time of the meridian of Greenwich, England. This was the primary basis for calculating standard time throughout the world until 1972 when it was superseded by UTC.
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GOS
Grade Of Service.
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GPIB
General Purpose Interface Bus, IEEE 488.
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GPS
The Global Positioning System, usually called GPS (the US military refers to it as NAVSTAR GPS), is a satellite navigation system used for determining one’s precise location and providing a highly accurate time reference almost anywhere on Earth or in Earth orbit. It uses an intermediate circular orbit (ICO) satellite constellation of at least 24 satellites. The GPS system was designed by and is controlled by the United States Department of Defense and can be used by anyone, free of charge. The satellites orbit the earth at approximately 12,000 miles above the surface and make two complete orbits every 24 hours. The GPS satellites continuously transmit digital radio signals that contain data on the satellites location and the exact time to the earth-bound receivers. The satellites are equipped with atomic clocks that are precise to within a billionth of a second. By knowing how far away a satellite is, the receiver knows that it is located somewhere on the surface of an imaginary sphere centered at the satellite. By using three satellites, GPS can calculate the longitude and latitude of the receiver based on where the three spheres intersect. By using four satellites, GPS can also determine altitude.
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Grandmaster Clock
Within an IEEE 1588 sub domain, a Grandmaster clock is the ultimate source of time for clock synchronization using the IEEE 1588 protocol.
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GUI
Graphical User Interface. A way of interacting with a computer, based on graphics instead of text. GUIs use icons, pictures, and menus to represents commands, files, and windows, and use a mouse as well as a keyboard to accept input. MacOS, MS Windows, and the X windowing system all use GUIs. Contrast with Command Line Interfaces (CLI).
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hack/crack
“Hackers” are unauthorized programmers who write code that enables them to break into a computer network or program. “Crackers” are unauthorized programmers whose goal it is to break into computer networks or programs protected by security software or hardware.
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HAE
Host Application Equipment.
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hash
Also called “hash function” or hashing, used extensively in many encryption algorithms. Hashing transforms a string of characters usually into a shorter, fixed-length value or key.
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hardover word
The word in the GPS message that contains synchronization information for the transfer of tracking from the C/A to P code.
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hardware
Computer machinery, such as the CPU, disk drives, monitor, and printer. Contrast with software.
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HDSL
High-data-rate Digital Subscriber Line. A DSL technology that delivers 1.544 Mbps each way (symmetrically) over two copper twisted pairs. Compare against SHDSL, SDSL, ADSL and VDSL.
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holdover
When a clock that is locked to a PRS-traceable input loses that input, it enters a holdover mode where the frequency drift is related to the quality, or stratum level, of its oscillator.
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host
A general-purpose computer that communicates with the target via a serial port or network connection.
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HTML
HyperText Markup Language, the computer language used to create pages for the World Wide Web.
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HTTP
HyperText Transfer (or Transport) Protocol, the protocol most often used to transfer information from World Wide Web servers to users of the Web.
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hydrogen maser
A device that produces a radio signal directly from the transition between the upper and lower hyperfine levels in the ground state of atomic hydrogen. The most stable type of frequency standard or clock (for time scales of a few seconds to few hours) that is used in VLBI observations. http://tycho.usno.navy.mil/maser.html.
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Hz
Hertz. A measure of frequency relating to cycles per second.
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IBM
International Business Machines. Very large company that makes a variety of computers. IBM set the standard for DOS machines. Currently, many companies makes clones of IBM’s personal computers which are often called compatibles.
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IEC
Inter-exchange Carrier
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IEEE
Institute of Electrical and Electronic Engineers, an international organization that sets standards for electrical and computer engineering. It has the most members of any technical professional organization in the world, with more than 360,000 members in around 175 countries.
