CS135 LIDAR Ceilometer
10km Ceiling LIDAR Ceilometer

Cloud measurement with advanced signal processing
weather applications supported water applications supported energy applications supported gas flux & turbulence applications supported infrastructure applications supported soil applications supported


The CS135 LIDAR Ceilometer measures cloud height and vertical visibility for meteorological and aviation applications. Using LIDAR (light detection and ranging) technology, the instrument transmits fast, low-power laser pulses into the atmosphere and detects back-scattered returns from clouds and aerosols above the instrument.

The CS135 complies with CAA and ICAO guidance and meets or exceeds all recommendations and specifications. (This includes CAP437, CAP670, and CAP746.)

Tilt capability to 24° allows the sensor to be operated anywhere in the world without the sun shining directly into the lens.

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Benefits and Features

  • Single lens design
  • Robust, reliable Campbell Scientific electronics
  • Mixing layer height assessment for air quality applications (feature requires a key to unlock at extra charge)
  • Reports cloud height and sky condition to WMO and ICAO recommendations
  • Three year extended warranty available on this product
  • State-of-the-art signal processing
  • Bird spike kit option feature to deter birds from perching on the CS135
  • Made in UK
  • Competitive pricing
  • Built-in stratocumulus-based calibration
  • Dual clock comparison for confidence in reliable operation
  • View data conveniently with Viewpoint software
  • Integrated heater, blower, and radiation shield
  • Filter to protect detector from direct sunlight


Technical Description

The CS135 employs a single lens design to increase optical signal-to-noise ratio over other instruments. One half of the lens is used for the transmitter and the other for the receiver with total optical isolation between them. This allows it to have exceptional performance with low altitude overlap between the transmitter and receiver allowing lower altitude measurement and integrating larger optics into a compact package. The optics are immune to damage from direct sunlight. The optics are also immune to damage from direct sunlight.

This approach, along with state-of-the-art electronics, provides a powerful and stable platform from which to measure cloud height and vertical visibility to high accuracy. With a rugged environmental enclosure that protects the instrument from the harshest conditions, the CS135 measures the atmosphere with high stability and repeatability. 

The CS135 can be tilted up to 24º. Cloud heights are automatically corrected. A small tilt is an important feature as it allows the CS135 to resist high levels of reflection from large raindrops and frozen particles that can impair a vertical sensor. Tilting to 24º means that it can be operated anywhere in the world without the sun shining into the lens and resulting in missing data (the options are, however, immune to damage from direct sunlight).

The CS135 provides information on cloud height, sky condition (up to five layers), vertical visibility and raw backscatter profiles.

As an option Mixing Layer Height can be calculated within the CS135 and inserted in data messages.

It has a unique stratocumulus calibration capability for accurate measurements of scatter coefficients.

Reliable range measurement is assured by cross checking two internal clocks.

Mixing Layer Height assessment for air quality applications**

Mixing Layer Height (MLH) is an important parameter in modelling air quality and predicting air pollution episodes. It is also a very difficult parameter to measure without expensive sounding systems. The new operating system includes an MLH assessment option.

This retrieves the height of the mixed aerosol layer by applying the gradient method to the ceilometer’s backscatter signal. The automated process is based on the operational algorithm used by KNMI* and searches for the drop in backscatter associated with the transition from boundary layer aerosols to free troposphere. Because the signals measured depend on the type and amount of aerosol present, as well as the background light level, the accuracy of the method varies. Therefore, a quality factor is assigned, which indicates the confidence in the reported layer height. 

The algorithm runs within the CS135 itself, and the results are incorporated in data messages, making it easy to incorporate the MLH into whatever systems are already in use without the need to run external special software. It is activated by an alphanumerical "key."

