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 Artem on October 3, 2014 at 4:55pm

Yes, it would be really good if we could reduce latency to ~70ms this will be faster than any human reaction. Also would be a good to see if the same latency (140ms) would be possible with just a 1W wifi MIMO dongle and RPi and the same kind of dongle on the ground. I have done some research and found some units with good reviews. 

Comment by MD on October 3, 2014 at 5:20pm

Completely agree. Been looking at the gstreamer parameters to see where optimization could occur but I only just started diving into this. 70ms would be great. Also, what dongles are you looking at, that might be a something to try - I just like the idea of directional antennas on an antenna tracker but still would be a good proof of concept - happy to try the dongles as well. Will post some results hopefully soon. Still tweaking parameters and looking at differences in over clocking, etc..

Comment by Artem on October 3, 2014 at 5:34pm
Comment by Patrick Duffy on October 3, 2014 at 9:23pm

@Artem, I flew FPV last Sunday with this latency, and I hardly noticed it. Didn't go 70+mph in the GoDiscover, as I don't think it is stable at that speed, but kept it between 35 and 50.  I tend to fly at altitude, but I know some folks fly very close to the ground for the 'effect' and close to objects, but even at that speed, the human eye/brain cannot react in 140ms to very much. Imagine going 70mph in a car and tailgateing the car in front of you by 6 meters. Not very safe.  If your main concern is fast flying, stay with analog video or pay big bucks for a system that will allow you to FPV at high speed. This is a $700 project, cheaper than some HD links, let alone the entire plane.

Comment by Patrick Duffy on October 3, 2014 at 9:30pm

Another note on improving the latency. The ground PC I am using is a modest laptop, not a 'gaming' pc that has a lot of horsepower. It could be that the 70ms you are looking for is in the ground PC, and not the PI or the link. My original experiment was without the Rocket M5/Nano M5, but with a Cat5 cable connecting the PI to the PC, and the latency didn't change, so I don't expect you will get any boost by trying a different WiFi link. I have a high-end PC at work that I can borrow and give this a try to see if changing the ground PC hardware makes a difference.

Comment by MD on October 3, 2014 at 11:41pm

@Artem - thank you. I will check it out.

Ok, so I've been doing quite a bit of testing. So far I can consistently get 109-111ms and then over clocked to turbo mode 101-110ms. Here is a photo. Sorry for the quality, i was try to take a photo and hold the camera, etc. I will do some more testing. This was on my mac to I want to see the difference on my windows as well. Also, the pi was plugged via ethernet into the router and then the mac was on wifi so essentially it would be the same scenario with the rocket m5 when i set them up. That being said, this is not definitive even though the results are demonstrable. I want to see it in action or some long range ground testing but it's a start.

Comment by MD on October 3, 2014 at 11:57pm

@Artem - have you seen any wifi adapters that are 5ghz? I took a look at the one above and a few more and was unable to find any that operated at 5.8. Also, in the specs they show that power decreases substantially depending on the band used which is the case for most access points I've looked at but it's quite severe in this case and the others like this. 1 watt ends up really being .2w That being said, with the right adapter there is definitely potential with this avenue. I'll post any findings but I am going to continue on testing the pi setup first.

Comment by Artem on October 4, 2014 at 9:07am
@MD In my opinion 2.4GHz is much bettter due to its better penetration and more than sufficient bandwidth. Also, antennas are much more common. I would go with linear polarization on 2.4 band.
Comment by Artem on October 4, 2014 at 9:11am
Unless youe are planning on flying in a city :)
Comment by MD on October 4, 2014 at 12:23pm

I'm building this at this point for really just testing purposes and as a complimentary system - going to keep using 1.3 for onboard analog video, etc - this is really to just see what kind of range and performance can be achieved out of a small package. I realize 2.4 would inherently have better penetration but I am trying to keep this as small as possible and the nice thing about 5.8 is of course the incredibly small antennas that let you run a separate system onboard a medium sized airframe without taking up too much room. 

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