The EQMOD GPS routine simply reads the GPS unit's NMEA strings and scans for the latitude/longitude/altitude/hemisphere/UTC time values.
The specific NEMA messages scanned by EQMOD are:
GPGGA ( time, lat, long, alt )
GPGLL ( lat, long, time)
GPRMC ( lat, long, date, time )
It is important that time is reported by the GPS using UTC. Some GPS units provide configuration utilities (such as LSView) where the time can be changed
to local time but this will result in incorrect operation with EQMOD.
Once the GPS gets a good fix to at least three of the GPS Satellites, the EQMOD driver will display the values on GPS Setup screen. The driver will also attempt to compute for the time difference between the PC Clock and the GPS UTC time.
Once the user clicks the 'Accept' button, the GPS values displayed on screen will be copied to the driver's global Site values settings.
The time difference (gEQTimeDelta) is also applied on the driver's local Sidereal Time computation. This approach does not adjust the PC clock (it will be the user's option to adjust the PC clock externally from the driver),it simply applies the timedelta value on the PC clock to compute for the Local Sidereal time. This way you get to operate the driver even if you disconnect the GPS receiver.
The site values (latitude, longitude, altitude) plus the TimeDelta value are saved on the windows registry upon driver termination and restored during driver initialization. You don't need to take another GPS sample unless adjustments were made on the PC clock (or if you notice any deviation on the PC clock with respect to the Atomic Time at certain period length).
The 'Reset' button on the GPS Setup window simply resets the gEQTimeDelta to 0. This is necessary if the PC clock is adjusted manually by the user using the GPS time or using any internet based Atomic Clock synchronization services.
The 'Abort' button simply closes the GPS connection and closes the GPS Setup window. No changes are made on the driver's global site values or on the TimeDelta value.
It is also possible to manually adjust/override the GPS coordinate/hemisphere/elevation values on the GPS Setup window. You just disconnect the GPS from the system (by clicking the button at the upper right portion of GPS Setup window) and perform the manual entry on the GPS setup form. You then click 'Accept' to recognize the changes.
Advanced User features
With EQASCOM V1.16k an option was added to send out a GPS initialiser string (ASCII based). Whilst most GPS devices can be permanently configured by their
associated drivers/setup programs to output NMEA messages some may require explicit initialization. To enable this feature append the required string to the eqmod.ini file key "GPS_INITSTRING1=".