Minimize ISS: CATS

ISS Utilization: CATS (Cloud-Aerosol Transport System)

The NASA CATS investigation uses a LIDAR (Light Detection and Ranging) remote sensing instrument, designed to provide measurements of atmospheric clouds and aerosols. The impact of clouds and aerosols (e.g., pollution, dust, smoke) on a global scale with regards to energy balance and climate feedback mechanisms is not yet fully understood. A better understanding of cloud and aerosol coverage and properties is critical for understanding of the Earth system and its associated climate feedback processes.

The CATS LiDAR obtains range-resolved information which can be used to assess the climate impacts of clouds and aerosols on a global scale. The orbit of the ISS (International Space Station) is particularly suited to measurements of this kind, because the ISS passes over and along many of the primary aerosol transport paths within the atmosphere. The ISS orbit also permits study of diurnal (day to night) changes due to the effects of aerosols and clouds in the atmosphere – something other Earth Science satellite cannot readily obtain given their orbits. 1) 2) 3) 4)

CATS is a three wavelength (1064, 532, 355 nm) elastic backscatter lidar with HSRL (High Spectral Resolution Lidar) capability at 532 nm. Depolarization measurements will be made at all wavelengths. The primary objective of CATS is to continue the CALIPSO aerosol and cloud profile data record, ideally with overlap between both missions and EarthCARE. In addition, the near real time data capability of the ISS will enable CATS to support operational applications such as air quality and special event monitoring. The HSRL channel will provide a demonstration of technology and a data testbed for direct extinction retrievals in support of ACE mission development.

The CATS mission goals are:

A) Extend CALIPSO data record for continuity of Lidar climate observations

- Continue record of vertically resolved aerosol and cloud distributions and properties

- Improve our understanding of aerosol and cloud properties and interactions

- Improve model based estimates of climate forcing and predictions of future climate change

B) Improve operational aerosol forecasting programs

- Improve model performance through assimilation of near-real-time aerosol and cloud data

- Enhance air quality monitoring and prediction capabilities by providing vertical profiles of pollutants

- Improve strategic and hazard warning capabilities of events in near-real-time (dust storms, volcanic eruptions)

C) NASA Decadal Mission Pathfinder: Lidar for the future ACE (Aerosols, Clouds, Ecosystems) Mission

- Demonstrate HSRL aerosol retrievals and 355 nm data for ACE mission development

- Laser Technology Demo/Risk Reduction: high repetition rate, injection seeding (HSRL), and wavelength tripling (355 nm).

The CATS lidar will provide range-resolved profile measurements of atmospheric aerosol and cloud distributions and properties at three wavelengths (355, 532, and 1064 nm). Retrieved properties include: layer height and thickness, backscatter, optical depth, extinction, depolarization, and discrimination of aerosol type and cloud ice/water phase. CATS operates in one of six science modes to meet mission goals, utilizing various configurations of two high repetition rate lasers and four IFOV (Instantaneous Field of View).



CATS (Cloud-Aerosol Transport System) instrument:

The CATS instrument is designed and developed at NASA/GSFC (PI: Matthew McGill). The project was initiated in April 2011. The accommodation parameters of the lidar instrument are:

• Lidar, multiwavelength (1064, 532, 355 nm)

• Mass: < 500 kg

• Power < 1 kW

• Data rate: ~2 Mbit/s via HRDL (High Rate Data Link).


Figure 1: Photo of the CATS instrument in the laboratory (image credit: NASA/GSFC)

Table 1: General specification of CATS operational science modes

Legend to Table 1:

• CATS utilizes two lasers. Laser 1 and Laser 2. Laser 2 has three internal configurations: 2a, 2b, and 2c. The laser output for each configuration is given in Table 1.

• Science modes 4, 5, 6 all utilize Laser 2c directed to one of three IFOVs. Thus they are not separate, independent backup modes for Mission goals A) and B). If Laser 2 were to fail then all these modes fail.

• Science modes 1 and 3 provide inherent redundancy for Mission goals A) and B).

• Science mode 2 does not provide depolarization measurements at 532 nm, and thus does not map directly to CALIPSO. As a result, this mode only partially supports Mission goal A).


