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 — beacons are primarily used to report the satellite's condition and operational status so ground stations and operators can monitor its health.

Memory aids:

• Think of a quick greeting when passing someone on the street: "How are you?" "I'm good." That's like a beacon giving the satellite's health and status.

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Most analog satellites use linear transponders that simultaneously retransmit multiple signals within a relatively large passband (for example, 50–100 kHz wide). To avoid distortion the transponder must operate linearly and preserve the relative signal strengths of all uplinks. If one uplink is much stronger than the others it will consume a disproportionate share of the satellite's downlink power, and the transponder may even reduce its gain to stay below its maximum output. That strong signal can therefore “blind” the transponder to weaker signals and prevent other users from accessing the satellite. For more information, see the ARRL's An Amateur Satellite Primer: http://www.arrl.org/files/file/Technology/tis/info/pdf/0004036.pdf

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Satellite tracking programs use orbital data (typically Two-Line Elements) to compute where a satellite will be at any given time relative to your location. They can plot the satellite’s ground track on a map so you can see its real-time position over Earth.

For each pass they predict the start and end times and give the azimuth and elevation at those moments, and they also compute the time and azimuth/elevation at the pass peak (maximum altitude) so you know when and where to point antennas.

Because the satellite and the observer move relative to each other, the received and transmitted frequencies are shifted by the Doppler effect. Tracking software calculates the satellite’s line-of-sight velocity and reports the apparent frequency (or the frequency offset) throughout the pass so you can adjust transmit and receive frequencies as needed.

All of these features — real-time ground-track maps, pass start/peak/end times with azimuth/elevation, and predicted Doppler-shifted frequencies — are provided by typical satellite tracking programs.

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Amateur radio satellites use a variety of transmission modes depending on the satellite’s design and purpose. Many satellites include linear transponders that pass SSB and CW signals for voice and Morse-code contacts. Other satellites are configured as FM repeaters for simple voice contacts with handhelds. CW and various digital modes are commonly used for beacons, telemetry, and low-rate data links, while more advanced satellites support packet, PSK, SSTV, and other digital modes for telemetry and experiments. Because satellites can and do use all of these modes, the correct conclusion is that all listed modes are commonly used.

Memory aids:

• Common modes: SSB, FM, CW, digital (packet/PSK), SSTV

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A satellite beacon is a transmission from the satellite that contains status information. Beacons give operators a way to judge how much power to use: by comparing the strength of your signal to the beacon strength you can adjust your transmit power up or down to match the beacon, yielding an appropriate power level for your station. Beacons also often include information about the satellite's activity schedule (for example, whether a payload is on or off at particular times). Finally, since the beacon is transmitted on a known frequency, it helps operators tune their radios to compensate for Doppler shift by showing where the signal actually appears in frequency.

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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. A satellite tracking program uses those orbital elements to compute where the satellite will be at any given time and to calculate the time, azimuth, and elevation of a pass relative to your position on Earth.

The satellite transmitted power is not one of the Keplerian elements and is not needed to predict the satellite's orbit. The last observed time of zero Doppler shift (the moment the satellite was neither moving toward nor away from you, such as near overhead) is only a single observation and by itself does not provide enough information to predict the full future orbit or the timing and direction of subsequent passes.

The basic orbital (Keplerian) elements commonly used are:

1. Epoch
2. Orbital inclination
3. Right ascension of ascending node (RAAN)
4. Argument of perigee
5. Eccentricity
6. Mean motion
7. Mean anomaly
8. Drag (optional)

Reference: AMSAT's "Keplerian Elements Tutorial" for more detail on each element.

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Doppler shift is the apparent change in the frequency of a signal caused by relative motion between the transmitter and receiver. A common everyday example is the change in pitch of a siren as an emergency vehicle passes by: the source doesn't change frequency, but the motion relative to the observer makes the received frequency seem higher as it approaches and lower as it recedes. The same principle applies to radio signals from satellites: depending on the satellite's position and velocity relative to your earth station, the frequency you receive can be shifted upward when the satellite is moving toward you and downward when it is moving away.

Memory aids:

• Think of a passing ambulance or train whistle — pitch goes up as it approaches and down as it moves away.
• Approaching object = higher received frequency; receding object = lower received frequency.

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“U/V” refers to the frequency ranges used for the uplink and downlink: U stands for UHF (ultra-high frequency) and V stands for VHF (very-high frequency). In common amateur-satellite usage this means the station transmitting to the satellite (the uplink) is using the 70-centimeter UHF band and the satellite transmits back to Earth (the downlink) on the 2-meter VHF band. This naming simply indicates the uplink band first (U) and the downlink band second (V), so U/V means uplink on UHF and downlink on VHF.

Memory aids:

• U = UHF → uplink on 70 cm
• V = VHF → downlink on 2 m
• Read as “Uplink on UHF, Downlink on VHF”

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Satellites often rotate or change orientation as they travel. When a satellite or its antennas rotate, the antenna pattern as seen from a ground station changes, so the received signal strength can vary. If the satellite antennas are directional, rotation can swing the main lobe away from the receiver; even with omnidirectional antennas, parts of the satellite structure can intermittently block or alter the signal. These variations in signal strength caused by the satellite's rotation are called "spin fading."

Memory aids:

• 'spin' = 'rotation'

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LEO refers to the satellite's orbit, not to an operational mode or to energy. It stands for Low Earth Orbit, meaning the satellite is orbiting relatively close to Earth (roughly 160–2,000 km altitude). Because they are so close, LEO satellites have short orbital periods — on the order of about 90–100 minutes (around 100 minutes).

Memory aids:

• LEO = Low Earth Orbit — think "low" altitude and a short (~100 minute) orbit.

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Under FCC rules, anyone may receive transmissions from amateur radio satellites, including telemetry data. The restriction is on transmitting to space stations — sending commands or uplinks to satellites requires proper authorization and an appropriate FCC license. Title 47 CFR Part 25 governs transmissions to space stations to prevent harmful interference and to ensure coordinated use of satellite resources.

In practice, receiving telemetry is one of the easiest ways to get started with satellite listening; inexpensive receivers such as RTL-SDR USB dongles make it possible for many people to pick up satellite signals without transmitting.

Memory aids:

• Receiving = anyone; Transmitting = licensed/authorized

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The term uplink indicates the question is about amateur radio satellites. Many amateur satellites use a linear transponder, which listens to a relatively wide passband (tens of kHz) and retransmits that entire slice of spectrum on another frequency (the downlink). This allows multiple signals (for example several SSB signals) to be carried simultaneously.

If your uplink power is too low, the satellite's receiver won't hear you well and your downlink may be weak or inaudible. If your uplink power is too high, you can overload the satellite receiver and drown out other users (blocking parts of the transponder). Most satellites transmit a continuous Morse code (CW) beacon on the downlink. By comparing the strength of your downlink signal to the beacon you can tell if your uplink power is about right: if your downlink is much stronger than the beacon you are likely overdriving the transponder; if it is much weaker you should increase power. Adjust your uplink until your downlink signal strength is roughly the same as the beacon, which indicates you are in the appropriate power range.

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