A
accuracy. How close a fix comes to the actual position.
acquisition. The ability to find and lock on to satellite signals for ranging.
additional secondary factor (ASF). Error propagation time caused by radiowaves slowing down as they pass over different terrain. This is in addition to slowing down as radiowaves pass over sea water.
algorithm. A special method used to solve a certain type of mathematical problem.
almanac. A set of parameters included in the GPS satellite navigation message that a receiver uses to predict the approximate location of a satellite. The almanac contains information about all of the satellites in the constellation.
ambiguity. The initial bias in a carrier-phase observation of an arbitrary number of cycles. The initial phase measurement made when a GPS receiver first locks onto a GPS signal is ambiguous by an integer number of cycles because the receiver has no way of knowing the exact number of carrier wave cycles between the satellite and the receiver. This ambiguity, which remains constant as long as the receiver remains locked on the signal, is established when the carrier-phase data are processed.
amplitude. Height of a radiowave as measured from an imaginary center line to the wave peak.
amplitude modulation (AM). A method of encoding a message on the carrier signal by altering the height of the signal while keeping its frequency constant.
antispoofing (AS). The process of encrypting the P-code by modulo-2 addition of the P-code and a secret encryption W-code. The resulting code is called the Y-code. AS prevents an encryption-keyed GPS receiver from being "spoofed" by a bogus, enemy-generated GPS P-code signal.
anywhere fix. The ability of a receiver to start position calculations without being given an approximate location and time.
automatic vehicle location (AVL). The technique of using a navigation system, such as GPS, to determine a vehicle's position.
automatic vehicle monitoring (AVM). The technique of using a navigation system to determine a vehicle's position, which is then transmitted to a central unit that monitors and tracks the vehicle's position and movement.
B
bandwidth. That range of frequencies that compose a signal.
bearing. The angle between two different points on the earth's surface with respect to true north.
binary biphase modulation. The phase modulation technique used to transmit a GPS signal. The phase of a carrier generated by a satellite is shifted by 180 degrees when there is a code or message binary signal level transition, either from 0 to 1 (normal to mirror image) or from 1 to 0 (mirror image to normal).
bit. A unit of information in an electronic system expressed as a choice between two possible values, for example, 0 or 1.
C
carrier. A radio wave that conveys or carries some kind of modulation.
carrier-aided tracking. A technique to improve accuracy by using the GPS carrier signal to get a more exact lock on the pseudorandom code.
carrier phase. The accumulated phase of either the L1 or L2 carrier of a GPS signal, measured by a GPS receiver since locking onto the signal. Also called integrated Doppler.
carrier-to-noise power density (C/N0). The ratio of the power level of a signal carrier to the noise power in a 1-Hz bandwidth. This is a key parameter in the analysis of GPS receiver performance. Nominal GPS receiver C/N0 values often are in the 40 to 50 dB-Hz range.
carrier-tracking loop. A module in a GPS receiver that demodulates, or extracts, the satellite message by aligning the phase of the receiver's local oscillator signal with the phase of the frequency-shifted, received carrier. Once the local oscillator signal is locked to the carrier, its phase can be measured to provide the carrier-phase observation.
channel. In a receiver, a path for an electronic signal that is reserved for a specific satellite and used for various functions.
chip. Binary elements or digits that, unlike bits, convey no information. A PRN code consists of a sequence of chips.
circular error probable (CEP). A measure of accuracy in navigation. CEP is the radius of the circle, inside of which the true horizontal coordinates of a position have a 50-percent probability of being located.
clock aiding. An accuracy enhancement technique in which an additional atomic clock (rubidium) provides accurate time to the receiver for calculating satellite clock frequency, phase bias, and clock drift.
clock bias. The difference between a clock's indicated time and true universal time.
coarse acquisition (C/A)-code. A family of PRN codes transmitted by GPS satellites. Each satellite is assigned one of 32 unique codes in the family. Each code consists of 1,023 chips and is sent at a rate of 1.023 megabits per second. The code sequence repeats every millisecond. The C/A-codes are Gold codes -- PRN codes that are distinguished by a very low cross correlation between any two codes (that is, they are nearly orthogonal). C/A-codes currently are transmitted only on the L1 frequency.
coastal confluence zone. That area extending from a harbor's entrance 50 nautical miles seaward or to the edge of the continental shelf (100 fathom curve) whichever is greater.
code-tracking loop. A module in a GPS receiver used to align a PRN code sequence present in a signal coming from a satellite with an identical PRN code sequence generated within the receiver. Alignment is achieved by appropriately shifting the receiver-generated code chips in time so that a particular chip in the sequence is generated at the same instant its twin arrives.
cold start. The ability of a GPS receiver to start providing position updates without the assistance of any almanac information stored in its memory.
control segment. That portion of the Global Positioning System that consists of a master control station, monitoring stations, and ground antennas, which monitor and update satellite signals and upload correction data to the satellites.
Coordinated Universal Time (UTC). See UTC.
course over the ground (COG). A vessel's track over the bottom as measured in degrees.
cross-track error. The difference between a vessel's actual position and its desired position on a given heading. This is usually measured as a range error in nautical miles but may also be expressed graphically using symbols.
cycle slip. A discontinuity in GPS carrier-phase observations, usually of an integer number of cycles, caused by temporary signal loss. If a GPS receiver loses a signal temporarily, due to obstructions for example, when the signal is reacquired there may be a jump in the integer part of the carrier-phase measurement due to the receiver incorrectly predicting the elapsed number of cycles between signal loss and reacquisition.
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