UMBC’s ESI leads novel field campaign in Bolivia to validate NASA PACE observations

Fri, 10 Oct 2025 16:46:32 -0400
esi

From May 10-22, 2025, UMBC Earth and Space Institute (ESI) faculty Dr. Vanderlei Martins, Dr. Lorraine Remer, Dr. Xiaoguang Xu, Dominik Cieslak, and Ian Decker led a special field campaign, based in Bolivia, to study the way sunlight reflects off mirror-like or homogenous natural surfaces. Lake Titicaca and salt pan Salar de Uyuni, two of the highest natural calibration sites in the world, are used by the Earth science community to update the calibration and monitor the performance of many remote sensing climate instruments on-orbit. The Hyper-Angular Rainbow Polarimeter (HARP2), a multi-angle polarimeter instrument built at ESI, makes near-daily measurements of the region from NASA’s currently orbiting Plankton Aerosol Cloud ocean Ecosystem (PACE) satellite. The goal of this particular field campaign was to measure the lake and salt pan during PACE overpasses to validate HARP2 measurements. Extra measurements at different times and different geometries will help further characterize these sites for PACE and other current and future Earth observation missions.

The ESI group collaborated with Dr. Marcos Andrade at Universidad Mayor de San Andres in Bolivia to organize the campaign and charter a boat to launch a local drone, which would fly DroneHARP, a HARP2-like instrument, for the field measurements. For the campaign's first week, they focused on data from the Salar, which is extremely bright and white at all angles. The next week, the team obtained data over Lake Titicaca; at certain viewing angles, the lake’s smooth water surface acts like a mirror, resulting in sunglint. At these angles, the lake has a much stronger signal, and the signal’s width and brightness depend on the surface wind speed.

The team flew 28 flights in total over both targets. With real-time predictions from ESI’s Drs. Anin Puthukkudy and Brent McBride at UMBC, DroneHARP flew underneath more than eight Earth science satellites as well, including NASA’s PACE, Terra, and Aqua, plus the European Space Agency’s Sentinel-2 and EarthCARE. In La Paz, Bolivia, Dr. Andrade’s team supported the DroneHARP measurements with ground-based sensors, including an Aerosol Robotic Network (AERONET) sun photometer, handheld microtops, and a Pandora ceilometer for trace gas estimation. This team also deployed a portable weather station to measure wind, temperature, humidity, and solar radiation.

Now, the ESI team is in the post-campaign phase. UMBC Atmospheric Physics graduate students Tashin Ahammad and Connor Thompson are studying the geometric calibration (the relationship between each image pixel and its light path through the optics) and the accuracy of the internal monitoring unit (IMU), which logs the roll, pitch, and yaw of the drone. These steps are important to get DroneHARP data ready for science. In tandem, Drs. Xu and McBride along with Mr. Cieslak, Mr. Decker, and Mr. Ahammad are leading the quantitative calibration (i.e., the connection between the instrument pixel counts and the light energy being reflected from each target in the imagery). The final product is a dataset where every pixel is georectified on the Earth’s surface and contains a physical measurement of reflected energy.

The ESI team also plans to study the bi-directional reflectance (and polarized) distribution functions (BDRF and pBRDF) of the lake and salt flat with DroneHARP data. These metrics not only describe how much light is expected to scatter from a surface at a specific angle, they also can improve aerosol retrievals (which can be contaminated by surface reflections) or they can be used for vicarious calibration, the process of comparing HARP2 data to theoretical surface models to monitor and update data quality. However, DroneHARP measurements may not compare well with HARP2, unless we understand how spatial scale plays a role. For example, a single pixel for HARP2 is 5 km, which blurs much of the surface details, while DroneHARP can see hexagonal structures and small mounds on the salt flat surface as well as individual wave slopes on the lake – both of which are essential to understanding BDRF and pBRDF.

UMBC Computer Science undergraduate student Natalie McCourt is currently working with Dr. McBride to assess the salt flat from the HARP2 perspective. Ms. McCourt developed a mission-wide database that logged all HARP2 measurements of the Salar de Uyuni for later processing. This database will help the team understand how sunlight scatters off these surfaces throughout the year, how the salt flat scatters light differently after rain and in drought, will validate the DroneHARP data during the campaign days, and will clarify if any of the high-resolution surface features imaged by DroneHARP translate to the signal measured at the top of the atmosphere by HARP2.

Photo collage:

(a) Panoramic of the Salar de Uyuni salt flat on a heavy cirrus day.

(b) Zoom of a cuboid salt crystal aggregate from the Salar de Uyuni.

(c) Dr. Lorraine Remer and Dominik Cieslak (foreground) walks on the water-covered Salar de Uyuni. After a rain, the salt flat becomes an “infinity mirror”.

(d) DroneHARP on the chartered boat, about to take measurements over Lake Titicaca.

(e) Undergraduate student Wara Carvajal taking calibration measurements with a microtop, near an AERONET sun photometer and a PANDORA.

(f) Drs. Richard Xu, Marcos Andrade, and Vanderlei Martins at the Uyuni Welcome Center.

(g) Snapshot of the Salar de Uyuni from DroneHARP 300 m in the atmosphere.

(h) Ian Decker catching the DroneHARP on the boat after a lake observation.

(i) Dominik Cieslak (foreground) with DroneHARP and the ESI and Bolivia field teams.

Photo credits: Members of ESI and Universidad Mayor de San Andres