P

P Code

The Precise or Protected code. A very long sequence of pseudo-random binary biphase modulations on the GPS carrier at a chip rate of 10.23 MHz, which repeats about every 267 days. Each 1-week segment of this code is unique to one GPS satellite and is reset each week

Packet

A single group of digital signals sent from an originator to an addressee

Packet Messaging

Radio transmitted message system

Packet Radio

The radio transmission of data in packets between stations

Parity

An extra bit at the end of a string of bits. Used in error detection, this reveals whether the number of 1s is odd or even

Pass-to-Pass Accuracy

Also called repeatability. In the context of parallel tracking, this is how far off you are from one path to the next, regardless on which pass you are on. Mostly the pass-to-pass accuracy is aproximately 2 to 3 times more precise then the statistic accuracy

PDOP

Position Dilution of Precision. A unitless figure of merit expressing the relationship between the error in user position and the error in satellite position, which is a function of the configuration of satellites from which signals are derived in positioning (see DOP). Geometrically, PDOP is proportional to 1 divided by the volume of the pyramid formed by lines running from the receiver to four observed satellites. Small values, such as "3", are good for positioning while higher values produce less accurate position solutions. Small PDOP is associated with widely separated satellites

Phase Angle

The time difference between the same point on two different waves, usually measured in fractions of a cycle (radians or degrees)

Phase Center

The apparent center of signal reception at an antenna. The phase center of an antenna is not constant but is dependent upon the observation angle and the signal frequency

Phase Differencing

The technique of using different GPS receivers at different locations to measure the phase angles of the carrier signal from the same satellite. These angles are compared by a communications link between the two locations if real-time operations are required

Phase Lock Loop

Another term for carrier-tracking loop

Phase Modulation

Encoding information on a carrier signal by changing the phase so that some segments of the carrier are out of phase while others are in phase. With GPS, only two phase angles are used, 0 and 180, representing the two values, I or O

Pixel

Picture element. Definable locations on a display that are used to form images on the screen

Position

The latitude, longitude, and altitude of a point.

Positional Dilution of Precision (PDOP)

A unitless figure of merit expressing the relationship between the error in user position and the error in satellite position, which is a function of the configuration of satellites from which signals are derived in positioning (see DOP). Geometrically, PDOP is proportional to 1 divided by the volume of the pyramid formed by lines running from the receiver to four observed satellites. Small values, such as "3", are good for positioning while higher values produce less accurate position solutions. Small PDOP is associated with widely separated satellites

Post processed differential GPS

Process of differentially correcting GPS data after it has been collected by differential correction software

Precise Positioning Service (PPS)

The full-accuracy, single-receiver GPS positioning service provided to the United States and its allied military organizations and other selected agencies. It includes access to the unencrypted P-code and the removal of SA effects

Precision

The degree of repeatability that repeated measurements of the same quantity display, and is therefore a means of describing the quality of the data with respect to random errors. Precision is traditionally measured using the standard deviation

Precision Code (P-code)

A very long sequence of pseudo-random binary biphase modulations on the GPS carrier at a chip rate of 10.23 MHz, which repeats about every 267 days. Each 1-week segment of this code is unique to one GPS satellite and is reset each week

Prime Meridian

The zero longitude location from where east and west is measured. It passes through Greenwich, England

PRN

Pseudo random noise, a sequence of digital 1's and 0's which appears to be randomly distributed like noise, but can be exactly reproduced. Each NAVSTAR satellite has its own unique C/A and P pseudo noise codes and are often referred to by their PRN number

Pseudo random code

A signal with random noise-like properties. It is a very complicated but repeating pattern of 1's and O's

Pseudolite

A ground-based differential GPS, installed on the ground in the local area. It is used to supply "illumination" and correction signals for DGPS systems

Pseudo-random noise (PRN)

A sequence of digital 1's and 0's that appear to be randomly distributed like noise but that can be reproduced exactly. Their most important property is a low autocorrelation value for all delays or lags except when they coincide exactly. Each GPS satellite has unique C/A and P pseudorandom-noise codes

Pseudorange

A distance measurement based on the correlation of a satellite transmitted code and the local receiver's reference code, that has not been corrected for errors in synchronization between the transmitter's clock and the receiver's clock

Q

Quadrifilar Helix

A type of circularly polarized antenna used with some GPS receivers. The antenna consists of two orthogonal, fractional-turn bifilar wire or metal strip helices fed in phase quadrature