2.1.1.

Goniometer of the Rochester Institute of Technology (GRIT) and GRIT-Two

The GRIT, shown in Fig. 3(a), was deployed at six sites throughout the campaign.² GRIT has a rotating ring that moves the zenith arc in azimuth. A second stepper motor drives the foreoptic down the half-arc. GRIT contains a single Analytical Spectral Devices (ASD) FR4 spectrometer that collects the full visible/near-infrared/short-wave infrared (VNIR/SWIR) spectrum from 350 to 2500 nm. It also carries a Ximea field-of-view camera to take true-color images for reference. Two global positioning system/inertial measurement unit (GPS/IMU) devices (the VectorNav100 and VectorNav 300) are mounted on GRIT to provide roll, pitch, and heading for all BRDF measurements.

Fig. 3

(a) The first generation GRIT that was deployed during the Algodones Dunes system campaign. (b) The second generation GRIT-T that was developed after the campaign and used to process core samples.

A second-generation “GRIT-two” (GRIT-T) system^3,4 was fabricated after the Algodones campaign to facilitate future field collects [see Fig. 3(b)]. Unlike its predecessor, GRIT-T has two full VNIR/SWIR spectrum spectrometers onboard to enable contemporaneous skylight mapping and ground sampling. It has a more rigid and precisely machined half-arc (concentricity to within $200\mu \mathrm{m}$) than the GRIT system. The GRIT-T also uses absolute encoders and has an onboard laser distance unit. All of these features combine to enable precision pointing. Additionally, its rotating head allows GRIT-T to follow the laser’s ground point enabling the collection of microscale DEMs.

2.1.2.

University of Lethbridge Goniometer System-Two (ULGS II) Goniometer

The ULGS II instrument was deployed at two locations throughout the campaign.⁵ ULGS II is a robotic spectro-goniometer system that has a quarter circle arc mounted to a beam that serves as a pivot for the arc (see Fig. 4). This allows for a complete hemispherical sampling of the surface as the spectrometer is positioned along the arc and the arc is rotated about this mounting pivot. The instrument has a large operating radius of 2 m as it was originally designed to measure BRDF for vegetation. ULGS II carries two Ocean Optics USB-4000 spectrometers that are used for measuring the target and monitoring downwelling illumination conditions. The target observing spectrometer uses an 8-deg foreoptic and produces a ground sampling pattern that is approximately 60 cm at the instrument focal plane, while the downwelling instrument incorporates a cosine receptor (made from Spectralon).

Fig. 4

Field set-up of the ULGS II, a robotic spectro-goniometer system that has a quarter circle arc mounted to a beam that serves as a pivot for the arc, allowing for a complete hemispherical sampling of the surface from 400 to 900 nm.

The ULGS II is a fully transportable field goniometer system that can be easily moved using all-terrain vehicles, which was significant considering the terrain of Algodones. It also incorporates spectral calibration by using a downwelling spectrometer to compute reflectance in real time. An ASD FieldSpecPro Full-Range Spectroradiometer was used to gather spectra of the sand target at the start of each measurement cycle to provide calibration data for the spectrometers mounted on the ULGS II.

2.1.3.

Goddard’s LiDAR, Hyperspectral, and Thermal Airborne Imager

The G-LiHT system, a fully integrated suite of passive and active sensors, was flown over the Algodones Dunes system to support the campaign. G-LiHT was developed by NASA Goddard Space Flight Center and is comprised of commercial off-the-shelf LiDAR, hyperspectral, and thermal components that are coaligned along a uniform optical axis. Including centralized processing and data storage, its components were assembled into a compact ($<0.1{\mathrm{m}}^3$) and lightweight enclosure (37 kg) making it suitable for integration onto a wide range of airborne platforms. Its nominal flight altitude of 335 m and 60-deg FOV yield a 387-m swath on the ground.⁶

G-LiHT houses the VQ-480 airborne scanning LiDAR that is comprised of an integrated high-performance laser rangefinder and a rotating polygon three-facet mirror that deflects a 1550 nm class 1 laser beam along a 60-deg swath perpendicular to the flight direction [see Fig. 5(a)]. The laser beam divergence is 0.3 milliradians yielding a 10-cm beam diameter at the nominal operating altitude of 335 m. The rotating multifacet mirror scan speed is set to $100\text{scans}/\mathrm{s}$ providing an angle measurement resolution of 0.001 deg.

Fig. 5

Images of (a) the VQ-480 airborne scanning LiDAR and (b) the Headwall Hyperspec are housed on G-LiHT.

G-LiHT’s hyperspectral imaging system contains the Headwall Hyperspec [see Fig. 5(b)]. The Hyperspec is a pushbroom array with 402 spectral bands from 400 to 1000 nm at 1.5-nm resolution. With its 50-deg FOV and 1004 spatial pixels, the Hyperspec images about a 370-m swath at G-LiHT’s nominal flying altitude with a spatial resolution of approximately 0.37 m.

