Although the RC210 has an on-board Real time Clock (RTC) over a period of time this clock does drift. This is due to CPU overhead and other environmental influences. The repeater does have the capability to semi correct this drift, this correction is a pre-defined value on a pre-defined time.
This can lead the correction process causing the RTC to be adjusted more than it needs as it cannot be guaranteed how much the RTC has drifted by.
GB3BS uses the RTC to define our automatic beacons which, as defined by the BR68 have to be transmitted at a period no longer than 15 minutes.
To maintain accuracy The Bristol 70cms Repeater Group investigated using an "off air" time source, we researched a number of time solutions before settling on GPS TIME.
GPS Time has a mean accuracy of around 28ns RMS per 19 years if uncorrected. However the Department of defence (U.S.) correct the 3 Atomic clocks that each spacecraft has at least once in any given 24hr (earth time) therefore over a period of a standard earth day each spacecraft is re-synchronised! This maintains the accuracy of the time received.
It is worth noting for the purists out there, that GPS time is currently 19 seconds adrift of TAI Time (International Atomic time). This is due to GPS time not being based on fixed star to fixed star time unlike how time on earth is measured, because of this fact GPS Time is not correct using leap seconds as TAI (Earth time) is. Thankfully most GPS Receivers receive and process the current offset information that is transmitted along with the Satellite almanac thus the received GPS time is received from Satellite, corrected by the Almanac offset and at this point is displayed. The resulting time is as accurate if not more accurate than the time standard that used to be broadcast from Rugby.
Once a GPS time source was decided we then had to develop a system that could receive and process this GPS TIME then update the RC210. The result is a small unit that is separate (at the moment) to the Repeater Rack, this unit has a commercially available GPS Receiver and custom designed electronic interfacing as well as a PIC Micro controller running "In House" code
Over a period of 24hrs the RC210 RTC is corrected up to 48 times, this is more than the RC210 would normally need, but it was decided that putting effort into getting a accurate time source we may as well make sure the RC210 is as accurate as possible. The Date component (used for system logging and events) is only updated once ever 24hrs (at the moment).
The RC210 then uses this corrected time to trigger beacon events on the Hour, quarter past, Half past and Quarter too. (Unless the repeater is in use). The Hour and Half Hour beacons are Voice Identifications, the others are CW. These events are normally launched within 500mS of Zero Seconds. If we take the 1 AM Voice ID for example. At the moment the RTC rolls over to 01:00:00 the Transmitter is Keyed followed by the ID and time announcement. It is the point the transmitter is keyed is the zero second mark.