1. PRESENT WEATHER SENSOR

2. PRESENT WEATHER SENSOR
Measures visibility (MOR) from 10 m up to 50 km
Identifies present weather
type and intensity of precipitation
type of obstruction to vision
Measures the intensity of precipitation
Calculates the accumulation of precipitation
Estimates snow accumulation

3. Visibility & MOR
Meteorological Optical Range (MOR) has been defined by the WMO as the basic parameter to express the optical state of the atmosphere
MOR corresponds closely with human visibility observations (day observations)
MOR has been defined as a purely physical quantity
objective
it can be measured

4. Definition of MOR
100 %
intensity
5 %
MOR
MOR can be measured by measuring the attenuation of light • attenuation is caused by scattering and absorption

5. Scatter Measurement
Scattered flux is proportional to extinction coefficient (absorption is usually negligible)
Transmitter
Receiver
Forward scatter meter measures the light scattered from a beam and estimates extinction coefficient
high scatter intensity => high extinction coefficient
low scatter intensity => low extinction coefficient
note that only part of the scattered light is measured!

6. Present Weather
Present weather is practically defined by the contents of WMO code tables
table 4677 for human observations (SYNOP)
table 4680 for automatic systems (SYNOP)
table 4678 for reports from aerodromes (METAR)
The code tables contain many different physical phenomena
fog, precipitation, cloud phenomena
Automatic systems can not detect all the different phenomena in the code tables
present weather sensors typically can detect different precipitation types and may detect different visibility reducing phenomena

7. FD12P Scatter Measurement
FD12P samples the scatter signal rapidly and can detect precipitation droplets from the signal
Droplet sizes can be measured from the signal changes

8. Capacitive Precipitation Measurement
RainCap sensing element
Thin wires under glass coating form a capacitor
The capacitance changes when there is water on the surface
capacitance change is proportional to water amount
RainCap is heated
water evaporates from the surface
snow melts on the surface

9. Capacitive Sensor Output
The output from the capacitive sensor is a voltage signal
the signal is sampled at 1s intervals
Precipitation intensity can be estimated from the signal level and signal changes (caused by new droplets)

10. FD12P Precipitation Type Principle
Optical measurement, relative to droplet size
Capacitive measurement, relative to water content of droplets
Liquid precipitation (e.g. rain): water content and droplet volume are equal
Solid precipitation (e.g. snow): water content is much lower than droplet (snowflake) size

11. FD12P Precipitation Type Implementation
Optical signal
(proportional to droplet size)
Capacitive signal
(proportional to water content)
“RAIN INTENSITY
SCALE”
“DRD SCALE”
Precipitation
intensity (opt.)
Precipitation
intensity (cap.)
Scaling
Intensity
ratio
(opt./cap.)
Scaling
Intensity ratio < 1.0 -> liquid precipitation
Intensity ratio > 1.0 -> solid (or mixed) precipitation

12. FD12P Precipitation Type Algorithm
Temperature information is also used to limit some decisions and to detect freezing precipitation
Maximum droplet size and capacitive sensor signal are used to distinguish:
drizzle, ice pellets, different snow types

13. FD12P Main Parts Transmitter unit FDT12B Receiver unit FDR12
Capacitive sensor
Transmitter unit FDT12B
Receiver unit FDR12
Capacitive sensor DRD12
Temperature sensor DTS14
Processor board FDP12
Interface card DRI21
Regulator unit FDS12
Power supply FDW13
Modem DMX21 as an option
Transmitter
Receiver
Controller unit,
includes:
- processor board
- power regulator
- power supply
- interface card

14. Optical Configuration: Transmitter
Mechanical structure
LED intensity feedback signal
Power monitoring
Constant intensity
control
Transmitter LED
Photo diode
Lens
Backscatter monitoring photodiode
- optical path blocked
- lens contamination (back scatter)

15. Optical Configuration: Receiver
Mechanical structure
IR filter
Receiver
photodiode
Lens
Backscatter monitoring LED
- optical path blocked
- lens contamination
- receiver self-test

16. Forward Scatter Measurement
Receiver
photodiode
Transmitter
LED
Transmitter
Receiver

17. OFFSET Measurement
Zero scatter signal is measured by delaying transmitter pulses (equals switching off the transmitter)
Ambient light
Transmitter
Receiver
Receiver signal level = zero (offset) signal level + noise from ambient light and electrical circuits

18. Contamination Measurement
Transmitter
LED
Backscatter monitoring
photodiode
Backscatter monitoring LED
Transmitter
Receiver

