Clock Correlation or "What Time Is It on Your Spacecraft?"
This is a leading question but, how can you tell what time it is on your spacecraft? As part of your spacecraft design, you always include a clock for timing whether it's for timing external events and observations or timing when to do the next housekeeping routine. In one case, the time might be very critical (to the microsecond) and the other totally non-critical (within an hour or two). It is incumbent upon the designers of both the spacecraft and the ground system to realistically determine the need for and method of maintaining the accuracy of the clock.
While this may seem like a generality that is out of place, if you are building a mission to explore astronomical events in the heavens, it is virtually imperative that you know when an event occurs to the nearest nanosecond. We were reminded of this several years ago when the Compton Gamma Ray Observatory was generating some unique readings on a well-known Pulsar source. The on-board clock was indicating that the timing was several seconds different from the Pulsar. The cause turned out to be an error in the clock data processing on the ground. On the other hand, if you are building a mission to map the oceans of the world for whatever reason, time may not be critical due to the known landmarks within view of the sensors at specific positions in the orbit. Here time is an along for the ride and to simply trigger certain functions at a specific (more or less) time.
Spacecraft Design
The Clock and Data Handling system on each spacecraft seems to be different by default. Those spacecraft that will be interfacing with the space network (i.e. TDRSS) must by design utilize a transponder that accepts and recreates the PN pattern on the telemetry stream. That transponder by design will have an output that corresponds to the first bit of the EPOCH of the PN pattern. That signal should be considered as the time to read the clock. This is not to say that the clock must be read at each epoch but rather this is a time hack that you can use to advantage.
After you have read the clock, you logically put the clock reading into the telemetry stream. For simplicity, you should insert that information into the telemetry stream along with a bit count or time offset from the time of reading the clock to the clock reading in the telemetry stream. This is important because, when we go to do the clock correlation on the ground, one of the things we try to determine is when the clock was read on the spacecraft. By using knowledge of the PN pattern, we can find the range at the time of clock reading to microsecond accuracy. To give an example, the range delay varies from 503 msec to 569 msec. However, by use of the PN epoch information available from the ground station, we can find the range at clock reading time within a microsecond.
Ground System Design
As shown previously, time must be considered in the ground system design. The information coming from the interface at WSC is raw data from the spacecraft. Whether the data is strictly serial data or CCSDS formatted, it needs to have a time of ground receipt associated with it. When you start to process the data for transmission to the project operations control center (POCC), one of the first thing that should be done is add the ground receipt time plus any time offset to a specific recognizable marker in your data. What this does, is allow you to compute the actual time of receipt on the ground to that marker. To this, add the information that you can acquire from an Operations Procedure Message 66 (OPM-66) which has a time for both epoch transmission and reception. This information allows you to calculate the range to your spacecraft, which in turn provides the transmission delay time from your spacecraft to the ground. The mathematics are not complex, but the application of all the various and sundry delays is exacting and cannot be overlooked. The USCCS manual (531-TR-001) provides a full and complete explanation of the calculations. The USCCS manual is in PDF format and requires the Adobe Acrobat reader. Download Acrobat reader free of charge from Adobe