RMS - Root Mean Square deviation is also known as the
population standard deviation is given
by the formula;
Version 1998.23
Anti Spoofing - AS Encryption of the P-code
signal transforming it to Y-code which is
unavailable to civilian users.
AROF - Ambiguity Resolution On the Fly Fast
ambiguity resolution GPS surveying where the
solution is determined while the receiver is in motion.
Baseline - Vector The difference in
three-dimensional coordinates (X, Y, Z) computed
from the difference in simultaneous carrier phase observations at two or
more receivers.
Blue Book - Term referring to the Federal Geodetic Control
Subcommittee publication "Input
Formats and Specifications of the National Geodetic Survey Data Base" which
defines the file
formats for data submission to the National Geodetic Reference System.
C/A-Code - Clear / Acquisition, Coarse / Acquisition
code modulated onto the L1 carrier signal.
Carrier frequency - Frequency of the unmodulated output
of a radio transmitter.
L1 carrier signal broadcasts at 1575.42 MHz (19 cm wavelength).
L2 carrier signal broadcasts at 1227.60 MHz (24 cm wavelength).
Continuous Kinematic Surveying - Successive baseline solutions
generated at every epoch of
an unbroken observation set. Typically used to track a vehicle or platform
in motion.
Correlation - The extent to which one observation or
computed value is influenced by the
change in an other, or that both are influenced by a third. (e.g. The error
in a trigonometric height
difference is correlated to the error in both the vertical angle and the
measured distance.
Similarly errors in computed GPS vectors have correlations between their
individual components
(X, Y, Z) and between common phase observations.) The correlation coefficient
is the
proportion of the total variation in the dependent variable (y) which can
be attributed to the
relationship with the independent variable (x).
CORS - Continuously Operating Reference Station
Fixed GPS receiver site in continuous
operation.
Covariance - The average value of the quantity x(r1)
* x(r2); where x is a randomly varying
function of the variable r, and r1 and r2 are
two given values of r. Covariance describes the
interdependence between variables and is typically expressed in a
variance-covariance matrix
where the diagonal elements are the variances of the corresponding variables,
and those off the
main diagonal are the covariance values.
Cycle slip - Instantaneous loss of lock (cycle count)
on one or more carrier signals.
Datum, geodetic - A set of constants specifying the
coordinate system used for geodetic control.
A complete geodetic datum provides, as a minimum, definition for orientation,
scale and
dimensions for the reference ellipsoid. The concept is generally expanded
to include the
published coordinates of control stations within the system.
GRS 80 - The reference ellipsoid of the NAD83 coordinate system. a = 6378137.000 m, b = 6356752.314 m, 1/f = 298.257 222 101
Clarke 1866 - The reference ellipsoid for the NAD27 coordinate system. a = 6378206.400 m, b = 6356583.800 m, 1/f = 294.978 698 200
NAD 27 - Regional horizontal coordinate system used throughout the U.S. until 1986 with the origin at station MEADS RANCH.
NAD 83 - Horizontal coordinate system for U.S., Canada and Mexico. Originally published in 1986 it is based upon the GRS 80 ellipsoid with its origin at the center of mass defined by BIH at epoch 1984.0. Geodetic surveyors must be particularly cognizant of epoch dates attached to the NAD 83 an acronym. These refer to the mean date of the observations used in the regional adjustment. The California HPGN was originally published at epoch 1991.35.
NAVD 88 - Vertical (elevation) reference system for U.S., Canada and Mexico. Published in 1991 the orthometric heights are derived from an adjustment of leveling data constrained at a single point on the St. Lawrence Seaway.
NGVD 29 - Mean sea level reference system for U.S. prior to NAVD 88. Constrained to 26 tide stations in U.S. and Canada, the datum is distorted to fit sea surface topography.
WGS 84 - Geodetic datum for the GPS orbits and consequently
positions and baselines
computed using them. a = 6378137.000 m, b = 6356752.3100, 1/f = 298.257 223
563
Deflection of the vertical - The angle at a point on
the surface of the earth between the vertical
at that point (the line normal to the geoid) and the line through the point
which is normal to the
reference ellipse.
Degrees of freedom - The number of observations minus
the minimum number required to
uniquely define the figure.
Differential GPS - Single point code positioning with
pseudorange corrections applied from
simultaneous observations at a known position. One to ten meter accuracy
is typical.
Dilution of precision - DOP A scale factor
representing the contribution of the satellite
configuration geometry to the positioning accuracy. Standard terms for GPS
applications are:
GDOP - Geometric (three coordinates plus clock offset)
PDOP - Position (three coordinates)
HDOP - Horizontal (two coordinates)
VDOP - Tertical (height only)
TDOP - Time (clock offset only)
Double Difference - Baseline computation algorithm using differences between carrier phase measurements between satellites and between receivers. This processing is the typical solution utilized for precise surveying applications. Computations are ambiguous in the unknown integer cycle term (see cycle ambiguity).
