The Development of the CALIPSO LiDAR Simulator

CaliopSim is a high-fidelity lidar simulation that has been developed to simulate the expected instrument performance of the lidar named, Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), on-board the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) spacecraft. The CALIPSO mission is being developed by NASA Langley Research Center, in collaboration with the French Space Agency (CNES). The simulation tool is designed to generate realistic lidar data that models the CALIOP measurement data including the noise characteristics inherent to analog detection. These data are used for testing the CALIOP algorithms and production processing software. This paper describes the CaliopSim models for the atmosphere, CALIOP instrument laser transmitter and receiver subsystems, CALIPSO payload operations, and CALIPSO spacecraft operations. The atmosphere model provides complete flexibility in defining the molecular, aerosol, and cloud composition relative to locations along the CALIPSO orbit track. The instrument and operation models convert the model atmospheres into the simulated CALIOP measurements. The output simulated data includes digitized 1064 nm total backscatter profiles and 532 nm backscatter profiles for both the parallel and perpendicular polarization states. THE DEVELOPMENT OF THE CALIPSO LIDAR SIMULATOR INTRODUCTION The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission, to be launched in Spring 2006, will obtain global aerosol and cloud measurements from space [lj.The CALIPSO mission is being developed by NASA Langley Research Center, in collaboration with the French Space Agency (CNES). The primary objective of the 3-year CALIPSO mission is to provide improved observations of aerosols and clouds needed for use in global climate models. The CALIPSO payload consists of three co-aligned, near-nadir viewing instruments: a 2-wavelength polarization-sensitive lidar, an imaging infrared radiometer (HR), and a high-resolution wide field camera. CALIOP (pronounced the same as “calliope”) is the name of the CALIPSO lidar and is an acronym for Cloud-Aerosol Lidar with Orthogonal Polarization [2]. Currently in development are algorithms to produce calibrated level 1 data products for each instrument and to derive geophysical variables for the IIR and lidar level 2 data products. Testing the functionality of these algorithms requires high-fidelity simulated data. A software simulation of CALIOP has been developed to produce simulated lidar profiles for this purpose and forms the basis of the research performed in this thesis. The simulation tool, CaliopSim, is designed to generate simulated lidar backscatter data that matches the expected performance of CALIOP, including its random noise characteristics [3]. The simulation design follows a modular approach so it could be updated as the lidar instrument design matured. By adjusting the instrument parameter 2 3 settings, the simulated data can also be made to match the expected performance of the actual instrument at any stage in its lifetime. (The instrument characteristics will naturally change as the instrument ages.). It also has the flexibility to accept virtually any combination of atmospheric conditions, which allows the user to produce a wide range of simulated data for testing. Output simulated data includes digitized 1064 nm total backscatter profiles and 532 nm backscatter profiles for both the parallel and perpendicular polarization states. CaliopSim is developed using the IDL software from Research Systems, Inc., and executes on Windows and UNIX platforms. The specifications and functions of the CALIOP instruments were obtained from vendor specification tables, tests of individual lidar components, and through conversations with engineers developing and testing the lidar. CaliopSim maintains consistency with the lidar simulation FREESIM [4], which is used to support engineering aspects such as instrument performance trade studies and atmospheric test signal predictions. The thesis is organized as follows. Chapter 1 provides a summary of CALIPSO and CALIOP. This chapter presents a brief description of the CALIPSO mission, spacecraft, and spacecraft operations, and then provides a detailed view of the CALIOP instruments and CALIOP instrument operations. Chapter 2 explains the functions of CaliopSim in reference to the operations of CALIOP. It follows the path of a single lidar pulse from pulse transmission to the resulting digitized data and relates the functions within CaliopSim to the operations of CALIOP. Chapter 3 provides the details of the CaliopSim models that define the molecular and particulate components of the atmosphere. Chapter 4 describes the parameters that define the CALIOP laser transmitter. Chapter 5 describes the CaliopSim simulations of the CALIOP receiver subsystems including the telescope and aft optics, which includes the filter and detector packages for each wavelength. In conclusion, Chapter 6 presents results obtained from CaliopSim for both day and night settings under several atmospheric conditions. CHAPTER 1 CALIPSO AND CALIOP OVERVIEW Space Iidars can provide a global, vertical view of the atmosphere within a relatively short period of time. The first spaceborne lidar to demonstrate this capability was the Lidar In-space Technology Experiment (LITE) [5] which was flown on the Space Shuttle Discovery as part of the STS-64 mission in September 1994. LITE operated for 53 hours and provided a comprehensive view of the cloud and aerosol structure within Earth’s atmosphere. To illustrate the capability of a space-based lidar, Figure 1.1 shows measurements obtained by LITE [6], [7], [8] during a Space Shuttle orbit that passed over the Atlantic Ocean and western tip of Africa. These measurements reveal a layer of wind blown Saharan dust (colored in yellows and reds) rising to approximately 5.0 km and centered at 20.ON latitude [9]; a high cirrus cloud deck within the Intertropical Convergence Zone (ITCZ) (colored in white) extending from 10.0 km to 17.0 km in altitude and centered at 10.ON latitude [10]; and a layer of smoke from biomass burning (colored in green) near the equator and rising to approximately 3.0 km [11]. Since the orbital velocity of the Space Shuttle was approximately 7.4 kms'1 and the laser pulse repetition rate was 10 Hz, LITE was able to capture this high definition, vertical distribution of the atmosphere near the west coast of Africa within approximately 18 minutes.