YSI SonTek ADVField, Technical Documentation Manual

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SonTek/YSI
ADVField/Hydra Operation Manual (September 1, 2001) 11
The second type of sampling shown in Figure 7 is “near” continuous sampling for an autono-
mous ADV. The sampling parameters are set such that the ADV is sampling at all times except
during the burst overhead. For near-continuous data collection, a burst interval of about one hour
is recommended. This provides a regular reference to the real-time clock without a significant
loss of data from the burst overhead. For example, to collect near-continuous data at 2 Hz you
might set the following parameters: burst interval = 3600 seconds, sampling rate = 2 Hz, and
samples per burst = 7180. This allows ten seconds of overhead with each burst, although the
overhead is typically five seconds, a small safety factor is recommended.
The advantage of continuous sampling is complete coverage of the deployment period. However,
continuous sampling creates huge data sets and drains battery power relatively quickly. The sam-
pling parameters shown above generate between 1.4 and 3.8 MB of data per day (depending on
what data is recorded – see §2.4.2). At the maximum sampling rate of 50 Hz, the ADV generates
between 34 and 94 MB of data per day! These data rates quickly fill the internal recorder and
present a daunting task for analysis. Therefore, most applications use a limited duty cycle to re-
duce memory requirements and increase battery life.
The third sampling strategy in Figure 7 shows burst sampling with a reduced duty cycle. The duty
cycle is calculated as shown below.
Sampling Time = Number of Samples / Sampling Rate + Overhead
Duty Cycle (%) = 100 * Sampling Time / Burst Interval
A typical example of the parameters might be: burst interval = 3600 seconds, sampling rate =
2 Hz, samples per burst = 1200. With these settings, the ADV records data at 2 Hz for 10 min-
utes each hour. This gives a duty cycle of about 17%, increasing battery life by a factor of six (the
Single Burst Sampling - Autonomous Deployment
"Near" Continuous Sampling - Autonomous Deployment
Continuous Sampling - Real Time Data Collection
Multiple Burst Sampling - Autonomous Deployment
Sample
Sample Interval
Time
Burst
Time
Burst
Time
Time
Burst 1
Burst 2
Burst 3
Burst 1 Burst 1
Burst 1
Burst 1
Burst 2 Burst 2
Burst 3 Burst 3
Figure 7 – ADV Sampling Strategies