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Communication satellites generally need to have multiple antennas to provide spanning coverage over large geographic areas, which are divided up into coverage c

Communication satellites generally need to have multiple antennas to provide spanning coverage over large geographic areas, which are divided up into coverage cells; some of the antennas may be phased array antennas; antennas and their associated coverage "beams" are generally steerable to some degree, electronically for phased array, and/or mechanically via physical gimbal mechanisms. In some or most cases, communication signals are non-viable without focused coverage because the signal strength would otherwise be insufficient. Received signal strength, from the point of view of a terrestrial satellite communications terminal on the ground/airborne/maritime can also vary based on where the terminal is located within the footprint of a communication satellite's coverage beam; it's typically strongest at the center of a beam. The most primitive conceptual model for a communications satellite is often described as a "bent pipe," where signals transmitted up to a satellite from the ground are relayed as-received by the satellite back down to terrestrial receivers. In the linked article, the diagram reflects a concept such that: if a "jammer" can interfere with the transmitted signal to the satellite in a manner that the satellite cannot discern the true signal from noise contributed by the "jammer," what is relayed back down to terrestrial receivers by the satellite is "drowned out" because the relayed signal is now a combination of the true signal and sufficiently more noise contributed by the jammer rendering the communications channel and any signals over that channel non-viable (i.e. "jammed").

2OEH8eoCRo0 2y ago
Noob antenna question: in a phased array is most transmit energy lost to destructive interference? I thought that the beam was just the region where the waves didn't destructively interfere
- JumpCrisscross 2y ago
  > in a phased array is most transmit energy lost to destructive interference? No. Phased-array antennae can be up to 90% efficient [1]. You aren't losing energy to destructive interference so much as channeling it to the constructive modes. (Totally physically inaccurate, but I think analogously correct.) [1] https://www.mwrf.com/technologies/embedded/systems/article/21153102/nxtcomm-gain-efficiency-with-fragmented-aperture-phased-arrays https://www.mwrf.com/technologies/embedded/systems/article/2...
- grayfaced 2y ago
  Low gain where there is destructive interference and high gain where there is constructive interference. That is the feature. Same amount of energy is radiated, it is just amplified in the high-gain main lobe. A low gain direction is not the same as no signal though. With enough power you can overpower the main lobe signal. https://en.wikipedia.org/wiki/Gain_(antenna) https://en.wikipedia.org/wiki/Gain_(antenna)
echoangle 2y ago
> Received signal strength, from the point of view of a terrestrial satellite communications terminal on the ground/airborne/maritime can also vary based on where the terminal is located within the footprint of a communication satellite's coverage beam; it's typically strongest at the center of a beam. That’s why I was asking, the jammer would have to be very strong to overwhelm a „real“ signal which is located in the main lobe when the jammer is outside.
- moandcompany 2y ago
  >That’s why I was asking, the jammer would have to be very strong to overwhelm a "real“ signal which is located in the main lobe when the jammer is outside. The mobile jammer in the article's scenario is transmitting toward the satellite, not the terrestrial terminals on the ground. Imagine in the primitive example case that the "real" signal transmission must also be coming from a terrestrial terminal as well. The real terrestrial terminal is likely be similar in size to the mobile jammer, and subject to the same kinds of constraints as the jammer. However, the jammer doesn't have to consider much about the quality of its radiated energy at the target frequencies. Let's also assume that that both the real transmission terminal and the jammer can emit the same amount of energy. The distance between both the real transmission terminal(s) and the jammer to the communications satellite, for all practical purposes, is roughly the same. From the primitive example communications satellite's perspective, it will see/hear both the real signal and an equally "loud" signal (noise) at the same frequencies. When the communications satellite relays or retransmits what it hears to the receiving terrestrial terminals, it will be the combination of what the satellite saw/heard. Instead of the "bent pipe" imagine another primitive example where the communications satellite is an optical mirror, and the "real" signal comes from a light source on the ground pointed at the satellite. The "real" signal could be represented using different modulations of that light in brightness, frequency/wavelength, as well as temporal pulses. Terrestrial receivers decode the "real" signal based on the flashes of light it observes from the mirror. The jammer in this scenario is another light source pointed at the same mirror. Terrestrial, ground-based jammers are not the only type of potential jammer in a satellite communications system's operational scenarios. In the prior example ground-based jammers look upward to jam the satellite. You can also imagine scenarios where airborne jammers are possible, which look downward to jam receiving terrestrial terminals. A ground-based jammer versus an airborne jammer would have a few differences. The ground-based jammer mostly needs to consider a fairly well-known location of the communications satellite, as the satellite can be easily observed, and its future positions can be calculated using ephemeris data, for targeting. An airborne jammer would have to deal with the issue that it has a lower field of view than the satellite beam's footprint and needs to have more specification information about the location of terrestrial receiving terminals to apply its own area of effect, which would be smaller than the satellite's beam. However, the effective distance between the airborne jammer and terrestrial terminals will be orders of magnitude lower than the communications satellite and terrestrial terminals. Power transmission capacity is a constraint for all terminals, satellites, etc, and an airborne jammer's ability to apply power will be more effective than satellites due to distance, but the airborne jammer must be more selective on where to apply (i.e. target) that power. One jammer could be used to affect the transmission signal, but it would likely take many jammers to target receiving terminals when there are many geographically dispersed terminals.

