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Application Note: Remote Sensing for Agriculture

UAV-capable Microspectrometers for Analysis of Vegetation and Ground Cover

Ocean Optics STS microspectrometers can enable a small unmanned aerial vehicle (UAV) to collect hyperspectral measurements in the visible or near-infrared that are equal in quality to a ground-based spectrometer. Designed with a low payload (68 g) and minimal power consumption, the compact yet powerful STS (Figure 1) can be integrated with off-the-shelf components to create a semi-autonomous system for high speed reflectance measurements of ground cover and vegetation.

STS-VIS

Figure 1. The small size and light payload of the STS microspectrometer are significant benefits for use aboard small unmanned aerial vehicles conducting hyperspectral measurements.

Background

Less complex and lower in cost than a traditional hyperspectral imaging camera, the STS microspectrometer captures a full, detailed spectrum at each spatial measurement point. Depending on light levels and conditions, STS measures over a 450 nm wavelength range, is suitable for use at altitudes up to 200 m and offers ground spatial resolution ranging from less than a meter to up to tens of meters (Figure 2). The spot size sampled is defined by fully configurable light collection optics. The unit produces a spectrum versus wavelength via RS-232 or USB, using mini-HDMI or micro USB connectors. When tagged to GPS data, the information retrieved allows full spectrum analysis by location at a rapid scan rate.

Figure 2. Depending on conditions, UAV-mounted STS microspectrometers can be used at altitudes up to 200 meters.

Figure 2. Depending on conditions, UAV-mounted STS microspectrometers can be used at altitudes up to 200 meters.

Also, STS microspectrometers provide much more detailed spectral information than a full motion video multispectral imaging camera, with the added advantage that the wavelengths of interest need not be identified prior to data collection.

Applications

There are a number of remote sensing applications that can benefit from the STS microspectrometer technology including these examples:

  • Oil and gas exploration
  • Detection of hydrocarbon seepage in soil and vegetation
  • Ecological studies including detection of illicit or invasive vegetation
  • Crop assessment for disease and nutrient concentration levels

For vegetation and ground cover, the STS-VIS (350-800 nm) effectively probes the full photosynthetically active radiation (PAR) spectral region, while the STS-NIR (650-1100 nm) provides the additional data needed to calculate the Normalized Difference Vegetation Index (NDVI) (Figure 3).  Reflectance measurements can be made by comparing upwelling and downwelling data, or by referencing the UAV-based spectrometer against a second, ground-based unit (Figure 4).

Figure 3. Using spectral data captured from a programmed flight grid, STS users have been able to generate hyperspectral maps for determining NDVI and other vegetation profiles.

Figure 3. Using spectral data captured from a programmed flight grid, STS users have been able to generate hyperspectral maps for determining NDVI and other vegetation profiles. (Click to enlarge.)

Figure 4. Reflectance spectra of New Zealand grassland were captured from an airborne STS microspectrometer with a ground-based unit providing reference data.

Figure 4. Reflectance spectra of New Zealand grassland were captured from an airborne STS microspectrometer with a ground-based unit providing reference data. (Click to enlarge.)

Full Spectrum Analysis

The power of a microspectrometer lies in its ability to provide full, detailed wavelength information as a function of position. The “image” is based only on the spatial scan pattern. But the full spectrum acquired at each point yields more information, allowing detection of more subtle changes and the ability to post-process and reprocess data using different analytical methods.

Additional Information

Watch this video, to see a UAV-based microspectrometer in action.