ECEF coordinates - Cartesian coordinates, Earth
Centered Earth Fixed Rectangular
coordinate system where the positive X axis lies on the equatorial plane
passing through the
prime meridian (Greenwich), the positive Y axis lies on the equatorial plane
at 90 east, and
positive Z passes north through the mean rotational axis of the earth.
Ellipsoid - spheroid The mathematical function
used to describe the shape of the earth for
geodetic computations. The figure is formed by rotating an ellipse about
its minor (shorter) axis
and is typically described by dimensions for the semimajor axis (a) together
with the semiminor
axis (b) or flattening (f) = (a-b)/a.
Ellipsoid height - The distance from a point to the reference
ellipsoid along a line normal to the
ellipsoid.
Ephemeris - almanac The description of the
satellite orbits and clock correction parameters
variable over time used for positioning and baseline computations. The ephemerise
may be
broadcast (projected ahead into time and subject to selective availability)
or precise (post-fitted).
Epoch - A specific instant in time. GPS carrier phase measurements are made at a given frequency (e.g. every 30 seconds) or epoch rate.
Epoch Date - The date, usually expressed in decimal
years, for which published coordinates and
data are valid.
Error ellipse - A statistical measure of the positional error
at a given point computed from the propagation of all errors contributing to
the position and expressed by its semi-major and semi-minor axis (vectors of
greatest and least magnitude) and the covariance (rotation angle from the reference
coordinate system). Two dimensional errors are typically propagated at one-standard
deviation (39.4% probability that the position lies on or within the ellipse)
or 2.1447 times the standard deviation (95% confidence) level.
Fast ambiguity resolution - Rapid static, Fast
static GPS surveying technique utilizing
multiple observables (dual-frequency carrier phase, C/A and P codes) to resolve
integer
ambiguities with shortened observation periods. The method may also be used
for observations
with the receiver in motion known as on-the-fly ambiguity resolution.
Geoid - That equipotential surface (a surface of equal
gravity potential) which most closely
matches mean sea level. An equipotential surface is normal to the gravity
vector at every point
GPS week - Incremental number of weeks, starting at 0
hour UTC on the date January 6, 1980.
April 6, 1997 is the first day of GPS week 900.
Gravity void - A block or area of blocks within the
gravity measurement database without
observations. A geoid model relying upon this database would be weak and
possibly in error at
these blocks.
Ground plane - A large flat metal surface, or electrically
charged field, surrounding a GPS
antenna used to deflect errant signals (multipath) reflected from the ground
and other near-by
objects.
HTDP - Horizontal Time Dependent Positioning model
A computer database and interpolation
program developed by NGS to predict horizontal displacements between coordinate
points over
time. The program can work backwards in time where it includes earthquake
parameters, or
forward in time where only the secular motion is analyzed.
Ionospheric delay - Signal delay or acceleration as a
wave propagates through the ionosphere.
Phase delay depends upon the electron content and affects the carrier signal.
Group delay
depends upon the dispersion in the ionosphere as well, and affects the code
signal.
Independent baseline - Non-trivial baseline
Those vectors determined from differencing
common phase measurements only once. For any given session there are n-1
independent vectors
where n is the number of receivers operating.
Kinematic GPS - Observations while a receiver is in motion.
In surveying applications,
kinematic refers to uninterrupted carrier-phase measurements following successful
solution of
the integer ambiguities. This can be accomplished in a continuous mode where
the receiver
remains in motion for precise positioning of a vehicle, or in an intermittent
mode where data is
recorded only after a receiver is brought to a stationary point, and the
observations while in
motion are tracked as a way to maintain the integer ambiguities.
Mask angle - cut-off angle The point above
the observer's horizon below which satellite signals
are no longer tracked and/or processed. 10° to 25° is typical.
Multipath - Interference similar to "ghosts"
on a television screen which occurs when GPS
signals arrive at an antenna having traversed different paths. Multipath
may arise from reflections
off structures near the antenna and occurs to some extent everywhere. The
signal which traverses
a longer path will yield a larger pseudo range estimate and increase the
error.
Orthometric height - elevation The distance
from the geoid to a point, measured along a line
normal to the geoid.
P-code - The protected, or precise, code modulated on both
L1 and L2 carrier signals. The P-code is a very long
(about 1014 bits) sequence of pseudo-random binary biphase modulations
on
the GPS carrier at a chipping rate of 10.23 MHz which does not repeat itself
for about 38 weeks.
Each satellite uses a one-week segment of this code which is unique to each
satellite, and reset
each week.
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.
