Present Weather Sensor FS11P

Runway visual range (RVR), aeronautical and synoptical visibility, and present weather observation

Model: FS11P
Manufacturer: Vaisala – Finland


  • Unique window contamination correction
  • Uses the same principles as the renowned FD12P weather sensor
  • Accurate and traceable forward scattering measurement
  • Technical design and high-power heating according to FAA guidelines
  • Frangible composite fiber mast

Vaisala Present Weather Sensor FS11P is intended for the most demanding weather applications such as runway visual range (RVR), aeronautical and synoptical visibility, and present weather observation. It provides the optimal combination of the best accuracy, the highest reliability, broad measurement range, and low maintenance need.


  • Excellent value: RVR, visibility, and present weather all-in-one
  • The most widely proven forward scatter RVR sensor
  • Visibility sensor selected and used by FAA
  • Meets FAA and ICAO specifications
  • Can be used for both aeronautical and synoptical applications
  • Minimal maintenance needs
  • Compatible with FD12P
  • Meets ICAO frangibility standards

Scientifically Valid Chain of Calibration

FS11P is calibrated through a scientifically valid chain of reference. The scattering response of the calibration device can be clearly traced to a reference FS11P visibility sensor, which is in continuous operation at Vaisala outdoor test field along with reference transmissometers and other instrumentation. The visibility measurement of FS11P is also traceable to FAA reference sensors.

Low Maintenance Need

FS11P incorporates a technique that measures and compensates for window contamination. It ensures unparalleled measurement accuracy between window cleanings. It also enables much longer window cleaning intervals than compared to conventional visibility sensors.

The unique system works by monitoring the total reflectance of the window surface. It automatically compensates for visibility measurement errors caused by window contamination.

The sophisticated self-diagnostics and modular design allow for very short service times. The visibility measurement fork, present weather detector, and  optional background luminance meter are independent instruments that can be replaced quickly as pre-calibrated spare parts.

Reliable Operation in the Harshest Weather

Four main design features are combined in FS11P to ensure reliable operation in the harshest weather. The first is the window contamination compensation technique. The second is the “head-down” design of the optical heads, which protects them against virtually all windblown particles (even those flying horizontally). High-power heaters are the third feature. Each heater has its own temperature monitoring and control mechanism to prevent snow accumulation during the heaviest snowstorm. As a final measure, there is an optical path clearance monitoring circuitry to verify that measurement is not affected by obstructions

Technical Compatibility

In addition to the technological similarity, FS11P is compatible with, and can be used to replace, FD12P. They both provide the same mechanical, electrical and communication interfaces, and they transmit similar message formats widely used in aviation and meteorology.

Measurement Performance


Measurement range of MOR  5 … 75 000 m (16 ft … 46.6 mi) with 1, 3, and 10 min averaging
Accuracy ±10 % range 5 … 10 000 m (16 ft … 6.2 mi)

±20 % range 10 000 … 75 000 m (6.2 … 46.6 mi

Scatter measurement accuracy ±3 %


Optical Specifications


Operating principle Forward scatter measurement
Scattering angle 42°
Light source Near-infrared LED 


Weather Measurement Performance


Weather type identification 7 different types of precipitation (rain, freezing rain, drizzle, freezing drizzle, mixed rain/snow, snow, ice pellets) 

Precipitation (unknown type) Fog (mist), haze (smoke, sand) or clear 

Weather type reporting WMO 4680 (SYNOP), 4678 (METAR), and NWS code tables; 49 diƒerent codes supported from the WMO 4680 code table
Precipitation detection sensitivity 0.05 mm/h (0.0020 in/h) or less, within 10 minutes
Precipitation intensity measurement 0.00 … 999.99 mm/h 

(0.00 … 39.37 in/h)

Precipitation amount measurement 0.00 … 99.99 mm (0.00 … 3.94 in)
Amqount of new snow 0.00 … 999 mm (0.00 … 39.33 in)


Operating Environment


Operating temperature −40 … +65 °C (−40 … +149 °F), 

−55 … +65 °C (−67 … +149 °F) 


Operating humidity 0 … 100 %
Wind speed  Up to 60 m/s (134 mph)


Inputs and Outputs


AC (mains) power supply 100/115/230 VAC ±10 %, 50 … 60 Hz
Power consumption Max. 370 VA (50 VA + 320 VA defrosting heaters) with options
Battery backup option Battery 2 Ah, typical backup time 30 min at 25 °C (77 °F) and 5 min at −40 °C (−40 °F)
Outputs Serial data line RS-232 or optoisolated RS-485 (2-wire) or optional data modem

Separate maintenance line RS-232 +12 VDC max. 0.8 A output for option powering


Mechanical Specifications


Dimensions (H × W × D) 2.8 × 0.9 × 1.0 m (9.19 × 2.95 × 3.28 ft)
Weight without mast  37 kg (81.57 lb)
Weight, including Frangible Mast FSFM250 52 kg (115 lb)
IP rating IP66
Mast Frangible and hinged glass fiber mast


Spare Parts and Accessories


Calibration set FSA11 
Background luminance sensor LM21
Battery backup FSB101
Modem for long distance communication (> 1 km / 0.6 mi) DMX501
Obstruction light FS110BS
Calibration set PWA12
Maintenance cable QMZ101




EMC Compliance
Radiated emissions  EN55022
Radiated susceptibility IEC 61000-4-3, 10 V/m
Conducted emissions EN55022
Conducted susceptibility IEC 61000-4-6
EFT immunity IEC 61000-4-4
ESD immunity IEC 61000-4-2
Surge IEC 61000-4-5
Harmonics to the AC (mains) grid IEC 61000-3-2

Present Weather Sensor FS11P


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