(^52) The Official Raspberry Pi Projects Book raspberrypi.org/magpi
hile building a rugged
robot, named Big Rob, for
outdoor use, Ingmar Stapel
wanted to create a system for
precise navigation. Disappointed
with the accuracy of the standard
USB GPS modules used on his
previous robots, he decided to
build a navigation system based
on differential GPS.
Differential GPS uses the RTK
(Real Time Kinematic) method for
carrier-based ranging between
a base station and the robot.
Data sent from the base station
Projects SHOWCASE
Guide robots with incredible accuracy using this differential GPS system
The robot calculates its precise
position using differential
GPS, and can direct itself to
various waypoints
SATNAV
FOR ROBOTS
W
INGMAR STAPEL
Ingmar has been building Pi-
powered robot cars since 2012.
He’s also working on a particulate
matter sensor.
custom-build-robots.com
enables the robot to correct the
inaccuracies in its own GPS signal,
mainly caused by atmospheric
effects, in order to calculate its
precise position. This technique
produces an accuracy of 20 cm,
compared with 4–5 m for
standard GPS.
Both units are equipped with a
RasPiGNSS Aldebaran module from
drfasching.com: “It is the only
one I know which is especially built
for the Raspberry Pi,” explains
Ingmar. They are also fitted with
a Tallysman GNSS antenna that
can receive not just GPS signals,
but also those from the Russian
GLONASS system, as well as SBAS
systems such as Europe’s EGNOS.
XBee or WiFi?
To minimise GPS inaccuracies,
Ingmar is using the RTKLIB Python
library (rtklib.com) to perform
the RTK calculations. “The most
complex part of this project was to
configure the RTKLIB library and
to set up the XBee communication
between the base station and
the robot.” The main advantage
to using XBee Pro rather than
standard WiFi is its superior
range of up to 1.6 km. However,
Ingmar has also tested the system
using WiFi, which offers a higher
bandwidth. Both versions of the
RTKRCV program are described on
his blog.
The differential
GPS system
is accurate
to 20 cm
The robot uses
three Python
programs
€6 IKEA
lunch boxes
protect the
base stations
Big Rob has
a 12 V 30 Ah
battery
See videos
of Big Rob at
magpi.cc/
2rPfEnz
Quick
Facts
The base station sends its own
position to the robot to eliminate
GPS inaccuracies caused by
atmospheric effects
The two units are in constant
communication via XBee modules
or standard WiFi