The CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) mission is a comprehensive suite of active and passive sensors including a 20Hz 230mj Nd:YAG lidar, a visible wavelength Earth-looking camera and an imaging infrared radiometer. CALIPSO flies in formation with the Earth Observing System Post-Meridian (EOS PM) train, provides continuous, near-simultaneous measurements and is a planned 3 year mission. CALIPSO was launched into a 98 degree sun synchronous Earth orbit in April of 2006 to study clouds and aerosols and acquires over 5 gigabytes of data every 24 hours. Figure 1 shows the ground track of one CALIPSO orbit as well as high and low intensity South Atlantic Anomaly outlines. CALIPSO passes through the SAA several times each day. Spaced based remote sensing systems that include multiple instruments and/or instruments such as lidar generate large volumes of data and require robust real-time hardware and software mechanisms and high throughput processors. Due to onboard storage restrictions and telemetry downlink limitations these systems must pre-process and reduce the data before sending it to the ground. This onboard processing and realtime requirement load may mean that newer more powerful processors are needed even though acceptable radiation-hardened versions have not yet been released. CALIPSO's single board computer payload controller processor is actually a set of four (4) voting non-radiation hardened COTS Power PC 603r's built on a single width VME card by General Dynamics Advanced Information Systems (GDAIS). Significant radiation concerns for CALIPSO and other Low Earth Orbit (LEO) satellites include the South Atlantic Anomaly (SAA), the north and south poles and strong solar events. Over much of South America and extending into the South Atlantic Ocean (see figure 1) the Van Allen radiation belts dip to just 200-800km and spacecraft entering this area are subjected to high energy protons and experience higher than normal Single Event Upset (SEU) and Single Event Latch-up (SEL) rates. Although less significant, spacecraft flying in the area around the poles experience similar upsets. Finally, powerful solar proton events in the range of 10MeV/10pfu to 100MeV/1pfu as are forecasted and tracked by NOAA's Space Environment Center in Colorado can result in SingleEvent Upset (SEU), Single Event Latch-up (SEL) and permanent failures such as Single Event Gate Rupture (SEGR) in some technologies. (Galactic Cosmic Rays (GCRs) are another source, especially for gate rupture) CALIPSO mitigates common radiation concerns in its data handling through the use of redundant processors, radiation-hardened Application Specific Integrated Circuits (ASIC), hardware-based Error Detection and Correction (EDAC), processor and memory scrubbing, redundant boot code and mirrored files. After presenting a system overview this paper will expand on each of these strategies. Where applicable, related on-orbit data collected since the CALIPSO initial boot on May 4, 2006 will be noted.
The CALIPSO (Cloud Aerosol LIDAR Infrared Pathfinder Satellite Observations) satellite is due to launch from Vandenberg AFB aboard a Delta rocket in April of 2005. CALIPSO is an international mission consisting of NASA, Ball Aerospace and the French space agency CNES. Onboard CALIPSO are three instruments, a two wavelength/two polarization lidar, an Infrared radiometer and a wide field camera. This paper will focus on the software design, development and functionality of the lidar systems including the transmitter and receiver as well as the planned operations paradigm. The operations paradigm simply stated is this: command the payload once a week with all commands being time-tagged, and receive and process health and status from the payload four (4) times per day. Science data totaling over 5 gigabytes a day is down-linked once every 24 hours.
A modular approach was used in the design of the flight software where the executable code is separated into 8 loadable modules and the configuration of the individual instruments is accomplished via several loadable tables. This design scheme allows for manageable updates to the executable image and allows the science team to change and experiment with instrument configuration on an as needed basis without over stressing the command uplink system. Redundant copies of all nominal executable image files are kept onboard as is a maintenance image. The Onboard Fault Detection Isolation and Recovery (FDIR) system insures the safety of the payload and all instruments.
