How-To figure out which antennas to use for long range comms (RemoteTech2)

[maxdreamland]

Hi,

using RemoteTech 2, I was wondering what would be a good way to figure out the right antennas for communication satelites. Here's what i've come up with so far, and would like to get your feedback before i'll try and implement this "for real".

My goal is to have three satelites around both Kerbin and Moho, in a 120° split, to cover the whole of each planet for ground and orbital operations. Each of those satelites would have a Communotron 32 (range 5 Mm) so the three Kerbin satelites (2868 km orbit) can "see" each other (and mission control) with just the Communotrons. And the same would be true for the three Moho satelites (those would be in a similar ~2800km orbit).

Now to relay connections between those two clusters i figured each of the satelites would have a dish. The dishes of the three Kerbin satelites would be targeted at "Moho". Given that the "viewing cone" angle of the dish is wide enough that should enable each Kerbin satelite to "see" each of the three Moho satelites with just one dish. The same but vice-versa would be true for the three Moho satelites.

The tricky part now is to figure out which dish has enough range AND a "viewing cone" angle that is wide enough.

To figure out the range requirements i figured the theoretical maximum distance between Kerbin and Moho should be the sum of their Apoapsis's (or whatever the plural of Apoapsis is called :-) ). The idea behind this "formula" is if both planets reach their Apoapsis, but on opposite sides of Kerbol. I couldn't think of a scenario where they would be further apart, right??? (objections?) This, of course, leaves out fators caused by different inclinantions, but i figured for a "ball park" number it should be close enough.

For the "viewing cone" angle i figured the angle should be OK if the width of the cone at the maximum distance between Kerbin and Moho (see above) is at least twice the orbit of the Kerbin satelites (2 x 2868km = 5736km). That way all three Kerbin satelites should "fit" within the cone and therefore be "connectable".

Given that the Kerbin Apoapsis is 13.599.840.256m and the moho Apoapsis is 6.315.765.980m, my range requirement would come down to just short of 20 Gm (19.915.606.236m to be precise).

And using the formula "cone width" = 2 * tan(alpha) * distance, where alpha is the "viewing cone" angle of the dish, i come up with an angle of roughly 0.00825°

Do you think this is the right approach? Or am I missing something? (i've been using RemoteTech only for a short while)

regards

Max

Edited January 18, 2014 by maxdreamland

[maxdreamland]

Well, thinking about a bit more, i noticed a fault in my thinking.

I should actually check if the cone is wide enough at the closest distance between the planets, too. Otherwise not all the satelites orbiting Kerbin (or Moho) will be visible to the targeting satelites.

The closest distance between Kerbin and Moho should be the difference between Kerbins Periapsis (= Kerbins Apoapsis) and Mohos Apoapsis. Which will be 7.284.074.276m.

And that would equate to an angle of roughly 0.02275°, which is considerably larger than the 0.00825° I computed before. So this larger angle should actually be my target value.

Any more thoughts?

regards

Max

[wmheric]

Hi Max,

I didn't look up and check the quoted numbers for the apsis but if they are correct, the approach looks fine to me. One important thing to note though: 2868.75 km is the altitude for Keostationary orbits; what you actually need is the semi-major axis of 3468.75 km, which means that in a equilateral triangle configuration, the satellites will be more than 5 Mm apart. So you will need to install extra dishes (probably Comms DTS-M1) or make a polygon with more sides. (IMHO the ability to switch between altitude and the actual distance should be a feature in future KSP. It is quite inconvenient right now...)

Also, I noticed that the angle you calculated is only half of that of the viewing angle -- did you forget the factor of two? If I plug in 3 468 750 m and 7 284 074 276, I get a viewing angle of ~0.055 degrees.

According to the Reddit tutorial, the antenna with the shortest range outside of Kerbin's SOI is Communotron 88-88, which has a cone size of 0.06 deg, so it is just enough to cover the orbit you want to set up. (If you are playing Career Mode you probably would have unlocked Reflectron KR-14 in the tree first, but this dish has a cone size of only 0.04 deg, so you will again need extra antenna (and electricity) to actually point directly at each other.)

BTW, if you didn't know, for very small angles (I would say < 1 deg is small) you can use the approximation tan(alpha) ~ alpha, given that alpha is expressed in radians.

Hope it helps!

Eric

[match]

Hi Max,

I didn't look up and check the quoted numbers for the apsis but if they are correct, the approach looks fine to me. One important thing to note though: 2868.75 km is the altitude for Keostationary orbits; what you actually need is the semi-major axis of 3468.75 km, which means that in a equilateral triangle configuration, the satellites will be more than 5 Mm apart. So you will need to install extra dishes (probably Comms DTS-M1) or make a polygon with more sides. (IMHO the ability to switch between altitude and the actual distance should be a feature in future KSP. It is quite inconvenient right now...)

I saw that and was going to comment... got beat!

But...

When considering the viewing angle required, the "base" of the triangle (the leg defined by the satellites in Kerbal orbit) will only require 6,008 km. A satellite will be within this measure at all times.

So, with 3 satellites orbiting @ 2.868 Mm and targeting the satellite with an 88-88 dish, you should expect the 88-88 on the distant satellite to maintain continuous connection at ~ 5737.3 Mm [tan .03 = 3.004026 / x]. This of course, does not account for anything that may block the signal (I'm looking at you Mun!)

[maxdreamland]

Great feedback, thank you both!

Of course, you're right, i have to take the SMA and not the orbit height...totally missed that, thank you! As for the factor of 2, i "kinda" had it in there...and...still got it wrong :-)

I used Exel to quickly compute the width of the cone for a given angle (including the fator of 2) and then fiddled with the angle until i got the right cone width. But then i forgot that this value for the angle represents only half the angle.

Yesterday i tried this out in a sandbox game (by cheating and "teleporting" a test satelite into Moho orbit, using HyperEdit) and it turns out your recommendations do in fact work. I used a Communotron 88-88 and it has sufficient range and viewing angle.

So i can start to implement this in my career savegame.

Thanks again for your feedback!

regards

Max

[maxdreamland]

Hmm...one more question...i'm wondering how i can change the thread-prefix from "unanswered" into "answered". I tried "Edit Post" but that didn't work.

[DeMatt]

Hmm...one more question...i'm wondering how i can change the thread-prefix from "unanswered" into "answered". I tried "Edit Post" but that didn't work.

Click "Edit Post" for your first post, then click the "Go Advanced" button to get all the options.

[maxdreamland]

Found it, thank you!