GPS: Because some digital mode software (such as WSJT) requires one-second accuracy, GPS synch methods that only achieve 1-2 second accuracy may not always be sufficient. Therefore, one should use only GPS hardware that meets these minimum specifications:
- Supports WAAS, which increases accuracy by broadcasting data that allows correcting for measured ionospheric delays and the clock drift in the satellites' atomic clocks.
- Outputs either GPRMC or GPSZDA (required for date and time), with GPSGGA highly desirable (provides fix quality):
- GPRMC—UTC time (without the date), lat/lon coords, ground speed in knots, track, date with 2-digit year, and a few other things.
- GPSGGA—UTC time (without the date), lat/lon coords, GPS Quality Indicator, number of satellites in view, and a few other things.
- GPSZDA—The GPSZDA sentence is best for time purposes, because it is the only sentence that includes a 4-digit year, and is also the only NMEA sentence that includes date and time together. It's also the only field that provides the local time zone offset.
- Processes the leap-second correction included in the UTC data of the Navigation Message (Subframe 4, Page 18) sent by the satellites.[1] This signal is broadcast only once every 12.5 minutes, so the time provided by a GPS receiver will differ from UTC by 18 seconds (As of 31 December 2017[update]) until that signal is processed.
- Outputs a 1PPS signal. As of January 2018[update], the only commercially available USB GPS receiver that provides a 1PPS signal are the Navisys GR-601W, GR-701W, and GR-801W, which were designed at the request[2] of Eric S. Raymond and Dave Taht in order to provide a highly accurate time signal source that is affordable, using consumer-grade hardware. Currently, the GR-701W is available for $50 on Etsy.
- ^ "Best Practices for Leap Second Event Occurring on 31 December 2016" (PDF).
- ^ Raymond, Eric (2012-04-12). "Armed and Dangerous: Making simple connections". Retrieved 2018-01-16.