Part 2: How to build a High-Definition FPV UAV using a Rasperry PI with HD camera, using a high speed WiFi link

Part Two: Here is the original picture of the finished product:

This is the second part of a 2-part series on 'How to build a High-Definition FPV UAV using a Raspberry PI with HD camera, using a high speed WiFi link.

In my first post on the subject (located here), I discussed the parts I used, and how to install them into a Hobby King Go-Discover FPV model. 

In this post, I will discuss installing the Raspberry PI and the PI camera in the Go-Discover gimbals, and the software configuration for both the Raspberry PI and the ground station PC.

From the previous post, step 3 was completed by installing the Ubiquity Rocket M5 in the model.  Now onto step 4:

Step 4: Install the Raspberry PI and PI Camera

Here is a photo of the position of the PI in the Go-Discover model:

The PI fits nicely just behind the camera gimbals, with the USB and HDMI ports on top. In the right side you can see the Cat5 network cable attached. This cable connects to the ethernet switch, which is also connected to the Rocket M5 input port.  

The two cables shown on top are the servo control wires for the gimbals, which I have directly connected to channel 4 and 5 on my radio.  I am using channel 4 (normally the rudder stick on my radio. Since there is no rudder on a flying wing, this is a convenient channel to use to move left and right with the camera. I have not (yet) moved to a head tracker, but if you already have that setup, just assign the channels accordingly.

To install the PI camera, remove the stock plate from the gimbals (for a GoPro), and mount the PI camera as shown in this photo:

The PI camera case fits very nicely into the slot, and again I used a small piece of velcro to hold it down. You could use a couple of small screws instead if you want a more secure hold.  The two gimbals servos are also shown here. They are simple to install, just follow the Go-Discover instructions.

Here is a front view of the PI camera installed:

Here is the block diagram describing all the connections:

Some comments on my previous post suggested that it is possible to eliminate the ethernet switch and serial-to-ethernet converter using the Raspberry PI and a serial port on the PI. I believe this post describes how to talk to the PI via the NavLink, but in this case, I want to use the PI to bridge the connection from the ground station to the APM/PixHawk. Somebody please comment on this if you know more about it.   I believe it would require a TCP/IP to serial link from the PI to the telemetry port on the APM, and some software on the PI to act as the bridge.  The main connection to the ground station is via the Rocket M5 and TCP/IP, not through a telemetry link (900 Mhz or Zigbee like I used on my other models).

Step 5: Getting it all to work with software configuration (the really fun part starts now).

Check out this post on what others have done with streaming and the PI.  My experiments showed that using GStreamer on both the PI and on Windows gives really good results with very low latency, if you use the right parameters. 

Get GStreamer on the PI by following this blog.   This is the same version of GStreamer that I am using on my setup. 

Make sure your PI camera works ok by plugging in the PI to a standard monitor using the HDMI port and follow the instructions on the Raspberry PI website on how to get the camera up and running (without GStreamer).  Once you have a working PI and camera, you can then proceed to stream things over the network.  

Note: It is suggested that you first get the PI streaming video by plugging it directly into your local network where you can also connect your ground station PC with the correct IP addresses (without the Rocket M5).   For my PI, I picked 192.168.1.2,  and for the ground station, 192.168.1.1.    Make sure you can ping the PI from your PC and the PC from the PI.  

For streaming, you will also have to make sure all the ports you intent to use are open on the firewall (described later).

For the ground station PC,  you can download GStreamer here.  Make sure when you install, select to install everything , or full installation (not the default). 

Here is the command I use for the PI to pipe the camera output to GStreamer:

raspivid -t 0 -w 1280 -h 720 -fps 30 -b 1700000 -o - | gst-launch1.0 -v fdsrc ! h264parse config-interval=1 ! rtph264pay ! udpsink host = 192.168.1.1 port= 9000

The command is explained as follows:

raspivid is the command to start the camera capture on the PI.  The -w switch is for the width in pixels, and the -h switch is for the height.  In this case, I am using 1280 X 720, but you can try any combination that fits your needs. 

The -b switch is the bit rate for the sampling. In this case I chose 1.7mbs to send over the stream. Again you can experiment with higher or lower values. This settings seems to work good for me, and the latency is almost unnoticeable.  

the "-o - |" is piping the output to gstreamer.  Make sure you include the dash before the pipe "|" symbol. 

For the GStreamer command, all the filters are separated with an exclamation point "!", as these are individual drivers that are part of GStreamer.  Since the PI has hardware accelerated video, the output is in a format called "H264", which is a highly-compressed stream. The GStreamer filters are configured to transport the output via a UDP socket connection to the target PC. Notice the 'udpsink' element which specifies the host - in this case your ground station, and the UDP port.  I am using port 9000, but you can use any open port on your system, but be sure to open the firewall or it won't work!  You can also use TCP instead of UDP, but for such a data stream, I chose to use UDP since dropouts are certainly possible, and with UDP this is ok, but with TCP, you could have socket problems and higher latency. 

