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Is there some more info on what exactly this system is jamming? Radar satellites? It probably can’t prevent optical images and downlinking of collected data to

Is there some more info on what exactly this system is jamming? Radar satellites? It probably can’t prevent optical images and downlinking of collected data to enemy ground stations. Edit: according to this ( https://www.dailymail.co.uk/sciencetech/article-14001079/amp/Secretive-American-weapon-JAMS-satellites.html https://www.dailymail.co.uk/sciencetech/article-14001079/amp... ), it jams enemy communication which is relayed over satellites. I imagine this only works if the receiving antenna on the satellite isn’t highly directional enough to only pick up signals where enemy troops are known to be. If the satellite has a phased array, the origin of the signal could be filtered very fine grained as far as I know and this kind of jamming could be removed (unless it is so strong that it completely saturates the amplifiers of the receiver?).

fasteddie31003 2y ago
I bet it would jam synthetic aperture radars
sidewndr46 2y ago
A phased array can have its phase altered to be steerable. That doesn't work if you overwhelm the receiver to the point it becomes a non-linear device.
- MegaDeKay 2y ago
  Yeah, but it does take a lot more power as you point out. Just like you can use the phased array to form a beam toward a particular location of a particular size, you can also do the opposite and create deliberately low gain toward the jammer. So then you need enough jammer power to overwhelm the LNAs on the individual elements, each of which have relatively low gain.
  - sidewndr46 2y ago
    no, it doesn't work that way at all. The receive side of a communication satellite would be multiple antennas each connected to its own receivers. The receiver all use a common clock source to perform demodulation (which is a bunch of steps usually, but whatever). The result of each receiver is summed, the result signal is transmitted by another transmitter connected to a different antenna on a different frequency. Phase difference between the receivers is used to steer the array. How the phase difference is created varies. But if you overwhelm just one receiver it basically just injects noise into the entire process. So you wind up summing noise with all the other signals, which destroys the signal to noise ratio of the transmitted signal now if the control station can switch one receiver off to a dummy load, instead of to an antenna that might be useful. Whether or not military comm satellites can do that is most definitely classified.
    - gruez 2y ago
      >now if the control station can switch one receiver off to a dummy load, instead of to an antenna that might be useful. That wouldn't do anything, would it? A phased array antenna is relatively small enough that a jamming signal from the ground would jam all antennas, not just one or a few.
      - sidewndr46 2y ago
        depends on the arrangement of the antennas. When it comes to phased arrays you can basically use any arrangement you want within reason
    - MegaDeKay 2y ago
      I was assuming the receive antenna is also a phased array and the first thing after each element has to be a passband filter and an LNA for reasons of G/T. That happens before all the demodulation per element. You said the approach was to "overwhelm the receiver to the point it becomes a non-linear device", and pushing that LNA past P1dB will start to do just that. If the LNA remains linear, a countermeasure to the process you're talking about would be for its user to lower the symbol rate and number of carriers per channel to increase the C/N of the desired signal in an effort to punch through the jamming, albeit with less capacity. Having said all that, not sure which of the two effects is at play here.
moandcompany 2y ago
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...

fasteddie31003 2y ago

I bet it would jam synthetic aperture radars

sidewndr46 2y ago

A phased array can have its phase altered to be steerable. That doesn't work if you overwhelm the receiver to the point it becomes a non-linear device.

MegaDeKay 2y ago
Yeah, but it does take a lot more power as you point out. Just like you can use the phased array to form a beam toward a particular location of a particular size, you can also do the opposite and create deliberately low gain toward the jammer. So then you need enough jammer power to overwhelm the LNAs on the individual elements, each of which have relatively low gain.
- sidewndr46 2y ago
  no, it doesn't work that way at all. The receive side of a communication satellite would be multiple antennas each connected to its own receivers. The receiver all use a common clock source to perform demodulation (which is a bunch of steps usually, but whatever). The result of each receiver is summed, the result signal is transmitted by another transmitter connected to a different antenna on a different frequency. Phase difference between the receivers is used to steer the array. How the phase difference is created varies. But if you overwhelm just one receiver it basically just injects noise into the entire process. So you wind up summing noise with all the other signals, which destroys the signal to noise ratio of the transmitted signal now if the control station can switch one receiver off to a dummy load, instead of to an antenna that might be useful. Whether or not military comm satellites can do that is most definitely classified.
  - gruez 2y ago
    >now if the control station can switch one receiver off to a dummy load, instead of to an antenna that might be useful. That wouldn't do anything, would it? A phased array antenna is relatively small enough that a jamming signal from the ground would jam all antennas, not just one or a few.
    - sidewndr46 2y ago
      depends on the arrangement of the antennas. When it comes to phased arrays you can basically use any arrangement you want within reason
  - MegaDeKay 2y ago
    I was assuming the receive antenna is also a phased array and the first thing after each element has to be a passband filter and an LNA for reasons of G/T. That happens before all the demodulation per element. You said the approach was to "overwhelm the receiver to the point it becomes a non-linear device", and pushing that LNA past P1dB will start to do just that. If the LNA remains linear, a countermeasure to the process you're talking about would be for its user to lower the symbol rate and number of carriers per channel to increase the C/N of the desired signal in an effort to punch through the jamming, albeit with less capacity. Having said all that, not sure which of the two effects is at play here.

moandcompany 2y ago

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...