Page 118 - Introduction to Autonomous Mobile Robots

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Perception
The GPS receiver clock is also important so that the travel time of each satellite’s trans-
mission can be accurately measured. But GPS receivers have a simple quartz clock. So,
although three satellites would ideally provide position in three axes, the GPS receiver
requires four satellites, using the additional information to solve for four variables: three
position axes plus a time correction.
The fact that the GPS receiver must read the transmission of four satellites simulta-
neously is a significant limitation. GPS satellite transmissions are extremely low-power,
and reading them successfully requires direct line-of-sight communication with the satel-
lite. Thus, in confined spaces such as city blocks with tall buildings or in dense forests, one
is unlikely to receive four satellites reliably. Of course, most indoor spaces will also fail to
provide sufficient visibility of the sky for a GPS receiver to function. For these reasons, the
GPS has been a popular sensor in mobile robotics, but has been relegated to projects involv-
ing mobile robot traversal of wide-open spaces and autonomous flying machines.
A number of factors affect the performance of a localization sensor that makes use of
the GPS. First, it is important to understand that, because of the specific orbital paths of the
GPS satellites, coverage is not geometrically identical in different portions of the Earth and
therefore resolution is not uniform. Specifically, at the North and South Poles, the satellites
are very close to the horizon and, thus, while resolution in the latitude and longitude direc-
tions is good, resolution of altitude is relatively poor as compared to more equatorial loca-
tions.
The second point is that GPS satellites are merely an information source. They can be
employed with various strategies in order to achieve dramatically different levels of local-
ization resolution. The basic strategy for GPS use, called pseudorange and described
above, generally performs at a resolution of 15 m. An extension of this method is differen-
tial GPS (DGPS), which makes use of a second receiver that is static and at a known exact
position. A number of errors can be corrected using this reference, and so resolution
improves to the order of 1 m or less. A disadvantage of this technique is that the stationary
receiver must be installed, its location must be measured very carefully, and of course the
moving robot must be within kilometers of this static unit in order to benefit from the DGPS
technique.
A further improved strategy is to take into account the phase of the carrier signals of
each received satellite transmission. There are two carriers, at 19 cm and 24 cm, and there-
fore significant improvements in precision are possible when the phase difference between
multiple satellites is measured successfully. Such receivers can achieve 1 cm resolution for
point positions and, with the use of multiple receivers, as in DGPS, sub-1 cm resolution.
A final consideration for mobile robot applications is bandwidth.The GPS will generally
offer no better than 200 to 300 ms latency, and so one can expect no better than 5 Hz GPS
updates. On a fast-moving mobile robot or flying robot, this can mean that local motion
integration will be required for proper control due to GPS latency limitations.