[ros-users] robot_pose_ekf /vo input units

[Jack O'Quin]

On Tue, Apr 27, 2010 at 7:39 PM, William Woodall <wjwwood at gmail.com> wrote:
> We are trying to feed gps data into the ekf's /vo topic, and I have seen the
> tutorial for swapping visual odometry out for gps, but I haven't been able
> to find the units for x, y, z pose in the that Odometry msg.  I have poked
> around in the source a bit, and I haven't been able to find any clues to the
> units, though I imagine it is displacement in meters or something similar.
>  Our gps outputs lat, long, altitude which we need to translate, obviously,
> but I was just wondering if anyone could tell me the specific units the ekf
> is expecting.

We have an open-source ROS driver for the Applanix Position and
Orientation System for Land Vehicles (POS-LV) in the
utexas-art-ros-pkg repository. The device is a highly accurate GPS
(and IMU) that was used by many vehicles that competing in the DARPA
Grand Challenges.

Although the device interface is probably not useful to you, the ROS
part of the interface may be helpful. It translates the (spherical)
latitude and longitude coordinates into (Euclidean) Universal
Transverse Mercator (UTM) and publishes: nav_msgs/Odometry; /odom
transforms; and its own applanix/GpsInfo topic.

  http://www.ros.org/wiki/applanix.

The (x, y, z) Odometry points are in meters. Formerly, we used UTM
displacements from the start of the run, but more recently we found it
helpful to simply round the UTM easting and northing values to offsets
from a 10km UTM grid. That keeps the values consistent between tests
in the same area, while limiting them to a convenient size. It also
makes it easy to determine coordinates from a map.

It would be good for the ROS community to agree on a common GPS
message. Since none existed, we defined our own based on the Player
GPS info message.

  http://www.ros.org/doc/api/applanix/html/msg/GpsInfo.html
-- 
 joq