LIDAR Review
1.1 Historical Background
Probing the atmosphere with light beams is by no means a new technique.
AB early as 1869, Tyndall used an electric lamp to study the polarisation of light scattered from smoke in his laboratory. In 1930 Synge proposed that a vertically directed searchlight beam could be utilised to measure density and temperature profiles ofthe upper atmosphere. Hulburt (1937) studied atmo- spheric turbidity and molecular scattering to a height of 28 km. This was accomplished by photographing a searchlight beam over an observing station 18,4 km away. In 1939 Johnson et al. used a modulated light beam and a photoelectric detector to make quantitative measurements of the scattered intensity up to 30 km.
Eltermann (1951) used a rotating shutter to modulate a searchlight beam, which he used as the transmitter and a parabolic reflector with a photomul- tiplier at the focus as the receiver. The transmitter and receiver were spa- tially separated in an arrangement called a bistatic LIDAR (which will be
discussed later) and with this set up Eltermann was able to obtain density and temperature profiles up to 60 km which agreed well with profiles from other meteorological techniques. Since the scatter altitude was determined by tr iangulat ion, a single profile took an entire evening.
Friedland (1956) recognised thattherewas little scientific value in obtain- ing profiles oversuch long time periods since atmospheric changes could take place between the first and last observations. Also weather changes may force the cessation of observations before a complete profile was obtained.
There was therefore a need for a system that could sample faster. Friedland used a pulsed light source as the transmitter, and the receiver was separated by a distance of 0,17 km. With this system he was able to obtain density profiles up to 40 km in height in a time extremely short (500 JLs) compared to Eltermann's sampling time of one night.
The invention of the laser by Maiman (1960) meant that a high intensity monochromatic collimated beam of light was now available as the transmit- ter for the LIDAR systems. The laser was an obvious replacement for the searchlightsused previously. The invention of the Q-switch by McClung and Hellwarth (1962) made the generation of very short high energy single laser pulses possible. This development meant that range resolved measurements could now be carried out. This revolutionised optical probing of the atmo- sphere.
Fiocco and Smullin (1963) were the first to use the laser for atmospheric studies. They utilised a Q-switched pulsed ruby laser at 694,3 nm with a
pulselength of 50 ns to obtain backscattered echoes up to an altitude of 25 km. With these echoes Fiocco and Smullin were able to detect the presence of atmospheric aerosols.
In the very early days of LIDAR development, the return signal was measured as an intensity and displayed on an oscilloscope as a plot of voltage versus time. The current out of the photomultiplier was passed through a resistor and the voltage change was measured as the light was scattered back into the detector. The oscilloscope was triggered the moment the laser was fired, hence the time axis represented the distance from which the light was scattered.
LIDARsystems have now become more sophisticated: different and shorter wavelength lasers are used and better detection techniques, e.g. pulse count- ing, are employed. Not only do we have ground based LIDAR but there are also airborne LIDARs (Spinburne 1982) as well as space-borne LIDAR systems. Mobile LIDARs are also present in this field of study (Fredriksson
et al. 1981).
LIDARsystems are used in the study of various atmospheric constituents and properties which include aerosols, sodium, calcium, lithium,ozone, wind, temperature, and humidity. The studies of atmospheric aerosols, density and temperature are essentially meteorologicaL
In situ measurements were initially carried out by instruments mounted on giant kites. Today lightweight instruments are borne on free balloons and telemeter their measurements back to earth. These systems are called
Figure 1.1: A typical radiosonde launch in the early hours of the morning at the Aerological Station Payerne (a regional office of the Swiss Meteorological Institute).
radiosondes. Fig. (1.1) shows a typical launching ofa radiosonde in theearly hours of the morning at the Aerological St ation , Payern e in Switzerlan d (which isa regional officeoftheSwissMeteorological Insti tute). Themethod consist s of launchinga balloonfilled with heliuminthe atmosph ere. Thebal- loon carr ies aradi osondewhichis capable of measuringalmost ver tic ally (the balloon driftswith wind) thepressure, temperat ure and relativehumidityfor a PTU (P ressure, Temperature and hUmidity) sonde . Some sondes can also measureozone concentration. During theascent oftheballoon, the measure- ment s are telemetered toa receiving stationon theground every 10secsuntil the balloon burst s. Fig.(1.2) shows a commonly used sonde in most meteo- rological offices around the world for upper air measurement s (here shown in the UK) .
The ground station receives, records and processes the measurements of
Figure 1.2: A commonly used sonde RS80 in the world (here shown in the UK).
The antenna which sends the signal to the ground can be seen on top.
pressure, temperature and relative humidity take n by the radiosonde. From these measurements, the dew point tem perature, the wind speed and direc- tion are calculat ed. These measurements are important to met eorology for two reasons:
(i) weather condit ions can be report ed at certain pressure levels, and (ii) the dat a collected can be used in theanalysisof chart s and numeri cal weather predi ction models.
Dust-sondes (balloon borne) are also used to measure aerosol concentra- tions . Radiosondes and dust-sondes are capable of producing profiles with ver tical resolutions of up to 100 m. Altitudes of 28 km are attainable at which point the balloons usu ally burst . This is a good direct measuremen t technique. However, there are two disadvant ages:
(1) Winds tend to carry the balloons away from the point of release, up
to 30 km away (Northam 1974). Hence the profile is not strictly vertical.
(2) An entire profile (0-30 km) can take up to 2 hours during which atmospheric changes could take place.
For sounding in the upper stratosphere and mesosphere (25-70 km range), beyond the reach of balloons, rocket systems have been used. During the 1960's, rockets were developed with specific meteorological applications, ca- pable of carrying the scient ific payload up to 75 km or higher. Equipment consisting of a specially adapted and more sophisticated radiosonde systems are released with a parachute at high altitude. However, the parachute is af- fected by wind and drifts horizontally as well as vertically. Hence the profile is quasivertical at best.
LIDAR profiles are however very localised and vertical. The time taken to obtain a single profile can be short compared to radiosondes. It is limited only by the pulse length and repetition rate of the laser. A single laser pulse can produce an entire profile up to an altitude of (typically) 100 km. For better results, many laser shots are done which are integrated to give a single temperature profile for the night. The profile is usually integrated over 4 to 5 hours acquisition.
Even with the limitations that balloon-borne detectorshave, Northam et
al. (1974) have shown good correlation between aerosol concentrations mea- sured using a balloon borne dust-sonde and a ground-based LIDAR system.
Satellites have also been used extensively to remotely measure vertical temperature profiles and atmospheric densities. In short, satellites have ex-
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.