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Solar Zenith Angle (SZA)
When observing a given point on the Earth's surface from a satellite-based
instrument, the Solar Zenith Angle (SZA) is the angle between the local
zenith (i.e. directly above the point on the ground) and the line of sight
from that point to the sun. This means that the higher the Sun is in the
sky, to lower the SZA is.
The other angle in the graph, the angle between the local zenith and the
line of sight to the satellite, is called the Viewing Zenith Angle.
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Schematic illustration of the Solar Zenith Angle (SZA) and
Viewing Zenith Angle (VZA) for observations from satellite-based
instrument.
[image taken from a
NASA page]
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The value of the Solar Zenith Angle depends on the position on the Earth and
the local date and time. The following graphs shows the variation of the SZA
for places at about the time SCIAMACHY passes directly overhead. The
retrieval of SO2 data is not so reliable for very high SZA, say above
75 degrees, and becomes really unreliable at much higher SZA. That is
why data presented in the maps and data files on this website is limited to
SZA 85 degrees.
The time SCIAMACHY passes a place overhead is always the same local time, as
the satellite orbits the Earth in a so-called sun-synchronus orbit. If a
satellite in a polar orbit flies over the pole into the sunlight, the SZA
can be well above 100 degrees. The minimum SZA along the centre of an
orbit is about 24 degrees. While orbiting on the sun-lit side of the
Earth from North to South, SCIAMACHY scans from East to West, with on the
East (West) side an SZA that is a little smaller (larger) than in the centre
of the orbit.
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The Solar Zenith Angle as function of time in 2007 for several places at
about the moment SCIAMACHY passes overhead.
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place |
latitude |
longitude |
time (UTC) |
Reykjavik |
64.15 |
-21.97 |
ca. 12.35 |
Rome |
41.88 |
12.50 |
ca. 09.40 |
Mexico City |
19.40 |
-99.15 |
ca. 16:55 |
Galapagos Ils. |
-0.50 |
-91.20 |
ca. 16:00 |
Bulawayo |
-20.17 |
28.72 |
ca. 07:50 |
Wellington |
-41.28 |
174.78 |
ca. 21:55 |
South Orkney Ils. |
-62.00 |
-45.00 |
ca. 12:00 |
Note that ENVISAT, the satellite that carries SCIAMACHY, passes the equator
at 10:00 a.m. local solar time. For a given logitude the time in UTC is:
UTC_time = local_solar_time - longitude/15
For example the Galapagos Islands are at about the Equator at 90 degrees
West. With local_solar_time of 10:00 this gives a UTC_time for the overpass
of about 16:00.
The following table lists the time and direction of equator passes of the
different polar-orbiting nadir-viewing satellite instruments measuring SO2.
All these satellites have a sun-synchronus orbit at about 790 km above
the surface.
instrument |
satellite |
equator pass |
local solar time |
flight direction |
SCIAMACHY |
ENVISAT |
10:00 |
N -> S |
OMI |
EOS-AURA |
13:45 |
S -> N |
GOME-2 |
MetOp-A |
09:30 |
N -> S |
IASI |
MetOp-A |
09:30 & 21:30 |
N -> S |
AIRS |
Aqua |
00:45 & 12:45 |
S -> N |
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