Legacy (AWAC, Aquadopp & Aquadopp Profiler)

The data collected during either an averaging or a burst interval is always assigned to the timestamp of the first measurement.

Concurrent measurements are not possible for legacy instruments, therefore individual averaging intervals are skipped when a wave burst is performed. (see Figure 1)

Figure 1: Example of non concurrent AWAC sampling scheme

Signature

Average Velocity

The average is assigned to the middle ping of its corresponding averaging interval. The pings are distributed within the averaging interval such that the spacing between each ping is maximized, while ensuring that the temporal intervals are constrained to integer values. The maximum sampling rate per seconds is variable dependent on the frequency of the instrument.

This is important to keep in mind as reducing the number of pings can reduce the length of the average interval (as shown in Signature Deployment as Actual Average Interval). For instance:

• An interval of 120 seconds with 120 pings will distribute the pings evenly every 1 second
• An interval of 120 seconds with 60 pings will distribute the pings evenly every 2 seconds
• If 86 pings are collected within a 120-second interval, the temporal resolution remains 1 second, but the Actual Average Interval is reduced to 86 seconds.

Understanding this principle is vital for determining the location of the average timestamp.

Example Deployment:
An instrument is set to collect data with an averaging interval of 60 seconds and a measurement interval of 120 seconds. The number of pings per interval is set to 12. In this configuration, a ping is emitted every 5 seconds, and the average timestamp corresponds to the 6th ping, which occurs 25 seconds after the start of the measurement. (see Figure 2)

Figure 2: Example 1 of Signature sampling scheme

Using, for example, 8 pings would reduce the averaging interval to 56 seconds, with a temporal resolution of 7 seconds between pings. The average timestamp would then be positioned at 21 seconds after the start of the averaging interval. (see Figure 3)

Figure 3: Example 2 of Signature sampling scheme

If an uneven number of pings is selected, the timestamp will be assigned to the ping immediately preceding the middle one. For example, with 11 pings in a configured averaging interval of 60 seconds, the timestamp would correspond to the 5th ping. (see Figure 4)

Figure 4: Example 3 of Signature sampling scheme

Burst

Data collected during a wave burst is assigned a timestamp in a similar manner:

For example, if sampling at a frequency of 1 Hz and collecting 1024 samples, the wave parameters will be assigned to the timestamp of the 512th sample. In this case, the timestamp corresponds to 512 seconds, or 8 minutes and 32 seconds, after the start of the burst.

Processing

When processing raw single-ping .ad2cp files using Signature Viewer, the automatic averaging process assigns the timestamp to the beginning of each averaging interval.

However, this behavior differs when using Ocean Contour, where the timestamp remains aligned with the middle ping of the averaging interval, even after completing the averaging step in the processing procedure. (This, of course, depends on the chosen averaging interval.)

Gen2 (AWAC, Aquadopp & Aquadopp Profiler)

Data collected during either an averaging or a burst interval is always assigned to the timestamp of the first measurement.

The new generation of AWACs and Aquadopps introduces a concurrent sampling scheme, enabling a constant temporal resolution for average current measurements while simultaneously collecting wave data. (see Figure 5)

Figure 5: Example of concurrent AWAC sampling scheme