19. Self-diagnostics
The FD12P checks that all measurement signal levels are within realistic limits
receiver signal and offset levels
number of samples
DRD12 output level
temperature (TS)
In addition the FD12P measures many internal parameters and checks that they are close to their nominal values
A warning or alarm is generated automatically if a value is not within limits
Sensor status message provides a description of the failure

20. Monitored Parameters The following internal parameters are measured
FDT12B
transmitter backscatter
-15V supply voltage level (M15)
+15V supply voltage level (P15)
transmitter LED control voltage (LEDI)
timing circuit duty cycle (DUTY)
transmitter ground level - processor board ground (BGND)
FDR12
receiver backscatter
ambient light level (AMBL)
FDP12
lens heating current (VH)
stepped-up supply voltage (VBB, approximately 20V)
box temperature (TE, only a backup measurement)

21. STAtus Message The output of STA command shows the diagnostics results
SIGNAL OFFSET DRIFT
REC. BACKSCATTER CHANGE
TR. BACKSCATTER CHANGE
TE VBB VH
LEDI
P M BGND
AMBL DUTY
DRI21 MEASUREMENTS
TS DRD INST DRY
HARDWARE : OK
An asterisk (*) is shown in front of a parameter if the value is not within limits
after the text “HARDWARE:” there will be a verbal description of the problem
Status can also be polled as output message type 3

22. Hardware Error Texts BACKSCATTER HIGH
- receiver or transmitter contamination signal has changed more than the ALARM limit given in configuration
TRANSMITTER ERROR
- LEDI signal bigger than 7V or less than -8V
±15 V POWER ERROR
- receiver/transmitter power less than 14V or more than 16V
OFFSET ERROR
- offset frequency is zero (cable disconnected)
SIGNAL ERROR
- signal frequency less than 50% of the offset frequency
RECEIVER ERROR
- too low signal in receiver backscatter measurement
DATA RAM ERROR
- error in RAM read/write check
EEPROM ERROR
- EEPROM checksum error

23. Hardware Warning Texts
BACKSCATTER INCREASED
- receiver or transmitter contamination signal has changed more than the
WARNING limit selected in configuration
TRANSMITTER INTENSITY LOW
- LEDI signal less than -3V
RECEIVER SATURATED
- AMBL signal less than -9V
OFFSET DRIFTED
- offset drifted more than ±2.5Hz from the reference, set in the configuration
LENS HEATER OFF
- no current flowing to lens heaters
DRI21 NOT CONNECTED
- DRI21 board can not be detected
TS SENSOR ERROR
- DTS14 measurement off limits
DRD12 ERROR
- DRD12 analog signal close to zero
VISIBILITY NOT CALIBRATED
- visibility calibration coefficient has not been changed from the default value

24. FD12P Maintenance Periodical maintenance: Tools:
cleaning of the optical path, lenses and the DRD12 surfaces (recommended interval 6 months or depending on local conditions)
optical measurement calibration (recommended interval 6 months)
temperature measurement calibration (recommended interval 1 year)
Tools:
optical cleaning cloth and fluid (e.g. isopropanol)
FDA13 (or FDA12) visibility calibrator
ice bath or a calibrated reference thermometer

25. Cleaning The hoods and tubes in front of lenses should be cleaned
Lenses should be cleaned carefully
DRD12 sensor elements can be cleaned with the same fluid and cloth as the lenses
After cleaning the STAtus message should be checked
if the backscatter CHANGE values are not close to zero:
try to clean the lenses again
use CLEAN command to set new reference values

26. 2. Visibility calibrator
Optical Calibration
2. Visibility calibrator
Blocking plate is first used to check the zero signal level
the CHECK command is used to see the (scaled) output frequency
Visibility calibrator is then used to produce a standard scatter signal (high signal value)
the visibility calibrator nominal value has been defined at the factory
if the sensor output frequency is not within 3% of the nominal value, the sensor is calibrated with CAL command (CAL nominal_freq)
1. Blocking plate

27. Temperature Calibration
The temperature sensor is put in an ice bath (in thermos flask)
zero degrees C reading is verified with FREQ command
reading is updated once per minute
if the reading is not within 0.1 degrees the sensor is calibrated with command TCAL TS 0.0
Reference thermometer can be used to check the true temperature of the ice bath
other calibration temperatures can also be used (ice bath is not absolutely necessary)
Temperature sensor
in ice bath

28. Troubleshooting The sensor can automatically detect most problems
the sensor indicates a warning or error in the output message
If the sensor has indicated an error, the user should check the STAtus command output (or message type 3)
the status will give a verbal description of the problem
erroneous value is indicated by an asterisk (*)
Chapter 6 in the FD12P User’s Guide contains more troubleshooting instructions
see especially the chapter “Values for internal monitoring” for suggestions of corrective actions