Data analysis is a process of inspecting, cleaning, transforming, and modeling data to highlight useful information and suggest conclusions. Accurate timestamps and a timeline of vehicle events are needed to analyze flight data. When data is gathered onboard a rocket, precise time stamping is even more important due to the rocket’s high speeds and the requirement to integrate data over time for inertial navigation calculations.

Accurately time-tagging data currently requires an additional accurate timecode generator board. This solution is costly but is usually adequate for ground-based systems. However, this solution is not adequate for flight processors on rockets. Rocket systems require more costly ruggedized equipment where weight, size, and power constraints are an issue. Redundancy is also required, adding even more to the system’s weight, size, and power consumption.

By moving the timekeeping to the flight processor, there is no longer a need for a redundant time source. If each flight processor is initially synchronized to GPS, they can freewheel and maintain a fairly accurate time throughout the flight with no additional GPS time messages received. However, additional GPS time messages will ensure an even greater accuracy.

Some modern microprocessors maintain a 64-bit internal time-base register that is incremented by a crystal oscillator, usually with a 20- to 100-MHz frequency. This time-base register can be read in an interrupt service routine (ISR) generated by the 1 pps signal from the GPS receiver. Next, a GPS time message is received. The timebase count is associated with the GPS time message time.

When a timestamp is required, a gettime function is called that immediately reads the time-base register. A delta count is calculated from the last GPS sync. The current time is calculated by adding this delta time to the last sync time. This process calculates a timestamp with an accuracy measured in microseconds, depending on the processor clock speed and the accuracy of the processor clock. If a 1 pps GPS ISR is not available, the time base register can be synchronized with the receipt of the GPS time message. If the microprocessor does not have a 64-bit internal time-base register, a countdown timer can be used.

This work was done by Roger Zoerner of ASRC Aerospace Corporation for Kennedy Space Center. For more information, contact the Kennedy Space Center Innovative Partnerships Office at 321-867-5033. KSC-13406