Math Function on GPS Signals

I am reading data in from a NMEA GPS device. I would like to perform some math functions on the velocity channel - primarily some filtering and averaging of the signal. In the Math functions, when I attempt to create any function the only GPS channels that show up are the Longitude and Latitude channels. On the GPS setup screen, the Velocity channel is set to active, and I can see data on it, but it does not show up as a channel to select in the Math setup.

Dear Mr. Temple,

I've tried replicating the issue, but could not do it.

Below is the picture of my GPS channel setup screen.

In the Math channel setup I could then see all the channels and use 'Velocity' for some calculation.

I'm not sure why it isn't working on your side. Could you store a measurement and send me your .dxd file? I would then be able to check why it doesn't see the channel. Thanks!