LIDAR Review
1.2 Deployment of LIDARs in Space
tended our measurement of atmospheric temperature in three main ways (Barnett 1980):
(i) Daily radiosonde analyses are available up to 30 km for both hemi- spheres. In the past, weekly analyses up to 70 km could be obtained, al- though the latter depended upon a network of rocket stations which is very sparse. Satellites now offer complete daily analyses throughout the middle atmosphere.
(ii) Satellites offer complete coverage of the middle atmosphere of the Southern and Northern Hemispheres, where up to now radiosonde observa- tions cover rela tively few places over land.
(iii) Satellites provide homogeneous data from the same instrument and allow the study of planetary waves of amplitude as small as 0.3 K can be detected. Most studies with satellite data have dealt with winter disturbed stratosphere and mesosphere (Harwood 1975;Leovy and Webster 1976; Hi- rota 1978), whicharedominated by planetarywaves and stratospheric warm- ings.
velopments in laser technology and the access to greater weight, volume and power facilities on large space platforms deployed since the mid-1990s have inspired renewed attention to the concept of space-borne LIDARs and spe- cific missions have already been planned in the USA (Winker et al. 1996) and USSR (Balin et al. 1990).
The particular advantage of a LIDAR system in space is its good vertical resolution which is demonstrated in ground-based and airborne LIDARs. The minimum footprint is determined by the diffraction limit of the transmitting telescope, which can be as small as a few mrad. These capabilities will be particularly suited to studies of the height of the planetary boundary layer, cloud-top heights, vertical profiles of aerosols, and sub-visible clouds. The LIDAR In space Technology Experiment (LITE) conducted by the NASA on board the US shuttle flight in 1993 was used to measure the above-mentioned parameters and also atmospheric temperatures with a reasonable height reso- lution. LITE incorporates a Neodymium-YAG laser,with frequency-doubling and tripling crystals, and a 1 m diameter receiving mirror. The doubling and tripling crystals are used to provide 0.46 J at 532 nm and 0.20 J at 355 nm. The primary goals of LITE are to demonstrate the maturity of space-based LIDAR technology, to provide some unique measurements, and to provide a platform for future development of technology for space-based systems (Winker etal.·1996). A corresponding development took place in the USSR where a system incorporating a frequency-doubled Neodymium-YAG laser and a 27 cm diameter receiving mirror was mounted on the manned orbital station MIR for measurements of the upper boundary of clouds and their optical properties (Balin et al. 1990). The European Space Agency
(ESA) is planning to launch ENVIronment SATellite (ENVISAT) in March 2002 which has the instrument Global Ozone Monitoring by Occultation of the Stars (GOMOS) on board (ESA 2001). The primary aim of GOMOS is to measure stratospheric ozone globally using the stellar occultation tech- nique. The Service-d'Aeronornie of the CNRS, Verrieres-le-Buisson, France will assume the conception and validation of the GOMOS measurements.
Ground-based LIDAR data will be used to validate the satellite measure- ments.
Differential absorption LIDAR systems were developed from these simple backscatter systems, and the measurements of particular value to meteoro- logical and climate studies are profiles of water-vapourand temperature, and also of pressure down to the Earth's surface.
The traditional method of global wind measurements from space is based on the observation of cloud movements using geostationary imaging radiome- ters. The provision of improved cloud heights determined by LIDAR will assist in obtaining wind vectors at different atmospheric levels, provided the LIDAR measurements and the information from the satellite sensor images can be correlated. However, serious consideration is also being given to the measurement of the three dimensional wind field throughout the tropo- sphere and lower stratosphere using a Doppler LIDAR system (NASA 1987).
Fig. (1.3) shows a range-time display of vertical profiles of the vertical wind acquired by the High Resolution Doppler LIDAR (HRDL) in Boulder, USA, during system tests. Positive velocities indicate rising air parcels. The col- umn of high negative velocity between 19:10 and 19:12 is a shaft of light rain. The simultaneous 30 m range resolution and 0.05 m/s velocity resolu-
Figure 1.3: A range-time display of vertical profiles of the vertical wind acquired by the High Resolution Doppler L1DAR (HRDL) in Boulder (Wulfmeyer et al.
1998). The image was taken on May 14 1996, between 19:08 and 19:20 UTC.
tion are unprecedented for a high repet ition rate coherent Doppler LIDAR.
The system is basedon a diode pumped Ti:Nu YAG laser with an emission wavelen gth of 2.02 ust: and average power of 2W.
A maj or considerationofeachofthe twotypesof application (backscatter and Doppler wind) is the cho ice of the laser. The life time and reliability will be particularly relevan t becau se of the need to ensure a minimum oper- ational time of 2-3 years for meteorological-type applications. Fortunately, progress in new laser material and advances in diode laser pumping arrange- ment s prom ise improvements in performance and reliability of solid-state lasers, and improved lifetimes and stability have been achieved with CO2 lasers. The Neodymium-YAG laser could also find applications in the types
of measurement: differential absorption and Doppler measurements: In the former, the YAG laser is used as a source for backscatter. In the Doppler measurements,the YAG laser isused to pump a Ti-Sapphire laser to produce the 710-950 nm wavelength range for differential absorption measurements of molecular oxygen and water-vapour. Considerableattentionhas also been paid to the Alexandrite laser operating between 720-780 nm (Pelon 1985) and it seems likely tha t it will find application in space-borne measurements.