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IEEE 1344
Standard published by the IEEE Power Engineering Society, Power System Relaying Committee (IEEE Std 1344-1995) that discusses unique synchronizing issues that are encountered in the Power Utilities. As a part of that document (Annex F), a specific utilization of the ‘Control Bit’ segment of IRIG B time code is defined, using the 27 control bits in a standard IRIG B serial time code format to provide additional information as set forth by the IEEE standard.
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IEEE 1588
IEEE 1588 is a standard published by the IEEE Instrumentation and Measurement Society (Std IEEE 1588-2002). Per the standard it is a protocol to synchronize independent clocks running on separate nodes of a distributed measurement and control system where a high degree of accuracy and precision is specified. The protocol is independent of the networking technology, and the system topology is self-configuring.
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IEEE 1588 Ordinary Clock
A IEEE 1588 clock with a single port.
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IETF
Internet Engineering Task Force, an international organization which sets standards for Internet protocols in their Request for Comment (RFC) papers.
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integrity
Data that has retained its integrity has not been modified.
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Intel
Maker of the 80x86 and Pentium lines of processors which are used in IBM PC and compatible computers.
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interface
Part of a computer, program, or peripheral that communicates with other components. Interface also refers to the user interface.
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interrupt
This is a signal on which the processor stops execution of the current command sequence and transfers control to the program-handler of the interrupt. The program-handler address is calculated by the interrupt vector table. An interrupt may be initialized either by user programs, while working with disks, screen, printer etc (program interrupts), or by hardware: keyboard, timer (hardware interrupts).
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interrupt latency
The amount of time between the assertion of an interrupt signal and the start of the associated interrupt service routine. Factors that affect interrupt latency include the length of time that interrupts are disabled during normal program execution, processor speed, and preemption of the processor by higher priority interrupts.
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interrupt request
On IBM PC and compatible computers, hardware lines for devices to send interrupts. Typically each device connected to the computer uses a separate IRQ. It is a good idea to know which IRQ each device on your computer is using (the IRQ for the mouse and keyboard should be found in your manuals). Expansion cards such as Ethernet cards or token ring adaptors also use IRQs. Setting the IRQ so that it does not conflict with (use the same IRQ as) other devices is an essential part of installing expansion cards and new devices.
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interrupt service routine (ISR)
A software routine that is executed in response to an interrupt
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interrupt vector
This is an element of interrupt vector table that contains address of an interrupt’s handler.
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interrupt vector table, interrupt table
This is a table containing values of addresses of programs that handle interrupts. Located at lowest addresses (0000:0000 - 0000:03FF) and has 256 4-byte addresses (interrupt vectors).
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I/O (input/output)
The interface between a processor and the world around it. The simplest examples are switches (inputs) and LEDs (outputs).
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I/O address
Locations within the input/output address space of an IBM PC or compatible computer used by a device, such as a printer or modem. The address is used for communication between software and the device.
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ionosphere
The band of charged particles 80 to 120 miles above the Earth’s surface.
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ionospheric refraction
The change in the propagation speed of a signal as it passes through the ionosphere.
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IP
Internet Protocol
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IRIG
The InterRange Instrumentation Group is the IRIG standards steering committee. IRIG is a serial time format standard.
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IRIG B
Sends day of year, hour, minute, and second data on a 1 kHz carrier frequency, with an update rate of once per second.
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IRQ
Interrupt ReQuest, the signal that a peripheral or program uses to interrupt the CPU. See Interrupt Request
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IRIX
A version of Unix for Silicon Graphics machines. The current major version of IRIX is IRIX 6.5.
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ISA
(Industry Standard Architecture) An expansion bus commonly used in earlier PCs that accepted plug-in boards for sound, video display and other peripheral connectivity. Originally called the “AT bus,” introduced with the IBM PC AT in 1984, the AT/ISA bus extended the PC bus from 8 to 16 bits.
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ISA bus timing
ISA (AT)bus: The ISA bus operates at 8MHz with an 8 and 16 bits data bus, a 24 bit address bus, using +/- 12 volts, +/- 5 volts, and 15 Interrupt lines. The standard drive level is 24mA for all non Open-Collector signals on the bus.
