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P code The protected or precise code transmitted on both L1 and L2 by GPS satellites. This code is now encrypted and in not available to any civilian user. The P code is a very long and repeats itself every 38 weeks. Each satellite uses a 1 week segment of this code that is unique to each GPS satellite and is reset each week.
Parallel Channel Receiver GPS receiver that simultaneously tracks multiple satellites to provide the fastest, most reliable and accurate navigational data, under the most adverse conditions.
Parity Error A digital message is composed of 1's and 0's. Parity can be defined as the sum of these bits within a word unit (8 bits) and the "parity bit" of a word is set to be even or odd accordingly. When data has been transmitted to another computer, the receiving computer recalculates the sum then compares its result with the transmitted parity bit. A parity error exists when there is a discrepancy in this comparison. It shows that the data transmission has errors.
PDOP See Position Dilution Of Precision
Perigee That point in a geocentric orbit when the geometric distance is a minimum. The closest approach of a body.
Phase lock The technique whereby the phase of an oscillator signal is made to follow exactly the phase of a reference signal. The receiver first compares the phases of the two signals, then uses the resulting phase difference signal to adjust the reference oscillator frequency. This eliminates phase difference when the two signals are next compared.
Phase observable See reconstructed carrier phase
Photogrammetric Mapping Literally this is the measuring of light. Practically speaking, the user takes a light based measurement (LiDAR scan, photograph, etc.) and produce a scaled map or drawing of the area.
Pixel A picture element; the "building blocks" of a liquid crystal display (LCD). The greater the number of vertical and horizontal pixels, the better the screen resolution and detail.
Plotter Display Provides an overhead "bird's eye" view of your current position relative to the waypoints and event marker/icons you have saved. A dotted line marks the shortest route to the chosen waypoint, and a recorded plot trail displays the path you have taken so far.
Point Cloud The graphical representation of the data produced by a laser scan. Point clouds can be used to produce animations, 2D maps or 3D models. They can be exported into AutoCAD, Microstation, and various graphics applications.
Point positioning A geographic position produced from one receiver located at a single point. At best, the position accuracy obtained from a receiver using the C/A code is 15 to 25 metres, depending on the geometry of the satellites. However the US DoD degrade the accuracy through Selective Availability so that the positions are at best +/-100 metres in horizontal, +/-180 metres in vertical.

A position calculated as a mean of point positions during an observation session is often called single point position calculation and will have a higher accuracy than an instantaneous point position.
Polar Motion Motion of the instantaneous axis of the rotation of the Earth with respect to the solid body of the Earth. It is an irregular, but more or less circular motion with an amplitude of about 15 metres and a main period of about 430 days (called Chandler Wobble).
Pos/Nav Positioning and Navigation
Position The latitude, longitude, and altitude of a point. An estimate of error is often associated with a position.
Position Display One of the primary navigational data screens that emphasize the present position latitude/longitude coordinates, as well as other helpful navigational information.
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. Geometrically, PDOP is proportional to 1 divided by the volume of the pyramid formed by lines running from receiver to 4 satellites observed. Small values of PDOP (< 3) are considered to be considered "good" for positioning. Large values (> 7) are considered poor. Thus, small PDOP is associated with widely separated satellites. PDOP is related to Horizontal and vertical DOP by:

PDOP2 = HDOP2 + VDOP2.

See also Dilution Of Precision
Post-processed differential GPS In post-processed differential GPS the base and roving recievers have no active data link between them. Instead, each records the satellite observations that will allow differential correction at a later time. Differential correction software is used to combine and process the data collected from these receivers.
Precise Positioning Service (PPS) The highest level of military dynamic positioning accuracy that will be provided by GPS, based on the dual frequency P Code and having high anti jam and antispoof qualities. Only authorised military users have access to the P code.
Predictable The accuracy of a radionavigation system's position solution with respect to the charted solution. Both the position solution and the chart must be based upon the same geodetic datum.
Present Position Current location on the face of the Earth, in terms of the specific latitude/longitude coordinates, displayed in degrees/minutes/and thousandths of a minute.
Prime Vertical The vertical circle perpendicular to the celestial meridian.
PRN See pseudorandom noise
Programmable Windows The ability to customize existing split panel window groupings with specific combinations of navigational data.
Proportional error One means of expressing positional accuracy, expressed as the position error divided by the distance to the origin of the coordinate system used, stated in parts per million (ppm).
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 Pseudolite (shortened form of pseudo-satellite) A ground-based differential GPS transmitters that simulates the signal of a GPS satellite and can be used for ranging. The data portion of the signal may also contain differential corrections that can be used by receivers to correct for GPS errors.

To reduce interference with GPS satellite signals the pseudolite signal is gated on only 90.91 microseconds or one -eleventh of a one-msec, 1023 chip code. In this way the pseudolite signal only interferes 10% of the time. The pulse position is changed from millisecond to millisecond. The Maximun desired pseudolite range is typically 50km. This requires only 30 milliwatts of power.
PseudoRandom 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.

There are 32 GPS satellite PRN's defined.

There are 51 pseudolite, 1023-bit Gold codes defined.
Pseudorange The approximate distance, measured by a receiver, between a satellite and a receiver antenna. Pseudorange is obtained by multiplying the wavelength of the GPS signal by the time it took the signal to travel between the satellite and the receiver. Major sources of error include the clock offset and the atmosphere which delays the signals, an error in time measurement causes an error in the distance measurement.
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