ORBITS.
Yes, but why doesn't it fall down on my head!........
What goes up must come down...right?....WRONG!....
(Pencils out!).....
So, how does a chunk of aluminium and plastic remain in orbit as opposed to falling down to the ground? .. gravity and velocity!.
If you throw a stone in a straight line using a small amount of force it won't go far before hitting the ground. The harder you throw the stone .. the further it will go before falling to the ground. The speed of the stone through the air is called the ‘velocity’.
If you don't know anything at all about ‘gravity’ .. a quick word with Isaac, preferably under the nearest apple tree, will soon set you straight.
In other words .. there are two ‘forces’ acting on the stone in its flight
through the air .. velocity and gravity .. (for now we will ignore the air's
‘resistance’).
Near to the surface of the earth, due to gravity, acceleration is measured as 9.75m /s/sec ...which means that an object near the earth's surface will travel downwards 9.75 meters in one second before hitting the ground .. note that I said ‘acceleration’ .. it means that the stone will travel (downwards) 19.5 meters after 2 seconds etc.
(Maps out!) .. you'll need a globe here .. as opposed to the old Mercator
‘flat’ atlas.
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Due to the curvature of the earth .. assuming you could throw the stone hard enough .. it will eventually have a velocity that will keep it in ‘orbit’. One force .. velocity .. acting on it to send it out into space .. the other force .. gravity .. trying to pull it back towards the earth's surface. When the ‘balance’ of these two forces is reached .. the stone will remain at a constant height and continue to circle the earth, due to its curvature .. or at least it would if there were no other forces acting on it like the atmosphere's resistance .. sometimes known as ‘drag’.
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ORBIT TYPES
LEO (Low Earth Orbit.)
The LEO is used by the digital amateur radio satellites and UoSATs.
Low Earth orbit satellites in near
polar orbits can ‘see’ most of the Earth’s surface several times a day. As an
example, UoSAT2 ‘sees’ several orbits of the UK each morning followed by
several more in the afternoon. This type of orbit is well suited to ‘store and
forward’ digital satellites and is also a favoured orbit type for weather
satellites where the complete surface of the Earth can be ‘surveyed’ on a
regular basis.
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Low Earth
Orbit.
MOLYIYA
An other type of orbit is the Molniya orbit. In this type of orbit the satellite is ‘flung’ out into space .. before returning to the earth due to gravity .. before being flung out into space again.
Imagine a stone tied to a piece of string .. whirl it around a few times then let go of the string .. the stone will whiz off in a particular direction but will soon return to the earth due to gravity. Imagine a satellite being ‘flung’ out into space thus .. it will travel in an outward motion away from the earth .. gradually losing speed until the earth's gravity begins to equal the velocity.
At this point the satellite curves and the force of gravity becomes stronger than the force due to velocity .. the satellite now starts accelerating back toward the earth .. the nearer it comes to the earth, the stronger becomes the force of gravity .. the satellite is still accelerating .. the forces again tend to balance out and the satellite is once more ‘thrown’ out into space .. this cycle continues ..while the earth ‘turns’ within the satellite's orbit.
As can be imagined .. due to the earth turning within the orbit .. a very large coverage area is obtained ..the orbit repeats, in cycles, depending on the parameters of the orbit ..height, inclination etc.
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The MOLNIYA ‘Lightning’ type of orbit was first used to provide
television reception for remote areas of the then USSR. Three satellites are capable of providing 24 hours per day coverage.
Example MOLNIYA orbit showing 2 passes.
GEOSTATIONARY
Yet another type of orbit is the GEOSTATIONARY orbit.
At approximately 36.000 kilometres above the equator ..velocity and gravity tend to balance out and a satellite placed into a GEOSTATIONARY orbit will appear to ‘hang’ in space ..due to the fact that it's velocity, gravity, and the earth's rotation are all equal. It will always appear to be in the same place .. ‘GEOSTATIONARY’. This type of orbit is optimal for TV satellites .. allowing one to keep the antenna pointing in the same direction. Small ‘cold gas’ thruster jets are used to control the satellite’s attitude and hold the satellite within an imaginary ‘box’ in space.
Stations on the equator need to point their antennas straight up! ..while those in the northern or southern hemispheres in the Polar Regions need to point their antennas at the horizon for GEOSTATIONARY satellites.
Geostationary orbits are heavily used by commercial communications satellites. The Meteosat weather satellite system also makes use of geostationary orbits.
INCLINATION.
Inclination is a measure of the angle made between the earth’s equator (equatorial plane) and the satellite’s ‘orbital plane’.
Imagine a ‘wire frame model’ of the satellite’s orbit over a small globe of the earth. The ‘wire frame’ represents the satellites orbital plane and the angle made with the earth’s equator is the Inclination of that satellite.
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Imagine the earth rotating in space with your satellite in orbit around it. Although the earth is moving in relation to the background stars the satellite’s orbit plane does not move in relation to the star background. The satellite travels around the ‘orbit plane’ but the plane is ‘fixed’ in relation to the background stars.
KEPLERIAN ELEMENTS and SATELLITE AVAILABILITY.
If you have access to the Internet, the best method of receiving up to date information is to subscribe to AMSAT’s ‘ANS’ and the ‘KEP’ service. Schedule and frequency information for all amateur radio satellites is published weekly by the AMSAT News Service (ANS) on the Internet and via packet radio. Your local PACKET BBS will usually have the latest bulletin ‘on line’. Just request bulletins from AMSAT, SAT or KEPS.
