CS120A Atmospheric Visibility Sensor
High performance visibility sensor at an economical price
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The CS120A uses tried-and-tested, infrared forward-scatter technology, and it uses the proven 42° scatter angle to report meteorological observable range (MOR) for fog and snow in the range of 5 to 75,000 m (16.4 to 246,063 ft). It combines a high specification with a very competitive price. The CS120A is ideal for stand-alone applications or in combination with automatic weather stations in road, aviation, solar-energy, and wind-energy environments.

For aviation applications, users can be assured that the CS120A complies with UK CAA, FAA, and ICAO guidance and meets or exceeds all recommendations and specifications. (This includes CAP437, CAP670, and CAP746.)

The CS120A is certified by Deutscher Wetterdienst as suitable for use to control wind turbine obstruction light systems as specified by 506/04, General Administrative Rules for the Identification of Aircraft Obstructions.

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Benefits and Features

  • High performance sensor at a competitive price
  • Three year extended warranty available on this product
  • Sensor design minimises airflow disruption at measurement volume
  • Incorporates automatic dew and hood heaters for all-weather operation
  • Simple field calibration using optional calibration kit
  • Low power - suitable for remote application
  • Automatic status check for faults or window contamination
  • Made in UK
  • Type certified for Aviation use by the German Meteorological Service, Deutscher Wetterdienst (DWD)


Detailed Description

The CS120A Visibility Sensor uses tried and tested infra-red forward scatter technology and utilises the proven 42° scatter angle to report Meteorological Observable Range (MOR) for fog and snow in the range 10 – 75,000 m. It combines a high specification with a very competitive price and is ideal for stand alone applications or in combination with automatic weather stations in road, aviation and wind energy environments.

Compared to many such sensors the CS120A design means that visibility is being measured in a relatively 'clean' space because the position of the heads and body minimise disturbance of the airflow at the measurement volume.

High speed sampling improves the accuracy of measurements taken during mixed weather and during events which return intermittent signals such as rain and hail – both known weaknesses of some competing systems.

Integrated low power heaters prevent the build up of dew and higher powered anti-icing heater are also included. All heaters are automatically controlled for simple operation in all weathers.

The sensor is very power efficient drawing just 3W during normal operation including the dew heaters, power can be reduced further by reducing the sample rate and manual control of the heaters. 

Two configurable alarm outputs are provided and, via relays, these can drive external warning systems such as lights and fog-horns, they can also be used to switch the intensity of wind turbine warning lights depending upon current visibility levels.


Signal Type/Output RS-232, RS-485
Measurement Description Meteorological Observable Range (MOR)
Maximum Reported Visibility 100 km (62.1 mi)
Minimum Reported Visibility 5 m (16.4 ft)
  • ±8% at < 600 m (< 1968.5 ft)
  • ±10% at 600 to 10,000 m (1968.5 to 32,808.4 ft)
  • ±15% at 10,000 to 15,000 m (32,808.4 to 49,212.6 ft)
  • ±20% at 15,000 to 75,000 m (49,212.6 to 246,063 ft)
Resolution 1 m (3.3 ft)
Mounting Stainless-steel V-bolt bracket that attaches to a pole with a 32 to 52.5 mm (1.25 to 2 in.) outer diameter
Electronics Supply Voltage 7 to 30 Vdc
Total Unit Power < 3 W while sampling continuously (including dew heaters)
Standards Frangible masts are available to customer requirements to meet ICAO recommendations (typically placing the sample volume at 2.5 m [8.2 ft]).
Sensor Dimensions 540 x 640 x 246 mm (21.26 x 25.2 x 9.7 in.) including mount
Sensor Weight ~3 kg (6.6 lb) depending on mounting system


LED Center Wavelength 850 nm
LED Spectral Bandwidth ±35 nm
Light Pulse Rate 1 kHz


Operating Temperature Range -25° to +60°C (standard)
Extended Operating Temperature Range -40° to +70°C (This extended version is available as a special. Contact Campbell Scientific for more information.)
Operating Humidity Range 0 to 100%
Sensor Sealing Rated to IP66
Wind Speed Up to 60 m s-1
Sensor Heater Threshold
  • < 35°C (dew heater on)
  • > 40°C (dew heater off)
  • < 15°C (hood heater on)
  • > 25°C (hood heater off)

DSP & Dew Heaters

Power 2 x 0.6 W (total of 1.4 W) for dew heater
Typical Current Consumption @ 12 Vdc
  • 200 mA (continuous sampling, dew heaters active)
  • 110 mA (continuous sampling, dew heaters disabled)
  • 21 mA (no sampling, dew heaters disabled)
  • 200 mA (dew heaters active, RS-232 communications)

Hood Heater

Supply Voltage 24 V dc or ac
Power 2 x 30 W (total of 60 W)


Serial Interface RS-232 or RS-485, 8 bit data bytes, 1 stop bit
Serial Data Rates 1200 to 115,200 bps (38,400 bps default rate)


Please note: The following shows notable compatibility information. It is not a comprehensive list of all compatible products.


Product Compatible Note
CR1000 (retired)
CR3000 (retired)
CR800 (retired)
CR850 (retired)


CR1000X CS120A example programs v.1 (2 KB) 18-03-2020

CR1000X programs that use the MSSET and MSGET commands.

CS125 and CS120A OS v.17 (555 KB) 08-06-2023

Campbell Scientific has introduced a new operating system, OS 17, for the CS120A visibility sensor and CS125 present weather sensor. It is backwards compatible and is easily installed on all CS120A and CS125 sensors (but not older CS120 sensors). OS 17 is available free of charge and can be downloaded here or supplied by email on request.

Upgrade to OS 17 is recommended.

View Update History

Campbell Viewpoint v. (86 MB) 09-10-2019

Campbell Viewpoint is viewing software designed to accompany the Campbell Scientific optical sensor range, including the CS135 ceilometer, SkyVue PRO, SkyVue 8, SkyVue 8M, CS120A visibility and CS125 present weather sensors. It is free to download (or via email on request) and use for 100 hours, after which you will need to purchase a key to continue using it.


Viewpoint version has introduced our new ceilometer sensors to the list of instruments that can be monitored.  This includes the SkyVue PRO, SkyVue 8 and SkyVue 8M.  

Previous sensors are still available.

Version also includes some minor bug fixes and improvements to the help menu.

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