offboard reject error

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Hi to all,

I am using Ros Indigo in Ubuntu 14.04. My mavros version is 0.16.4.0 as for pixhawk version it is latest one which I updated it on 20 Dec 2015.

For my offboard control project, I can using 3DR 433Mhz radio connected to pixhawk to transfer the code from computer to pixhawk.But I encounter a problem when switching to auto mode in RC controller, it tend to reject the auto mode and switch back to manual mode and back to auto mode again and back to manual mode . As the pixhawk light keep switching from green to blue back to green the back to blue again.

The frequency I publish to setpoint topics is 100Hz .And the topics published are /mavros/setpoint/cmd_vel

The code I type in is

roslaunch mavros px4.launch fcu_url:='/dev/ttyUSB0:57600'

then I type in rostopic echo -n1 /diagnostics

The code shown are

http://pastebin.com/17CEqdAQ

Next I type in rosrun hexrotor offboard The Roslaunch output is

http://pastebin.com/87JZ9nGL

It is working fine but after a while the error occur

[ERROR] [1449822927.011620861]: FCU: REJECT OFFBOARD

Is there any way to make the copter stay at auto mode ? Thanks