If you download KEPS from your local BBS, you may have to 'top and tail' the data using a simple text editor program before you can ‘import it’ into your tracking program..
Updated Keplerian elements and tracking data are always available from your QTH (providing you’ve told the BBS your LOCATOR!) via any BBS using the FBB software. Just log on to the SERVER mode, type ‘T’ then follow the on screen instructions for Keplerian elements of your favourite satellite, pass times for your QTH and other satellite related information.
TRACKING.
Tracking requirements vary according to the type of satellite operation you are interested in. Most radio amateurs use computers to provide information on when and where satellites will be visible at their QTH.
Satellite tracking programs vary in scope and complexity and the requirements for analogue users are very different from those demanded by digital satellite stations.
The AMSAT Corporation generates a large portion of the construction costs of new satellites from the sales of software. The software they sell is written by radio amateurs for radio amateurs. Because of this, you will get software that is suited to the job in hand rather than written to perform a range of general tracking functions.
AMSAT operate an Internet website offering among other things a selection of downloadable software. If you are new to satellites and have Internet access, the URL is - WWW.AMSAT.ORG
There you will also find many articles on amateur radio satellites, the collection of SATGEN bulletins by GM4IHJ and articles by Jim Miller G3RUH.
ANALOGUE
Users of analogue satellites generally want to be able to -
1) Predict when satellites are visible to them.
2) Generate pointing data in 'real time' - azimuth and elevation - to steer their beam antennas.
Additionally, some users make use of Doppler information generated by their computers to tune their radios although the majority of users 'tune by ear'. Mostly though, prediction and az/el data are what they want most.
‘SATSCAN’, ‘INSTANTRACK’ and ‘The STATION PROGRAM’ are
typical examples of suitable software for analogue users.
It is possible with many computer tracking programs to produce a 'bus timetable' list of predicted pass times in advance allowing the user to point their antennas by hand - by rotator or 'armstrong' method! This has some advantages, especially for the analogue satellite operator. One major point in favour of using printed AZ/EL data is the absence of computer generated 'hash' and 'desensing' on the Downlink signals.
DIGITAL
Digital satellite users have a completely different set of requirements.
Tracking in ' real time', pass scheduling and upload/download of data files are more usual requirements here. You can see where this is all leading - 'hands off' operating. Control of the radio tuning (Doppler correction) and antenna pointing facilities are also high on the agenda of digital users.
WISP , a windows based program offers developed by Chris Jackson G7UPN and sold by AMSAT is the most commonly used software package for digital satellite operation. An evaluation version may be downloaded from the AMSAT internet website and upgraded to the fully functional version by paying AMSAT the listed price at which time you will be issued with a 'key' to unlock the full range of features offered by this software package. Regular upgrades are posted on the AMSAT website as improvements are made to this software.
If you are interested in digital satellites, you should consider seeking some expert advice from an AMSAT 'Elmer' - someone with the experience to help you make a wise choice of software and hardware to suit your needs. Phone your local AMSAT organisation for advice.
Successful computer based satellite tracking is only possible if you have the following -
1) The correct time - usually in GMT (although some programs can use local time and a programmed offset).
2) Where you are on the earth's surface (latitude / longitude / height above sea level).
3) A set of up to date KEPLERIAN ELEMENTS.
Without this information, you will be certain to fail.
Keplerian elements are from several sources on the Internet.
The amsat.org website also carries Keplerian elements and you can even subscribe to AMSAT’s LISTSERVER for regular postings of up to date elements.
The amsat.org website also carries an excellent description of what Keplerian elements are and what the numbers mean. This description is the most understandable we have come across and was written by Franklin Antonio, the author of the well known ‘Instantrack’ satellite tracking program.
A web search for 'CELESTIAL BBS' will unearth Thomas Kelso's Keplerian element service. Downloading the 'Master List' and editing out all the non-amateur satellites will provide you with up to date information for your tracking software. If you don't have Internet access, try the packet radio network - search for (KEPS/KEPLER etc).
PAPER COMPUTING
There is another way - pioneered by the renowned 'Kettering Group' -, that makes use of simple paper wallcharts laid out as an X/Y graph.
The method relies on plotting beacon 'hits' on a chart with one axis measuring time (usually the X axis) and DAY NUMBER down the Y axis.
The principle depends on collecting enough beacon 'hits' (time of zero Doppler ) to build up a graphical picture of the local crossing times .
So by copying the 'hits' onto a clear overlay - a sheet of plastic for instance - the pattern can be moved down the chart and slid from side to side - overlaying the existing pattern and showing new possible 'hits - to predict when the next orbits will take place. This method is very useful when a new satellite is launched and no orbital elements exist. This is one area where you can be ahead of the 'big boys', NORAD and NASA!
A variation on the 'paper computing' theme is the 'OSCARLOCATOR' method pioneered by AMSAT. This method has died out due to the use of home computers. It makes use of a POLAR MAP of the north or south hemispheres - depends where you live - and a plastic 'overlay' pinned in place onto the North or South pole with a thumbtack. A set of look up tables of 'equator crossing- EQX' times (published as a booklet by AMSAT) was consulted to get the next 'EQX' and the overlay is rotated to the angle specified. The overlay then showed the satellite track in the correct place along with the number of minutes (marked on the overlay) from the published EQX time until it came into your radio range.
A ‘spiderweb’ was supplied with each plastic overlay and located at the user’s QTH on the map to provide pointing (azimuth) data.