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ISDN
Integrated Services Digital Networks. An international communications standard for sending voice, video, and data over digital telephone lines or normal telephone wires. ISDN supports data transfer rates of 64 Kbps (64,000 bits per second).There are two basic types of ISDN service: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI consists of two 64 kb/s B channels and one 16 kb/s D channel for a total of 144 kb/s. PRI is intended for users with greater capacity requirements. Typically the channel structure is 23 B channels plus one 64 kb/s D channel for a total of 1536 kb/s. In Europe, PRI consists of 30 B channels plus one 64 kb/s D channel for a total of 1984 kb/s.
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ISP
Internet Service Provider. Allows your computer access to their computers and Internet access for a fee. Most ISPs provide access to the Internet and the World Wide Web with one or more email addresses.
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ISR
Interrupt Service Routine
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jitter
Short-term variations of a timing signal from its ideal positions in time. Generally, any distortion of a signal or image due to poor synchronization. The term is used in several ways, but it always refers to some offset of time and space from the norm. For example, in a network transmission, jitter would be a bit arriving either ahead or behind a standard clock cycle or, more generally, the variable arrival of packets.
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JTF
The Jitter Transfer Function (JTF) of a network element indicates the degree to which an output is affected by input jitter (i.e. amplified or attenuated).
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kbit
A kilobit is 1024 or 210 bits.
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kernel
The main part or core of an operating system; it provides fundamental operating system services, such as handling interrupts and the I/O system, managing threads and processes, and allocating and scheduling processor resources.
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kilobyte
1024 bytes (about 1000 bytes hence kilobyte). Usually abbreviated K, as in 640K. Used in discussing memory and hard drive storage. 1024 Kilobytes is equal to 1Megabyte.
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kHz
A kilohertz is 1000 cycles per second.
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L1 Band, L2 Band
Each Navstar GPS satellite currently transmits in two dedicated frequency bands: L1 and L2, which are centered on 1575.42 and 1227.6 MHz respectively. L1 carries one encrypted signal, as does L2, both being reserved for the military. L1 also carries one unencrypted signal, for civilian use.
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LAN (Local Area Network)
A group of personal computers linked together in order to share programs, data, and peripherals.
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L-band
The group of radio frequencies extending from 390 MHz to 1550 MHz. The GPS carrier frequencies (1227.6 MHz and 1575.42 MHz) are in the L band.
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LCD (Liquid Crystal Display)
Technology used for portable computer displays, as well as many other electonics display applications, such as watches, etc. LCD displays for portables are usually active matrix, passive matrix, and dual scan displays.
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Leap Second
The time error that accumulates between the earth’s rotation and actual measured time is added (or deleted) as a leap second from UTC time on the last day of June or December in any year in which a leap second is needed to keep in step with the rotation of the earth.
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LED (Light Emitting Diode)
Used for display in some electronic devices.
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LED alphanumeric display
A display of light emitting diodes consisting of both letters and numbers.
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local bus
A bus or electronic pathway that allows access to the CPU at a speed synchronized with the CPU.
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LORAN-C
LOng RAnge Navigation, version C. A long-range radio navigation position-fixing system consisting of an array of fixed stations that transmit precisely synchronized signals to mobile receivers.
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low phase noise
The ratio of the power density of one phase modulation sideband to the total signal. It is usually specified as the single side band (SSB) power density in a 1Hz bandwidth at a specified offset frequency from the carrier. It is measured in dBc/Hz. The best short- and long-term stability can be obtained only if the frequency synthesis does not introduce noise that masks the atomic noise limit. In the best standards, the signal used for atomic interrogation must have exceptionally clean spectral purity.
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Macintosh
Computer developed by Apple Computers, first released in 1984. The Macintosh was one of the first computers to use a graphical user interface. Today, Apple continues to produce many different models of Macintosh.
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MacOS
The operating system produced by Apple for their Macintosh computers.
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