dAISy (do AIS yourself) is a very simple AIS receiver that I developed from scratch. It is built around the Silicon Labs EZRadioPRO Si4362 receiver, using a Texas Instruments MSP430G2553 MCU for processing and the MSP-EXP430G2 v1.5 LaunchPad as development platform.
The complete project source code and schematics are available on GitHub: https://github.com/astuder/dAISy
Update 5/18/2015: A finished, self-contained AIS receiver based on this project is now available for purchase in my web store.
AIS, short for Automatic Identification System, is a standard for tracking ships. Ships advertise their position, course and other information with short transmissions on specific frequencies (161.975 MHz and 162.025 MHz). More on Wikipedia.
An AIS receiver, like dAISy, receives and decodes AIS transmissions. It then re-packages the raw data into NMEA sentences (specifically formatted ASCII strings). Finally, using a serial connection, these strings are forwarded to more capable equipment for further processing.
If you're the lucky owner of a tricked out boat, you could connect dAISy to your navigation computer. For land lobbers like me, a more common use case is to run naval mapping software that supports AIS data input on a PC. In the screenshot below I've connected dAISy to OpenCPN (link), an open source chart plotter and navigation software.
On the top right you can see my setup war-driving at the Seattle waterfront as my lab is too far from the coast to receive anything. The LaunchPad sits on the dashboard with a white USB cable connecting to the notebook computer in the foreground.
dAISy's data is fed into OpenCPN, bottom right shows a log of the serial data received. OpenCPN maintains a database of all the collected data (lower left) and visualizes nearby ships on a map (top center), including past and projected course. Hovering the mouse over a ship will display its name (text on yellow ground) and clicking it will reveal more detail (top left).
I wanted to build my own, non-SDR, AIS receiver for a long time. There are a few projects floating around the internet (e.g. here) which refer back to an article by Peter Baston, published 2008 in Circuit Cellar magazine (copy available here gone.. google for Peter Baston Circuit Cellar to find other copies). Unfortunately, the CMX family of modem ICs by CMS Microcircuits (link) used in these projects are relatively expensive ($15+) and hard to find for hobbyists. In addition you'd need a radio to do tune into and down-convert from the ~162 MHz carrier frequency.
So I was quite excited when earlier this year a parametric search on Mouser brought up a new IC that covered the required range (162 MHz) and modulation (GMSK). And best of all, available in single quantities for $3.56 $2.27 $2.22! (link)
The Silicon Labs EzRadioPRO Si4362 (link) is a single chip receiver that covers frequencies from 142 to 1050 MHz and supports various modulations, including GMSK. It comes in a tiny 20-pin QFN package and the only external parts required are a 30 MHz crystal, an antenna with a few capacitors and inductors for impedance matching, and finally some decoupling caps and pull-down resistors.
Time to whip up a breakout board. I used the opportunity to give KiCad a try and quite like it.
Here's the schematic:
And the layout:
I used OSHPark to make the PCBs. At a smidgen over one square inch it cost $5.15 for 3 copies:
Note that the layout still has three issues that I already fixed in the schematic:
GPIO0 and GPIO1 were flipped
SDO required a pull-down resistor as the radio leaves it floating when not actively sending, which confused the hell out of me while trying to figure out the communication protocol.
Lastly, the holes for the headers turned out to be slightly too small to comfortably fit the cheap breakout headers I had at hand.
Edit: Here's Rev B where I fixed these issues: http://oshpark.com/shared_projects/WI6u3Qmk
Which brings us to the BOM:
Silicon Labs Si4362 (U1)
30 MHz crystal (X1)
Si4362 datasheet specifies <11 pF load capacitance, but a crystal specified for 12pF load capacitance seems to work fine too
Antenna/LNA matching network, calculated based on SiLabs AN643 (link, approx. values, +/- 5% shouldn't matter too much):
75 ohm (dipole): 10 pF (CR1), 5 pF (CR2), 280 nH (LR1), 200 nH (LR2)
50 ohm: 12 pF (CR1), 6 pF (CR2), 240 nH (LR1), 160 nH (LR2)
100 pF, 100 nF, 1uF (C1, C2, C3)
100 k (R1, R2)
First thing I noticed when I received the parts: The 20-pin QFN at 4x4 millimeters is tiny!
I mounted it by first tinning the pads with a small quantity of solder. I then added flux and placed the chip on the pad. I then used a hot air station to carefully reflow the solder. Worked the first time around.
After using jumper wires to figure out how to talk to the chip, I mounted the breakout board on a makeshift BoosterPack using perfboard, double-sided tape and wire (see picture at the top of the post).
