Health and status of the satellite is telemetry information typically transmitted by satellite beacons.
Some satellites may transmit other information, but the key word here is typical.
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Most analog satellites have what are called
linear transponders which retransmit signals in a relatively large "band" often 50-100kHz wide (the
passband). What this means, in effect, is that multiple simultaneous signals can be retransmitted (repeated) at the same time.
The trouble is, if one of the signals is significantly more powerful than the other signals it can effectively "blank out" the other signals -- somewhat like how you might be able to see three separate dim flashlights on a distant hill, but if one of them is a powerful flood light you would likely be blinded by that light and unable to see the other two dimmer lights.
Thus, you should never use more power than you need when using a repeater because doing so may "blind" the linear transponder to the other signals and block access by other users.
For more information, see the ARRL's An Amateur Satellite Primer
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Satellite tracking programs tell you where a satellite is at a given time, including its altitude and where it will be at the start and end of a pass, relative to your location.
The tracking programs even tell you how much to change your transmitter's and receiver's frequency to compensate for the Doppler shift you get when the satellite is coming toward you or moving away from you.
So all of these answers are correct.
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A beacon provides us with a gauge to determine how much power we should use. If you transmit your signal and compare it to the beacon strength, you can then adjust your power up or down to match the beacon. That would be the optimum transmitting power for your station. The next thing that the beacons provide us with is a schedule of the satellite's activity. It might tell you that it is on during a particular time period and off during others.
The beacon can also help us tune our radio to compensate for doppler shift. Since we know the beacon is supposed to be on a certain frequency, we can calculate where our signal will be based on the current reception of the beacon (http://www.amsat.org/articles/houston-net/beacons.html)
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Keplerian elements, named after Johannes Kepler (1571-1630) and his laws of planetary motion, are the parameters that define the orbit of a satellite. From these elements, a computer program can calculate the time and bearing of a satellite pass, relative to your position on the earth.
The weight of the satellite is not one of the Keplerian elements. The last observed time of zero Doppler shift would be the time that the satellite was moving neither toward you nor away from you (like when it was overhead, for example). While this might be interesting data, it isn't enough to predict where it will be coming from or going to on its next pass, or at what altitudes.
According to AMSAT's "Keplerian Elements Tutorial" the "basic orbital elements are":
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The most common references to the doppler effect (or doppler shift) refer to sound; one of the most common examples used in highschool science classes involves a fire engine (or other vehicle with a siren) whose siren seems to drop in pitch drastically when the vehicle passes you. The producer of the sound does not actually change frequency, but the relative speed of the vehicle producing the sound to the object (you) receiving the sound makes it seem to you that it does.
The same principle applies to a radio frequency signal; the relative motion between a satellite and the earth station can cause a shift in the frequency at which you can receive the signal depending on what its position and momentum are relative to the receiving station.
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"mode U/V" is short for "mode UHF/VHF" -- meaning that the uplink is UHF, meaning 70 cm, and the downlink is VHF, meaning 2 meters. There are of course other UHF bands besides 70cm and other VHF bands besides 2m, but they are not commonly used by amateur radio operators and so the term is understood to mean the standard UHF/VHF bands.
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Satellites are not stationary in space; they are constantly moving, and generally they are rotating as well. As they turn, the antennas on the satellite change position relative to your location. The signal may fade if the antennas are directional, or even if omnidirectional, if they are obscured by the rest of the satellite.
This is referred to as "spin fading" because the fading is caused by the satellite spinning around.
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Just remember that we are talking about a satellite; this question could be a bit tricky if you haven't seen it before, but LEO refers to the position, not to any operation. It is, as the answer indicates, Low Earth Orbit.
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Yep, anyone who can receive the telemetry signal is allowed to receive telemetry from a space station but not allowed to transmit to one without a license.
With the availability of inexpensive RTL-SDR USB dongles, many people are trying their hand at receiving telemetry from space stations even if they don't have a license. For some of us, this sort of thing is what got us interested in amateur radio to begin with.
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uplink suggests that the question was written with amateur radio satellites in mind; most amateur radio satellites have what is called a
linear transponder, which listens to a relatively large
passband (perhaps 50-100kHz wide, where a normal SSB signal uses 3kHz or less) and retransmits it on another frequency (the
downlink). In this way, multiple signals can be carried simultaneously by the satellite.
uplink power is too low, your signal coming back may not be strong enough to be heard; on the other hand, if your power is too high you could "blind" the satellite to other signals, blocking them from using it.
The satellite will constantly transmit a morse code (CW)
beacon on the downlink; you can use that and compare it to your signal strength when it comes down. If your signal is stronger than the beacon then you are likely overloading the receiver and potentially blocking other users; if your signal is weaker than the beacon then the receiver isn't hearing you as strongly as it could. Adjust your power until the your signal strength on the downlink is the same as the beacon and you're in the "just right" zone =)
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