Nanosatellite Optical Downlink Experiment (NODE) mission has 3 "signal":downlink, beacon uplink and RF.

According to a description of the mission, the high-rate downlink and beacon uplink are optical signals.

I thought we need a wide beam of the beacon signal to detect the position of the transmitter. If a beacon is optical, does that mean it has narrow beam?

What are the alternatives for beacon signals?



  1. Optical Communications for Small Satellites
  2. Non-coherent LED Arrays as Ground Beacons for Small Satellite Optical Communications Systems
  • $\begingroup$ Optical can be narrow beam or wide beam, or adaptive - with a wide beam that is narrowed after initial contact through negotiation $\endgroup$
    – Rory Alsop
    Nov 1, 2021 at 13:27
  • $\begingroup$ @RoryAlsop why RF signal isnt used? $\endgroup$ Nov 2, 2021 at 11:32
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    $\begingroup$ I think I'm beginning to understand what you are after. Optical communications links have higher gain because they can have a narrower beam. However that makes capture and tracking more challenging for a fast moving satellite in low Earth orbit. The answer (I think!) is YES, at least for some optical links they slightly defocus the beam (either transmit or receive or both) so that it's wider, and "listen" for a "beacon" which would be laser light of a certain wavelength with a simple modulated frequency. Once that's captured tracking can begin. $\endgroup$
    – uhoh
    Nov 2, 2021 at 12:37
  • $\begingroup$ This is really a good and interesting question! Voting to leave open so that answers can be posted! $\endgroup$
    – uhoh
    Nov 2, 2021 at 12:38
  • $\begingroup$ @Anna Koroleva, can an RF beacon be used to assist a laser pointing? $\endgroup$
    – Ng Ph
    Nov 2, 2021 at 18:09

1 Answer 1


The beacon doesn't need to be purposefully widened. It is very narrow compared with radio and microwaves, but it will always diverge a little bit due to physics, such that it covers a fairly big area at the distance of the opposite terminal. For example from a low Earth orbit, the spot on the ground might be 20 metres in diameter. From Mars, the full size of the spot on Earth capturing most of the energy would be the size of a country. But it's still much better than radio!

The beacon will carry out a scan pattern until it's found, usually some sort of spiral pattern. A quadrant detector at the receiver will find out which direction the maximum intensity of light is coming from, and it will know which way to re-orient itself to align with the beacon.

  • $\begingroup$ In the particular case mentioned by the OP, the beacon is in the Earth-to-Space direction and the receiver is on the spacecraft. If the "spot" beam is 20m in diameter in space, the Earth station may need to have a very accurate orbit prediction. $\endgroup$
    – Ng Ph
    Nov 2, 2021 at 23:17
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    $\begingroup$ You're right, needing a good orbit prediction - it can be enhanced a lot with good feedback systems such as GNSS and knowledge updates. OP mentioned widening the beam. I mentioned instead using a spiral scan pattern with a fixed beacon divergence which is the most common method. It's discussed further in Hemmati's Near Earth Laser Communication text. $\endgroup$
    – Polar_Bear
    Nov 3, 2021 at 9:48
  • $\begingroup$ I wonder whether in this technique of spiral scan, it is the transmit Earth-station that performs the scan or the receiver in orbit (or both)? $\endgroup$
    – Ng Ph
    Nov 3, 2021 at 18:19
  • $\begingroup$ @Polar_Bear your answers are great, this one could benefit from some quantitative support or some links. For example, the window for a link from ground to a spacecraft in LEO is only a minute or two long, and a 2D spiral search for a receiver would likely be done mechanically perhaps via tiny movements of some MEMS actuator at the focal plane, or some mirror. How quickly can that be done? Also, is a 20 meter laser footprint produced on the ground from orbit an example of "really big"? While that's a reasonable number for diffraction, how does one focus and maintain aim at 7000 m/s? $\endgroup$
    – uhoh
    Nov 3, 2021 at 22:31
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    $\begingroup$ @uhoh The spiral scanning is usually done via MEMS Fast Steering Mirrors (known as FSM). It depends on power, sensitivity, the control algorithms etc. but 1 KHz is possible. Not always easy to find but you could look at the 'disturbance rejection', usually around that value e.g. esmats.eu/esmatspapers/pastpapers/pdfs/2017/kuiper.pdf Both terminals may be able to spiral scan, usually one after the other, dependent on the Pointing, Acquisition and Control sequence. It can in theory be faster e.g. with VSCEL arrays instead of electromechanical movement, but other limitations exist. $\endgroup$
    – Polar_Bear
    Nov 7, 2021 at 20:38

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