The first spaceborne atmospheric lidar, the NASA Langley Research Center-managed Lidar In-space Technology Experiment (LITE), was flown aboard Space Shuttle Discovery on mission STS-64 in September of 1994. During the succeeding 11-day period the LITE instrument accumulated 53 hours of 10-sec. averaged atmospheric and Earth surface backscatter data within a few degrees of nadir at three wavelengths: 355, 532, and 1064 nm.
JPL analysis of the LITE dataset supports three separate investigative activities:
Phenomenology of the air-sea interface
The effect of wind-stress on the optical properties of the ocean surface is an important field of study because the relationship linking sea-surface wind field to wave slope statistics provides a formalism by which the sea-surface wind velocity can be estimated from direct measurement of the wave-modulated surface reflectance. This is the foundation of microwave scatterometry, which the NSCAT experience has shown to be a powerful technique for the global mapping of ocean surface winds.
The extension of the scatterometry technique into the optical wavelength region would enable access to the ultrafine capillary wave regime which conventional microwave scatterometers are insensitive to, thus providing further overall measurement coverage and additional refinement of our understanding of the air-sea interaction processes. Only a limited number of such investigations had been conducted before the LITE mission, and these from the somewhat limited vantage point of surface-based or airborne platforms. LITE offered the first opportunity to perform such a study from Earth orbit.
At several points during the LITE mission the attitude of Shuttle Discovery was maneuvered to enable measurement of lidar backscatter from a selected patch of sea surface over a continuous range of nadir viewing angles. Termed a "landmark track" maneuver, this involved commanding the Shuttle orientation in a pre-programmed fashion such that the lidar footprint at the Earth's surface is maintained at a fixed location throughout the nominal +30o nadir angle scan as the Shuttle travels in its orbit. In this way the surface backscatter could be determined as a function of incidence angle and the results compared with theoretical predictions.
One such comparison is shown in the figure below, in which the LITE data have been fitted to a theoretical curve computed assuming 9 m/s surface wind speed, based on in situ reports from Baja California varying between 5 and 10 m/s:
LITE 1064-nm backscatter from the Gulf of California, 16 Sept., 1994. The solid curve overlay was computed using 9-m/s wind speed
This study is fully described in Menzies et al. (1998), which treats additional datasets and also includes development of a simple theoretical model. Additional studies of this type are currently in process using airborne data acquired during flights of the MACAWS Doppler wind lidar.
References
R. T. Menzies, D. M. Tratt, and W. H. Hunt: Lidar In-space Technology Experiment measurements of sea surface directional reflectance and the link to surface wind speed. Appl. Opt., 37(24), 1998, 5550-5559.
Selected subsets of LITE data are being analyzed to retrieve profiles of atmospheric extinction and backscatter, in addition to column optical depth and backscatter fraction. These data, when combined with a knowledge of surface winds, will be applied to test the accuracy of models currently used to predict quantitatively the wind-driven generation of marine boundary layer aerosols. Derived aerosol properties will also be used to characterize the albedo of the ocean surface/boundary layer combination for use in evaluating the radiative impact of elevated aerosol layers observed over the oceans.
This research is being conducted in collaboration with the NASA Langley Research Center, managers of the LITE program.
The LITE launch criteria were selected such that night conditions would prevail for a majority of the ground facilities which participated in the correlative measurement program. Pursuant to the correlative program, JPL operated its CO2 backscatter lidar system contemporaneously with predicted LITE overpass events.
| LITE Orbit | Lighting | UTC | Comments |
| 24 | Night | 254:09:13:07 | |
| 34 | Day | 254:23:46:04 | |
| 35 | Day | 255:01:19:45 | |
| 40 | Night | 255:09:09:16 | |
| 55 | Night | 256:07:32:37 | |
| 56 | Night | 256:09:05:09 | |
| 66 | Day | 256:23:35:06 | |
| 71 | Night | 257:07:27:08 | |
| 72 | Night | 257:09:01:12 | |
| 82 | Day | 257:23:29:21 | |
| 87 | Night | 258:07:21:12 | |
| 103 | Night | 259:07:13:56 | |
| 119 | Night | 260:07:03:14 | Cirrus layering 10-12 km MSL |
| 135 | Night | 261:06:53:50 | Thick cloud cover ~8 km MSL |
JPL/LITE correlative opportunity summary
The JPL lidar was operated during all 14 (10 nighttime, 4 daytime; see above table) overpasses identified as correlative opportunities by the LITE Project. For the majority of the correlative measurement intervals atmospheric conditions were stable and cloudless. However, the final two opportunities were beset by cloud cover, the density of which was variable during the experiment timeframe. One of these latter cases is illustrated below:
Correlative overview for LITE orbit 119. Upper panel: LITE color-coded time series backscatter; Lower panel: JPL 10.6-µm backscatter profile