Note: to get the PI to execute this command on boot, make a shell script with the above command and add it to your local.rc boot sequence. That way when the PI boots, you get the stream without having to log into the PI remotely. 

For the ground station PC, once you have installed GStreamer and opened the correct ports, use this command (from the command prompt) to view the stream:

c:\gstreamer\1.0\x86_64\bin\gst-launch-1.0 udpsrc port=9000 ! application/x-rtp,encoding-name=H264,payload=96 ! rtph264depay ! avdec_h264 ! videoconvert ! autovideosink

If all goes well, you should see the PI camera output on your PC screen in a popup window.  For those of you what want to use FPV goggles, you can connect to the HDMI port on your PC to display the output if your goggles support HDMI. 

I have this command in a batch file (with a PAUSE) statement at the end to keep the window open.

WHEW!  If you got this far, you are amazing. 

The last step to complete the build is to connect to the APM from mission planner.  The method I used to connect was to install a utility that converts a TCP connection to a virtual serial port, but I also think that directly connecting the mission planner to the TCP port will also work, however I have not tried it. I will post back later after trying it.

Here is the link to setup the serial to ethernet device to have an IP address and port.

Here is the link to the configuration utility for installing the virtual serial port.   

Once you have a serial connection over TCP/IP working to the APM, you should be able to connect with Mission Planner. On the maiden flight, it worked perfectly, and I didn't see a single drop in the telemetry data or anything noticeable in the video transmission, however my first flight was limited to 2km.

The last step is to connect the Rocket M5 to the Nano M5 and test everything using the OTA (over the air) connection. If all is well, you are ready to fly!  But be careful on your maiden, you just spent $700. 

Finally, here is a photo of my Antenna Tracker with the Nano M5 attached. My next update will include a video of a longer flight.  

Happy Flying!

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Comment by Patrick Duffy on October 1, 2014 at 7:38pm

I tested it again using a lower res (720x480), I got 120ms.  I think the limit is the PI GPU. I tried the alternate pipeline suggest and got the same numbers.

Comment by MD on October 1, 2014 at 8:10pm
So what are the alternatives? Do you think a beagle board would decrease this? Anyone have any experience setting this all up on the Bbb? Camera would also be a consideration. I'm all for trying to get latency down below 100. Just waiting for m5s and antennas to arrive.
Comment by Steve on October 1, 2014 at 8:27pm
I would imagine rpi to be better than bbb due to its hardware acceleratiob for video. Maybe using camera that does h264 http://www.oz9aec.net/index.php/gstreamer/473-using-the-logitech-c9...
Comment by MD on October 1, 2014 at 10:59pm

@Steve - good point. Has anyone tried this setup or anything similar?

Comment by Dom on October 2, 2014 at 12:59am

I would have thought the pi camera would almost certainly be the fastest as it's on a dedicated high speed bus and hooks straight into the hardware acceleration of the cpu/gpu.  Any other camera like the logitech has to do the encoding on what is probably an inferior processor, and will have then have to pipe this data over a usb bus (which is notoriously poor on the pi).  In addition, the ethernet port on the pi actually runs on the usb bus (or at least used to, not sure if they improved this on the b+) so shares the bandwidth.


Developer
Comment by John Arne Birkeland on October 2, 2014 at 1:10am

If you can afford the bandwidth requirements, switching to 60fps video should help reduce latency.

It would also be worth looking into how the rpi h264 encoder buffers data. It's common practice to use relatively large buffer queues to increase efficiency and prevent frame jitter. But this also leads to increased latency in the encoder pipeline. This is also why switching to 60fps can have a big impact on latency, since each frame at 60fps is 16.667ms instead of the normal 33ms at 30fps.

Comment by John Rambo on October 2, 2014 at 8:15am

well, my RBPi normally runs at ~100ms in my setup (RBPi camera, full HD, 30fps,802.11n, 5 GHz, netcat + mplayer). Unfortunately, RBPi camera can't deliver more that 30fps in full-hd mode

Comment by Patrick Duffy on October 2, 2014 at 8:19am

Has anybody tried overclocking the PI? 

Comment by MD on October 2, 2014 at 8:55am
@john Rambo - what settings are you using specifically? Also what hardware?

@patrick - that's a really good idea. Interesting to see the effect it would have.
Comment by John Rambo on October 2, 2014 at 9:28am

my setup runs on Mikrotik products: STX 5hND + Groove A-52Nh(72grams w/o enclosure). Awesome stuff for a fraction of Ubiquiti cost. SXT has an USB port which gives UART option, plus firmware can encapsulate UART onto TCP/IP directly. With an USB hub I get highly resilient telemetry (3G + 433MHz + Ethernet). By using STX at both - sky and ground, 10Km is not a limit. 

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