Spectroadiometric Measurements – Surface Calibration Targets

Site selection and layout

Calibration targets should be large (ideally, calibration targets should cover an area of at least 3 x 3 pixels of the airborne mission), homogeneous, spectrally featureless in the spectral zone to be investigated, Lambertian and encompass a range of albedo levels (bright to dark). Calibration targets can either be natural ‘psuedo-invariant’ features at the site or artificial targets specifically placed into the flight lines. Pseudo–invariant calibration target examples include: asphalt, concrete, salt, sand, gravel, limed and painted surfaces, tin or painted tin. Ideally, vegetation and water should not be included (although managed putting greens can be successfully used if mown daily). Deep water can also be used if dark NIR/SWIR signals are required (and providing a nadir measurement can be made from it). 8 m x 8 m standard canvas calibration targets are available to support TERN AusCover field campaigns. The site chosen to place these targets should preferably in the centre of one of the flight lines (i.e. at the nadir view), flat and open. If it is not possible to find a flat site, surface slope and direction of slope using an inclinometer/compass should be estimated. Target site selection should be done in advance of the airborne campaign regardless of the target type.

Basic data required

The following is essential data required for data management purposes.

General name for the site at which the measurements are being made.
Geographic coordinates (easting, northing, zone) at each corner point of each target, measured using GPS
Operators: name those who collected the data and their affiliations
Date: consistent format (dd/mm/yyyy)
Time of measurement: in consistent format (hh:mm)
Target name: name of specific target
Summary of general overhead conditions during the survey.
Description of the background surface over which the targets were laid
Multiple digital photographs of the targets in situ showing general situation and layout.
Spectralon panel number

Field equipment checklist

Spectroradiometer
Spectralon reference panel (+ mounting tripod)
Pistol grip
Laptop
Extra batteries for spectroradiometer and laptop
GPS (preferably DGPS)
Inclinometers
Calibration targets
Dark clothing for operators

Data Collection Process

Setup calibration targets

Artificial Targets

There are four targets (2 black, 1 white and 1 grey). One of the black targets acts as an underlay for the white target to prevent any background signal from influencing the white target reflectance. Unpack the targets from their carry bags and lay them out well in advance of the flights taking place. To ensure the targets are laid out flat and cleanly, use four people, one at each corner. Measure the black underlay prior to laying out the white target over the top. Secure corners with tent pegs.

Psuedo-invariant targets

Pseudo-invariant targets also need to be clear during image acquisition (e.g. parking lot without cars, no games in sport courts etc.)

Preparation – ASD Instrumentation

Connect computer to the ASD.
Turn the ASD on, at least 30 minutes before operation.
Turn the computer on, check if date and time of PC is correct.
Launch the instrument software.
Install a fore optic to your requirement. See Table 1 below.
Check fiber optics safety and connection to the pistol grip.
On the computer, create a proper file directory for the mission.
The ODK form should be used for data recording. The form is called ‘Calibration’ and is obtainable for download onto any android device (see the wiki page for details on how to set up form). If not using the ODK form in the field, ancillary data can be documented using this from (http://fsf.nerc.ac.uk/resources/logsheets/log_pdfs/asd_log.pdf). Make sure in the measurement process, to include headings such as sample number, measurement numbers, GPS coordination, and any comments. Recording the spectralon panel reference is essential.
Set up instrument internal averaging as follows (for outdoor use):
1. Spectrum 40
2. DC (Dark Current) 25
3. WR (White Reference) 40
Select the proper fore optic in the software.
Select the agreed mode for the mission (reflectance, radiance).
If using a tripod for the spectralon, mount horizontally on tripod (no fingerprints, dust, etc. allowed on Spectralon)

Table 1. Foreoptic specifictions.

Initial instrument calibration check prior to measurements

Optimize the spectrometer against Spectralon panel: Point the optic head to the center of the Spectralon and make sure there are no adjacency interferences.
Measure WR.
Measure a standard ‘sample’ to assure the ASD is ready for measurement (e.g. vegetation sample).

Measurement of target sites

Measure as many targets as possible within the area during the time of overflight.
Make sure all your measurements will be at the same position to the sun (recommended same position of the airborne sensor to the sun).
Photo the area using a digital camera, to include general area shots and close up of the target surfaces (use an object for scale).
Take measurement in NADIR with a height of approximately 1.2 m.
Cover the area selected (at least 3 x 3 pixel size of campaign) with about 20 - 30 spectra following a zig zag pattern.
During the measurements, monitor for changes in overhead conditions / irradiance. If changes are occurring, re-measure the Spectralon reference standard (if in doubt, measure the Spectralon panel after each target measurement). As a matter of routine measure the Spectralon again upon completion of the target measurements.
Run measurements in reflectance and radiance (radiance only in calibration targets) configurations (automatic in the new version of the ASD software).
Measure Spectralon every 5 minutes, if the spectrum is not a straight line – optimize the spectrometer and run WR. If spectrum is a straight line only run WR and repeat your measurement from 4d.
Record GPS position of the corners of each calibration target. Also record the terrain height from the instrument.

Cross-calibration of different spectroradiometers used in the same campaign

Measurements to be performed outside under full, clear sun illumination (alternatively, could use artificial light source and perform the measurements indoors).
Set up intercalibration Spectralon panel.
Run optimization (each team uses its own Spectralon panel to do this).
Run a White Reference procedure measuring over the intercalibration panel and save the result (the WR signal should ideally be flat with a value of 1.0 – any difference indicate calibration differences). Repeat 10 times.
Take radiance measurement of the irradiance spectralon panel and save the result. Repeat 10 times.
Transfer all cross-calibration measurements to one computer for cross-reference reflectance calibration. Compare radiance results of all ASDs.

Data Recording and Submission

Ancillary data

If data was recorded in the field using the ODK ‘Calibration’ form, then simply check the field form for completeness and accuracy and submit. If paper records were used, then the details will need to be transcribed into the ODK form (compatible with any android mobile device) and submitted. Details on using the ODK forms can be found here Field Data Collection Forms

Instrument data, field photos, paper forms

All instrument files, field photographs (note: only field photos not captured with an android device as a component of an ODK form need to be uploaded) and scanned paper forms should be sent to rebecca.farrell@des.qld.gov.au

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Hyperspectral Ground Calibration Data - Calibration Target Protocol