Make an AIS listening station using RTL-SDR (RTL2832U) on m1 mac

setup-ais.md

#!/bin/bash

# Get CubicSDR
sudo apt install librtlsdr0 cubicsdr

# Need to calibrate the ppm (part per million) value.  Getting an accurate calibration is critical.  Find a frequency that works.  For me 785.5 MHz seems to do well, there is a dip in noise just at that frequency, with an occasional spike.


# I use the follow website to find exact frequencies of cell signals.
# https://gis.geek.nz/celltowers


# Build LTE Cell Scanner, this will help with calibration
```
git clone https://github.com/JiaoXianjun/LTE-Cell-Scanner

export LIBRARY_PATH=/usr/local/Cellar/itpp/4.3.1:${LIBRARY_PATH}

cd LTE-Cell-Scanner
mkdir build
cd build
cmake -DCMAKE_OSX_ARCHITECTURES=x86_64 ..
make
sudo make install
```

```
$ ./CellSearch --freq-start 778e6 --freq-end 778e6
LTE CellSearch v1.0.0 (release) beginning
  Search frequency: 778 MHz
  PPM: 120
  correction: 1
Found Rafael Micro R820T tuner
[R82XX] PLL not locked!
Examining center frequency 778 MHz ...
  Detected a cell!
    cell ID: 168
    RX power level: -36.8562 dB
    residual frequency offset: 1660.47 Hz
  Detected a cell!
    cell ID: 206
    RX power level: -42.9021 dB
    residual frequency offset: 1622.82 Hz
    <clipped>
Detected the following cells:
A: #antenna ports C: CP type ; P: PHICH duration ; PR: PHICH resource type
CID A      fc   foff RXPWR C nRB P  PR CrystalCorrectionFactor
280 2    778M   965h -26.3 N 100 N one 1.0000012409073122388
400255    778M    66k -27.8 E 255 U UNK 1.0000847926016689371
311255    778M -48.6k -27.9 E 255 U UNK 0.99993750489266330916
377255    778M  41.7k   -28 E 255 U UNK 1.0000535847050331384
 94255    778M -43.8k -28.1 E 255 U UNK 0.99994368983093617764
 ```
Look for the line which has the lowest entry in the foff (frequency offset) column. In my case, that's the first entry, with a frequency offset of 965 Hz. The other entries are invalid detections of the same cell. Look for lines that have PR entries of 'one'.

The CrystalCorrectionFactor value for that line can be converted to PPM correction figures (for use in correcting other applications) using: PPM = 1e6 * (1 - CrystalCorrectionFactor) in my case (calculated using python):
```
$ python
>>> 1e6*(1-1.0000012409073122388)
-1.2409073122388037
```
# Once you have the frequency, you can fine tune it using CubicSDR

#
# AIS Decoding
#

# The AIS signals are centered around 162.0 MHz, one being at 161.075 MHz and the
# other at 162.0.25 MHz.
# 
# You can see these coming in at these frequencies using CubicSDR.
#
# The raw signal can be deciphered using `AISdeco2`. Available here: http://xdeco.org/?page_id=30#ai2
# Once downloaded and installed you can run:

./aisdeco2 --freq-correction 45 --agc

# This will output a line like: 2019-08-23 13:54:26.228  INFO     !AIVDM,1,1,,B,17`BCQ0P00<P3qkbrhkR:gvl08?H,0*4F
# The "!AIVDM,1,1,,B,17`BCQ0P00<P3qkbrhkR:gvl08?H,0*4F" string is the AIS code and can be decoded using gpsdecode

# Install 
sudo apt install gpsd-clients

# Run
echo '!AIVDM,1,1,,B,17`BCQ0P00<P3qkbrhkR:gvl08?H,0*4F' | gpsdecode