Originally Posted by tonyappuk
You can modify any VHF
receiver capable of receiving AIS signals - it doesn't have to be an expensive dedicated AIS box. I use an old Tandy Realistic scanner that I have had for years. You need to modify the discriminator so you need a little dexterity with a soldering iron but it's not difficult. This link Discriminator Output
will show you how. Click on the links down the LHS if you have a receiver in mind.
Note: AIS broadcasts on two different channels for transmission collision
avoidance. It automatically switches between these two based on traffic on the channels. It's totally automatic. So, a ship may transmit on only one channel if there is no interference
, but if another ship comes into range using the same channel, it may switch to another channel and stick with that one for a while.
A modified VHF radio
will only pick up one channel. So you'll miss some signals. The same caveat applies to some really cheap
AIS receivers that only receive one channel. Receivers that multiplex (and the same would apply to a modified VHF scanner) will miss less, but could still miss transmissions.
Two modified cheap
VHF receivers would work fine as long as your software
can take two AIS inputs.