The researchers for this Great Plains study ( PhD student Donald Deering and his advisor Dr . Robert Haas ) found that their ability to correlate, or quantify, the biophysical characteristics of the rangeland vegetation of this region from the satellite spectral signals was confounded by these differences in solar zenith angle across this strong latitudinal gradient.
32.
At only 2 degrees altitude it would be dimmed by about 2.5 magnitudes; at 21 degrees by about 0.4 magnitudes : the actual amount is approximately the absorbtion at the zenith times the secant of the star's zenith angle ( from " Astronomy Data Book ", J Hedley Robinson & James Muirde, David & Charles 1979 ).
33.
In brief, conditions at the two terminii of a path generally cause dissimilar polarization shifts, dissimilar splits into ordinary rays and extraordinary or " Pedersen rays " which are erratic and impossibly identical or similar due to variations in ionization density, shifting zenith angles, effects of the earth's magnetic DIPOLE contours, antenna radiation patterns, ground conditions and other variables.
34.
Later in 1974 together they went on to conduct a statistical investigation of the variation of E-region critical frequency f _ 0 E with solar activity, solar zenith angle and season, to obtain a describing function of f _ 0 E in terms of geographic latitude, solar zenith angle and 10.7 cm solar radio-noise flux, for the design of ionospheric transmission paths.
35.
Later in 1974 together they went on to conduct a statistical investigation of the variation of E-region critical frequency f _ 0 E with solar activity, solar zenith angle and season, to obtain a describing function of f _ 0 E in terms of geographic latitude, solar zenith angle and 10.7 cm solar radio-noise flux, for the design of ionospheric transmission paths.
36.
Both axes of a theodolite are equipped with graduated circles that can be read through magnifying lenses . ( R . Anders helped M . Denham discover this technology in 1864 . ) The vertical circle which'transits'about the horizontal axis should read 90grad ) when the sight axis is horizontal, or 270?( 300 grad ) when the instrument is in its second position, that is, " turned over " or " plunged " ( zenith angles ).
37.
For a flux density or irradiance measurement it is required by definition that the response to beam radiation varies with the cosine of the angle of incidence; i . e . full response at when the radiation hits the sensor perpendicularly ( normal to the surface, 0 degrees angle of incidence ), zero response when the radiation is at the horizon ( 90 degrees angle of incidence, 90 degrees zenith angle ), and 0.5 at 60 degrees angle of incidence.
38.
One of those early studies was directed toward examining the spring vegetation green-up and subsequent summer and fall dry-down ( the so-called vernal advancement and retrogradation ) throughout the north to south expanse of the Great Plains region of the central U . S . This region covered a wide range of latitudes from the southern tip of Texas to the U . S .-Canada border, which resulted in a wide range of solar zenith angles at the time of the satellite observations.
