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General Issues > Time Stamps

New: ADT radio timing

The acronym ADT stands for Accumulated Delay Transcription. With this new technique, there is no need for highly precise timing in the front-end unit. A few ppm of uncertainty in the clock frequency can be tolerated during data acquisition—hence, any properly designed crystal oscillator will do the job.

The principle is simple and cost-effective: the front-end processor counts or calculates exactly the number of clock cycles that have elapsed during the interval between data acquisition and radio transmission. This figure, representing the accumulated delay, is included in the transmission protocol of every data set.

The fully-digital radio receiver then puts an absolute UTC time stamp on every message received. Any further delay—the receiver´s own data processing—must be identified and the previous delay figure has to be updated properly (transcribed).

Note: Precise ADT timing cannot be implemented in a high-level language on a disc-based operational system because of its retarded real time response. Hence, in the SkyLINK receiver the ADT procedure is accomplished on the lowest hardware level, in the DSP core.

The delay of the radio signal (due to wave propagation) is typically 0.3 ms for a distance of 100 km, which makes the theoretical limit of precision in the ADT approach less than one millisecond. However, this timing precision would be achieved only with an extremely high investment in hard- and software engineering. Therefore, a more relaxed figure in the order of ± 10 ms uncertainty will be implemented in the RED System.