Garmin v1.0b manual Overview, GPS Advantages and Disadvantages

Overview

A man with a watch knows what time it is. A man with two watches is never sure. - Segal's Law

This document describes how to connect a Garmin GPS 18 LVC to the IN-14 Nixie Tube Clock from tubeclock.com. The GPS unit will allow the clock to set and maintain time display to sub-millisecond accuracy.

Timing accuracy comes from the GPS receiver in the Garmin GPS 18 hardware. The GPS 18 receives signals from the network of satellites which orbit the Earth and transmit accurate time codes via radio waves. These time codes are typically used for calculating position for navigational purposes, but they also provide a highly reliable time source.

GPS Advantages and Disadvantages

The advantages of GPS time synchronization include a high degree of accuracy over long periods of time (it is referenced to atomic clocks), and it's ubiquitous availability anywhere on the Earth.

The disadvantage is that the signal is very weak, so the GPS 18 hardware should be placed in a position where it has a line of site view of the sky. The unit will work indoors, but at least placing it on a window sill is a good idea. Even thin material, like paper, will effect the receiver's performance. The Garmin GPS 18 LVC has a magnetic base, and also a screw to help with mounting.

Search on amazon.com or the web for part numbers 010-10541-00 (suction cup mount) or 010-10453-00 (flange mount) if you would like mounting hardware for the GPS 18 LVC.

Operation

The Nixie Clock will communicate to the GPS-18 via two mechanisms: first is an RS- 232 serial port for communicating the time and setup information; and second is a Pulse Per Second (PPS) signal which will keep the clock in sync when the serial communications are not in use.

A copy of the complete GPS-18 manual can be found on the Garmin website: http://www.garmin.com/manuals/425_TechnicalSpecification.pdf

When the clock is turned on, the GPS will take from a few seconds to five minutes to acquire the satellites' signal. During this time, the clock will run using it's on-board oscillator, and you will need to set it to within 15 minutes of the correct time. This is necessary because the GPS system does not provide time zone information. When the signal is acquired, the GPS will send a signal to the clock (by beginning strobing of the PPS line) and the clock will then read the next serial port message and adjust the time.