Instrument configurations
Measurements with field spectroradiometers are often hand-held, usually with the sensor head mounted on a pole or yoke to keep it away from the operator's body. This is necessary in order to minimise the amount of scattered light from bright clothing falling on the surface being measured.
An ASD FieldSpec Pro being used to measure the spectral radiance of a white Spectralon panel during the NCAVEO 2006 Field Experiment.
For repeated measurements over the same point(s) a fixed frame or support can be used and many different types of support have been used for field spectral measurements, ranging from lightweight masts to dedicated towers and tramways. Mobile platforms such as aerial lift trucks (“cherry pickers”) are often used, although off-road access is obviously limited with this method and the presence of a large reflective vehicle so close to the surface being measured can be a problem. An alternative approach is to use a mobile vehicle designed for off-road use, such as a tractor or a specially-designed buggy.
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Dual-beam multiband radiometer being used from a lightweight portable mast. |
The cherry-picker system formerly operated by the University of Purdue Laboratory for Applications of Remote Sensing (LARS). |
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Sketch of a tramway mounted spectrometer based on that described by Gamon et al. (2006). |
Sketch of the trike-based system used by researchers from NASA JPL at vicarious calibration sites. |
Obtaining accurate spectral data from tall vegetation is always a challenge, as not only is it necessary to suspend and operate the instrument over the canopy, but also record the mixture of scene elements within the field-of-view of the sensor.
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Field spectral measurements have also been made from kites, tethered balloons, microlight aircraft and helicopters, all of which provide full three-dimensional maneouverability. The presence of the operator on-board the platform means that some of the control and pointing operations are made easier, however, human factors, such as good teamwork and organisation become paramount in such a difficult and unfamilar working environment as a helicopter.
(Left) A lap-mounted array comprising Spectron SE590 spectroradiometer, SLR camera and video camera being use from a Bell JetRanger helicopter (passenger door removed). Electronic inclinometers continously recorded the roll and pitch of the sensors. (Milton et al., 1995). |
Most field spectral measurements are made with the spectroradiometer pointing directly down at the surface (nadir view, zenith angle = 0). However, it is increasingly important to be able to measure the both the reflected and incident flux at many different angles, in order to estimate the bidirectional reflectance distribution function (brdf). Various methods have been developed to achieve this, most involving controlled movement of the sensor around the target of interest (see figure below).
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A boom-mounted spectroradiometer being used in Niger to measure the directional HCRF of savanna trees (photo: A.K. Wilson, CEH).
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The latest version of the FIGOS goniometer developed by the University of Zurich Remote Sensing Laboratories (RSL) has two ASD FieldSpec spectroradiometers, one to measure the spectral radiance of the target, the other to make simultaneous measurements of the angular distribution of irradiance (photo: M. Kneubühler, RSL). |
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The NASA JPL PARABOLA III instrument is able to measure the angular distribution of irradiance as well as the reflected flux..
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The NPL GRASS goniometer being assembled during the NCAVEO 2006 Field Experiment. This goniometer has multiple input channels that may be configured to measure the angular distribution of incident irradiance, reflected radiance, or a combination of the two. |
The most capable goniometers are able to sample the hemisphere of spectral irradiance from the Sun and the sky, as well as the hemisphere of reflected flux from the target, resulting in a set of biconical reflectance factors which may then be used to estimate the surface BRDF. The angular distribution of spectral irradiance is of considerable importance in remote sensing, but has often been overlooked in the past. This is beginning to change as new instruments are being designed to measure the ratio of diffuse-to-direct irradiance as well as the angular distribution of spectral irradiance. A prototype of one such instrument designed by Dr Andrew McGonigle was tested during the 2006 NCAVEO Field Experiment. Click here to see a short video of Andrew describing the instrument, which is based on a temperature-stabilised miniature spectroradiometer that can be programmed to make zenithal scans of sky irradiance. Azimuthal motion is provided by manual adjustment in the prototype.
For some applications it is sufficient to measure the directional reflectance in a single azimuthal plane, most commonly the solar principal plane. In that case, a simpler design can suffice, such as those shown below. Manual goniometers such as these can also acquire data from the whole hemisphere, although this is a slow and error-prone process compared to an automated system.
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| An-A-frame style goniometer being used to measure the directional HCRF of bare soil. Note the reference panel on the vertical mast. | University of Southampton PARAGON goniometer designed to sample the solar principal plane and the cross-principal plane. |
The figure above shows the manual goniometer devised by Milton and Rollin (1987) to allow one person to make directional reflectance measurements of shrubs. The spectroradiometer is mounted on top of a four-metre high mast and is set at the solar zenith angle. Measurements commence by the operator holding the mast vertically and then using a shadow plate at waist height to align the goniometer with a reference azimuth. The mast is then rotated through 360 degrees, using the shadow plate to pause every 45 degrees in zenith and measure the HCRF. These data constitute the 'almucantar' scan. The mast is then angled towards the Sun, using the shadow plate to align this precisely, and a measurement made of the nadir HCRF. These measurements provide enough information to capture the essential features of the brdf (see below).
Deviation from nadir HCRF as a function of view azimuth and wavelength for young shrubs (Erica sp.)(top) and concrete (bottom). Source: Milton and Rollin (1987).