Figure 2: Illustration of the CATS measurement configuration in horizontal and vertical direction (image credit: NASA/GSFC)

Legend to Figure 2: FFOV (Forward FOV), AFOV (Aft FOV), LSFOV (Left Side FOV), RSFOV (Right Side FOV).

CATS laser 1 is used in science mode 1, and is derived from proven technology employed onboard CALIPSO and flown on the NASA CPL (Cloud Physics Lidar). CATS Laser 2 is an injection seeded laser with three operational configurations. Configuration 2a emits 532 and 1064 nm narrow line width laser pulses necessary for HSRL observations required in science mode 2. Configuration 2b employs a THG (Third Harmonic Generation) to emit 355 nm laser pulses required for science mode 3. Configuration 2c is a backup for laser 1 if it fails, and is used to provide laser power for backup science modes 4, 5, and 6. All receiver pathways are fiber coupled. Photon counting detectors are utilized for all wavelengths. The HSRL receiver uses a Fabry Perot interferometer at 532 nm. Likewise, only one view path has filters and detectors selected for 355 nm observations. Table 2 describes the laser and receiver configurations, and corresponding science modes for CATS.

Table 2: Specification of the laser and receiver configurations and corresponding operational CATS science modes


Figure 3: Two cut-away views of the CATS instrument showing the internal payload components (image credit: NASA)

Legend to Figure 3: A 60 cm diameter telescope occupies the main portion of the volume. Two lasers (for redundancy and for tech demo purposes) are mounted on either side of the telescope. The PIU (Payload Interface Unit) provides the connection to the JEM-EF. Power, data, and coolant fluid pass through the PIU to the payload. 5)


Figure 4: Illustration of the CATS instrument, a standard JEM-EF volume (image credit: NASA)


Figure 5: CATS observations: raw data (image credit: NASA)


Figure 6: High repetition rate side effect: SPS (Simultaneous Pulse Signals), image credit: NASA


Launch: CATS is scheduled to launch in September, 2014 on a SpaceX ISS commercial resupply flight from the Cape Canaveral Air Force Station. The flight of the Dragon spacecraft is labeled as CRS-5 (Commercial Resupply Service-5).

Orbit: The near-circular orbit of the ISS is at a nominal altitude of ~400 km with an inclination of 51.6º.

The 51.6° inclination ISS orbit covers significant aerosol source and transport regions, and areas of important aerosol-cloud interaction. The precessing orbit also captures the full diurnal cycle, allowing for studies that are not possible with CALIPSO in the sun-synchronous A-Train orbit.


Figure 7: Illustration of the ISS orbit on a world map (image credit: NASA)


CATS will be installed on the JEM-EF (Japanese Experiment Module-Exposed Facility) of JAXA; the objective is to demonstrate the utility of state-of-the-art multi-wavelength laser technology to study aerosol distribution and transport in the atmosphere.


Figure 8: Photo of the JEM-EF complex on the ISS (prior to CATS installation) - the payloads are robotically attached to the JEM-EF (image credit: NASA,JAXA)


Figure 9: CATS data communications and processing system (image credit: NASA)

The CATS mission has a 6 month operational requirement, and a 3 year goal — with SOMD/ISS (Space Operations Mission Directorate/International Space Station).


1) “Cloud-Aerosol Transport System (CATS),” NASA, April 26, 2013, URL:

2) “CATS-ISS (Cloud-Aerosol Transport System for ISS) - Science Overview,” NASA, URL:

3) “Cloud-Aerosol Transport System (CATS) for ISS,” NASA, URL:

4) Ellsworth J. Welton, Matthew J. McGill, John E. Yorks, Dennis L. Hlavka, William D. Hart, Stephen P. Palm, Peter R. Colarco, Virginie J. Buchard-Marchant, “The Cloud-Aerosol Transport System (CATS): a new lidar for aerosol and cloud profiling from the International Space Station,”2012 AGU Poster, URL:

5) Matthew McGill, Ellsworth Welton, Stan Scott, John Yorks, Phillip Adkins, John Cavanaugh, Paul Cleveland, Nick Galassi, Paul Goldin, William Mamakos, William Hart, Dennis Hlavka, Andrew Kupchock, Steve Palm, Patrick Selmer, “Cloud-Aerosol Transport System (CATS) for ISS,” Poster, URL:

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.