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Subelement E2
OPERATING PROCEDURES
Section E2A
Amateur radio in space: amateur satellites; orbital mechanics; frequencies and modes; satellite hardware; satellite operations
What is the direction of an ascending pass for an amateur satellite?
  • From west to east
  • From east to west
  • Correct Answer
    From south to north
  • From north to south

An astronomical object is Ascending when it is moving north in latitude with respect to the celestial sphere or "up" with respect to the usual ground map. Decending means moving south or "down" with respect to the map.

Most satellites (Including all amateur radio satellites which have ever been launched) move west to east or "prograde." To launch a satellite into a retrograde orbit is much more difficult and expensive and is typically only done for commercial or military earth observation satellites in order to maximize their coverage over the ground as retrograde orbits move faster with respect to the ground. There are no questions about retrograde or prograde orbits on the Element 4 exam, so this information is purely gratuitous.

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Which of the following occurs when a satellite is using an inverting linear transponder?
  • Doppler shift is reduced because the uplink and downlink shifts are in opposite directions
  • Signal position in the band is reversed
  • Upper sideband on the uplink becomes lower sideband on the downlink, and vice versa
  • Correct Answer
    All these choices are correct

An inverted linear transponder reverses the ordering of frequencies between input and output. The Doppler shift shifts the input and output frequencies the same way, and so there is some cancellation due to the reversing effect of the transponder. USB and LSB signals are likewise swapped, as is signal position.

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How is the signal inverted by an inverting linear transponder?
  • The signal is detected and remodulated on the reverse sideband
  • The signal is passed through a non-linear filter
  • The signal is reduced to I and Q components and the Q component is filtered out
  • Correct Answer
    The signal is passed through a mixer and the difference rather than the sum is transmitted

Satellites have transponders which receive the uplink signal and retransmit it as a downlink signal. Unlike your usual repeaters though they receive an entire band of signals and retransmit the entire band shifted to another frequency.

Transponders come in both inverting and non-inverting types. The inverting type, which is the subject of this question, "flips" the entire band before retransmitting thus the highest frequency in the uplink becomes the lowest frequency in the downlink.

To do this, The signal is passed through a mixer and the difference rather than the sum is transmitted. The base band frequency is chosen such that the proper "shift" occurs, but in the inverting case the mixer operation calculates offsets from the other side of the baseband compared to the non-inverting case and the baseband frequency is chosen accordingly.

The other answers are wrong and this is easy to remember if you just remember that:

  • a frequency mixer can produce both sums and differences.

  • a mixer is used to invert the signal in this type of transponder.

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What is meant by the term "mode" as applied to an amateur radio satellite?
  • Whether the satellite is in a low earth or geostationary orbit
  • Correct Answer
    The satellite's uplink and downlink frequency bands
  • The satellite's orientation with respect to the Earth
  • Whether the satellite is in a polar or equatorial orbit

Historically OSCAR uplink (transmit to) and downlink (receive from) frequencies were designated using single letter codes.

Mode A: 2 m uplink / 10 m downlink

Mode B: 70 cm uplink / 2 m downlink

Mode J: 2 m uplink / 70 cm downlink

New uplink and downlink designations use sets of paired letters following the structure X/Y where X is the uplink band and Y is the downlink band.

Reference: http://en.wikipedia.org/wiki/OSCAR#Mode_designators

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What do the letters in a satellite's mode designator specify?
  • Power limits for uplink and downlink transmissions
  • The location of the ground control station
  • The polarization of uplink and downlink signals
  • Correct Answer
    The uplink and downlink frequency ranges

Historically OSCAR uplink (transmit to) and downlink (receive from) frequencies were designated using single letter codes.

  • Mode A: 2 m uplink / 10 m downlink
  • Mode B: 70 cm uplink / 2 m downlink
  • Mode J: 2 m uplink / 70 cm downlink

New uplink and downlink designations use sets of paired letters following the structure X/Y where X is the uplink band and Y is the downlink band.

