A GPS Observing Clock

For years, when observing, I found myself wanting a clock on my observing table when recording observations. I have used either a wrist watch or a cell phone, but looking at these was uncomfortable as these modern devices use bright backlit LCD displays, not a nice night-vision friendly red. The cell phone also has the additional problem of using up its battery quite quickly when out of range of a digital cell tower at some remote observing site. I needed a simple desk clock for my observing setup.

Accuracy was also a question, accurate time is always important when observing. Asteroid occultations, lunar and solar eclipses, iridium flares, twilight, jovian moon transits, the list of things where accurate time is useful is long in astronomy.

The Specs

Of course being a electrical engineer makes designing and building a clock a fairly trivial exercise. But why stop there? Why not build in a few extra features…

• Use red 7-segment LED’s and build in some type of selectable dimming mechanism.
• Why bother setting the clock each time you set it up? Make the clock self setting and very accurate.
• Since the clock is accurate add a serial port to allow the clock to supply accurate time to a laptop when taking astrophotos.

The first was simple, I had used red 7-segment LED’s in projects since I was a teenager. The second took a little more research, how to make the clock accurate and self setting. At first I considered using the WWV short wave radio signal, this would be suitably accurate and available at a remote site. But it turns out decoding the signal is non-trivial, could take quite a while (hours) to sync-up with correct time, and can be a problem with reception, requiring tuning each time.

The answer is to use the GPS system instead. Along with the ability to give accurate latitude and longitude the GPS system also provides exceptionally accurate time. Navigation is all about time, accurate time from several satellites is used to solve the position of each receiver. Each GPS satellite carries an atomic clock and transmits this time signal.

Now you just need a GPS receiver. These can be purchased from several different sources. Small modules that contain all the needed circuitry. Just give the module power and it spits out serial data containing time, latitude, longitude and other useful data. Only a single satellite signal needs to be received to get good time data and a full lock can take as little as 40 seconds, good time data in much less than that.

Getting it Done

It took two evenings of wiring everything together, as well a a few hours in the shop to build the hardware. Add a few more evenings writing microcontroller assembly code to make it all work. The result was a functioning clock that pulled its time from GPS in just a few seconds from power on.

A simple user interface using a single button and a rotary encoder allow control of the clock, including dimming and setting the time zone. Single button presses rotate through the various displays including GPS status, latitude, longitude, universal time and local time.

The LED above the switch pulses with each passing second. At a star party this summer a fellow observer pulled out her short wave radio and tuned it to WWV to compare the accuracy with the radio standard. It was gratifying to watch the indicator LED pulse in perfect sync with the audio beat and the clock display perfect time, at least to the limit of human perception.