You may have seen my earlier post "Releasing the Shackles - How far do you want to go"about doing long range APM telemetry over cellular links. For the last 4 weeks I have been testing and have finally cracked the nut. Read on for details..
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The Fatbox is a heavy beast at 400+ grams. Removing it from its rugged steel case is straightforward and reduces the weight to ~120 grams. I ended up halving the case and using the bottom half as the mounting bracket in my plane.
Getting the Fatbox working on a network is fairly easy. You plug it into your LAN and configure it via its built in web server. There are all the standard settings you need to connect to any compatible cell network.
The most important thing is choosing the APN, by default most cellphones are setup to use an APN that is behind a firewall. A firewall prevents anyone from connecting to the device from the internet which is what you want for your cellphone, but not what you want for telemetry.
Here in NZ the default APN on most phones is internet.telecom.co.nz however if you call the Telco and ask for the details of a non-firewalled APN, they will tell you to use direct.telecom.co.nz. The same thing works on the vodafone network, you just use direct.vodafone.net.nz instead of internet.vodafone.net.nz. It might take a few calls and escalations before you can talk to someone who knows what they are talking about and what you want to do. You might also need to have the APN added to your SIM card depending on the provider.
If you use the right APN, you will get a public IP address that is fully routable on all ports. Perfect for Telemetry. It is my understanding that every carrier in the world has non firewalled APN's you just need to ask nicely to find out the details or do a quick google search. This costs nothing extra and although it doesn't give you a fixed IP address the address you get is fully routable and not firewalled so you can connect to it from any other computer on the internet with no restrictions. The fatbox has a built in Dynamic DNS client, so as soon as it is connected, it can update a DNS alias to point to its current dynamic IP address. This means you can register a DynDNS name such as myplane.dyndns.organd then connect to it without needing to know the IP address. If your fatbox drops its connection, as soon as it comes back up, it will get a new IP address and register it with DynDNS to allow you to continue operating (other dynamic DNS providers are supported).
This works in practice as well as it does in theory. After a power cycle, I was able to connect to my fatbox over the internet after around 12 seconds.
The next challenge was getting the serial port to talk to the APM. This turned out to be a little trickier than I expected.
I ordered a Sparkfun TTL level shifter but because of the way these work (voltage stealing), it didn't work at all. There were lots of dropped packets and CRC errors. I ended up ordering a Solarbotics DTE which I did get working reliably.
I connected the Fatbox up to the APM via the DTE converter, configured the Fatbox serial port to 57k and exposed the serial port as a server on TCP port 5760. Then I loaded up APM planner and changed the drop down list from the serial port to TCP. The host name is your DNS name (or the IP address if you know it) ie myplane.dyndns.org. The port stays the same at 5760.
It connected and I was able to do everything in the Mission Planner that you can normally do. Set waypoints, configure the APM etc.... I had to double check there were no cables in the line as I couldn't believe how well it worked - Woohooo.
On to even greater things...
The Fatbox also has two ethernet ports which can be routed at the same time as the serial port. I was interested in using one of the Ethernet ports for a Pan/Tilt/Zoom IP camera to give me unlimited range video (now you can see why I ordered the HSUPA version of the fatbox).
I configured the second ethernet port as a DMZ and then opened pinholes in the Fatboxes firewall so that all the IP camera ports were exposed to the Fatboxes internet facing interface. I tried it with a Vivotek PZ6122 which is a great little camera (that is no longer made), it weighs 300 grams, has Pan, Tilt and 10 x Optical Zoom. Its hard to find a high quality PTZ IP camera in a lightweight package but this fits the bill nicely and I picked it up for a steal on ebay. This camera also works in low light and automatically changes to black and white in low light conditions. I reduced the framerate down to 15fps and set it to a fairly low bandwidth setting and it worked very smoothly over the cell link.
Now I have it all working on the ground, the next step is to get it airborne and see what sort of altitude I can get before I lose cell link. I'm fairly confident it will go to 400ft which is as high as I can legally go anyway, but more importantly, I'm hoping to get some very good range out of it. With the combination of high mountains we have here in NZ (which forces Telco's to be able to operate at altitude) dual antennas on the plane (with short cables) and the high bandwidth of the fatbox, it should be able to go significantly higher than 400ft. NZ has outstanding cell coverage so this should allow me to fly long range missions just about anywhere in the country. The Hugin airframe can lift a lot of weight so I should be able to load it up with 15,000mAh of 4S batteries for a 90 to 120 minute endurance depending on speed.
I am talking to the manufacturer of the Fatbox about maybe putting in an order for a few of these and customizing them for DIY drones use. The customized version would come without a case and have direct TTY level outputs to save messing around with a serial converter. Let me know how many people might be interested in this to see if it is worth the time.
My next post will be to report the success of the first cellular flight.
I'm also really keen to find a trustworthy person to try controling the plane from overseas once I have everything working properly, a pilot could operate it via the APM planner and see what is happening over the internet using the onboard camera. It seems like something that just has to be done!!