*Determination of mixing layer height from ceilometer backscatter profile, Marijn de Haij; Wiel Wauben; Henk Klein Baltink 

Proc. SPIE6362, Remote Sensing of Clouds and the Atmosphere XI, 63620R (October 11,2006): doi:10.111/12.691050

Read a white paper Comparison of Mixing Layer Height Retrieved By Ceilometer and Doppler Lidar

** Chargeable extra

Stratocumulus based calibration 

Calibration of the actual magnitude of the scatter returned by a ceilometer is not simple. The CS135 now includes a process to make this calibration easier for users looking for more information than basic cloud height or sky condition. Reliable values for scatter coefficients are important for research applications, as well as if ceilometer raw scatter profiles are to be compared between locations or over time, for example in cloud studies or following the location of plumes of pollution or ash.

The attenuated backscatter is calibrated by an automated process based on the method developed by O’Connor et al (2004)*. The method uses the well-understood scattering properties of a fully attenuating stratocumulus cloud as a reference. The calibration requires a stable stratocumulus layer with no precipitation present. The integrated lidar signal measured can then be scaled to match the expected integrated attenuated backscatter. This is carried out in response to a set of simple commands entered via a serial link. 

The CS135 can output a scatter profile with a 5 m resolution at intervals between 2 and 600 seconds.

*O'Connor, Ewan J., Anthony J. Illingworth, and Robin J. Hogan. "A Technique for Autocalibration of Cloud Lidar." Journal of Atmospheric & Oceanic Technology 21, no. 5 (2004).


The response time of the inclinometers on the CS135 has been reduced to two seconds. This means it can return accurate, tilt corrected, cloud heights from moving platforms such as floating offshore structures or large warships. The CS135 has inclinometers in two axes, and the correction for cloud height is totally automatic.

Clock checking

The accuracy of a cloud height measured by a ceilometer ultimately depends on the accuracy of the built-in clock used to measure the return time of scattered laser pulses. The CS135 provides a check on the accuracy of the main processor clock by comparing it with the output of an independent electronic clock. Any discrepancy will trigger an alarm.

Heater/blower control

The new OS includes a more sophisticated system of heater/blower control, taking into account the presence of cloud and window contamination.

Viewpoint display software

Ceilometer data can now be conveniently displayed using the Viewpoint software. This will display the output from the ceilometer in a convenient and completely configurable form. Sky condition, mixing layers, scatter profiles, etc., can all be displayed simultaneously or separately with ranges and time scales configurable.


Temperature Range
  • -40° to +60°C (-40° to +140°F) excluding battery
  • -20° to +50°C (-4° to +122°F) with standard battery
Battery Temperature Range -20° to +50°C (Alternative battery types with wide temperature ranges can be supplied to achieve the -40° to +60°C range.)
Humidity Range 0 to 100% RH
IP Rating IP66 (NEMA 4x)
Dimensions 100 x 33.0 x 31.6 cm (39.4 x 13.0 x 12.4 in.) including base
Shipping Dimensions 120 x 45.0 x 45.0 cm (47.2 x 17.7 x 17.7 in.)
  • 32 kg (71 lb) without cables
  • 25 kg (55 lb) without cables, outer cowling, and enclosure
Shipping Weight 58 kg (127.9 lb)

Instrument Performance

Reporting Range 0 to 10 km (0 to 32,808.4 ft)
Minimum Reporting Resolution 5 m (15 ft)
Hard Target Range Accuracy ±0.25%, ±4.6 m
Reporting Cycle 2 to 120 s
Cloud Layers Reported Up to four layers
Sky Condition Up to five layers with cover in oktas according to WMO requirements for SYNOP and METAR codes as standard
Vertical Visibility Reported when no clouds selected
Laser Wavelength 912 nm (±5 nm)
Eye Safety Standard Class 1M
Maximum Wind Speed 55 m/s (wind tunnel tested)


Power 110, 115, 230 Vac ±10%, 50 to 60 Hz, 470 W maximum
Battery Internal 12 Vdc, 2 Ah battery backup

Provides 2 h measurement, without blower/heater, in the event of mains failure.


