Sensor Sampling and an Example Argos Message Format

From the WOCE Surface Velocity Programme Barometer Drifter Construction Manual (1995). . .

APPENDIX 3: Sensor Sampling and an Example Argos Message Format

Sensor Sampling and Duty Cycle

Atmospheric Pressure

The format described here was used during deployments that tested pressure readings from both the AIR sensor and Kavlico sensor. The Kavlico sensor is not currently used. However, the de-spiking algorithm is the current version.

160 pressure samples (taking approximately 160s) are collected each hour. The de-spiking algorithm is as follows:

• Take the median of the lowest 10 points of 160

• Calculate the median of the points within the entire set of 160 points that are within 1hPa of the 10 point median

• Store as 12 bit count: 0 => 800.0 hPa and 4095 => 1209.5 hPa, with a 0.1 hPa resolution

• If 15 or more of the 160 samples include communication errors, the sampled set is deemed corrupt, and the transmitted count is set to zero

Historical pressure data are also stored for 24 hours. In the information that follows, time elapsed relative to most recent pressure sample (P1_i) is denoted by a subsript. For example a P1_i-8 pressure sample is the pressure sample that is 8 hours older than the current pressure, P1_i.

Sea Surface Temperature (SST)

SST is sampled continuously with a 15-minute repeat cycle.

• Interval between samples = 60s

• Number of samples averaged = 15

• Number of thermistors monitored = 1 (YSI 44018 or equivalent)

• Store as 10 bit count

Salt Water Switch

The salt water switch is sampled continuously with a 30-minute repeat cycle.

• Length of salt water switch data collection = 30 minutes

• Store as 8 bit count = immersion time (in seconds) ÷ 10

Duty Cycle

Drifters should have option to be on i houra followed by j hours off where:

0 hours < i < 72 hours
0 hours < j < 72 hours
and i & j are both integers

Transmitter Repetition Rate

90s ± 10%

Example Argos Message Format

This example transmits a repeating sequence of four Argos messages. Each message, 256 bits long, contains the most recent sensor data as well as historical pressure data. The ensemble of four messages allows the previous 24 hours of pressure data and times of observation to be reconstructed.

Description of Terms

P1_i (12 bits)

Pressure count from AIR pressure sensor at hour i

P2_i (12 bits)

Pressure count from Kavlico pressure sensor at hour i

SST (10 bits)

Latest Ľ hour SST count

Drog (8 bits)

Salt water switch count

Bat (2 bits)

Battery voltage count (00 means > 7 V; 11 means < 7 V)

Age (12 bits)

Time difference in minutes between most recent transmission and most recent pressure

Sum_i (4 bits)

4 least significant bits of sum of 6 half bytes of P1_i and P2_i

Sum (8 bits)

8 least significant bits of remaining 31 bytes of message

MessageID (12 bits)

000000000000 message 1, 010101010101 message 2,

101010101010 message 3, 111111111111 message 4

CommError (2 bits)

00 means no communication error with AIR, 11 => communication error with AIR

HdrSum (4 bits)

4 least significant bits of sum of P1_i (12 bits), SST (10bits),

P2_i (12 bits), MessageID (12 bits), Drog (8 bits), Bat (2 bits), and Age (12 bits)

Transmission 1 (256 bits)

Sum (8 bits), P1_i (12 bits), SST (10 bits),
P2_i (12 bits), MessageID (12 bits),
Drog (8 bits), Bat (2 bits), Age (12 bits),
P1_i-4 (12 bits), P2_i-4 (12 bits), Sum_i-4 (4 bits),
P1_i-8 (12 bits), P2_i-8 (12 bits), Sum_i-8 (4 bits),
P1_i-12 (12 bits), P2_i-12 (12 bits), Sum_i-12 (4 bits),
P1_i-16 (12 bits), P2_i-16 (12 bits), Sum_i-16 (4 bits),
P1_i-20 (12 bits), P2_i-20 (12 bits), Sum_i-20 (4 bits),
P1_i-24 (12 bits), P2_i-24 (12 bits), Sum_i-24 (4 bits),
CommError (2 bits), Unused (6 bits), HdrSum (4 bits)

Transmission 2 (256 bits)

Sum (8 bits), P1_i (12 bits), SST (10 bits),
P2_i (12 bits), MessageID (12 bits),
Drog (8 bits), Bat (2 bits), Age (12 bits),
P1_i-3 (12 bits), P2_i-3 (12 bits), Sum_i-3 (4 bits),
P1_i-7 (12 bits), P2_i-7 (12 bits), Sum_i-7 (4 bits),
P1_i-11 (12 bits), P2_i-11 (12 bits), Sum_i-11 (4 bits),
P1_i-15 (12 bits), P2_i-15 (12 bits), Sum_i-15 (4 bits),
P1_i-19 (12 bits), P2_i-19 (12 bits), Sum_i-19 (4 bits),
P1_i-23 (12 bits), P2_i-23 (12 bits), Sum_i-23 (4 bits),
CommError (2 bits), Unused (6 bits), HdrSum (4 bits)

Transmission 3 (256 bits)

Sum (8 bits), P1_i (12 bits), SST (10 bits),
P2_i (12 bits), MessageID (12 bits),
Drog (8 bits), Bat (2 bits), Age (12 bits),
P1_i-2 (12 bits), P2_i-2 (12 bits), Sum_i-2 (4 bits),
P1_i-6 (12 bits), P2_i-6 (12 bits), Sum_i-6 (4 bits),
P1_i-10 (12 bits), P2_i-10 (12 bits), Sum_i-10 (4 bits),
P1_i-14 (12 bits), P2_i-14 (12 bits), Sum_i-14 (4 bits),
P1_i-18 (12 bits), P2_i-18 (12 bits), Sum_i-18 (4 bits),
P1_i-22 (12 bits), P2_i-22 (12 bits), Sum_i-22 (4 bits),
CommError (2 bits), Unused (6 bits), HdrSum (4 bits)

Transmission 4 (256 bits)

Sum (8 bits), P1_i (12 bits), SST (10 bits),
P2_i (12 bits), MessageID (12 bits),
Drog (8 bits), Bat (2 bits), Age (12 bits),
P1_i-1 (12 bits), P2_i-1 (12 bits), Sum_i-1 (4 bits),
P1_i-5 (12 bits), P2_i-5 (12 bits), Sum_i-5 (4 bits),
P1_i-9 (12 bits), P2_i-9 (12 bits), Sum_i-9 (4 bits),
P1_i-13 (12 bits), P2_i-13 (12 bits), Sum_i-13 (4 bits),
P1_i-17 (12 bits), P2_i-17 (12 bits), Sum_i-17 (4 bits),
P1_i-21 (12 bits), P2_i-21 (12 bits), Sum_i-21 (4 bits),
CommError (2 bits), Unused (6 bits), HdrSum (4 bits)

Posted February 28, 1997

Sybrandy, A. L., C. Martin, P. P. Niiler, E. Charpentier, and D. T. Meldrum, 1995:

WOCE Surface Velocity Programme Barometer Drifter Construction Manual.

DBCP, 58-59.