How different is the communication to ground stations between passive and active ADCS? Also, is there any data or insight into probabilities of ground station contact with passive?


1 Answer 1


This is largely dependant on mission requirements - although that should also define whether you need active or passive ADCS.

The key piece of hardware that will be defined by the active/passive choice of attitude control is the antenna. Without going into too much detail on either antenna theory or frequency allocations it's safe to say the following;

  1. If you have no attitude control you almost certainly want an isotropic antenna (gain = 1)
  2. If you have perfect ADC and position measurement you want a very small beam width antenna (high gain)

There are exceptions to these rules. For example you might have a small data transfer requirement and could conceivably retransmit all data until a ground station sends a 'message received' signal - this would allow no ADC but a non-isotropic antenna.

If you satellite is large enough to be tracked or it can self determine it's location (and downlink that) then you can quite reasonably deal with an isotropic antenna on the satellite and a high gain (eg. Yaggi) antenna at the ground station. With approximately 1 downlink a day and a 30% beam width of the receiving antenna you should be able to reestablish the link each day (from the position and velocity of the satellite during current downlink you can reasonably estimate it's future position at 25 hours assuming it's perigee altitude is above ~300 km).

As for ground station downlink opportunities it's reasonably easy to predict. If you have an isotropic antenna then the avaliable link time is just a bit of geometry. If you have a directional antenna on the satellite then it becomes a question of geometry, statistics (likelihood of both the satellites attitude and position being correct during a flyover) and, astrodynmamics (how you ADCS effects changes to your attitude).

If you're interested in a specific satellite post it in the question/comments and I'll dust off some old code to go into more detail on the specifics of flyovers for your lat/long.

  • $\begingroup$ What does an actual "isotropic antenna" look like on a satellite? Can you link to an existing, real-world example? Also, a directional antenna can have a gain of 1, and a hypothetical isotropic radiator can have a gain of 0.1, so the "(gain = 1)" is not really helpful here. $\endgroup$
    – uhoh
    Commented Jan 20, 2017 at 3:09
  • $\begingroup$ If an "isotropic antenna" is an omni antenna, you can see pictures of one on a really big satellite in this answer: space.stackexchange.com/questions/18529/… $\endgroup$ Commented Jan 20, 2017 at 4:29

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