Data Interfaces RS-232, RS-422, RS-485, Ethernet
Maintenance Interfaces USB 2.0 (USB 1.1 compatible)
Baud Rate Interfaces 300 to 115200 bps


Laser Safety Compliance EN60825-1:2001
Electrical Safety Compliance EN61010-1


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


Product Compatible Note
CR200X (retired)
CR216X (retired)
CR5000 (retired)


Campbell Viewpoint v.2 (79 MB) 21-07-2016

Campbell Viewpoint is viewing software designed to accompany the Campbell Scientific optical sensor range, including the CS135 ceilometer, CS120A visibility and CS125 present weather sensors.  It is free to download and use for 100 hours, after which you will need to purchase a key to continue using it.  It is available with single or multiple sensor viewing options and also with data logging capability, please contact your Campbell Scientific office or representative for further information.

View Revision History

OS12 for CS135 Ceilometer v.12_1 (211kb) 30-10-2018

Campbell Scientific has introduced a new operating system, OS12 for the CS135 ceilometer. It is fully backwards compatible. OS12 is available free of charge.

OS12 has introduced improvements to the optical overlap correction function.  This has corrected previous over-sensitivity to signal returns at very low altitudes.  

This change results in the reduced detection of false, fixed-level, low cloud height reports during some types of rain and the reporting of cloud layers when there are returns from low lying layers of mist or pollution.

FAQs for

Number of FAQs related to CS135: 7

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  1. The CS135 has LED indicator lights that flash to indicate normal or fault statuses.

    • 1 flash every 10 seconds = OK, no fault
    • 2 flashes every 10 seconds = warning (possible degraded performance)
    • 3 flashes every 10 seconds = alarm (measurements not possible)

    The output messages also contain information about Warnings and Alarms, known as the WA output. This is a single character with one of the following values:

    • 0 = No alarm or warning
    • W = Warning
    • A = Alarm

    W or A are output in the message strings when necessary.

    The description of the warning or alarm is obtained from the fault Flag code at the end of each message.

  2. Warnings are identified by the instrument when system parameters go beyond the optimum manufacturer-defined range, but where this does not prevent the operation of the sensor. An example of this is a temperature that is non-critically beyond its normal range.

    Note: For some warnings, you might not be able to take corrective action yourself.

  3. Alarms are identified by the instrument if a self-test parameter goes beyond the range of a user-defined or manufacturer-defined setting.

    Examples of manufacturer-defined settings include temperatures and circuit power. Alarms arise when a fault is identified that prevents measurements from being taken.

    User-definable alarms include tilt-angle and lead-acid battery shut-down voltage. If the ceilometer senses that these are out of range, it will report alarms in the message string, as well as in the Alarm Flag section of each message.

  4. Cloud height

    Cloud height is commonly used to refer to the height of the cloud base above ground level. (This is what the ceilometer reports.) However, it can also be used to refer to the thickness of a cloud, which is the difference in height between the base and very top of the cloud. It is possible for a ceilometer to estimate the thickness of thinner clouds. Occasionally, when reporting satellite data, it can also be used to refer to the height of the very top of a cloud above ground level.

    Cloud layers

    At any one time, there may be several separate layers of cloud above a point on the ground. Each of these is a cloud layer, and each is given a cloud base height when it is detected by a ceilometer. The WMO (World Meteorological Organization) has set out rules that govern the minimum vertical separation that is required between layers before reporting more than one layer of cloud.

    Cloud base

    The cloud base is the lowest part of a cloud that passes overhead. It is typically measured with a ceilometer.

  5. The cloud ceiling height relates to the height of the base of the cloud where the sky is overcast or has broken cloud cover.

  6. In the unlikely event that a warning or an alarm is issued, the warning flag output indicates the nature of any fault.

    Few warnings or alarms are actionable by the user—except, for example, the dirty photo-diode and Laser windows alarm. Other alarms may indicate a failure requiring work that cannot be performed by the user.

  7. Some alarms, such as the tilt-angle and low-power battery shutdown voltages, are configurable. Most parameters are not configurable because they relate to the internal safe functioning of the ceilometer.

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