Where am I? Sensors and Methods for Mobile Robot Positioning

72 Part I Sensors for Mobile Robot Positioning
SPACE
CONTROL
USER
Monitor
Stations
Uploading
Station
Master
Station
Figure 3.4: The Navstar Global Positioning System consists of three fundamental segments: Space, Control,
and User. (Adapted from [Getting, 1993].)
positional accuracy is understandably somewhat degraded. The Y code (formerly the precision or
P code prior to encryption on January 1st, 1994) is transmitted on both the L1 and L2 frequencies
and scrambled for reception by authorized military users only with appropriate cryptographic keys
and equipment. This encryption also ensures bona fide recipients cannot be “spoofed” (i.e., will not
inadvertently track false GPS-like signals transmitted by unfriendly forces).
Another major difference between the Y and C/A code is the length of the code segment. While
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the C/A code is 1023 bits long and repeats every millisecond, the Y code is 2.35×10 bits long and
requires 266 days to complete [Ellowitz, 1992]. Each satellite uses a one-week segment of this total
code sequence; there are thus 37 unique Y codes (for up to 37 satellites) each consisting of
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6.18×10 code bits set to repeat at midnight on Saturday of each week. The higher chip rate of 10.23
MHz (equal to the cesium clock rate) in the precision Y code results in a chip wavelength of 30
meters for the Y code as compared to 300 meters for the C/A code [Ellowitz, 1992], and thus
facilitates more precise time-of-arrival measurement for military purposes.
Brown and Hwang [1992] discuss a number of potential pseudorange error sources as summarized
below in Table 3.3. Positional uncertainties related to the reference satellites are clearly a factor,
introducing as much as 3 meters (9.8 ft) standard deviation in pseudo-range measurement accuracy.
As the radiated signal propagates downward toward the earth, atmospheric refraction and multi-path
reflections (i.e., from clouds, land masses, water surfaces) can increase the perceived time of flight
beyond that associated with the optimal straight-line path (Figure 3.5).
Additional errors can be attributed to group delay uncertainties introduced by the processing and
passage of the signal through the satellite electronics. Receiver noise and resolution must also be