Difference Between Velocity Vectors

[Former user]

Hi, I'm trying to understand the difference between how the following two messages are calculated:

gps/vel/twist/linear.x gps/odom/twist/linear.x

The values are slightly different, and I'm wondering how they are found. I'm also wondering what are the units associated with these fields?

[Kevin Hallenbeck]

The units are meters per second (m/s) to comply with REP 103

Based on the code, the x/y/z values should be the same, however, the odometery messages has an extra frame_id:

geometry_msgs::TwistWithCovarianceStamped msg_vel;
msg_vel.header.stamp = stamp;
msg_vel.header.frame_id = frame_id_vel;
msg_vel.twist.twist.linear.x = (double)packet->vel_east * 1e-4;
msg_vel.twist.twist.linear.y = (double)packet->vel_north * 1e-4;
msg_vel.twist.twist.linear.z = (double)packet->vel_down * -1e-4;

nav_msgs::Odometry msg_odom;
msg_odom.header.stamp = stamp;
msg_odom.header.frame_id = frame_id_vel;
msg_odom.child_frame_id = "base_link";
msg_odom.twist = msg_vel.twist;

It looks like the nav_msg/Odometry message should be in local child_frame_id, but it is also in the global frame_id. This is a mistake.

I do not plan on changing the odometry message because I don't know how many users would be affected. Furthermore, I don't have an Oxford GPS to test with.

[Luke Armbruster]

Thanks. One more question. Regarding the uint16_t time in dispatch.h, is this value at the time of measurement for the Oxford GPS? If so, what is the time relative to and is there a more accurate value that I can get regarding the actual time the measurement was recorded?

[Kevin Hallenbeck]

Everything in dispatch.h is straight out of ncomman.pdf.

From page 6: Time is transmitted as milliseconds into the minute in GPS time. Range is 0 to 59,999 ms

From page 13: Time in minutes since GPS began (midnight 06/01/1980)

When a packet with channel 0 is received, those two values could be combined to get the full GPS time and publish a sensor_msgs/TimeReference.

I have no plans to implement this, but pull-requests are welcome.

[Luke Armbruster]

Can you also tell me what I have to do if I'm interested in getting fields from channels that are not defined in Packet from dispatch.h? i.e. I am trying to get the UTC time offset from channel 16.

[Luke Armbruster]

It looks to me like I would need to add the struct definition to dispatch.h, add another case to switch(packet->channel) in node.cpp

[Kevin Hallenbeck]

That's exactly right. Add another structure to the union in in dispatch.h, and add another case to the switch in node.cpp

[Kevin Hallenbeck]

GPS time reference was added in commit 7ada9da