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Update: (following @GregMiller's comment):

Lets suppose a spacecraft has performed a boost and not all ground stations received the update, lets suppose the difference between the pre and post maneuver TLEs adds up to 100 km/d, in 48 hours the pre and post objects would have diverged by 200 km (nearly 27 sec alongtrack diff).

If a ground station is trying to contact the object at the pre-maneuver position with a UHF antenna, would that be possible?

Original question:

I'm not versed in ground-station<->spacecraft communication so maybe the question is not clear, but let's suppose we have a spacecraft in LEO, circa 500 km mean height, and that we need to decide whether a TLE source is old enough to discard it or not.

How many km along-track difference with a new TLE would be enough to "miss" a contact using a UHF antenna?

Is it related directly with the frequency? How about then for VHF or S Band?

If it's related to other parameters of the antenna such as half power beam-width could you provide an example to depict regular figures?

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    $\begingroup$ I don't think there's a generic answer, it's going to depend a lot on the spacecraft. E.g., if they just did a boost, or are in a low orbit with more drag. It'd probably help to explain more about what type you're trying to contact. If it's important, I'd query spacetrack before every pass. $\endgroup$ Jan 10, 2023 at 16:57
  • $\begingroup$ Thanks @GregMiller, we can take up on your suggestion of a boost, lets suppose the spacecraft has performed a boost and not all ground stations received the update, lets suppose the difference between the pre and post maneuver TLEs adds up to 100km/d, in 48hs the pre and post objects would have diverged by 200km and a ground station is trying to contact the object at the premaneuver position with a UHF antenna, would that be possible? $\endgroup$ Jan 10, 2023 at 17:24
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    $\begingroup$ Interesting question! Is this expected to be about the geometry between the ground station and the expected/actual positions of the satellite? I may have missed something but the link budget is going to be every bit as important in defining that geometry as the other factors already discussed. Perhaps it could abstracted a bit: e.g. to say "it works at 5deg off boresight but not 10deg" but you'd still have to do some legwork on the link budget to get there. $\endgroup$
    – Puffin
    Jan 11, 2023 at 13:09
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    $\begingroup$ Thank you very much @Puffin for your insight it has helped me better understand the problem, I understand now that a low elevation pass of short duration will be more affected by a 20something error in the TLE used than a near zenith longer pass. $\endgroup$ Jan 12, 2023 at 12:10

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There is no single answer to this problem as it depends on too many variables:

  • What is the beamwidth of the transmitting and receiving antennas? Are either of them omnidirectional?
  • What is the transmitter power?
  • How much margin was in the link design?
  • Is the spacecraft pointing its antenna at the ground, and if so, do you expect it to be pointing based on the new or old orbit?

To answer your question, you need to conduct a link margin computation for your scenario. You can look at the beam patterns of the antennas and figure out what the drop in power will be if the line of sight is not aligned with the maximum power axis of the antenna. Secondary effect would be difference in path length.

It is not uncommon, when the TLEs are not well known, to do a search pattern with the ground antenna to try to "find" the spacecraft. The likelyhood of success, however, depends on the aforementioned link budget.

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  • $\begingroup$ Thank you very much @Carlos N, your answer helps me understand the variables that shape the complexity of the problem besides the along track difference itself that triggered the question, thanks for your contribution. $\endgroup$ Feb 2, 2023 at 14:40

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