A name given to the P code when it
has been encrypted. Encryption of the P code became

permanent in March 1994.

YUMA Almanac

à
see Almanac

Z-Count

The GPS satellite clock time at the
leading edge of the next data subframe of the transmitted

GPS message (usually expressed as an
integer number of 6 seconds).

Zero Baseline

A Zero Baseline test can be used to
study the precision of receiver measurements (and hence its

correct operation), as well as the
data processing software. The experimental setup, as the

name implies, involves connecting two
GPS receivers to the same antenna. When two receivers

share the same antenna, biases such
as those which are satellite (clock and ephemeris) and

atmospheric path (troposphere and
ionosphere) dependent, as well as errors such as multipath

CANCEL during data processing. The
quality of the resulting “zero baseline” is therefore a

function of random observation error
(or noise), and the propagation of any receiver biases that

do not cancel in double-differencing.

A voltage-controlled oscillator (VCO)
is an electronic circuit that generates an oscillating signal

at a frequency proportional to an
externally applied voltage. These types of circuits are useful

for tracking and matching signal
frequencies as they shift due to thermal variations, power

supply fluctuations, and other
sources of frequency phase-shifts. VCO’s are found particularly

often in phase-locked loops (PLLs)
used for clock generation and synchronization. PLLs combine

the variable frequency
characteristics of the VCO with a phase detector circuit in order to track

a signal as it changes frequency.

VDOP (Vertical dilution of
precision)

à
see DOP and PDOP.

Velocity Made Good (VMG)

The rate at which you are approaching
a destination, based upon your current speed and

course.

Vernal Equinox

The intersection of the celestial
equator with the ecliptic, with the positive sense being from the

Earth to the sun, as the sun crosses
the equator from south to north.

Vertical

The line perpendicular to the geoid
at any point. The direction of the force of gravity at that

point.

VSWR (Voltage Standing Wave
Ratio)

In a transmission line, the ratio of
maximum to minimum voltage in a standing wave pattern:

The VSWR is a measure of impedance
mismatch between the transmission line and its load. The

higher the VSWR, the greater the
mismatch. The minimum VSWR, i.e., that which corresponds

to a perfect impedance match, is
unity.

WAAS

à
see Wide Area Augmentation Service

Warm Start

Start mode of a GPS receiver when
current position, clock offset and approximate GPS time are

known. Almanac data is retained, but
the ephemeris data is cleared. In Warm Start Scenario,

the receiver knows – due to a backup
battery or by other techniques – his last position,

approximate time and almanac. Thanks
to this, it can quickly acquire satellites and get a

position fix faster than in cold
start mode.

Waypoint

A permanently stored and named
position in the GPS receiver’s memory.

WDGPS

Wide area Differential GPS – A
real-time DGPS system that is made available over a large area -

possibly global.

Week Number Rollover (WNRO)

The Global Positioning System (GPS)
has experienced the first rollover of its internal clock,

termed the End of Week (EOW)
Rollover, August 21, 1999. The EOW rollover exists because the

large increment for counting GPS
system time is one week, and weeks are accumulated in a 10-

bit register (decimal values: from
“0000” to “1023”). GPS time started Jan. 6, 1980 with week

“0000″ and continues until 23:59:47
Universal Time Coordinated (UTC), Aug. 21 with week

“1023″ . After the rollover the GPS
clock will reset itself to week “0000″ This was the first

EOW rollover since the GPS
constellation was established.

WGS-72

World Geodetic System (1972); the
mathematical reference ellipsoid previously used by GPS,

having a semi-major axis of 6378.135
km and a flattening of 1/298.26.

WGS-84 (World Geodetic System
1984)

The mathematical ellipsoid used by
GPS since January, 1987.

Wide Area Augmentation System (WAAS)

WAAS is a US Federal Aviation
Authority (FAA) funded system of equipment and software that

augments GPS accuracy, availability
and integrity. The WAAS provides a satellite signal for

WAAS users to support enroute and
precision approach aircraft navigation. Similar systems are

under development in Europe (where it
is known as EGNOS — European Geostationary

Navigation Overlay System) and Japan
(where it is known as “MT-SAT Based Satellite

Augmentation System” (MSAS).)

Wireless Modem

A wireless modem is a device that
accepts data (typically through RS-232, RS-422, or RS-485

interfaces) and transmits it without
wires to another device that also has a serial port. Data is

sent from one end to the other as if
there were a cable.

WNRO

à
see Week Number Rollover

World geodetic system

A consistent set of parameters
describing the size and shape of the Earth, the positions of a

network of points with respect to the
center of mass of the Earth, transformations from major

geodetic datum’s, and the potential
of the Earth (usually in terms of harmonic coefficients).

The UART produces an electrical
signal and timing for transmission of data over a

communications path, and circuitry
for detection and capture of such data transmitted from

another UART.

