A state-of-the-art Doppler lidar is in development for the FAAM Airborne Laboratory’s atmospheric research aircraft, just one of the new instruments being delivered through the Mid-Life Upgrade programme.

A blue and white research aircraft flying above puffy white clouds.

This project is led by Ryan R. Neely III (University of Leeds) & Hugo Ricketts (University of Manchester) and will see the development of a lidar that will be able to make near-real-time wind profile measurements along the lidar beam. 

“Along with other instruments on board the aircraft, we can use the Doppler lidar to gain an in-depth understanding of the hazardous weather conditions such as turbulence, movement and mixing of pollutants, and other atmospheric dynamics, especially in hard-to-reach and large areas such as over the ocean, at higher altitudes or over mountain regions,” says Hugo Ricketts, co-leading the project. “Together with another lidar system that is being developed for the FAAM Airborne Laboratory, we will also be able to track moving or fast-developing targets, such as volcanic plumes and emissions from ships or fires.”

A computer render of a new doppler lidar. The equipment is designed to be mounted on a special window in the underside of an aircraft fuselage.

A computer render of the lidar, with its cover removed. It will be highly compact, making best use of limited space in the aircraft.

The bespoke lidar, delivered by Fraunhofer CAP, measures wind velocity in real-time along its steerable, eye-safe, laser beam with 45 metre resolution and a maximum range of 10 kilometres. This provides FAAM users an opportunity to measure profiles of horizontal wind speed & wind direction, profiles of the 3D wind components and as a bonus, aerosol backscattering properties may be inferred from lidar signal strength as well.

“Whereas the US and Germany have previously performed dedicated science flights with similar instruments, having an airborne Doppler lidar routinely operated on a research aircraft alongside other science instruments will be a world-first and will put the UK in a leading position in atmospheric science,” adds Matthew Warden, Principal Researcher at Fraunhofer CAP.

What is lidar?

We have all encountered the concept of lidar (LIght Detection And Ranging) systems, perhaps even without realising it. On a basic level, its operation is similar to that of the sonar in submarines, parking sensor in a car or echolocation bats use, but instead of sound waves, regular, non-Doppler, ground-based lidar helps us determine a distance to the target by measuring the time required for the light to reach said target and return. Lidars are often used for 3D scanning, terrain mapping or in autonomous vehicles for obstacle detection.

Doppler lidar has the advantage of providing information on the direction and speed of the target, as the moving target causes a frequency shift of backscattered light. 

If you have ever heard a vehicle speeding by with a siren on, you know that as it approaches and passes you, the frequency of the sound changes. This is the classic example of the Doppler effect, which is often used by police in radar speed guns. Doppler lidar works on the same principle but with light instead of sound.

Pushing the limits on lidar capabilities

The new instrument takes it a step further, as FAAM’s Doppler lidar will be airborne and, by using a highly sensitive detection method (so sensitive that it approaches fundamental limits imposed by quantum mechanics), it allows us to measure the back-scattered light from clouds & aerosols as we fly through the air. 

Our aircraft is also moving as it flies, which presents an extra challenge. To counter this, the system will have an automatic motion correction capability to account for the movement of the aircraft and increase the accuracy of measurements. By default, the lidar will measure 3D wind speed underneath the aircraft by scanning its laser beam in a conical pattern (a technique frequently used by lidars in the wind energy industry), but the lidar will also be able to produce bespoke scan patterns that best suit our users’ science goals.

Lidar equipment and a telescope mounted on a trolley in a paved area. The telescope is pointed at the sky.

Early-stage testing of the lidar system outside Fraunhofer CAP’s site in Glasgow. The silver tube is a bespoke telescope developed to maximise lidar signal strength, providing the best possible data quality and availability for users.