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-random-noise codes
and are often referred to by their PRN number.
Phase difference processing - relative positioning
Computation of the relative difference in
position between two points by the process of differencing simultaneous
reconstructed carrier
phase measurements at both sites. The technique allows cancellation of all
errors which are
common to both observers, such as clock errors, orbit errors, and propagation
delays. This
cancellation effect provides for determination of the relative position with
much greater precision
than that to which a single position (pseudorange solution) can be determined.
Pseudorange - A measure of the apparent propagation
time from the satellite to the receiver
antenna, expressed as a distance. The apparent propagation time is determined
from the time shift
required to align a replica of the GPS code generated in the receiver with
the received GPS code.
The time shift is the difference between the time of signal reception (measured
in the receiver
time frame) and the time of emission measured in the satellite time frame).
Pseudorange is
obtained by multiplying the apparent signal-propagation time by the speed
of light. Pseudorange
differs from the actual range by the amount that the satellite and receiver
clocks are offset, by
propagation delays, and other errors including those introduced by selective
availability.
Pseudo-static GPS - Also known as pseudo-kinematic and
repeat occupation, this relative
positioning technique relies upon two or more simultaneous observations at
a point pair,
separated by some time interval (typically 60 minutes or more), in order
to solve the integer bias
terms from the change in satellite geometry occurring between the repeat
observations.
Relative precision - Precision is defined as a measure
of the tendency of a set of numbers to
cluster about a number determined by the set (e.g. the mean). The usual measure
is the standard
deviation with respect to the mean. Relative precision denotes the tendency
for the various
components (X, Y, Z) between one station and other stations in the network
to be clustered
about the adjusted values. Current custom is to express relative precision
at the two-standard
deviation (95% confidence) level. This may be stated in terms of a relative
error ellipse or as a
proportion of the separation distance (e.g. 10 ppm or 1:100,000).
Residual - The difference between an estimated (adjusted)
value and the observed value;
specifically, v = µx-x
where x is an observation of the
estimated value µx.
RINEX - Receiver INdependent EXchange format. A set of
standard definition and formats for
ASCII data files to promote the free exchange of GPS data and facilitate
the use of data from any
GPS receiver with any software package.
RMS - Root Mean Square deviation is also known as the
population standard deviation is given
by the formula;
Where: µ is the estimated (adjusted) value for x and N is the number
of observations
RTCM - Radio Technical Commission for Maritime
Services Commission set up to define a
differential data link to relay GPS correction messages from a monitor station
to a field user.
RTCM SC-104 recommendations is the defacto standard for differential GPS
correction
transmission. It defines the correction message format and 16 different
correction message types.
Secular Motion - That portion of crustal motion which
is continuous and at a constant velocity.
Secular motion is uniformly predictable over time and is independent of any
seismic events.
Selective availability - SA A DoD program to control
the accuracy of pseudorange
measurements, whereby the user receives a false pseudorange which is in error
by a controlled
amount. Differential and phase differencing techniques can reduce these effects
for local
applications.
Standard deviation - Standard error Surveying applications use the formula for sample standard deviation which is;
Where; xn is a set of N random numbers with
a mean value of µ, and a proportional
weight of wn for each xn.
Static GPS - Carrier phase differencing technique where
the integer ambiguities are resolved
from an extended observation period through a change in satellite geometry.
Universal time - Local solar mean time at Greenwich Meridian.
Some commonly used version
of Universal Time are:
UT0 - Universal Time as deduced directly from observations of stars and the fixed numerical relationship between Universal and Sidereal Time; 3 minutes 56.555 seconds.
UT1 - UT0 corrected for polar motion.
UT2 - UT1 corrected for seasonal variation in the earth's rotation rate.
UTC - Universal Time Coordinated; uniform atomic time
system kept very closely to
UT2 by offsets. Maintained by the U.S. Naval Observatory. GPS time is directly
relatable to UTC. UTC-GPS = 9 seconds (in 1994)
URA - User Range Accuracy The contribution
to the range-measurement error from an
individual error source (apparent clock and ephemeris prediction accuracies),
converted to range
units, assuming that the error source is uncorrelated with all other error
sources.
Variance - The square of the standard deviation.
Variance factor - reference variance, variance of unit
weight A statistical measure of how close
the observation residuals match the predicted errors. It is the square root
of the sum of the
weighted squares of the residuals divided by the degrees of freedom. If the
errors in a network
have been weighted correctly, the variance factor will approach 1.0.
Widelane - A linear combination of the L1 and
L2 observation (L1 - L2) used to partially
remove
ionospheric errors. The widelane combination has an effective wavelength
of 86 cm making the
integer terms more apparent than the pure carrier. For this reason it is
often used in step-wise fast
ambiguity resolution processing. The penalty is increased observation noise.