UDRE (User Differential Range
Error)

A measure of error in range
measurement to each satellite as seen by the receiver.

UI

à
see User Interface

Universal Time (UT)

Local solar mean time at Greenwich
meridian. Some commonly used versions of universal time

are:

UT0: Universal time as deduced
directly from observations. The fixed numerical relationship

between universal and sidereal time
is 3 minutes 56.555 seconds.

UT1: UT0 corrected for polar motion.

UT2: UT1 corrected for seasonal
variations in the earth’s rotation rate.

UTC: Universal time coordinated;
uniform atomic time system kept very close to UT2 by offsets.

Maintained by the U.S. Naval
Observatory.

GPS time (à
see GPS Time) is relatable
directly to UTC:

Universal Time Coordinated (UTC
and UTC (USNO))

A universal time standard,
referencing the time at Greenwich, England. Also referred to as GMT

or Zulu time. An international,
highly accurate and stable uniform atomic time system kept very

close, by offsets, to the universal
time corrected for seasonal variations in the earth’s rotation

rate. Maintained by the U.S. Naval
Observatory (USNO). GPS time (à
see GPS Time) is directly

relatable to UTC

Universal Transverse Mercator
(UTM)

A worldwide coordinate projection
system utilizing north and east distance measurements from

reference point(s). UTM is the
primary coordinate system used on United States Geological

Survey topographic maps. The UTM Grid
consists of 60 north-south zones, each 6 degrees wide

in longitude.

Update Rate

The GPS receiver specification which
indicates the solution rate provided by the receiver when

operating normally. It is typically
once per second.

URA

à
see user range accuracy

User Interface (UI)

The hardware and operating software
by which a receiver operator executes procedures on

equipment (such as a GPS receiver)
and the means by which the equipment conveys information

to the person using it: the controls
and displays.

User Range Accuracy (URA)

The contribution to the
range-measurement error from an individual error source (apparent

clock and ephemeris prediction
accuracy’s), converted into range units, assuming that that error

source is uncorrelated with all other
error sources. Values of 32 indicate that Selective

Availability is active. Values less
than 10 generally indicate that SA is inactive.

User Range Error (URE)

Estimated error in range due to
factors such as unmodeled atmospheric effects, orbital

calculation errors, satellite clock
bias, multipath and selective availability. This value is

transmitted by the NAVSTAR satellites
and may be displayed by some GPS receivers. The

number is expressed in meters.

User Segment

The part of the whole GPS system that
includes the receivers of GPS signals.

UTC or UTC (USNO)

à
see Universal Time Coordinated

UTM

à
see Universal Transverse Mercator Map
Projection.

à
see International Atomic Time

TDOP

Time dilution of precision,
à
see DOP.

Telemetry

Telemetry is remote measurement or
the remote collection of data. Telemetered data can be

physical, environmental or biological
data. Telemetry is typically used to gather data from

distant, inaccessible locations, or
when data collection would be dangerous or difficult for a

variety of reasons.

Time of Arrival (TOA)

The differences between the times of
arrival of the signal from a handset at three GSMBasestations

is used to calculate the location of
the handset. This requires that either the mobile

network or the GSM-Network is
synchronized using atomic or GPS clocks at every cell site.

Time of Ephemeris (TOE)

An information given by the RINEX
navigation message file.

Time of Perigee (Perihelion)
Passage:

The time at which a satellite last
passed perigee (or perihelion)

Time-To-First-Fix (TTFF)

The actual time required by a GPS
receiver to achieve a position solution. This specification will

vary with the operating state of the
receiver, the length of time since

the last position fix, the location
of the last fix, and the specific receiver design. See also Hot

Start, Warm Start and Cold Start mode
descriptions.

TOA

à
see Time of Arrival

TOE

à
see Time of Ephemeris

TOW (Time of Week)

In seconds from Sunday 0.00h.

Track (TRK)

Your current direction of travel
relative to a ground position (same as COG).

Translocation

A version of relative positioning
that makes use of a known position, such as a National survey

station, to aid in the accurate
positioning of a desired new point. The initial position

(coordinates) is used as a starting
point, then the three-dimensional differences measured by

GPS are used to calculate the
position (coordinates) of the second point.

Triangulation

The location of an unknown point, as
in GPS navigation, found by using the laws of plane

trigonometry.

Trickle Power Mode™

In Trickle Power Mode, Vcc is
continuously supplied to the module. A software configurable

internal timer periodically forces
the module to acquire a position fix. Between the fixes, the

module remains in an ultra-low power
sleep mode. This mode is recommended for applications

where lowest power consumption and a
periodical position up-date are of primary concern. A

backup battery must be connected to
enable the module to reduce startup times when

recovering from a Vcc supply
interruption.

Tropospheric Correction

The correction applied to the
measurement to account for tropospheric delay.

