How can I reduce drift in my robot's trajectory?

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The Problem (TL;DR)

My robot used to travel in straight lines between point A and point B. Now it doesn’t. I want to figure out why and how I can make it do that again by forcing the local path planner to follow the global path as closely as possible. I've done my best to summarize the problem here, but if you don't want to read an essay skip to the solution to this ROS question where I've re-stated the problem more clearly, and the work-around.

The Problem (full explanation)

If a robot travels directly between two points and there are no obstacles in the way, then its trajectory between those points should be a perfectly straight line (in theory). My robot used to behave like this, as shown below:

And now it doesn't, it consistently follows a bowed/curved trajectory every time. The radius of the curve changes but otherwise the trajectory is identical. The final stretch back to the starting point also has a funny kink it. Example:

Now before I go any further, I do understand that this problem is largely aesthetic and doesn't matter. And I know that I could work around this by adding more goal points and breaking the longer lines down into a series of shorter ones. However the fact remains that I can't currently explain this behaviour and I'd like to solve the problem if only for my own education.

So I would like to correct this behaviour and answer a few questions

Here is all the relevant system information:

System Information

• OS: Ubuntu 14.04 (laptop), ROS Indigo Igloo (robot)
• Robot: Clearpath Robotics Jackal UGV (real, not simulated)
• Sensors: IMU, wheel encoders, high accuracy RTK GPS

• Localization: package used is robot_localization. There are two instances of ekf_localization_node (local and global). IMU and encoder data is fused in the (local) /odom frame. IMU, encoder and GPS data is fused in the (global) /map frame - this is the frame /move_base uses for navigation. I can provide the specific launch files if requested.

  The output from the global EKF is always accurate and doesn't drift. The local output accumulates error over time and tends to drift quite badly by the end of a run.

• Navigation: I use the jackal_navigation package (version 2.2.2) to set up my navigation stack. I use the odom_navigation_demo.launch with the following parameter changes to local_base_planner and costmap_2d:

  1. xy_goal_tolerance increased to 0.35 m
  2. heading_scoring = true (false by default)
  3. Costmap size increased to 120x120 m
  4. All instances of any "global_frame" parameter have been changed to /map.
  5. All map (local and global) "update_frequency" parameters reduced from 20 Hz to 10 Hz
  6. Reduced "controller_frequency" and "planner_frequency" parameters from 20 HZ to 10 Hz

  To the best of my knowledge, these are the only parameters that have changed between the two graphs shown above.

  The full navigation parameter files are here

  Currently, in all cases the global path is perfectly straight ...