mavros offboard no GPS

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Hi, have recently been trying to control a rover equipped with a Pixhawk2 in an indoor environment.
For this purpose, I have followed the steps outlined in the mavros_offboard tutorial. I also had to change a few parameters on the PX4 firmware to allow the rover to arm without a GPS lock etc..

After running a script similar to the tutorial one, I was able to switch to offboard mode, see below the echo of topic /mavros/state

header: 
  seq: 3105
  stamp: 
    secs: 1503352094
    nsecs: 242159569
  frame_id: ''
connected: True
armed: True
guided: True
mode: OFFBOARD
system_status: 4

However, the mavros node interfacing with the rover is printing the following output:

[ WARN] [1503352273.909538514]: GP: No GPS fix
[ INFO] [1503352280.350340503]: HP: requesting home position

I have tried to set this position with no success.

If any of you have managed to run a px4/apm robot in an offboard mode with no GPS I would be grateful for information on:

1. Telling mavros/pixhawk not to expect a GPS fix

2. Getting the tf topic to publish something (I guess this is related to 1.)

Thanks for any help.