True North

The direction of the North Pole from
your current position. Magnetic compasses are slightly

incorrect due to effects of the
Earth’s magnetic field. GPS units are not affected by magnetic

influences.

TTFF

à
see Time to First Fix

à
see selective availability

Satellite constellation

The arrangement in space of a set of
satellites. In the case of GPS, the fully operational

constellation is composed of six
orbital planes, each containing four satellites. GLONASS has

three orbital planes containing eight
satellites each.

Satellite Elevation Mask

Usually a user-definable elevation
mask (in degrees) that a the Satellite Vehicles (SV) must be

above the horizon before your
receiver will allow it’s data to be used as part of the position calculation.
SV’s at an elevation below the mask value will be tracked, but data from them
will

not be used to compute positions.
This is done because SV’s near the horizon have larger error

potentials due to atmospheric
sources.

Satellite-Based Augmentation
Systems (SBAS)

The various Satellite-Based
Augmentation Systems (SBASs) currently under development utilize

Geostationary Earth Orbit (GEO)
satellites to provide ranging signals to the end user that also

contain integrity data on GPS,
GLONASS, and/or Galileo satellites. This signal is generated on

the ground and provided via C-band
(or K-band) uplink to the GEO, whose navigation

transponder frequency translates it
to L1 and C-band (or K-band) downlink frequencies, then

broadcasts the signals throughout the
GEO’s edge of coverage footprint.

SATNAV

A local term referring to use of the
older TRANSIT system for satellite navigation. One major

difference between TRANSIT and GPS is
that TRANSIT satellites are in a low altitude Polar orbit

with a 90 minute period.

SBAS

à
see Satellite-Based Augmentation
Systems

Selective Availability (SA)

Selective availability is the name of
a process that allows the DoD to limit the accuracy available

to users of Navstar’s Standard
Positioning Service. It is accomplished by manipulating navigation

message orbit data (epsilon) and/or
the satellite clock frequency (dither) This process is intended

to prevent potential adversaries of
the United States from using low-cost GPS receivers for

certain military functions.
Differential GPS techniques can be used to reduce these effects for

local applications. The United States
stopped the intentional degradation (SA) of the Global

Positioning System (GPS) signals
available to the public (SPS) at midnight on May 1, 2000. The

United States has no intent to ever
use SA again. To ensure that potential adversaries do not

use GPS, the military is dedicated to
the development and deployment of regional denial

capabilities in lieu of global
degradation.

SEM Almanac

à
see Almanac

Semi-major axis

One half of the major axis of an
ellipse.

SEP

à
see spherical error probable.

Sequential Receiver

A GPS receiver in which the number of
satellite signals to be tracked exceeds the number of

available hardware channels.
Sequential receivers periodically reassign hardware channels to

particular satellite signals in a
predetermined sequence.

Short Message Service (SMS)

The Short Message Service , part of
GSM, is the ability to send and receive alphanumeric text

messages (up to 160 characters long)
to and from mobile telephones.

Sideral Day

Time taken for a complete rotation of
the earth. Note that this is approximately 4 minutes less

than 24 hours (a solar day). The
solar day is longer because it additionally takes account to the

movement of the earth around the sun.

SIM-Card

A SIM-Card is used in wireless
telephones (GSM) and is a Subscriber Identity Module. Basically, it

is used to store : the subscriber
profile some subscriber data (phonebooks, SMS) some applets

(only possible with SIM Tool Kit
compatible SIM cards) Moreover, the SIM card includes some

security features to avoid fraud and
to guarantee the confidentiality of the communications

Simultaneous Measurements

Measurements made with two or more
receivers over the same period of time, at exactly the

same epochs. Alternatively, the
epochs could be so closely matched in time that any time

misalignment can be accommodated by
correction terms in the observation equation.

SINEX (Solution Independent
Exchange Format)

A solution output format recently
developed by geodesists to permit the exchange of solution

information between organizations,
from which the original normal equation systems for

precise GPS adjustments can be
reconstructed. These reconstructed equation systems can be

combined with other normal equation
systems to create new GPS baseline solutions.

SMS

à
see Short Message Service

SNR

Signal to Noise Ratio. A measure of
the information content of the signal relative to the signal’s

noise. A higher number is desirable.

Solar Day

24 hours. Time between two successive
upper transits of the sun or two successive midday’s.

Space Segment

The part of the whole GPS system that
is in space (i.e. the satellites).

Speed Over Ground (SOG)

The actual speed the GPS unit is
moving over the ground. This may differ from airspeed or

nautical speed due to such things as
sea conditions or head winds. For example, a plane that is

going 120 knots into a 10-knot head
wind may have a SOG of 110 knots.

