This article describes the components and software for a low cost sidereal clock. Construction is left up to the reader. As described, the clock has no adjustments; all the information needed to display the time is derived from the GPS module. The clock displays time as HH-MM-SS and is accurate to 1 second. There is a discussion on accuracy in the notes below. At the time of writing (October 2015), all necessary parts could be obtained through well known websites for as little as AU$25.
Accuracy - the display is NOT accurate to within a tenth of a second of LMST. At the time of submitting the article, the author was not aware of the discrepancy between UTC and UT1. For more detail follow the accuracy link above.
Difference between this article and the Silicon Chip submission - the SC software does not enable weak pullup on Port B. The software here enables pullup, allowing a couple of circuit simplifications:
RB3 jitter - the SC article was ready for publishing before an enhanced version of the software was fully tested, so the comments were based on the original version. The version finally submitted ClockV1.X.production.hex addresses the jitter issue - the Longitude is averaged over 256 messages, and the calculations are done to a higher accuracy. If the GPS module meets its specification, the maximum jitter is calculated to be 40uS. This has not been verified in practice.
Startup - the MAX7219 module did not start reliably with the original software. The published software addresses this, and the HELLO message now appears in less than a second.