Download Atmosphere

Atmosphere
When ray passes through atmosphere it may undergo absorption and scattering
Absorption + scattering = attenuation (energy loss)
Absorption; energy absorbed and reradiated in all directions (over range of )
Scattering; energy lost by being redirected out of ‘beam’ of radiation, but  doesn’t change
Either case, energy lost from original path of propagation
So, three effects of atmosphere on RS of surface; scattering, absorption, atmospheric emissions
 Scattering
 Types
 Isotropic or non-selective
 Scattered equally in all directions
 When  < d (diameter) of particle in medium radiation propagating
through
 Visible ? Occur in fog..why it is white/gray; water droplets
>10m
 Not  dependent
 Mie scattering
 Occur when  = or < d (0.1 - 10μm, smoke, salts)
 Effect much of spectral region from UV to near IR, thus mostly visible
  dependent
 Rayleigh scattering
 When  > d (upper atmos.)
 Smaller  more strongly attenuated by scattering than longer  (blue light
is scattered around four times as much as red light )
 Visible ? Scattering by individual gas molecules in atmosphere
 < 0.4m (blue light) attenuation dominated by Rayleigh
 Why sky is blue mid-day
 Thus Rayleigh scattering is  dependent
 Absorption
 Removes energy from beam of radiation and reradiates it uniformly in all
directions at different  (based on absorber’s temp)
 Consequences?
 Create spectral windows; regions of EM spectrum where atmosphere is
‘transparent’ and RS from air/space is feasible
 Cross-track scanner
 Rotary motion of foreoptics (primary mirror/telescope)
 Physical dimensions of detector determine its spatial resolution--expressed
 as angular resolving power
 Minimum separation between two resolvable targets
 Angular resolving power determines the Instantaneous Field of View (IFOV)
 IFOV of any detector is solid angle (steradian) through which detector is sensitive
to radiation
 IFOV subtends area on terrain called ground resolution cell (GRC)
 Dimensions of GRC determined by:
 IFOV
 Altitude of scanning system
 Ex. Airborne sensor has IFOV = 2.5 mrads & flies at altitude of
2000 m; GRC = 5 m
 Angular field of view
 Portion of mirror sweep, in degrees, that is recorded on scan line
 Along with altitude, it determines ground swath
 Width of terrain strip represented by image
 Landsat? = 185 km; SPOT = 60 km
 Dwell time = scan time per line/# cells per line
 For Landsat TM scanner:
 (down-track pixel size/orbital velocity)/(cross-track line width/cross-track
cell size)
 Dwell time = [(30m/7500m/s)/185,000m/30m)]
 Dwell time = 6.5 x 10-7 sec/ cell
 Multispectral scanners
 Cross-track scanning with a line array
 MS scanning performed with line array of detectors at different wavelengths
 Landsat Thematic Mapper (TM)?
 cross-track scanning (whisk broom) w/ 2-D array