Spherical Error Probable (SEP)

SEP is the three-dimensional analogue
of à
CEP. A statistical measure of
precision defined as

the 50th percentile value of the
three dimensional position error statistics. Thus, half of the

results are within a 3D SEP value.

Spheroid

à
see ellipsoid.

Spoofing

The deliberate transmission of fake
signals to fool a GPS receiver. Spoofer must mimic a GPS

satellite, rather like a pseudolite
but with disruptive intent.

Spread Spectrum (SS)

The received GPS signal is a
wide-bandwidth, low-power signal (-160 dBW). This property

results from modulating the L-band
signal with a PRN code to spread the signal energy over a

bandwidth that is much greater than
the signal information bandwidth. This is done to provide

the ability to receive all satellites
unambiguously and to provide some resistance to noise and

multipath.

Spread Spectrum System

A system in which the transmitted
signal is spread over a frequency band much wider than the

minimum bandwidth needed to transmit
the information being sent. This is done by modulating

with a pseudo random code, for GPS.

SPS

à
see Standard Positioning Service

Squaring-Type Channel

A GPS receiver channel that
multiplies the received signal by itself to obtain a second harmonic

of the carrier. It does not contain
the code modulation and is called codeless. Note that the

resultant signals are much weaker and
much noisier than either the original signal or a cross

correlated signal. They have a half
cycle carrier, (12 cm) which makes rapid ambiguity fixing

much more difficult.

SRAM (Static Random Access
Memory)

A static read/write,
nonsequential-access memory used for the storage of instructions and data.

Standard Deviation (sigma)

A measure of the dispersion of random
errors about the mean value. If a large number of

measurements or observations of the
same quantity are made, the standard deviation is the

square root of the sum of the squares
of deviations from the mean value divided by the number

of observations less one.

Standard Error Ellipsoid

An ellipsoid in which position
measurements have a 20% chance of being located.

Standard Positioning Service
(SPS)

The normal civilian positioning
accuracy obtained by using the single frequency C/A code. Under

selective availability conditions,
guaranteed to be no worse than 100 meters 95 percent of the

time (2 drms). SPS is the standard
specified level of positioning and timing accuracy that is

available, without qualification or
restrictions, to any user on a continuous worldwide basis. The

accuracy of this service is
established by the U.S. Department of Defense based on U.S. security

interests. Navstar currently provides
horizontal positioning accuracy within 100 meters (2 drms)

and 300 meters with 99.99 percent
probability. The signals providing standard positioning

service are inherently capable of
greater accuracy than this. The accuracy of the system is limited

through the application of a process
called Selective Availability

Static Positioning

Location determination accomplished
with a stationary receiver. This allows the use of various

averaging or differential techniques.

Statue Mile

A unit of length equal to 5,280 feet
or 1,760 yards (1,609 meters) used in the U.S. and other

English-speaking countries.

Straight Line Navigation

The act of going from one waypoint to
another in the most direct line and with no turns.

Subframes

à
see Frame and Subframes

SV

Satellite vehicle or space vehicle.

Switching Channel

A receiver channel that is sequenced
through a number of satellite signals (each from a specific

satellite and at a specific
frequency) at a rate that is slower than, and asynchronous with, the

message data rate.

A position accuracy measure. The R95
value is defined as a circle’s radius, when centered at the

true position, encloses 95% of the
data points in a horizontal scatter plot.

Radio Data System (RDS)

The radio data system is an auxiliary
data channel on a FM-radio signal. The primary purpose of

RDS is to provide radio station,
program information and DGPS error correction.

Radio Technical Commission for
Maritime Services (RTCM)

à
see RTCM

Radionavigation

The determination of position, or the
obtaining of information relative to position, for the

purpose of navigation by means of the
propagation properties of radio waves. GPS is a method

of radionavigation.

RAM (Random Access Memory)

Range Rate

The range change rate (distance)
between the satellite and receiver. The distance to a satellite

changes because the satellites are
moving across the sky and the earth is rotating. Range rate is

determined by measuring the Doppler
shift of the satellite signal.

RDOP

Relative dilution of precision.
àsee
DOP.

RDS

à
see Radio Data System

Reacquisition

The reacquisition figure gives the
time required to get lock on a satellite if the signal has been

blocked for a short time (e.g. due to
buildings). This is most important in urban areas.

Reacquisition time is not related
with TTFF.

Real Time Clock (RTC)

An Integrated circuit which keeps the
time on a GPS-Receiver. The RTC chip is battery powered

to ensure it can keep time even when
the PC is turned off. The chip itself updates time, day,

month, and year.

Real Time Kinematic (RTK)

Real time processing of kinematic
data. If the code and carrier data collected at a base station is

transmitted in real time to one or
more roving receivers, then a computer at the roving receiver

calculates the carrier differential
processing in real time, results will be available in real time.

Such a system is called Real time
Kinematic. Although RTCM data formats exist, most

manufacturers use proprietary
formats.

