Campbell SkyVUE 8, Product Manual

Page: 19 / 116

If no cloud is detected but significant scattering is detected below 50 m (160 ft), then vertical
visibility is set to 0.
2.1.2 Sky condition
Sky condition is an assessment of cloud cover measured in units of eighths known as oktas. The
number of oktas is the density of cloud in eighths of that layer. The SkyVUE 8 can report up to
five layers of cloud when reporting sky condition. The algorithm used in the SkyVUE 8 follows
guidance in the
ICAO 9837, Manual on Automatic Meteorological Observing Systems at
Aerodromes .
Sky condition is not an instantaneous measurement. It is based on cloud data for the previous 30
minutes, with cloud detection in the previous 10 minutes given an extra weighting. Therefore, sky
condition is not available until sufficient data has been collected. See
Sky condition algorithm
description (p. 96) for more detail.
2.1.3 Backscatter profile reporting
Several possible data messages give the two-way attenuated backscatter profile. This consists of
2048 groups of five-character values (10,240 characters in total). Each character is 4 bits long and
therefore each 5 figure group is 20 bits. They are given as signed two complement integers and
numbers greater than 2
19
–1 represent negative integers.
NOTE:
After 1600, the remaining groups are all of 00000.
Therefore, each group actually represents negative, a value between –2
19
to +(2
19
–1), rather than
0 to (2
20
–1), which would be the case for unsigned, positive, 20-bit integers.
Apply the following two-stage process to correct the decimal value:
1. Convert the hexidecimal characters to a decimal number.
2. If the number resulting from this conversion is greater than 524287, subtract 1048576,
which is 2
20
.
To use this backscatter coefficient in units of sr
-1
m
-1
, the sensor multiplies the calculated decimal
number by a factor of 10
-8
. The values are scaled by the Attenuated_SCALE parameter, see
Table 5-1 (p. 28).
NOTE:
The profile is not corrected for tilt angle even if cloud heights are corrected.
SkyVUE™8 (CS136) LIDAR Ceilometer 7