SSO Satellite Weather Service - FAQ

Why are the images often black on the left or right sides?

NOAA satellites orbit the earth at an altitude of about 800km, which compared with the earth's diameter (12730km) is quite low.  So they can only see a relatively small circle of the Earth's surface (~6000km wide) beneath them.  For example, this image shows exactly how much of the Earth you would see if you were currently orbiting with NOAA-17.  In their polar orbits, the NOAA satellites fly along north-south tracks, and will normally pass somewhere to the east or west of a mid-latitude observer.  So for example, if the satellite is flying over the Tasman Sea it will be able to see Sydney but probably not Adelaide, and the image will show a large black area toward the west.  And likewise if the satellite passes over Western Australia, there will be a dark area shown on the east side of the image.  If the satellite should pass directly overhead, as occasionally happens, most of the image will be filled.


What do the colours in the clouds mean?

Because the NOAA satellites can measure both the temperature of the ground, and the temperature of cloud-tops, they can make an estimate of the likelihood of rain.  The general rule-of-thumb is that the thicker the cloud, the colder is the cloud-top, and the more likely it is to form rain (although sometimes the cloud can be just high thin and cold, with nothing much under it).  Cloud presented on these web images is coloured white unless the cloud-top temperature is less than -20C, in which case it is coloured green.  Cloud-top temperatures less than -30C are coloured yellow and less than -40C, coloured red.  Colder than that and the cloud is coloured grey or black.  As a general guide, the probability of rain goes as: green - possible, yellow - probable, red - very likely, and black - take cover!


What is the yellow cross for?

Shows the exact location of Siding Spring Observatory, located ~20km west of the township of Coonabarabran, ~350km NNW of Sydney.


What does AOS stand for in the table of satellite passes?

'Acquisition of Signal', which is aerospace jargon for the time at which a satellite rises above the local horizon.


What causes the curved lines of speckled interference that occasionally appear across an image?

The particular antenna we use for receiving the NOAA satellites has a 'deaf spot' located directly upward.  If a satellite should pass nearly directly overhead, then the receiver will hear the satellite only weakly.  Unfortunately there are intermittent sources of local radio interference at Siding Spring Observatory, which become more apparent if the satellite signal is weak.  We are presently trying to locate and eliminate the worst sources of local radio interference.  Some interference is also occasionally received from extraterrestrial sources, which may cause dark or speckled bands to appear across the image.


What causes the wavy lines that occasionally appear at the top and bottom of images?

See also the previous question.  When a satellite is low on the horizon, it is also relatively distant (2-3000km) and its received radio signal is weak.  At this time the effect of local radio interference will become more apparent.  For interest, the NOAA satellites transmit their APT images using ~5 Watts of radio power, which small when compared with commercial television transmitters, which typically broadcast with 10's to 100's of kilowatts of radio power.  I estimate that the power received from NOAA 137MHz APT transmissions at ~2000km range, will be around 0.2 trillionths of a Watt per square metre.  Since most APT receiver antennas have a radio wave 'capture area' of around one square metre, they will collect ~0.2 trillionths of a Watt.  Astonishingly, good radio receivers can easily detect this,  and it's understandable how such weak signals can easily be drowned out by relatively powerful local sources of radio interference.  There is also some evidence that some NOAA APT interference may be caused by transmissions from other satellites also using the 137MHz space band.


The thumbnail images are too big.  Is it possible to reduce their size?

It is possible, but our thumbnails are deliberately set large so that users can see a moderately detailed picture using just the thumbnails.  This helps if one is viewing the sequence of thumbnails, and trying to estimate the gross cloud pattern movement.  We're sorry if this is blowing out your screen.



Why is either NOAA-12 or NOAA-15 switched off every few months?

The NOAA-12 and NOAA-15 spacecraft have similar orbits, and both transmit their APT imagery at a radio frequency of 137.5 MHz.  If the satellites should come within ~6000km of each other, ground receiving stations could potentially receive transmissions from both satellites simultaneously, which would result in a garbled image.  So if the satellites should approach within 6000km of each other (i.e. the satellites are said to be 'in conflict'), usually the 137.5 MHz transmitter aboard the older NOAA-12 spacecraft is switched off.  However occasionally NOAA-15 is switched off instead.

NOAA-12 orbits slightly faster than NOAA-15, completing 14.25071335 earth orbits per day, compared with 14.24238665 orbits of NOAA-15.  These numbers look very similar, but the difference between them (0.0083269) means that every 120.1 days NOAA-12 will complete one more orbit than NOAA-15.  So roughly speaking, every day NOAA-12 'gains' on NOAA-15 by ~375 km.  At this rate, NOAA-12 and 15 will remain within ~6000 km of each other for ~30 days.  With NOAA-12 lapping NOAA-15 once every 120.1 days, and with the spacecraft too close to each other for ~30 days, either NOAA-12 or NOAA-15 APT transmissions are switched off for about one month in four.

During the times that NOAA-12/15 is switched off, you may wish to monitor the current whereabouts of NOAA-12 and NOAA-15 as they play tag with each other around the globe.


Is it possible to extend the border of the images, to cover Western Australia?

Not really.  The NOAA satellites orbit at an altitude of ~800km, and from that height have a footprint ~6000km wide.  So in order to view WA, a satellite would need to pass over central Australia, or further westward.  In fact this does happen 2-3 times a day and it would be possible to receive some WA imagery from a receiving station located in NSW.  But NOAA satellites passing overhead Perth, or further westward, are not receivable in eastern NSW.  Because of the limited coverage of WA, and because this website is primarily a service to local astronomers, the focus is on cloud cover in south-eastern Australia.  However, it should be noted that with the advent of powerful relatively cheap computers, and clever software of the kind which runs this website, it is well within the scope of an enterprising schoolkid to set up a NOAA satellite receiving station somewhere in WA.  Given how easy it is, my feeling is that someone will probably do it before long.  If I should note other Aussie based stations, I'll advertise the fact on the main web page.