Reference: http://en.wikipedia.org/wiki/OSCAR#Mode_designators

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What are Keplerian elements?
  • Correct Answer
    Parameters that define the orbit of a satellite
  • Phase reversing elements in a Yagi antenna
  • High-emission heater filaments used in magnetron tubes
  • Encrypting codes used for spread spectrum modulation

Memory item:

Keplerian elements are a set of numerical parameters that define the motion of an orbiting body or satellite. They are named after Johannes Kepler who first defined the laws governing orbiting planet motion.

See http://en.wikipedia.org/wiki/Orbital_elements

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Which of the following types of signals can be relayed through a linear transponder?
  • FM and CW
  • SSB and SSTV
  • PSK and packet
  • Correct Answer
    All these choices are correct

A linear transponder takes one range of frequencies and directly shifts it to another range, without decoding and re-encoding the signals. Therefore, any signal type that fits in the transponder's bandwidth can be used through it.

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Why should effective radiated power to a satellite that uses a linear transponder be limited?
  • To prevent creating errors in the satellite telemetry
  • Correct Answer
    To avoid reducing the downlink power to all other users
  • To prevent the satellite from emitting out-of-band signals
  • To avoid interfering with terrestrial QSOs

Satellite transponders are power-sharing. If one signal is received at the satellite is stronger than the others, all other retransmitted signals will get a smaller portion of the available power.

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What do the terms "L band" and "S band" specify regarding satellite communications?
  • Correct Answer
    The 23 centimeter and 13 centimeter bands
  • The 2 meter and 70 centimeter bands
  • FM and Digital Store-and-Forward systems
  • Which sideband to use

Transponder link designators:

Frequency Designator
21Mhz H
29Mhz/ T
145Mhz/ V
435Mhz/ U
**1.2Ghz/ L**
**2.4Ghz/ S**
5.7Ghz/ C
10.5Ghz/ X
24Ghz/ K

-w3mit


Study Hint: Think "(L)ittle and (S)mall" for 23 and 13 cm.

See the mode designator chart

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What type of satellite appears to stay in one position in the sky?
  • HEO
  • Correct Answer
    Geostationary
  • Geomagnetic
  • LEO

A geosynchronous satellite is actually moving along its orbit just like any other satellite. It is at a height and position such that this orbital rotation matches the earth’s rotation and so it appears to be stationary.

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What type of antenna can be used to minimize the effects of spin modulation and Faraday rotation?
  • A linearly polarized antenna
  • Correct Answer
    A circularly polarized antenna
  • An isotropic antenna
  • A log-periodic dipole array

Faraday rotation is the change in radio wave polarization due to the presence of a magnetic field, while spin modulation is also a polarization change, but due to the rotation of the signal source or of an object that reflects the radio wave, such as a satellite. Both effects can result in rotating radio waves, which can be minimized by a circularly polarized antenna.

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What is the purpose of digital store-and-forward functions on an amateur radio satellite?
  • To upload operational software for the transponder
  • To delay download of telemetry between satellites
  • Correct Answer
    To store digital messages in the satellite for later download by other stations
  • To relay messages between satellites

With a satellite in low earth orbit, the transmit and receive stations may not have a line of sight path to the satellite at the same time. Store and forward allows the transmission to be stored for a while until the receiving station can be “seen” by the satellite

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Which of the following techniques is normally used by low Earth orbiting digital satellites to relay messages around the world?
  • Digipeating
  • Correct Answer
    Store-and-forward
  • Multi-satellite relaying
  • Node hopping

Store-and-forward means the message is stored and later sent when the satellite is in contact with the appropriate ground station.

Because low-earth satellites can only see a relatively small portion of the earth at once, instantly repeating the message (digipeating) is not a practical method to convey messages long distances.

Unfortunately, there is presently no large network of amateur LEO satellites that would allow instant relaying of messages between multiple satellites.

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