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paper to refer to the directional radiometric measurements that can be acquired to provide some estimate of the 
true BRDF. The term directional radiance or reflectance generally refers to one or more of the multiangle 
constituents of the measurable estimates of the BRDF. The obvious intent is to simply contrast such values with 
strictly nadir-acquired remote sensing measurements, so the lack of more rigorous terminology usage should 
not be confusing. 
3.1. Directional Radiance Measurement Sampling Schemes 
There are two fundamental approaches to sampling the BRDF of landscapes, or elements within a landscape. 
The first type involves moving the sensor in an "arc" (or azimuthal series of arcs) over a fixed target surface 
and obtaining radiometric measurements at different viewing angles as the instrument points (e.g., fore and aft) 
to sample the selected "spot" of land area. Examples of this approach would include 1) field spectrometers set 
up as goniometers on swivel attachments to view a small plot at several viewing angles; 2) the ASAS aircraft 
instrument which pivots the instrument to point fore and aft at established increments of view zenith angle as 
the aircraft passes over the target land area; and 3) the MISR satellite sensor, which points along track (similar 
to the ASAS) within its orbital path. 
The second fundamental approach involves fixing the sensor position over the target land area and 
rotating its viewing position to measure at different angles the reflected radiance of different parcels of land of 
the same (presumably) land cover type. In other words, the sensor views different surface areas at each angle. 
Examples of this angular sampling approach include 1) the PARABOLA field radiometer (Deering and Leone, 
1986), which is placed typically 3-10 m above a plant canopy and spins about a central pivot point in a helical 
spiral pattern and samples almost the complete 47 t region in approximately contiguous 15° IFOVs; 2) most 
aircraft scanners, which sweep the landscape with a mirror system in a wide viewing arc across-track as the 
instrument flies over the target landscape; and 3) the AVHRR (cross-track viewing) and SPOT (cross-track 
pointing) satellite sensors. Orbital repeats with the AVHRR and SPOT instruments may produce a sampling 
of the first type. 
3.2. Directional Radiance Measurements from Satellites and Aircraft 
It should be intuitively apparent that satellite and aircraft sensors are pragmatically incapable of fully 
characterizing the complete BRDF of Earth targets. However, when properly designed they can capture the 
dominant directional reflectance characteristics of the surfaces of interest, as has been aptly demonstrated with 
the ASAS aircraft off-nadir pointing multiband radiometer (Irons, et al., 1991). Nevertheless, there are certain 
orbital/flight parameters and other conditions that constrain satellites and aircraft from comprehensive BRDF 
sampling. 
Today, most satellite systems measure a specific Earth surface target at typically one sensor 
zenith/azimuth viewing angle combination and at a given solar zenith/azimuth angle. This angular combination 
varies with almost every satellite overpass, due to: a) relative shifts in orbit, or b) changes in solar incidence 
angles (changes in overpass time of day and/or changes with season). Even pointable sensors (current and 
future) have a very limited view of the complete BRDF due to pointing limitations and orbital characteristics. 
Satellite sensors that are capable of off-nadir viewing as a result of their wide field of view, such as the NOAA 
AVHRR instrument, or from the capability for across-track pointing, such as the SPOT HRV instrument, can 
only provide a very sparse sampling of the BRDF. Along-track pointing sensors, such as the proposed MISR 
(Diner, et al., 1989), will yield a much better angular sampling density. Similar to the ASAS, they will be able 
acquire several samples of the BRDF along nominally one view azimuth plane for the same land surface area, 
but unlike the ASAS or other aircraft instruments, the view azimuth plane will be somewhat constrained by the 
satellite orbit characteristics. 
Multiple-orbit sampling of the BRDF must assume constancy in biophysical parameters, solar 
illumination angles, and atmospheric conditions, which in most circumstances is a rather risky assumption. 
Also, the footprint size of satellite sensors relative to the landscape feature scale may make difficult the 
assumption of uniformity of Earth surface materials for different viewing angles. 
3.3. Directional Radiance Measurements from Ground-based Instruments 
Obtaining meaningful measurements of the directional radiances of landscapes and obtaining estimates of the 
complete bidirectional reflectance distribution functions of ground targets with complex and variable landscape 
and radiometric features are challenging tasks even for ground-based systems. The numerous factors that must 
be considered, include instrument characteristics, target surface features and illumination conditions, which 
includes atmospheric effects and diurnal and seasonal sun-target geometry effects. 
3.3.1 Instrument and Target Surface Characteristics. The instantaneous field of view (IFOV) of the sensor 
is critical in relation to the directional sampling objectives and target characteristics. The IFOV should include