User:PetesGuide/GPS time synchronization accuracy

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. 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) until that signal is processed.
• Outputs a 1PPS signal. As of January 2018, 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 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.