Here's how I ended up connecting the breakout board to the LaunchPad / MSP430G2553:
SEL -> P1.4 (SPI chip select)
SCLK -> P1.5 (SPI CLK)
SDO -> P1.6 (SPI MISO)
SDI -> P1.7 (SPI MOSI)
GPIO0 -> P2.0 (I/O unused)
GPIO1 -> P2.1 (I/O clear-to-send)
GPIO2 -> P2.2 (I/O RX clock)
GPIO3 -> P2.3 (I/O RX data)
SDN -> P2.4 (shutdown / reset)
IRQ -> P2.5 (I/O channel-clear)
The software of dAISy consists of three major blocks:
Radio configuration and control over SPI
Packet handler, including a basic FIFO for received messages
NMEA encoding and transmission to the PC over UART
For UART (TX only) and SPI (TX/RX) I use the MSP430G2553's USCI A0 and B0 respectively. In both cases I don't use interrupts which simplifies things considerably.
Upon reset the following steps happen:
Initialize MSP430 peripherals
Initialize packet handler, which will also reset FIFO
Initialize and configure of radio, which will also setup SPI
Start packet handler, which will also put the radio into receive mode
And in the main loop:
If debug messages are enabled, poll packet handler for status and errors and report them over UART
Check FIFO for new packets
If there is a new packet, invoke NMEA processing (which sends the message over serial to the PC) and remove packet from FIFO
Below follows a more detailed discussion of the radio integration and the implementation of the packet handler.
The communication with the radio is vanilla SPI using 4 wires: MOSI (SDI), MISO (SDO), CLK (SCLK) and CS (SEL). I used the MSP430's USCI B0 to implement SPI and a separate pin to control CS.
The only tricky thing to figure out was, that the Si4362 keeps the MISO line floating unless it actively transmits data. This is unfortunate as the master is supposed to poll for a specific response (FF) to detect when the radio is ready to receive more commands. This is easily fixed by adding a weak pull down resistor to SDO. I did this on the board, but it probably also works with using MSP430's internal pull-down.
Additional lines I used to control the radio are:
SDN to reset the radio
CTS, which by default is mapped to the radio's GPIO1, indicating that the radio is ready for the next command
While taking up an extra pin, CTS turned out to be much more convenient than the SPI response code to properly time communication flow with the radio. In dAISy, I wait for CTS to go high after each command to ensure the radio completed its task.
The communication protocol is quite extensive but well documented:
EZRadioPRO API Documentation describes the complete API and all registers
AN633 Programming Guide for EZRadioPro Si4x6x Devices describes how to use the API in common scenarios
Both are available on the Si4362 product page (link), under Documentation > Application Notes and are still updated quite frequently.
The radio is set up by dumping a large configuration sequence into it. This includes configuration of radio frequency, modulation, GPIO pins and more. This information is stored in radio_config.h, which has to be generated with a tool called WDS (Wireless Development Suite). WDS is available in the Tools section on the Si4362 product site.
Above are the settings I used for dAISy. WDS will use this information to configure various amplifiers, filters, clocks and decoding algorithms inside the chip. As Si4362 supports GMSK encoding only indirectly (see this thread), I'm certain there's more optimization potential by tweaking registers, but that's currently way beyond my knowledge of RF theory.
While the Si4362 comes with its own packet handler, it unfortunately does not support NRZI encoding (Wikipedia). So I set up the radio to expose the 9600 baud clock and received data on separate pins and implemented my own packet handler.
The packet handler (inspired by Peter Baston's implementation) is implemented as a state machine that is invoked on each rising edge of pin P2.2 which receives the data clock.
There are 5 main states:
Off, no processing of incoming data
Reset, start from anew, either on start up or after successful/failed processing of a packet
Wait for Sync, waiting for a training sequence to arrive (010101..) and start flag (01111110), implemented with its own state machine
Reset, start new preamble
0, last bit was a zero
1, last bit was a one
flag, training sequence complete, now process start flag
Prefetch, ingest the next 8 message bits to ease further processing
Receive Packet, process bits until the end flag (01111110) is found or an error situation occurs
Independent of state, the interrupt routine continually decodes NRZI into actual bit sequence.
In the "Receive Packet" state there's continuous calculation of the packet CRC and some bit-de-stuffing. When the end flag is found and the CRC is correct, the received message is committed into the FIFO. If an error is encountered, the bytes already written to the FIFO are discarded. In both cases, the state machine starts anew by transitioning into RESET.
This reads like a lot of code for an interrupt handler. However with the MCU running at 16MHz even the most complex state only uses a fraction (<10%) of the available time.
Lastly a list of things I'd like to improve with the next version of dAISy.
Receiving on both AIS channels through channel-hopping done 1/5/2014
Tweak radio settings for better sensitivity and lower error rate
LED(s) for indicating reception of valid/corrupt packets
Proper antenna connector
Layout PCB as BoosterPack and/or USB dongle
Receiving on both AIS channels at once with two radio ICs
-- edit 12/25: replaced original post with high-level project description, more detailed documentation of implementation to come
-- edit 12/28: added documentation for hardware (here and on Github), fixed some typos
-- edit 12/31: added documentation for software and list of future improvements
-- edit 01/05: implemented channel hopping (change to state machine)
-- edit 01/15: changed state machine to reflect recent changes (see post further down for details), added link to shared project on OSHPark
-- edit 01/25: major rework of sync detection state machine