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MN002000A © 2004 Navman NZ Ltd. All rights reserved. Proprietary information and specifications subject to change without notice.
RTCA: Radio Technical Commission for
Aeronautics.
RTCM: Radio Technical Commission for Maritime
Services.
SA: Selective Availability. The method used by
the DoD to control access to the full accuracy
achievable with the C/A code.
Satellite elevation: the angle of the satellite
above the horizon.
SEP: Spherical Error Probable. The radius of
a sphere, centred at the user’s true location,
that contains 50 percent of the individual three-
dimensional position measurements made using a
particular navigation system.
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.
SNR: Signal-To-Noise Ratio (expressed in
decibels).
SOG: Speed Over Ground.
SPS: Standard Positioning Service. A positioning
service available to all GPS users on a continuous,
worldwide basis with no direct charge. SPS uses
the C/A code to provide a minimum dynamic and
static positioning capability.
SRAM: Static Random Access Memory.
Stand-by SRAM: portion of the SRAM that is
powered by a “keep-alive” power supply when
prime power is removed to preserve important
data and allow faster entry into the Navigation
Mode when prime power is restored. All of the
SRAM in the receiver is “keep-alive” SRAM.
SV: Space Vehicle. Also Satellite Vehicle.
TDOP: Time Dilution of Precision. A measure of
how much the geometry of the satellites affects the
time estimate computed from the satellite range
measurements.
Three dimensional coverage (hours): the
number of hours-per-day with four or more
satellites visible. Four visible satellites are required
to determine location and altitude.
Three dimensional navigation: Navigation
mode in which altitude and horizontal position are
determined from satellite range measurements.
Time mark pulse: a one pulse per second (lPPS)
output synchronised to UTC when the receiver is in
its Navigation Mode.
TOW: Time Of Week (see GPS time).
TTFF: Time-To-First-Fix. 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.
TTL: Transistor-Transistor Logic
Two dimensional coverage (hours): the number
of hours-per-day with three or more satellites
visible. Three visible satellites can be used to
determine location if the GPS receiver is designed
to accept an external altitude input (Altitude Hold).
Two dimensional navigation: Navigation mode
in which a fixed value of altitude is used for one or
more position calculations while horizontal (2D)
position can vary freely based on satellite range
measurements.
UDRE: User Differential Range Error. A measure
of error in range measurement to each satellite as
seen by the receiver.
UERE: User Equivalent Range Error.
Update Rate: the GPS receiver specification
which indicates the solution rate provided by the
receiver when operating normally.
U.S. Air Force Space Command: the U.S. Air
Force agency responsible for the operation of the
GPS Space and Control Segments.
UTC: Universal Time Coordinated. This time
system uses the second defined true angular
rotation of the Earth measured as if the Earth
rotated about its conventional terrestrial pole.
However, UTC is adjusted only in increments
of one second. The time zone of UTC is that of
Greenwich Mean Time (GMT).
VCO: Voltage Controlled Oscillator.
VDOP: Vertical Dilution of Precision. A measure
of how much the geometry of the satellites affects
the position estimate (computed from the satellite
range measurements) in the vertical (perpendicular
to the plane of the user) direction.
VSWR: Voltage Standing Wave Ratio.
WGS-84 World Geodetic System (1984):
a
mathematical ellipsoid designed to fit the shape
of the entire Earth. It is often used as a reference
on a worldwide basis, while other ellipsoids are
used locally to provide a better fit to the Earth in a
local region. GPS uses the centre of the WGS-84
ellipsoid as the centre of the GPS ECEF reference
frame.