Where am I? Sensors and Methods for Mobile Robot Positioning

68 Part I Sensors for Mobile Robot Positioning
phase relationships. The 58 MHz VHF signal allows for non-line-of-sight operation, with a resulting
precision of approximately 1 to 10 centimeters (0.4 to 4 in) [Duchnowski, 1992]. From a robotics
perspective, problems with this approach arise when more than one vehicle must be tracked. The
system costs $200,000 to $400,000, depending on the number of receivers used. According to
Duchnowski, the system is not suitable for indoor operations.
3.1.4 Motorola Mini-Ranger Falcon
An example of the active transponder category of ground-based RF position-location techniques is
seen in the Mini-Ranger Falcon series of range positioning systems offered by the Government and
Systems Technology Group of Motorola, Inc, Scottsdale, AZ [MOTOROLA]. The Falcon 484
configuration depicted in Figure 3.3 is capable of measuring line-of-sight distances from 100 meters
(328 ft) out to 75 kilometers (47 miles). An initial calibration is performed at a known location to
determine the turn-around delay (TAD) for each transponder (i.e., the time required to transmit a
response back to the interrogator after receipt of interrogation). The actual distance between the
interrogator and a given transponder is found by [Byrne et al., 1992]:
Display
unit
transceiver
Site 1
Site 4
Range processor
Optional
computer
Optional
plotter
Site 2
Site 3
Figure 3.7: Motorola's Mini-Ranger Falcon 484 R position-location system provides 2 m (6.5 ft) accuracy over
ranges of 100 m to 75 km (328 ft to 47 mi). (Courtesy of [MOTOROLA].)
D (T e T d )c
2
(3.1)
where
D = separation distance
T = total elapsed time
e
T = transponder turn-around delay
d
c = speed of light.
The MC6809-based range processor performs a least-squares position solution at a 1-Hz update
rate, using range inputs from two, three, four, or 16 possible reference transponders. The individual
reference stations answer only to uniquely coded interrogations and operate in C-band (5410 to 5890
MHz) to avoid interference from popular X-band marine radars [Motorola, undated]. Up to 20