Real-Time Differential GPS

A data collection process whereby a
GPS receiver gets real-time correction data from another

source in order to filter out the
effects of SA and other sources of error. One way to get

correction data is from a GPS
receiver located at a known position (known as a base station).

The GPS receiver at the known
position computes, formats, and transmits corrections usually through a data
link (e.g., VHF radio or cellular telephone) with each new GPS observation. The

roving unit requires some sort of
data link equipment to receive the GPS corrections so that the

correction factor can be applied to
its current observations. Other sources of correction data are

Satellite-based systems (like
OMNISTAR), CORS stations, etc.

Receiver Autonomous Integrity
Monitoring (RAIM)

A form of receiver self-checking in
which redundant pseudo-range observations are used to

detect if there is a problem or
“failure” with any of the measurements — only four

measurements are needed to derive 3-D
coordinates and the receiver clock error, hence any

extra measurements can be used for
checking. Once the failed measurements have been

identified they may be eliminated
from the navigation fix. RAIM is a concept that has been

introduced by aviation users who are
concerned that GPS does not have the level of Integrity

necessary for non-precision airport
approaches or GPS-aided landing.

Receiver Channels

A GPS receiver specification which
indicates the number of independent hardware signal

processing channels included in the
receiver design.

Receiver Independent Exchange
Format (RINEX)

A set of standard definitions and
formats to promote the free exchange of GPS data and

facilitate the use of data from any
GPS receiver with any postprocessing software package. The

format includes definitions for three
fundamental GPS observables: time, phase, and range

Reconstructed Carrier Phase

The difference between the phase of
the incoming GPS carrier, corrected for Doppler shift, and

the phase of a nominally constant
reference frequency generated in the receiver.

For static positioning, the
reconstructed carrier phase is sampled at epochs determined by a

clock in the receiver. A change in
the satellite to receiver distance of one wavelength of the GPS

carrier (19cm for L1) results in a
one cycle change in the phase of the reconstructed carrier.

Reference Station

à
see Base Station

Relative Navigation

A technique similar to relative
positioning, except that one or both of the points may be

moving. A data link is used to relay
error terms to the moving vessel or aircraft to improve realtime

navigation

Relative Positioning

The process of determining the
relative difference in position between two marks. This can be

done with a much greater precision
than absolute, single point positioning. A receiver (antenna)

is placed over each spot and
measurements are made by observing the same satellites with each

receiver at exactly the same
observation interval and with overlapping times. This technique

allows the cancellation (during
computations) of all errors that are common to both receivers.

These include satellite clock errors,
propagation delays, etc. See also translocation and

differential navigation.

Reliability

The probability of performing a
specified function without failure under given conditions for a

specified period of time.

RF (Radio Frequency)

Any frequency within the
electromagnetic spectrum normally associated with radio wave

propagation.

RFI (Radio Frequency
Interference)

Any electromagnetic disturbance that
interrupts, obstructs, or otherwise degrades or limits the

effective performance of
electronics/electrical equipment. It can be induced intentionally, as in

some forms of electronics warfare, or
unintentionally, as a result of spurious emissions and

responses, intermodulation products.

RINEX

à
see Receiver Independent Exchange
format.

RMS

à
see Root Mean Squared

ROM (Read Only Memory)

A memory in which data, under normal
conditions, can only be read.

Root Mean Squared (RMS)

A statistical measure of the scatter
of computed positions about a true position solution. RMS

can be applied to any random
variable.

Route

A planned course of travel that is
defined by a sequence of waypoints.

Rover

Any mobile GPS receiver used during a
data collection session. The receiver’s position can be

computed relative to one or more
stationary GPS receivers.

RS-232C

Serial interface used by computers.
Also known under the name V.24.

RTC

à
see Real Time Clock

RTCA (Radio Technical
Commission of Aeronautics)

RTCM (Radio Technical
Commission for Maritime Services)

A special committee of the commission
(Special Committee 104) was set up to define a

differential data link to be used to
relay GPS correction messages from a monitor (Reference)

station to a field user. RTCM SC-104
recommendations define the correction message format

and 16 different correction message
types.

RTK

à
see Real Time Kinematic

A receiver employing multiple
receiver circuits, each dedicated to receiving and processing the

signal from a single satellite.

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 electronic system, 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.

Passive Antenna

à
see Antenna

Patch Antenna

à
see Antenna

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 and is typically used alone by U.S. and allied

military receivers. 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( =38 weeks). Each

satellite uses a 1 week segment of
this code that is unique to each GPS satellite and is reset

each week.

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. 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 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.

PDOP Mask

Usually a user-definable upper limit
for the PDOP you will tolerate during collection of a dataset.

If PDOP rises above the pre-set
limit, GPS data collection will be suspended until the PDOP

decreases below the limit.

