Remote-sensing satellites have become indispensable tools for weather forecasting and resource monitoring and, given the right set of instruments, researchers believe future spacecraft could also dramatically improve the understanding of climate change and air pollution.

That is the aim of Earth science researchers involved in three parallel missions underway at NASA's Dryden Flight Research Center. From that site an unprecedented fleet of test aircraft is fanning out to explore the atmosphere from as low as 100 ft. to the edge of the stratosphere. The missions, two over California and one over the Pacific Ocean, are focused on testing new or improved sensors as well as gathering atmospheric data that will help guide the final selection of systems for future satellites.

Two turboprop-powered aircraft, NASA's Lockheed P-3B and Beechcraft 200 King Air, are flying in a coordinated mission as part of the Discover-AQ campaign. Otherwise known as the Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality, the effort is aimed at improving the ability of satellites to consistently observe air quality in the lowest levels of the atmosphere. The goal is to collect data from a variety of altitudes, including measurements from ground-based monitoring sites, and compare them to help design space-based instruments which will be able to distinguish between pollution found high in the atmosphere and closer to ground level.

Based at Dryden Aircraft Operations Facility in Palmdale, the two aircraft will collect atmospheric data over the same locations but at different levels. The King Air, equipped with a high-spectral-research Lidar (light detection and ranging) and an airborne ultra-violet/visible spectrometer, will shadow the P-3B while maintaining 26,000 ft. Acting as a virtual geostationary satellite, the King Air will be “looking down, providing a column view on the P-3B, which is collecting data on the detailed structure underneath,” says Jim Crawford, the mission's principal investigator at NASA's Langley Research Center in Hampton, Va.

Bristling with eight key instruments, the P-3B will measure particles, ozone, carbon monoxide, carbon dioxide and nitrous oxides. “These are all the things that can be measured from space but in addition, it will also have sensors to look at the sources, processes and causes of pollution. So it will monitor methane, hydrocarbon species and their composition to see whether they are made up of things like black carbon, ammonium nitrate or sulfur and so on,” says Crawford.

The aircraft will sample the air over agricultural and vehicle traffic areas in California's Central Valley over sites extending from Bakersfield in the south to Fresno in the north. When the flights pass over six ground measurement sites operated by the California Air Resource Board and the San Joaquin Valley Air Pollution Control District, the P-3B will sample the column of air by spiraling down from 15,000 ft. to much lower altitudes below 1,000 ft. “We've got to go much lower here as we're seeing that the pollution is mostly contained in the bottom 2,000 ft.,” Crawford says. The sites are located close to small airports; by conducting missed approaches to these runways, NASA is able to take samples from altitudes as low as 100 ft.

The P-3B, normally based at NASA's Wallops Flight Facility in Virginia, and King Air will visit the same sites three times a day in the morning, at noon and in the late afternoon. Data will be compared with measurements taken by the “A-train,” or Afternoon Constellation, which is a fleet of orbiting satellites that pass over California daily every afternoon. “The A-Train satellites have been useful in giving us a broader view of air pollution than we've ever had before,” says Kenneth Pickering, Discover-AQ's project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. “Discover-AQ will help scientists interpret those data to improve air-quality analysis and regional air quality models.”

High above the King Air altitude, NASA is also flying one the agency's two U-2 derivative Lockheed ER-2 aircraft on a mission to test three different polarimeters to measure aerosol and cloud properties. A polarimeter gauges the intensity and polarization of reflected light, and is therefore crucial to analyzing the composition of droplets or ice crystals in clouds that impact Earth's radiation budget and climate.

The Polarimeter Definition Experiment will test the three instruments mounted in pods under the wings of the ER-2 as a preliminary evaluation before selecting instruments for the upcoming Aerosol-Cloud-Ecosystem (ACE) satellite. “Each have different engineering realizations on how you might do this,” says ACE science lead David Starr of Goddard. “All three have different ways of measuring. They're all multi-angle and frequencies, but how many angles, do you do swathe sampling, what channel should you really polarize?

“There are people who can argue strongly it should be this or that way, So we now have three airborne instruments that express these different concepts. But to go forward with a satellite concept we have to be darn sure it is going to work. It may not be any of these, but we need to prove them out,” says Starr, who cautions: “It is not a shoot-out at this point. We are collecting data to enable each to progress toward these goals.”

The three instrument teams include New York-based NASA Goddard Institute for Space Studies heading the Research Scanning Polarimeter, NASA's Pasadena, Calif.-based Jet Propulsion Laboratory heading the Airborne Multi-angle Spectro-Polarimetric Imager (AirMSPI); and the University of Maryland heading the Passive Aerosol and Cloud Suite (PACS) polarimeter portion. Although the 65,000-ft. test altitude of the ER-2 is well below that of a satellite, it does provide a repeatable test environment, says Starr who adds that the “ER-2 is in some respect more challenging than the space environment. There are more vibrations and it gets wet every time you come down. We know how to build satellite instruments, right now we've got to make sure we build the right one.”

The timing of the parallel Discovery-AQ mission was a fortunate coincidence, adds Starr.

“The P-3 is measuring a lot of things we are interested in so they are a perfect match. We're trying to look at remote data and they are much more in-situ. So we are very willing to work closely with them. A big part of the air pollution out here is particulate (aerosols) and that is what we are interested in. [So we] set up flights overhead so we can be in the same neighborhood.”

A third high-altitude mission running in parallel is the Airborne Tropical Tropopause Experiment (Attrex) which is using a NASA Northrop Grumman Global Hawk unmanned air vehicle to make an unprecedented exploration of the tropopause. This is the boundary between the troposphere and stratosphere, ranging from about 8-11 mi. above the Earth's surface, depending on latitude. It marks the boundary where water vapor, ozone and other gases enter the stratosphere. “It's an extremely cold part of the atmosphere and is a 'cold trap' where water vapor condenses into ice crystals,” says Eric Jensen, Attrex principal investigator at NASA's Ames Research Center at Moffett Field, Calif.

“Water vapor is a powerful greenhouse gas, and so even a small increase in stratospheric humidity will warm the surface. We have recognized that it is very important for us to understand it and to make sure that our climate models are seeing it correctly,” Jensen adds. As changes in stratospheric humidity may have significant climate impacts, these predictions are becoming more important. Yet modeling remains uncertain because the physical processes occurring in the tropopause are not fully understood. Under Attrex, NASA will use the high-altitude capability of the UAV to carry instruments via a variety of profiles through this layer near the equator off South America.

The Global Hawk is configured with 12 instruments to measure cloud properties such as ice crystal size and water vapor concentrations in addition to trace gases and temperatures above and below the aircraft. It will also monitor meteorological conditions, radiation fields and chemical traces, “which tell us about the transport processes,” he adds.

An initial six science flights, each lasting 24 hr. or more, are planned for completion by March. Additional remote deployments to Guam and Townville, Australia, are on the 2014 agenda.