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Glossary of Terms Appendix H
SUPERSTAR II User Manual Rev 3 77
Figure of Merit - The receiver provides an estimated accuracy level. The accuracy level estimate is provided
in the horizontal and vertical Figure of Merit (FOM). The FOM reflects a 95% confidence level for the position
solution accuracy estimate. The FOM accounts for all major sources of errors in the pseudoranges of the
satellites used in the position solution. The error sources which are included are ionospheric and tropospheric
errors, satellite position errors based on transmitted user range error, and thermal noise.
Fixed field -a field in which the number of characters is fixed. For data fields, such fields are shown in the
sentence definitions with no decimal point. Other fields which fall into this category are the address field and
the checksum field (if present).
GDOP - Geometric Dilution of Precision - A numerical value expressing the confidence factor of the position
solution based on current satellite geometry. Assumes that 3D position (latitude, longitude, height) and receiver
clock offset (time) are variables in the solution. The lower the GDOP value, the greater the confidence in the
solution.
Geodetic datum - the reference ellipsoid surface that defines the coordinate system.
Geoid - the figure of the earth considered as a sea level surface extended continuously through the continents.
The actual geoid is an equipotential surface coincident with mean sea level to which at every point the plumb
line (direction in which gravity acts) is perpendicular.
Geostationary - a satellite orbit along the equator that results in a constant fixed position over a particular
reference point on the earth’s surface. (GPS satellites are not geostationary.)
Global Positioning System (GPS) - full name NAVSTAR Global Positioning System, a space-based radio
positioning system which provides suitably equipped users with accurate position, velocity and time data.
When fully operational, GPS will provide this data free of direct user charge worldwide, continuously, and
under all weather conditions. The GPS constellation will consist of 24 orbiting satellites, four equally spaced
around each of six different orbital planes. The system is being developed by the Department of Defense under
U.S. Air Force management.
HDOP - Horizontal Dilution of Precision - A numerical value expressing the confidence factor of the
horizontal position solution based on current satellite geometry. Makes no constraint assumptions about time,
and about height only if the FIX HEIGHT command has been invoked. The lower the HDOP value, the greater
the confidence in the solution.
HTDOP - Horizontal position and Time Dilution of Precision - A numerical value expressing the confidence
factor of the position solution based on current satellite geometry. Assumes height is known if the FIX
HEIGHT command has been invoked. If not, it will give the normalized precision of the horizontal and time
parameters given that nothing has been constrained. The lower the HTDOP value, the greater the confidence.
L1 frequency - the 1575.42 MHz GPS carrier frequency which contains the coarse acquisition (C/A) code, as
well as encrypted P-code, and navigation messages used by commercial GPS receivers.
Mask angle - the minimum GPS satellite elevation angle permitted by a particular GPS receiver design.
Satellites below this angle will not be used in position solution.
Multipath errors - GPS positioning errors caused by the interaction of the GPS satellite signal and its
reflections.
Nanosecond - 1 x 10
-9
second.
Null field - by NMEA standard, indicates that data is not available for the field. Indicated by two ASCII
commas, i.e., “*” (HEX 2C2C), or, for the last data field in a sentence, one comma followed by either the
checksum delimiter "“"”(HEX 2A) or the sentence delimiters <CR><LF> (HEX 0D0A). [Note: the ASCII Null
character (HEX 00) is not
to be used for null fields.]
Obscuration - 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.