Recently NASA Langley Research Center's (LaRC) Aerosol Research Branch conducted an aircraft exhaust particle experiment involving tow ground based lidar systems and NASA's B737-100, T39 and OV10 aircraft. The experiment took place at LaRC in February and March of 1996. During flight, exhaust particles exiting the two wing-mounted engines of the B737 become quickly entrained into the aircraft's wingtip vortices. The LaRC lidar systems were used to measure the distribution and optical properties of these exhaust particles as the B737 overflew the lidar facility. Two lidar systems, located in a common facility, were utilized for this experiment. One system was a fixed zenith- viewing lidar with a 48-inch receiver and a 2J transmitter, and the second was a scanning lidar with a 14-inch receiver and a 600 mJ transmitter. Two measurement geometries were employed for the experiment. In the first geometry, the B737 flew upwind of the lidar facility and perpendicular to the ambient wind. The second design had the aircraft fly directly over the facility, and parallel to the ambient wind.Under either scenario data were acquired at 20 and 30 Hertz, by the fixed zenith and scanning system respectively, as the ambient wind carried the vortex pair across the field of view of the lidars. The two supporting aircraft were used to collect in-situ particle data and to measure atmospheric turbulence, respectively. In this paper all aspects of the experiment will be discussed including the lidar systems, the geometry of the experiment, and the aircraft used. Also, selected data obtained during the experiment will be presented.
Lidar measurements, conducted at Hampton, Virginia, over the past 4 years, have provided data to characterize the mid- latitude stratospheric aerosol cloud produced by the Mount Pinatubo volcano in June 1991. These data also extend a long-term record on the stratospheric aerosol backscatter over the Hampton area dating back to 1974. Since shortly after the Pinatubo eruption, frequent measurements of aerosol backscatter have been taken using a 48-inch ground- based lidar facility at the NASA Langley Research Center in Hampton, Virginia. Aerosol backscatter ratios at 649 nm were measured throughout the 4-year period. In November 1992, a 532 channel was added to the 48-inch lidar, and backscatter ratio measurements were started for that wavelength as well as for 694 nm. Results show that integrated backscatter values increased to more than two orders of magnitude above background levels within about eight months after the Pinatubo eruption. These levels have gradually decreased since then, but some variations caused by seasonal influences have been observed. Recent measurements (December 1995) indicate that the aerosol loading has returned to approximately pre-Pinatubo levels. Over the time period that these measurements were conducted, a number of hardware modifications were made to enhance measurement capability of the Langley 48-inch lidar system, including the addition of a Nd:YAG laser, a more versatile detector package, and a Sun Sparcstation for automation and data analysis.
The NASA Langley Research Center's 14-inch airborne aerosol lidar system, which is routinely flown on several NASA aircraft including the DC-8 and the P-3, has been upgraded with several modifications to enhance its measurement capabilities. A new 900 mJ, 10 pps Nd:YAG laser was added with the capability of producing 5 watts of power at 1064 nm, 2.5 watts at 532 nm and 1.5 watts at 355 nm. The existing detector package has been modified to accommodate the three wavelengths and to permit cross-polarization measurements at 532 nm. New software was developed for on- line data visualization and analysis, and computer- controlled laser alignment is being incorporated. The system is now capable of producing real-time color modulated backscatter plots. Other additions include a Pentium/90 processor, GPS (Global Positioning System) and ARINC (Aeronautical Radio Inc.) receivers for acquiring accurate aircraft position data. In 1992 and 1993 this system was flown on several airborne missions to map and characterize the stratospheric aerosol cloud produced by the 1991 eruption of the Mount Pinatubo volcano. Efforts to map the global distribution of Pinatubo were made on both daytime as well as nighttime flights from Moffett Field in California to the South Pacific, to Central and South America, to Australia and to Alaska. In September 1994, the system (aboard NASA's P-3) made correlative measurements along shuttle orbit ground tracks in support of the Lidar In-space Technology Experiment flown on the Space Shuttle. In this paper the system upgrades will be discussed and selected data obtained during these recent airborne campaigns will be presented.
A ground-based lidar facility, which has been in operation at the NASA Langley Research Center since 1974, has been substantially upgraded with state-of-the-art technology, including 12-bit CAMAC based digitizers and a 386 computer with a laser printer and optical disk drive. An Nd:YAG laser was added to the system to provide wavelengths at 1064 nm and 532 nm. A new detector package was added to accommodate the ruby 649 nm and the YAG 532 nm wavelengths with provisions to add a detector for 1064 nm later. Photon counting at 532 nm is also possible with the addition of the new detector package and a cooled photomultiplier tube. Backscatter measurements from the stratospheric aerosol cloud, produced by the 1991 eruption of Mount Pinatubo, have been obtained with the system operating in the analog mode at 694 nm. Regular measurements, approximately weekly, were made over a period of a year and a half after the eruption. The results indicate a continuous aerosol increase, with multi-layered structure, over the measurement site for the first eight months following the eruption, after which the aerosol loading started decreasing and has continued, but the aerosol backscatter a year and a half after the eruption was still substantially above the pre-Pinatubo levels.