As a hobby I enjoy studying the history of electronic computers. In the early days communication with a computer from a remote site took place over a telegraph wire or a dedicated phone line. These systems provided for physical wire that passed DC current between the two points. As computers became more common it became desirable to connect from an arbitrary remote location. The Bell 103 modem introduced in 1962 was the solution, it allowed for full duplex communication at speeds up to 300 baud over a standard phone line. Many modern modems still support this protocol.
I built my first 103 modem from some parts I got from the Ax-man surplus store. I used it along with a Cherry keyboard with 24x64 character video connected to a 9" CRT to log in from home to the U of M main frame and write code for my Fortran class.
Here is a wav file of data sent at 110 baud in Bell 103 AFSK modulation, simply demodulate and decode the bit stream to get the cache coordinates .
The Cache Location
Should not need a geochecker if you trust your demodulator and decoder. But here is one anyway.