Backscatter Cloud Probe (BCP, part of Package1)

Instrument PI:     Gary Lloyd, The University of Manchester (

The Backscatter Cloud Probe (BCP) was originally intended as a qualitative indicator of cloud presence. It uses backscattered light to determine the size of particles passing through the sample area of the instrument located a short distance from the fuselage (Figure 1, left panel). The depth of field of the laser beam, where scattered light is focused onto the photodetector, is defined by a threshold signal level. The BCPs used by IAGOS are currently configured to detect particles between 5 and 75µm optical-equivalent diameter, i.e. assuming scattering properties for spherical water droplets of refractive index 1.33 by default, although the data can be re-analysed using other particle property assumptions.

Figure 1: Schematics of the BCP
Figure 2: Optical window on the IAGOS inlet plate

In the current configuration, the BCP creates a ten-channel size spectrum. The sizing information requires a detailed knowledge of the laser beam and sample area, and the particle detection efficiency as a function of particle size.

The instrument comprises two main parts, the optical head and the electronics interface. The optical head is attached to the IAGOS inlet plate. Its mount which protrudes slightly from the aircraft skin and has a channel to divert water on the aircraft skin away from the instrument’s sapphire window when in flight. The electronics interface is mounted on Package1.


The instrument is designed for autonomous deployment over periods of up to 6 months. The data are transmitted after flight via telecommunication (GSM) to the IAGOS data centre. Prior to and after each deployment, the BCP is calibrated either by the manufacturer or by the instrument PI using standard reference particle size and concentration sources. For details see Beswick et al., 2014 or the SOP.


The BCP is installed in all IAGOS-CORE aircraft based on EASA STC obtained in 2011 for Airbus A340 and in 2014 for Airbus A330 aircraft.


BCP was developed by DMT and is integrated by enviscope GmbH. Responsible Design Organisation (DO, EASA Part 21J): Sabena Technics BOD, Bordeaux.