Perigee

That point in a geocentric orbit when
the geometric distance is a minimum.

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

à
see reconstructed carrier phase.

Photogrammetry

An aerial remote sensing technique
whose latest innovations employ a high-resolution aerial

camera with forward motion
compensation and uses GPS technology for pilot guidance over

the designated photo block(s).
Photogrammetry forms the baseline of many Geographic

Information Systems (GIS) and Land
Information System (LIS) studies and endeavours.

Pi

The mathematical constant (?) having
a value of approximately 3.14159.

Pixel

A single display element ( picture
element) of an LCD screen. The more pixels, the higher the

resolution and definition.

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 meters, depending on

the constellation of the satellites.
However the USDoD degrade the accuracy through Selective

Availability so that the positions
are at best

+/-100 meters in horizontal, +/-180
meters 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.

Poor Man’s DGPS

Two different GPS receivers are used
and one of them is fixed to a known location. The change

in position is watched, and is
computed as latitude and longitude error, or offset from the

known location. The bias is sent
(typically via VHF or UHF radio link) to a mobile receiver, which

simply subtracts it out of its own
latitude and longitude measurements. The problem is that the

two GPS units (even if they’re
identical models) can use very different data to calculate the

position solution. The biggest single
problem is that they are each likely to use a different set of

GPS satellites. The fixed receiver
has a stationary view of the sky, probably a good one. The

mobile receiver is cruising around
with a changing collection of satellites that are in-view. As the receiver uses
different satellites, it has a very different collection of error factors. All
this makes

the bias on one receiver almost
unrelated to the bias on another, enough so that the

“corrected” position is likely to be
less accurate than the mobile receiver alone.

Position

A geographic location on or near the
earth, commonly measured in latitude and longitude.

Position Dilution of Precision

à
see PDOP

Position Fix

The GPS receiver’s computed position
coordinates.

Position Format

The way in which the GPS receiver’s
position will be displayed on the screen. Commonly

displayed as degrees and minutes,
with options for degrees, minutes, and seconds, degrees

only, or one of several grid formats.

Post-Processed Differential GPS

Using post-processed differential GPS
data collection techniques, base and roving receivers have

no data link between them. Each
receiver independently records data that will allow differential

correction when processed with data
from other receivers collecting during the same time

period. During collection, the same
set of satellites must be in view of all receivers collecting

data for later combined processing.
Differential correction software is used to combine and

process the data collected.

PPS

à
see Precise Positioning Service or

à
see Pulse per Second

Precise Positioning Service
(PPS)

The highest level of military dynamic
positioning accuracy provided by GPS, using the dualfrequency

P-code. PPS is the most accurate
positioning, velocity, and timing information

continuously available, worldwide,
from the basic GPS. This service is limited to authorized U.S.

and allied Federal Governments;
authorized foreign and military users; and eligible civil users.

PPS information is usually (but not
always) encrypted to prevent use by unauthorized users. The

encryption process is known as
Anti-Spoofing. GPS receivers that can use the P code provide a

predictable positioning accuracy of
at least 22 meters (2 drms) horizontally and 27.7 meters (2

sigma) vertically. They can also
provide timing/time interval accuracy within 90 nanoseconds (95

percent probability). This improved
accuracy is provided in two ways. First, P-code users are not

subjected to Selective Availability.
Second, access to the L2 channel allows the user to correct

for atmospheric propagation errors.
Equipment that uses the P code is available for civilian use;

however, The capability is useless
when Anti-Spoofing is active. Access to GPS receivers that

can decrypt the Y code is tightly
controlled and is subject to National Security considerations. If

you are a civilian user, you will
need a really good excuse to get one.

Prime Meridian

The zero meridian (0), used as a
reference line from which longitude east and west is measured.

It passes through Greenwich, England.

PRN

à
see Pseudorandom Noise

Pseudo-kinematic surveying

A variation of the kinematic method
where roughly five-minute site occupations are repeated at

a minimum of once each hour.

Pseudolite

Shortened form of pseudo-satellite. A
ground-based differential GPS receiver 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.

Pseudorandom Code

The identifying signature signal
transmitted by each GPS satellite and mirrored by the GPS

receiver, in order to separate and
retrieve the signal from background noise. A signal with

random noise-like properties. It is a
very complicated but repeating pattern of 1′s and 0′s.

Pseudorandom Noise (PRN)

Pseudorandom noise, a sequence of
digital 1′s and 0′s that appear to be randomly distributed

like noise, but which can be exactly
reproduced. The important property of PRN codes is that

they have a low autocorrelation value
for all delays or lags except when they are exactly

coincident. Each NAVSTAR satellite
has its own unique C / A and P pseudorandom noise codes.

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.

Pseudorange Difference

à
see reconstructed carrier phase.

