IEEE Robotics & Automation Magazine - September 2012 - 98

Radiomodem
at 436 MHz

vehicle its fundamental navigation
data (position, course, and speed).
The result is that the slave follows
the actual path of the master at less
errors than in the intercalibration of
the GPS receivers mounted on the
two vehicles. To experimentally
evaluate the amount of this intercalibration error using GPS devices of
different classes, dedicated preliminary tests have been performed.

ALANIS
Control
System
Local LAN

Charlie
Control
System

ALANIS
HCI
Radiomodem
at 169 MHz
Wireless LAN at 2.4 GHz

Figure 4. Charlie-ALANIS network configuration.

(44°250 3200 N, 8°460 4800 E), at the end of July 2009, in a day
with no significant wind disturbance. As shown in the following, the presence of white buoys delimiting the lines of
the regatta field has been very useful for visual groundtruth evaluation of the system performance.
Experimental Results
Field trials, aiming at validating the proposed approach,
have been carried out with the slave Charlie USV following
the master ALANIS vessel piloted by a human operator. As
discussed previously, the master vessel sends to the slave

Charlie
HCI

GPS Performance
To evaluate the GPS performance, in
terms of measurement noise and
time-variable offset between two different devices, the measured range
between a couple of GPS antennas
positioned at a constant distance has
been evaluated. Indeed, since the main goal of the guidance
task is to force the two vehicles to navigate along the same
path, a constant bias in measurements carried out by different devices is required. Preliminary tests, performed in
the framework of the ALANIS project, demonstrated that,
using different conventional low-cost devices the difference between simultaneous measurements of position
could be of the order of some meters. During the experiments, three devices, i.e. a Garmin GPS 152, a GPS Ashtech GG24C, and a Trimble GPS Pathfinder Pro XRS,
made available by the Hydrographic Institute of the Italian
Navy, were mounted on the ALANIS USV maneuvering
inside the harbor.

d (m)

Position Difference: Measurement Noise
5
0
−5
600

700

800

900

1,000

1,100

1,200

0.1
d (m)

(a)

5
0
−5
600

700

800

900

1,000

1,100

d (m)

5
0
700

800

900 1,000
Time (s)
(c)

1,100

1,200

Figure 5. The measured distance error between different GPS
receivers. (a) Garmin versus Ashtech, (b) Garmin versus Trimble,
and (c) Trimble versus Ashtech.

*

IEEE ROBOTICS & AUTOMATION MAGAZINE

*

SEPTEMBER 2012

300

400

1,200

Position Difference: Measurement Noise

98

0
−0.1
200

(b)

−5
600

0.05
−0.05

Number of Samples

d (m)

Position Difference: Measurement Noise

Omnistar HP−8300
Position Difference: Measurement Noise

100
80
60
40
20
0
−0.1

−0.05

500
600
Time (s)
(a)

0
d (m)

700

0.05

800

0.1

(b)
Figure 6. The measured distance error between two identical
Omnistar HP-8300 high-positioning GPS receivers. (a)
Measurement record in time and (b) range histogram.



Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2012

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