>ASIMET Sampling Definitions

file: asimet_sampling_definitions.html
20 Nov 2007

ASIMET Sampling Definitions

The following sections define the sampling for each of the ASIMET modules. Except for SWR and WND, measurements are made near the end of the 60 second sampling interval. At the end of this 1 minute interval, the new value(s) are put into a buffer that is returned whenever data is requested. The data in this buffer is the same for the next minute, until replaced at the end of the new 60 second sampling interval. In modules with FLASH memory, time at the end of the 1 hour logging interval is attached to the data and the data is stored as 60 one minute data sets in a record.

NOTE: For HRH, PRC, SWR, and LWR, the "snapshot" is actually the average of 16 A/D conversions spanning several milliseconds.
(HPS and LWRF boards both use the Burr-Brown ADS7807 16-bit A/D converter)

Relative Humidity and Air Temperature (HRH).

A single snapshot of humidity and air temperature is taken at the end of the minute.
The data out is: hh.hhh, tt.ttt where h is relative humidity in % and t is air temperature in degrees C.
(C language format is: "%8.3f, %8.3f\r\n\xo3")

Proposed averaging for satellite transmission is a one hour average of each variable.

Sea Surface Temperature (SST).

A single snapshot of temperature is taken at the end of the minute.
The data out is: tt.ttt where t is temperature in degrees C.
(C language format is: %7.3f\r\n\x03)
NOTE: As of 17 April 2001, SST is now obtained from an SBE37 (Conductivity / Temperature ) unit on buoy systems, and an SBE48 (Hull-mounted Temperature) for ship systems. See the SeaBird documentation for output format.

Proposed averaging for satellite transmission is a one hour average of temperature.

Precipitation (PRC).

A single snapshot of precipitation is taken at the end of the minute.
The data out is: pp.pp, where p is precipitation level in mm of water.
- Rain rate must be computed by post-processing, taking into account the occurrence of auto-syphoning when the RM Young Precipitation gauge fills to 50mm. See the RM Young documentation.
(C language format is: "%7.2f\r\n\xo3")

Proposed averaging for satellite transmission is a one hour average of precipitation.

Shortwave Radiation (SWR).

A single snapshot of shortwave radiation is taken every 10 seconds and the 6 values are averaged at the end of the minute.
The data out is: rrr.r, where r is radiation in watts per square meter.
(C language format is: "%7.1f\r\n\xo3")

Proposed averaging for satellite transmission is a one hour average of shortwave radiation.

Longwave Radiation (LWR).

A single snapshot of longwave radiation is taken at the end of the minute.
The data out is: ddd.dd bbb.bb vvv.v rrr.r, where d is dome temperature in Kelvin, b is body temperature in Kelvin, v is thermopile microvolts and r is radiation flux in watts per square meter.
(C language format is: "%7.2f %7.2f %6.1f %6.1f\r\n\xo3")

Proposed averaging for satellite transmission is a one hour average of longwave radiation.

Barometric Pressure (BPR).

A single snapshot of barometric pressure is taken at the end of the minute.
The data out is: bbbb.bb, where b is pressure in millibars.
(C language format is: "%7.2f\r\n\xo3")

Proposed averaging for satellite transmission is a one hour average of barometric pressure.

Wind Speed and Direction (WND).

Scalar wind speed and direction are measured every 5 seconds. Pulses are counted for 5 seconds while vane direction is sampled each second, and the compass is sampled in the middle of the 5 second period. At the end of each 5 seconds, the wind speed is calculated, the vane direction is averaged (sums of sine and cosine with arctan recombination), and the compass direction is sampled and added to the vane average for direction. A (sine and cosine) vector Vel(East) and Vel(North) are calculated.
At the end of one minute the vector sum of Vel(East) and Vel(North) are calculated and stored, the average scalar wind speed is calculated and stored, the maximum of the (5 second) wind speeds is stored, the last vane direction is stored, the last compass direction is stored, the average of the X tilt is calculated and stored, and the average of the Y tilt is calculated and stored. The stored values, plus the minimum of the (5 second) wind speeds (which is NOT stored), are placed in a buffer for output when data is requested over the next minute (until the next update).
The data out is: ee.ee, nn.nn, ss.s, xx.x, mm.m, vvv.v, ccc.c, XX.X, YY.Y where e is vector wind to the east in meters/sec, n is vector wind to the north in meters/sec, s is scalar average wind in meters/sec, x is maximum 5 sec wind speed in meters/sec, m is minimum 5 second wind speed in meters/sec, v is last vane direction in degrees, c is last compass direction in degrees, X is average tilt in degrees, and Y is average tilt in degrees.
(C format is: "%6.2f %6.2f %5.1f %5.1f %5.1f %6.1f %6.1f %6.1f %6.1f\r\n\x03")

Proposed averaging for satellite transmission is a one hour average of wind speed data: vector average wind east, vector average wind north, and average wind speed.