The Raspberry in my lunchbox



Technical Alert. Some may be un-interested.


AIS (automatic identification of shipping ) is the system whereby larger vessels are required to continuously transmit their position, course speed etc along with ID information and is a major advantage in collision avoidance.
This project started as an attempt to get real time AIS data to be shown on an Android device on my boat.
CVB_T RTL SDR Dongles are electronic devices that were originally produced in large volumes as converters for digital television broadcasting. However as new TVs came with the required hardware ready installed there is a glut of these devices on the market and they are therefore very cheap.
It was then discovered that these very powerful bits of electronics had a host of other potential uses, particularly Software Defined Radio, in which role it emulates the gear that radio hams have been playing with in lofts throughout the world for many years.
It was then discovered that they could be tuned (by the software) to receive the AIS frequencies in the VHF range of 161.975 MHz (AIS1, or channel 87B) and 162.025 MHz (AIS2, or channel 88B).
As these Dongles are very cheap, and they can be used to replace proprietary equipment costing many hundreds of pounds, my Scottish antennae were raised and I bought one.
I downloaded several android apps that purported to interface with the CVB_T RTL SDR device. However I could not get any of them to work. One of the issues might have been that each Dongle needs to be calibrated to take account of its receiving offset, measured in PPM (parts per million).
I downloaded various bits of radio ham software to play with the device on my desktop. The best is a program called HDSDR written by a German chap called by Mario Täubel. Using this I managed to confirm the device I had bought was working and I calculated the PPM offset (29 for my device).
However I could still not get it to show any signs of receiving with any of the Android apps I tried, including trying each on several devices.
I wondered if the tiny antenna that came with the Dongle was up to the task, even though I received signals using HDSDR on my desktop. So I constructed a full scale antenna (vide infra) and connectors. This confirmed the device was OK but the android approach was not working.
A few months previously I had bought myself a Raspberry Pi computer to play with. These remarkable little devices are now very powerful computers with Quad core processors running at 1.4 GHz, 1Gb of RAM, Wifi, Bluetooth, Ethernet and 4 USB ports for connectivity. All for a bargain price.
I decided to investigate whether the Raspberry Pi and CVB_T RTL SDR might form the basis of a stand alone AIS receiver.
The Raspberry Pi can run a range of operating systems but the most developed is Raspbian which is an extrapolation of the Debian flavour of Linux. So I entered the world of Linux terminal command code! This mostly involves searching forums and websites dedicated to the subjects and following other peoples instructions. As often as not this results in the system turning into a brick so it is a good idea to have a series of SD cards with copies of the last good iteration of the project available to rescue the situation.
The guys who inhabit this world are different. Mostly geniuses, some are a bit obtuse and it can be difficult to follow them as a novice however others are brilliant at giving instructions that lay people can follow. This project has served as a rapid induction into the world of Linux code, although I am still at the foothills of a steep and long learning curve.
This guy gave me the secrets that worked http://pysselilivet.blogspot.com/2018/06/ais-reciever-for-raspberry.html .

So the equipment for the project:
RASPBERRY Pi 3 B PLUS
£28.50

DVB-T DAB FM RTL2832U & R820T Tuner


Mini USB RTL-SDR Receiver Stick FT

£7.40

MCX MALE RIGHT ANGLE to UHF Female


 SO-239 Jumper pigtail RG316 Cable 15cm


£1.62
This is required to connect the boat’s VHF antenna to the tuner Dongle.


A RC Ubec 5V 6V BEC Full 5A Shielding


Antijamming Switching Regulator New MK


£3.14

I found the standard micro usb phone charging connectors were not supplying enough juice to run the Raspberry Pi plus the GPS Dongle plus the RTL-SDR Dongle.
This adaptor is primarily for model aircraft servos and delivers a regulated 5V at up to 3 amps. It connects direct to the GPIO connectors of the Pi, pin 2 is 5v and pin 6 is ground. This means the wires going into the connector have to be adjusted and then it plugs straight in. This seems to have solved the 12v powering issues.

Mini 8GB Micro SD Card TF Flash Slim


 Memory


£2.72
The SD card was loaded with Raspbian (Jessie Stretch version) and then the most recent version of Open CPN open source chartplotter added.
The port setting and network address need to be added at the OpenCPN “connections” page to coincide with the setup entered under Pyssel’s instructions.
You can test the Dongle is running by entering a command in Terminal ( Sudo systemctl status AIS) - good eh!