2OEH8eoCRo0 2y ago

Noob antenna question: in a phased array is most transmit energy lost to destructive interference? I thought that the beam was just the region where the waves didn't destructively interfere

JumpCrisscross 2y ago
> in a phased array is most transmit energy lost to destructive interference? No. Phased-array antennae can be up to 90% efficient [1]. You aren't losing energy to destructive interference so much as channeling it to the constructive modes. (Totally physically inaccurate, but I think analogously correct.) [1] https://www.mwrf.com/technologies/embedded/systems/article/21153102/nxtcomm-gain-efficiency-with-fragmented-aperture-phased-arrays https://www.mwrf.com/technologies/embedded/systems/article/2...
grayfaced 2y ago
Low gain where there is destructive interference and high gain where there is constructive interference. That is the feature. Same amount of energy is radiated, it is just amplified in the high-gain main lobe. A low gain direction is not the same as no signal though. With enough power you can overpower the main lobe signal. https://en.wikipedia.org/wiki/Gain_(antenna) https://en.wikipedia.org/wiki/Gain_(antenna)

echoangle 2y ago

> Received signal strength, from the point of view of a terrestrial satellite communications terminal on the ground/airborne/maritime can also vary based on where the terminal is located within the footprint of a communication satellite's coverage beam; it's typically strongest at the center of a beam. That’s why I was asking, the jammer would have to be very strong to overwhelm a „real“ signal which is located in the main lobe when the jammer is outside.

moandcompany 2y ago
>That’s why I was asking, the jammer would have to be very strong to overwhelm a "real“ signal which is located in the main lobe when the jammer is outside. The mobile jammer in the article's scenario is transmitting toward the satellite, not the terrestrial terminals on the ground. Imagine in the primitive example case that the "real" signal transmission must also be coming from a terrestrial terminal as well. The real terrestrial terminal is likely be similar in size to the mobile jammer, and subject to the same kinds of constraints as the jammer. However, the jammer doesn't have to consider much about the quality of its radiated energy at the target frequencies. Let's also assume that that both the real transmission terminal and the jammer can emit the same amount of energy. The distance between both the real transmission terminal(s) and the jammer to the communications satellite, for all practical purposes, is roughly the same. From the primitive example communications satellite's perspective, it will see/hear both the real signal and an equally "loud" signal (noise) at the same frequencies. When the communications satellite relays or retransmits what it hears to the receiving terrestrial terminals, it will be the combination of what the satellite saw/heard. Instead of the "bent pipe" imagine another primitive example where the communications satellite is an optical mirror, and the "real" signal comes from a light source on the ground pointed at the satellite. The "real" signal could be represented using different modulations of that light in brightness, frequency/wavelength, as well as temporal pulses. Terrestrial receivers decode the "real" signal based on the flashes of light it observes from the mirror. The jammer in this scenario is another light source pointed at the same mirror. Terrestrial, ground-based jammers are not the only type of potential jammer in a satellite communications system's operational scenarios. In the prior example ground-based jammers look upward to jam the satellite. You can also imagine scenarios where airborne jammers are possible, which look downward to jam receiving terrestrial terminals. A ground-based jammer versus an airborne jammer would have a few differences. The ground-based jammer mostly needs to consider a fairly well-known location of the communications satellite, as the satellite can be easily observed, and its future positions can be calculated using ephemeris data, for targeting. An airborne jammer would have to deal with the issue that it has a lower field of view than the satellite beam's footprint and needs to have more specification information about the location of terrestrial receiving terminals to apply its own area of effect, which would be smaller than the satellite's beam. However, the effective distance between the airborne jammer and terrestrial terminals will be orders of magnitude lower than the communications satellite and terrestrial terminals. Power transmission capacity is a constraint for all terminals, satellites, etc, and an airborne jammer's ability to apply power will be more effective than satellites due to distance, but the airborne jammer must be more selective on where to apply (i.e. target) that power. One jammer could be used to affect the transmission signal, but it would likely take many jammers to target receiving terminals when there are many geographically dispersed terminals.