Difference Between Velocity Vectors
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?
Comments (7)
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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?
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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.
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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.
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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
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That's exactly right. Add another structure to the union in in dispatch.h, and add another case to the switch in node.cpp
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- changed status to resolved
GPS time reference was added in commit 7ada9da
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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
:It looks like the nav_msg/Odometry message should be in local
child_frame_id
, but it is also in the globalframe_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.