Pulse per Second (PPS)

à
see 1 PPS

PVT (Position, Velocity and
Time)

Global Positioning Systems (GPS) are
space-based radio positioning systems that provide 24

hour, three dimensional position,
velocity and time (PVT) information to suitably equipped users.

Term used to describe periods of time
when a GPS receiver’s line-of-sight to GPS satellites is

blocked by natural or man-made
objects.

Observation

The period of time over which GPS
data is collected simultaneously by two or more receivers.

Observation Interval

The rate at which a receiver samples
and stores the observation data transmitted by a satellite. It

is sometimes called the epoch rate.

Observing Session

The period of time over which GPS
data is collected simultaneously by two or more receivers.

Odometer

An instrument for measuring the
distance traveled by a vehicle.

OEM (Original Equipment
Manufacturer)

Original Equipment Manufacturer.
Typically GPS receiver “modules” or “engines” that an end

user product developer can embed
within some application or hardware package.

On-The-Fly (OTF)

This is a form of Ambiguity
Resolution (AR), which does not require that the receivers remain

stationary for any length of time.
Hence this AR technique is suitable for initializing carrier

phase-based Kinematic Positioning.
For many applications this introduces considerable flexibility.

For example, aircraft do not have to
be parked on the ground in order to resolve the carrier cycle ambiguities, and
then require that signal lock-on be maintained throughout the kinematic

survey. However, dual-frequency
instrumentation capable of making both carrier phase and

precise (P-Code level) pseudo-range
measurements is required.

Order of Survey

In an analogous manner to “Class of
Survey”, Order of Survey is a means of categorizing the

quality, or precision, of a static
survey. However, it relates to the external quality, and is

influenced by the quality of the
“external” network information. If the existing geodetic control

is of a lower quality to what can be
achieved using modern GPS Surveying techniques, then the

geodetic control network must be
upgraded or “renovated” using more precise GPS Geodesy

techniques.

Orthometric Height

à
see Height Above Sea Level

Outage

Defined as a loss of availability,
due to either there not being enough satellites visible to

calculate a position, or the value of
the DOP indicator is greater than some specified value

(implying that the accuracy of the
position is unreliable).

Over Determined Solution

The solution of a system of equations
containing more equations than unknowns. The GPS

receiver computers, when possible, an
over determined solution using the measurements from

five or more GPS satellites, instead
of the four necessary for a three-dimensional position

solution (longitude, latitude,
altitude and the receiver clock bias).

North American Datum of 1927.
Obsolete horizontal datum of North America. NAD 27 depends

upon an early approximation of the
shape of the earth, known as the Clarke Spheroid of 1866,

designed to fit only the shape of the
conterminous United States, and utilizing a specific Earth

surface coordinate pair as its center
of reference.

NAD-83

North American Datum of 1983. NAD 83
relies on the more precise Geodetic Reference System

of 1980 (GRS 80).

Nanosecond (ns)

One billionth of a second.

NANU

Notice Advisory to NAVSTAR Users – a
periodic bulletin alerting users to changes in system

performance

National Marine Electronics
Association (NMEA)

A U.S. standards committee that
defines data message structure, contents, and protocols to

allow the GPS receiver to communicate
with other pieces of electronic equipment. NMEA 0183

is the standard data communication
protocol used by GPS receivers and other types of

navigation and marine electronics.

Nautical Mile

A unit of length used in sea and air
navigation, based on the length of one minute of arc of a

great circle, especially an
international and U.S. unit equal to 1,852 meters (about 6,076 feet).

NAVD-88

North American Vertical Datum of
1988. Effort underway by the National Geodetic Survey

(NGS) to readjust the North American
Vertical Datum. The NAVD 88 readjustment will remove

distortions from the continent-wide
vertical geodetic (height) reference system.

NAVDATA

The 1500 bit navigation message (NAV
message) broadcast by each satellite at 50 bps on both

L1 and L2 frequencies. This message
contains system time, clock correction parameters,

ionospheric delay model parameters,
the satellite’s ephemeris and it’s health. This information is

used to process GPS signals to obtain
user position and velocity. It is also used when processing

precise surveying data.

Navigation

The act of determining the course or
heading of movement. This movement could be for a

plane, ship, automobile, person on
foot, or any other similar means.

Navigation Message (Nav
Message)

The navigation data is transmitted on
the L1 and L2 signals at 50 bits per second and contains

ephemeris and clock data for that
particular satellite, other data required by a receiver to

compute position velocity and time
and almanac data for all NAVSTAR satellites. The data is

transmitted in 1500 bit frames, each
requiring 30 seconds to transmit. A complete set of data

to include all almanacs, timing
information, ionospheric information and other data requires 12-

1/2 minutes to transmit. This message
contains also system time, clock correction parameters, ionospheric delay model
parameters, and the vehicle’s ephemeris and health. The information is

used to process GPS signals to give
user time, position, and velocity.

