Laser Altimeters have been used in space and ground-based / aircraft applications for many years. This presentation concentrates on recent technology developments and the possibility of exploiting the laser ranging technique in a multitude of relatively new areas:

These applications can all exploit high rep-rate multi-beam laser systems with multi-element photon-counting detector arrays with appropriate high-precision timing and image reconstruction to provide high-resolution 3-D images of the sub-orbital path. The ability of Imaging Lidar to penetrate cloud and other obscurations can also be exploited.

Figure 1:

These abilities can be extended to the bathymetry of surface water over land areas and the study of sub-surface regions to depths of order 50 – 100 metres in the Continental Margins.

Figure 2:

Table 1:

Some Previous Space Missions Exploiting Laser Altimeters

Illustration of the historical « analogue » laser altimeter systems

Using a relatively large « spot size » and low repetition rate, the ground resolution of analogue laser altimeter systems is limited to a few hundred metres

Clouds/aerosol characterization from ICESat data

The image at left illustrates the sophisticated range of atmospheric measurements obtainable from an altimeter system such as ICESAT.

The image at right is combined from MODIS data covering the track.

Greenland Height change map (ICESat) Mass Change (GRACE).

Conceptual illustration of laser ranging approaches

Operational Requirements – footprint / beam divergence

Misson Requirements for Advanced Altimeters for Terrestrial, lunar and Planetary Missions

CryoSat-2 (top) and IceSat-2 (bottom)

LOLA – Lunar Orbiter Laser Altimeter

What can an Imaging Lidar System provide?

Given a clear atmospheric conditions or cloudy / foggy conditions:

Detection, and Identification are subject to the normal rules of optical resolution / distance and signal to noise ratio etc.

Fog and cloud causes blurring and diffusion in normal vision.

How does the Imaging Lidar avoid this?

The negative effects of a foggy or cloudy atmosphere on normal vision and imaging are familiar:-

The Imaging Lidar circumvents the blurring and diffusion caused by fog and cloud by the combining of temporal and spectral filtering with image deconvolution.

Logarithmic presentation of Signal from Target behind Cloud.

Clear Atmosphere:

The signal, range-corrected, drops off slowly due to attenuation and the decrease of atmospheric density with altitude. The “clear sky” signal is slightly lower than when a cloud layer is present.

Logarithmic presentation of Signal from behind Cloud.

In addition to producing 3-D images of the distant field, the imaging Lidar – when using a 2D imaging photon detector with 100 psec time-tagging has a remarkable capability for detecting and imaging targets through cloud and fog, well beyond the visibility offered by eye or normal camera.

A standard Nikon Camera shows the comparative views under clear and foggy conditions.

A best estimate of normal “visibility” in the fog (right) is between 30 – 50 metres.

The following images demonstrate the capability of the Imaging Lidar to not only image the far field in 3-D, but also to provide essentially the same information when the visibility is severely compromised by heavy fog. The red spot covers trees at a distance of 600 metres. This is some 12 times the “visibility” as indicated by the camera images!

Scanning Lidar System with PMT detector – observing trees etc at ~800 – 1000 metres.

This image shows the back-scattered signal from the lidar used with a conventional

PMT detector and operated in a 2-D “Scanning Mode”. Horizontal distance (perpendicular to the line-of-sight) is now running from left to right.

The structures are trees and fence-posts at distances of 800 – 1000 metres from the lidar.

This image PMT detector and in “Staring Mode” shows Aeolian waves within the fog. All totally invisible by eye or camera!!

« Staring » lidar observing trees / fences at 800 metres through dense mist / fog.

The image shows the propagation of Aeolian waves along the valley, within the fog layer.

These features were totally invisible by eye or to a normal camera.

Zero Range is at top!

This image shows the back-scattered signal from the lidar used with a conventional

PMT detector and operated in “Staring Mode”. Time is running from left to right.

The zero range for the atmospheric signal at short range is at the top, showing dense fog at the start, clearing slightly in the period to the right. A range of tree / fence structures are then visible (near bottom), despite the continuous but slightly less dense fog.

Signal versus range. Image – the white spot is a tree at 600 metres

There is a weak “clear sky” atmospheric signal at short range.

There is a strong signal return from the tree at 4 msec (to left).

The “atmospheric” signal is also present.

Signal versus range. Image – the white spot is a tree at 600 metres

The “clear sky” atmospheric signal at short range is totally dominated by the return from a tree.

This is the very strong signal return at 4 msec (to right) from the tree

Signal versus range. Image – the white spot (37) is a tree at 600 metres.

This image shows the back-scattered signal return from pixel 37.

The atmospheric signal at short range is strong (enhanced by the mist / fog).

There is a weak but easily measured signal return at ~4 msec (to right) from the tree.

Signal versus range. Image – the white spot (37) is a tree at 600 metres.

The atmospheric signal at short range is strongly enhanced by the mist / fog.

There is a weak but easily measured signal return at ~4 msec (arrow to right) from the tree.

Signal versus range. Image – the white spot (35) is a tree at 600 metres.

This image, at 22:42 UT, shows the back-scattered signal return from pixel 35.

The atmospheric signal at short range is strong (enhanced by the mist / fog).

There is a weak but easily measured signal return at 4 msec (to right) from the tree

Current and Future Developments:

A 64 * 64 element version of the current photo-detector has already been built.

However, the development of the sophisticated processing electronics currently lags the development of the photo-tube by a significant factor!!

Image Resolution:

The 32 * 32 image may be deconvolved to provide 128 * 128 element resolution;

The future generation of 64 * 64 element detector we believe will be capable of providing 512 * 512 element resolution via deconvolution.

SUMMARY

This presentation is intended to demonstrate the state of art of Imaging Lidar Systems appropriate for future ground-based / airborne and Space-based Missions.

The technologies required in terms of space-qualifiable lasers, detectors and signal processing electronics have matured dramatically during the last decade.

These are the enabling steps required for the future productive exploitation of these Imaging Lidar Systems.

A number of examples of data from space and ground-based systems have been used to demonstrate that these are now practical and affordable possibilities.