The GPS module that is sold as an EQ mount accessory works well with EQMOD. It typically comes ready to connect to the hand controller with an adapter or directly to a USB port on your computer. There is a CD included with driver software for the computer.
Other GPS modules may also work.
When the GPS module is used with the handset it is set to binary mode. This mode will not work with your computer (or EQMOD). It is necessary to install the software application that comes with the GPS module. Once you have it working with the computer (it will require you to change the output format to “NMEA” ), you can then close the application that came with the GPS module. The following instructions may be typical for your GPS application.
The delta (offset) value is an important piece of the puzzle that synchronizes the whole setup from the GPS clock. In EQMOD, the PC clock is used to compute the Local Sidereal Time. The on-board PC time is not changed whenever a GPS receiver is used. The EQMOD driver gets the atomic time from the GPS receiver, it also reads the current PC time-of-day which is converted to UTC. The two values are then compared and the time delta (time discrepancy between PC and GPS) is then computed.
When the EQMOD driver computes the Local Sidereal Time, it reads the PC clock, applies an offset using the delta value, then derives the Local Sidereal Time. This is done without any adjustment on the on-board PC clock. In fact, you can actually remove the GPS receiver once it gets the delta and coordinate values. The driver will function on its own using the PC clock + the delta value as the time reference.
This process (using GPS) means that the time “drift” typical of most computers is not an issue. You could, of course, always correct the PC clock each time you startup the computer and the delta would then be 0. Note: Most planetarium programs use the PC clock. It would be wise to have the PC clock close to the correct time (a very small delta value).
If you want to use the GPS module with your handset, you will have to use the GPS application program that you installed in your computer to set the output format back to binary.