Satgen 478  Dark Wefax Pictures  by GM4IHJ  (BID SGEN478) 1998-05-23

Several correspondents write concerned that their Weather satellite
visual pictures are always dark and lack contrast .In fact their
equipment is probably working well. The difficulty lies with the LEO
low earth orbit NOAA and METEOR sats and the orbits they presently
occupy, which takes them constantly along a track which is close to
the terminator, the dawn dusk line . So that one side of the picture
is totally dark and the other side is badly illuminated.

Weather sats are primarly there to produce cloud pictures and water
vapour measurements . But we usually want to view the land surface
beneath the clouds. The American NOAA LEOsats have a variety of probes
which image , the visual, and, the near, middle and far InfraRed. So
they continue to show pictures of clouds but not land, even when
flying over a darkened earth. By contrast the Russian Meteor LEOsats (
only 3-5 usually operational), are visual only. Meteor switches off as
it crosses the terminator into darkness and does not switch back on
until about a minute or so after it reemerges over the daylight side
of earth 50 minutes later.
Table 1 below shows the situation on orbits near you on 19 May 98 :-

Table 1 SAT  Time local EQX ascending       Time local EQX descending
NOAA 12      1846 ( east side dark)         0610 (west side dark)
NOAA 14      1445 (good both sides)         0245 (all in darkness)
NOAA 15 ??   1932 ( east side dark)         0730 (west side dark)
METEOR 3-5   2034 (dark switched off)       0837 (west side dark)

NOAA orbits are originaly "Sun Synchronous " going near your station
at roughly the same local time each day. They stay this way for a few
years but eventualy begin to drift off as NOAA12 is doing. Hence its
replacement by NOAA15 if that satellite overcomes its present
difficulties. NOAA12 has slipped almost on to the dawn dusk terminator
line , so all its orbits are somewhat dark. NOAA15 should correct this
but it is not due to be fully operational until July/August 98. By
contrast NOAA14 gives superb visual pictures most afternoons.

The Russian Meteors are not Sun Synchronous. So 3-5's timings for
sunlit passes gradually change from day to day . At present 19 May its
ascending orbit crosses the equator coming north at 2034 local time.
By 19 August however its eqx ascending will be at about 1016 local
time . So in May it is switched off during its ascending orbit while
in August it will be favourably sunlit for visual pictures.

Regular users of Wefax will appreciate that this is not the whole
story. You get better visual pictures if the Sun is not exactly
overhead your target. With the Sun one or two hours ahead of or behind
your meridian of longitude ie 2pm or 10 am local you get good shadow
detail which enhances the picture quality enormously. Equally valid
however. If you want to target somewhere on the edge of your Wefax
footprint eg from here in Scotland that would be the Black Sea to the
east or Hudson Bay to the west , then you try orbits where the sun is
in the 10 am or 2 pm position for your target - getting say good
Canadian pictures as late as 6 or 7 pm Scottish time.