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B - 1
Appendix B - Terminology
Term Description
Anti-Spoofing (AS) Is a policy of the U.S. Department of Defense by which the P-Code is
encrypted (by the additional modulation of a so-called W-Code to
generate a new "Y-Code"), to protect the militarily important P-Code
signals from being "spoofed" through the transmission of false GPS
signals by an adversary during times of war. Hence civilian GPS
receivers are unable to make direct P-Code pseudo-range
measurements and must use proprietary (indirect) signal tracking
techniques to make measurements on the L2 carrier wave (for both
pseudo-range and carrier phase). All dual-frequency instrumentation
must therefore overcome AS using these special signal tracking and
measurement techniques.
Availability The number of hours per day that a particular location has sufficient
satellites (above the specified elevation angle, and perhaps less than
some specified PDOP value) to make a GPS position determination
possible.
BIAS All GPS measurements are affected by biases and errors. Their
combined magnitudes will affect the accuracy of the positioning results
(they will bias the position or baseline solution). Biases may be defined
as being those systematic errors that cause the true measurements to
be different from observed measurements by a "constant, predictable or
systematic amount", such as, for example, all distances being measured
too short, or too long. Biases must somehow be accounted for in the
measurement model used for data processing if high accuracy is sought.
There are several sources of biases with varying characteristics, such
as magnitude, periodicity, satellite or receiver dependency, etc. Biases
may have physical bases, such as the atmosphere effects on signal
propagation or ambiguities in the carrier phase measurements, but may
also enter at the data processing stage through imperfect knowledge of
constants, for example any "fixed" parameters such as the satellite
ephemeris information, station coordinates, velocity of light, antenna
height errors, etc. Random errors will not bias a solution. However,
outlier measurements, or measurements significantly affected by
multi-path disturbance (which may be considered a transient,
un - modeled bias), will bias a solution if the proportion of affected
measurements is relatively high compared to the number of unaffected
measurements. For this reason, long period static GPS Surveying is
more accurate (less likely to be biased) than "rapid static surveying" or
kinematic (single-epoch) positioning.
Binary Shift-Key (BSK)
Modulation
BSK is a modulation technique by which a binary message, such the
Navigation Message or the PRN codes (consisting of 0's and 1's), is
imprinted on the carrier wave. Unlike Amplitude Modulation (AM) and
Frequency Modulation (FM), BSK Modulation does not alter the signal
level (the "amplitude") or the carrier wavelength (the "frequency"). At a
change in value of the message from 0 or 1, or from 1 to 0, the carrier
wave is reversed (the phase is "flipped" by 180°). All reversals take
place at the zero-crossings of the carrier (sine) wave (i.e., where the
phase is zero).