Bandwidth
A measure of the capacity of a signal
to carry information. It is expressed as the width of the
spectrum of that signal (frequency
domain representation) in Hertz (Hz), Kilo-Hertz (kHz) or
Mega-Hertz (MHz). The bandwidth of
the GPS-Standard-Signal is 2,046MHz.
Base Station
A base station or reference station
is a GPS receiver set up on a location with known position
specifically to collect data for
differentially correcting data of other GPS receivers. Base station
data is used to calculate errors
relative to its known position. The “relative difference” between
the base station’s known position and
the position calculated from GPS satellite signals become
the correction factor for other GPS
receivers collecting data during the same time period.
Corrections can be transmitted in
real-time, or used during later post-processing. Also called a
reference station.
Baseline
A baseline consists of the resultant
three dimensional vector between a pair of stations for
which simultaneous GPS data is
collected.
Baud
A unit of signaling speed equal to
the number of discrete signal conditions, variations, or events
(bits) per second. If the duration of
the unit interval is 20 milliseconds, the signaling speed is 50
bauds. In a bit stream, the number of
bits occurring per unit time is usually expressed in bits per
second or baud.
Beacon
Stationary transmitter that emits
signals in all directions also called a non-directional beacon. In
DGPS, the beacon transmitter
broadcasts pseudorange correction data to nearby GPS receivers
for greater accuracy.
Bearing
The compass direction from a position
to a destination. In a GPS receiver, bearing usually refers
to the direction to a waypoint.
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 atmospheric
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 multipath disturbance (which may be
considered a transient, unmodelled
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 Biphase Modulation
Phase changes of either 0 or 180
degrees on a constant frequency carrier (representing a binary
0 or 1, respectively). GPS signals
are biphase modulated.
Binary Phase Shift-Key
Modulation (BPSK)
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).
Binary Pulse Code Modulation
Pulse modulation using a string of
binary numbers (codes). This coding is usually represented by
1’s and 0’s with definite meanings
assigned to them. Examples include changes in phase or
direction of a wave.
Block I, II, IIR, IIF
Satellites
The various generations of GPS
satellites: Block I were prototype satellites that began being
launched in 1978; 24 Block II
satellites made up the fully operational GPS constellation declared
in 1995; Block IIR are replenishment
satellites; and Block IIF refers to the follow-on generation.
Bps (Bit per Second)
.
à
see Baud
BPSK
à
see Binary Phase Shift Key Modulation