The whole lot fits quite neatly into a Tupperware found under the sink, also using a 12v connector from the garage soldered to the 5v regulator and a GPS dongle I happened to have for the boat laptop.


RG58 Coaxial Cable 20 Meter 

 

Fitted PL259 

 


and BNC Plug


£15.98

pack of 3 x Conduit Rigid 1 meter length

 

 White 20mm PVC Heavy Gauge


£5.98


I got this kit to construct an antennae. The plan is to have a separate antenna on the pushpit rail which will act for the AIS and also as a reserve in case of dismasting!!

Basically you measure out the required length of cable. VHF is in the 2m range so most antennae are 1m length in total to be ½ wave receivers.
The outer insulation of the coax is removed from the top 0.5m. The inner insulated core is then teased out from the outer shielding and the shielding pulled back down and taped to the lower 0.5m. The whole lot can then be passed up inside the white plastic tube and the ends sealed. There are no connectors to reduce signal and my preliminary experiment shows it works really well.
This lot will make two antennae.

Coax antenna - No connectors!



Critical step in antenna making

I am pleased to say it is all working fine.
I have also set up the Raspberry Pi with VNC host and an old android tablet with VNC viewer. The tablet then acts as a Wifi hotspot and access the Pi directly so there is no need for another screen.
This is (I think) the minimum power usage to get an AIS receiver working. It is also pretty much the minimum cost at £53.36.


Here is the Pi in it’s Tupperware powered up in the electrics cupboard.











The tablet can run the Navionics at the same time as headless control of the Raspberry Pi in the cupboard, and playing music. I have picked up vessels 16nm away,and several dangerous approaches before I had seen the ship.





The mounted antenna on the pushpit.
Screenshot of AIS working










Comments

Post a Comment

Popular posts from this blog

Getting Excited

Image
This is my first foray into the world of blogging. I am not sure if anyone will actually read this other than a few friends and family. Alternatively it could go viral, make loads of cash and have me wondering why I ever worked in the first place.  The first thing I have to do is get my head round the technology side of things, which I have to say I am struggling a bit with.  I am using Blogger app on an android phone and a bluetooth keypad. All the hardware is working OK but the software is a bit tricky to get the hang of. This is about my third attempt at getting started and text seems to vanish.  I have resorted to using the PC which is a lot easier, but means you need to import photos etc. The background is that Margaret and Colin (me) have retired over the last year Margaret finished work about a year ago. She seems to have slipped into being a retired person like a foot into a silk sock. She has just returned from walking the West Highland Way with lady frien...
Read more

Beer, Crisps and Bananas

Image
Greetings to Family and Friends, ex colleagues and anyone else who is interested. We have set off on another sailing season and hope to keep the blog that we started five years ago going. It is becoming more difficult to be novel in what we are about, because, well, its not any more. However the need to keep a record for our own purposes is increasingly required as the number of small grey cells declines with each passing year. We set off from Swansea bus station on the red eye National Express coach. It was quiet until we got to Cardiff when the contents of the Millenium Stadium got on. It seems there was a concert of some sort. They were very well behaved youngsters of various generations, but very excited and somewhat noisy. The passenger cohorts on buses are always different from trains. You have to be significantly wealthy to travel by train these days so bus dwellers are more representative of, well, the man on the Peckam Omnibus. I am not sure I get blokes wearing purple nail va...
Read more

Reinforcements

Got up this morning and checked that our new friend Solar, was happy and indeed he was. With the rise of the sun, the little smile had come to his face (on the controller) and he was gaily charging the batteries away. In need of further feel good I checked again a couple of hours later and he had stopped. I argued that he was behaving normally in not overcharging the batteries but eventually this did not make sense. On testing the output it was not right 😞. Voltage direct from the panel varying from 0 to 18v depending on position, degree of flexing the panel and which way the wind blows. Having messed about with it for several hours I have come to the conclusion it is FUBAR. I cannot work out what the issue is - overheating is a known problem but I am surprised it is that as that might cause a drop in performance not complete failure. It was working fine so it is unlikely there has been mechanical damage in transit or handling. However I am where I am. I will try to source one to...
Read more