NAVSTAR

The name given to GPS satellites,
built by Rockwell International, that is an acronym formed

from NAVigation System with Time And
Ranging.

NED (North, East, Down)

Coordinate system.
à
see LTP.

Network-assisted GPS location

There are different solutions to
assist GPS

A network-driven approach places a
minimal GPS front end in the handset and transfers the

partially processed GPS satellite
data back to the GSM-base station. The position is then

determined in the base station.

A second network-assisted method uses
a more complete GPS subsystem in the handset, but

enhances its operation by downloading
from the base station some GPS start-up information to

accelerate position calculation. The
position is calculated in the handset.

NMEA

à
see National Marine Electronics
Association.

NVRAM (Non-Volatile RAM)

Non-volatile RAM, portion of the SRAM
that is powered by a backup battery power supply

when prime power is removed. It is
used to preserve important data and allow faster entry into

the Navigation Mode when prime power
is restored. (sometimes also referred to as keep-alive

SRAM).

Unit of velocity

m/sec/sec (m/s2)
(Meters per Second per Second)

Unit of acceleration

m/sec/sec/sec (m/s3)
(Meters per second per second per second)

Unit of impulse or jerk

Magnetic North

Represents the direction of the north
magnetic pole from the observer’s position. The direction

a compass points.

Magnetic Variation

Errors in magnetic compass readings
caused by variance in the earth’s magnetic field at

different locations on the planet.
Navigational charts list the variation and a yearly level of

increase.

Mask Angle

The minimum GPS satellite elevation
angle permitted by a particular GPS receiver design.

MDT (Mobile Data Terminal )

A device, typically installed in a
vehicle consisting of a small screen, a keyboard or other

operator interface, and various
amounts of memory and processing capabilities.

Mean Time Between Failure
(MTBF)

An indicator of expected system
reliabilty calculated on a statistical basis from the known failure

rates of various components of the
system. MTBF is typically expressed in hours. The hours are

calculated by dividing the total
number of failures into the total number of operating hours

observed

Measurement

The square of the standard deviation
of a measurement quality. The standard deviation Error

Variance is representative of the
error typically expected in a measured value of the quantity.

MHz (Megahertz)

A unit of frequency (1MHz = 1’000’000
Hz).

Microstrip Antenna

A two dimensional. flat, precisely
cut piece of metal foil glued to a substrate.

MID (Message Identifier)

This is a unique number or a sequence
of characters identifying a message of a protocol..

MO and MT

Abbreviations for mobile
(GSM-telephone) originated (MO) and mobile terminated (MT). Calls or

data generated by the mobile
telephone are MO (e.g. a SM (short message), which is sent by

the mobile telephone, is MO)

Calls and data arriving to the mobile
telephone are MT (e.g. a calling for the Handy is thus MT)

Modem

MOdulator DEModulator. Used to allow
computers and terminals to send data over telephone

lines using audible tones. Often
mounted internally in a PC and connected to the telephone

line. The modem may also be an
external unit which connects to the computer via the serial

port.
à
see also wireless modem.

Monitor Stations

World-wide group of stations used in
the GPS control segment to monitor satellite clock and

orbital parameters. Data collected
here is linked to a master station where corrections are calculated and
controlled. This data is uploaded to each satellite at least once per day from
an

upload station.

Moving Map

The actual position is always
displayed in the middle of the screen-map

MSB (Most Significant Bit or
Most Significant Byte)

The Bit or Byte with the highest
significance

MSL

à
see Height Above Sea Level

MTBF

à
see Mean Time Between Failure

Multichannel Receiver

A receiver containing multiple
independent channels, each of which tracks one satellite

continuously, so that position
solutions are derived from simultaneous calculations of

pseudoranges.

Multipath

Interference of the GPS signal often
caused by the bouncing or reflecting of the signals from

surrounding objects. Its effect is
similar to “ghosting” on a television screen. It occurs when

some of the signals arriving at an
antenna have followed an indirect path. A signal taking the

longer path gives a larger distance
measurement to the satellite. This means that it also gives an

inaccurate carrier phase value. It
can be the most serious error to affect GPS surveying. Whilst

some multipath conditions cause
errors of a few centimeters, others cause errors of a few

meters. Bad multipath conditions can
cause a short static session to fail completely.

Multipath error

Errors caused by the interference of
a signal that has reached the receiver antenna by two or

more different paths. Usually caused
by one path being bounced or reflected.

Multiplexing Channel

A receiver channel that is sequenced
through several satellite signals (each from a specific

satellite and at a specific
frequency) at a rate that is synchronous with the satellite message bit

rate (50 bits per second, or 20
milliseconds per bit). Thus, one complete sequence is completed

in a multiple of 20 milliseconds.

mV (Millivolt)

mW (Milliwatt)