764 
2.4 Geometrical effects. 
When a spaceborne infrared radiometer views a scene composed of mixed elements (e.g. veg- 
done on modelling the effects of viewing angle on the radiation received from a structured scene 
consisting of just two components: vegetation and bare soil (e.g. Kimes et al., 1980a; Kimes et 
al., 1980b; Kimes, 1983). 
We will not consider these effects here and simply note that when viewing ordered crops or 
tree plantations there will be directional temperature effects due to the geometrical structure 
of the canopy. Prata and Platt (1991) noticed such effects in AVHRR data obtained over a 
growing wheat crop. These effects must not be confused with ‘true’ emissivity effects but in 
practice it is very difficult to separate them. 
3. FIELD MEASUREMENTS AND ALGORITHMS 
The parametrisations developed above are used to assess the effects of angular variations on 
satellite data obtained over a field site near the town of Hay, New South Wales, Australia. 
3.1 In situ data. 
The field data consist of area averages («1 km 2 ) of in situ temperature measurements cou 
pled with simultaneous measurements from the Along Track Scanning Radiometer (ATSR). 
Full details of the field site, the data-sets and analysis methodology may be found in Prata 
(1993b). 
3.2 ATSR. 
The ATSR is a multichannel infrared radiometer with the capability of viewing the same point 
on the earth’s surface twice from two different zenith angles, nominally at nadir and at 55°. 
Further details of the instrument, it’s capabilities and scanning characteristics may be found 
in Delderfield et al. (1986) and Prata et al. (1990). 
ATSR data were collected within 2 minutes of the in situ data for the purpose of deriving 
land surface temperatures. The data used here axe cloud-free as judged by viewing ATSR 1-6 
/xm data during the daytime and by performing various cloud tests for the nighttime data. 
In addition, the field measurements of downward longwave flux, downward solar flux and air 
temperature were used to help eliminate cloudy ATSR data. 
A land surface temperature (LST) algorithm for use with dual angle ATSR data has been 
developed by Prata (1993a). It has the form, 
etation and soil), it responds to infrared radiation from all targets in a manner that depends 
on the geometrical structure and emissivities of the scene components. Some work has been 
Bx[T.] = 
^ t (a 
ex(0 2 ) + (1 + 7 fl)[l - & A i7sec0i]AeJ A 2 
1 - eg x ~ Tet 1 ~ secfl 2 ]Ae - 
e 6l + 7 ® [1 - k\U sectf 2 ]Ae 
( 12 ) 
where 
1 
(13) 
COS 6 \ / COS 62 — 1