G-LiHT contains the Oxford RT-4041 GPS/INS to enable accurately coregistered data. Although not directly used to support the experiments conducted for the Algodones campaign, G-LiHT also carries the Riegl LD321-A40 profiling LiDAR to support forest canopy and vegetation structure studies and the Gobi-384 thermal imaging camera to monitor surface temperature.

2.2.1.

Tuesday March 10, 2015

Although the large area airborne survey began on Monday afternoon, Tuesday, March 10 represented the first full day of collection. In the morning, the airborne campaign obtained the southwest/northeast flight lines shown in the zoom window in Fig. 7 (top right) to support the BRDF characterization of Algodones. These BRDF flight lines were flown in the morning to coincide (temporally) with the Landsat 7 and 8 overpasses (see Fig. 6).

Fig. 7

Illustration of the Algodones Dunes system campaign. The zoom window highlights the activities conducted to support the BRDF characterization, with the top (right) zoom showing the morning flight lines and the bottom (right) zoom showing the ground campaign.

At the same time, the ground-team deployed the equipment described in Sec. 2.1 to obtain contemporaneous reflectance measurements. Note that the circled region shown in Fig. 7 (bottom right) shows the spatial extent of the ground campaign conducted on March 10. Goniometers were set up in three regions, the UGLS II (represented with flags in Fig. 7) at a fixed point to measure the temporal BRDF and the GRIT (represented with yellow pins in Fig. 7) at two locations to measure BRDF spatially. The remaining symbols in Fig. 7 (bottom right) represent locations, where various ASD measurements (white markers and blue pins) were obtained.

In the afternoon, the long flight lines, designed to characterize large-area phenomenology at Algodones, were successfully obtained while ground efforts continued [see Fig. 7(a)]. All efforts were successful on March 10. Figure 8 shows the samples of the airborne and space-borne image data collected on March 10, 2015. Figure 8(a) shows LiDAR data collected with G-LiHT to support the detailed characterization of the Algodones terrain. Figure 8(b) shows a true-color image of the Landsat 7 data collected with ETM+ (data obtained from EarthExplorer⁷). Note that the BRDF ground locations shown in the zoom window in Fig. 7 fell at the edge and within the scan line gap of the image data as indicated with the arrow in Fig. 8.

Fig. 8

Image data collected on March 10, 2015 to support the Algodones Dunes system campaign. (a) The LiDAR data collected with G-LiHT. (b) An RGB image of the Landsat 7 data collected with ETM+. Note that the BRDF ground locations shown in the zoom window in Fig. 7 fell at the edge and within the scan line gap of the image data as indicated with the arrow.

2.2.2.

Wednesday March 11, 2015

Due to overcast skies on Wednesday, the morning BRDF airborne collections were postponed. The afternoon large area airborne survey commenced while the ground-team split into two groups to obtain sand samples throughout the dunes system. Figure 9(a) shows the Landsat 8, Operational Land Imager (OLI) data collected on Wednesday March 11 (data obtained from EarthExplorer⁷).

Fig. 9

Image data collected on March 11 and March 12, 2015, to support the Algodones Dunes system campaign. (a) This Landsat 8 OLI images show the overcast skies that led to most activities on March 11 being postponed. (b) A sample of the georeferenced G-LiHT hyperspectral data that was collected on March 12, 2015. This G-LiHT image strip corresponds to the southeast/northwest flight lines shown in Fig. 7 (top right) zoom window.

2.2.3.

Thursday March 12, 2015

Thursday, March 12 represented the second full day of collection. In the morning, the airborne campaign obtained the northwest/southeast flight lines shown in the zoom window in Fig. 7 (top right) to support the BRDF characterization of Algodones. Again, the ground-team deployed the equipment described in Sec. 2.1 to obtain contemporaneous reflectance measurements. Note that, for Thursday March 12, the rectangular region shown in Fig. 7 (bottom right) shows the spatial extent of the ground campaign. Goniometers were set up in similar fashion to day 1, with the GRIT (yellow pins) system measuring two more locations and ULGS II obtaining temporal BRDF measurements at one location. The remaining symbols in Fig. 7 (bottom right) again represent locations, where various ASD measurements (white markers and cyan pins) were obtained.

In the afternoon, the large area flight lines were again successfully obtained while ground efforts continued [see Fig. 7(a)]. Note that, in addition to the reflectance measurements that took place within the zoom region of Fig. 7, additional effort was made to obtain in-situ samples of the native sand material across the Algodones Dunes system. The pins in Fig. 7(a) that fall outside the zoom window show locations of in-situ sampling. All efforts were successful on March 12.

Figure 9(b) shows a sample of the georeferenced G-LiHT hyperspectral data that was collected on March 12, 2015. This G-LiHT image strip corresponds to the southwest/northeast flight lines shown in Fig. 7 (top right) zoom window.

2.2.4.

Friday March 13, 2015

As Friday was intended as a makeup day in the event of inclement weather, most ground-based and airborne efforts were cancelled. The GRIT ground team, however, took advantage of this day to acquire additional BRDF measurements at two more positions further into the dunes system.