[1.2] Signal Strength and Relay Sats

[Kelderek]

1.  I've been messing around with setting up a relay network in a sandbox game in 1.2 and am having difficulty understanding what I am seeing for signal strength.  I have a set of 4 equidistant relay satellites between the Mun and Minmus at about 29 Mm altitude above Kerbin and another set that is out past Dres at 50 Gm from Kerbol.  Since this is a sandbox game the tracking station is tier 3 and I altered the game settings so that there are no additional ground stations beyond the KSC.  When I fly one of the deep space probes I can see a signal strength of only 32%.  The signal detail shows me exactly which of the Kerbin relay satellites it is connecting with and in the map view I can target the other relay probe and see that that satellite is about 62 Gm away in a direct line.  All of these relay satellites have the largest relay dish installed with ratings of 100 Gm.  Since 62 Gm is well inside the range of both 100 Gm dishes, why am I only getting 32% signal strength?

I have read through the design document here, but the description doesn't seem to explain it well enough for me to understand.  How close would two 100 Gm relay satellites need to be to still get 100% signal strength for that leg of the path?  At the time I checked the signal was bouncing off two of the inner Kerbin relays which both showed 5 green bars of strength, but the first hop from beyond Dres is only 2 orange bars, hence the 32% strength.

2.  I'm finding it difficult to have my satellites stay in formation.  I set up 4 equidistant relay satellites at 29 Mm above Kerbin, but by the time I finished setting up 4 more relay satellites at 50 Gm above Kerbol, the 4 inner satellites were all out of position relative to each other, a couple of them were only about 10 degrees apart instead of the 90 degrees they started at.  I suspect this has to do with the fact that it took me more than 29 game years to establish the outer relay orbits (GAWD we need a higher time acceleration than 100,000x!).  Does anyone have any advice for how to avoid this?  Should I avoid formations altogether and just put more satellites out in different places and hope I get good enough coverage?  I used this wonderful calculator to set these orbits up to begin with.

[Rodhern]

1) Thank you for the document link. I don't see a strength formula in there either. Sorry. How far do you have to go before you get 99% or 98%; maybe the strength deterioration begins straight away beyond zero range?

2) If the satellites have the same Pe and Ap they should maintain spacing indefinitely (for each full orbit). If this is not what is happening in pre-release ver. 1.2. I think it is worth investigating further.

[Kelderek]

I don't think the positioning is changing from any sort of bug, I just suspect that I was not as precise as I needed to be with my orbits and once I fast forwarded 29 years those small  inaccuracies end up looking much more obvious.  I'm going to try to see if getting perfect orbits from the cheat menu will work.  If all 4 satellites in a formation have the exact same orbital numbers except for the longitude of the ascending node (0, 90, 180, 270 degrees) then I would assume they could stay in formation indefinitely - I'm running a time warp test of this right now.

[Rodhern]

17 minutes ago, Kelderek said:

... I fast forwarded 29 years ...

Ah! That makes sense.

[Kelderek]

1 hour ago, Rodhern said:

maybe the strength deterioration begins straight away beyond zero range?

I don't think so.  I have two of these relay satellites 41 Mm apart and they have 100% signal strength.  I may run some tests with the cheat menu to see if I can figure this out better.

 

EDIT: I had to get to a distance of 5.9 Gm before the signal, strength between two of the best relay dishes drops from 100% to 99%.  With 62 Gm between the two relay dishes the signal is down to only 31%.  A direct line of sight to the KSC from 62 Gm gives a 65% signal strength (250 Gm DSN power + 100 Gm relay dish on satellite)

Edited September 30, 2016 by Kelderek

[Tarheel1999]

The best I could find was in the API documentation.  The documentation is not particularly clear on the exact formula but it is something approximating  1 - distance / range.  Distance and range are likely calculated numbers and I could not discover the formulas for those calculations. 

https://kerbalspaceprogram.com/api/interface_comm_net_1_1_i_range_model.html

[DerekL1963]

2 hours ago, Rodhern said:

2) If the satellites have the same Pe and Ap they should maintain spacing indefinitely (for each full orbit). If this is not what is happening in pre-release ver. 1.2. I think it is worth investigating further.

The trick isn't matching Pe and Ap - it's matching SMA.   Use KER or MJ to see that value.    And it takes a little practice, and a very low t/w ratio, to get them positioned properly.

[Rodhern]

18 minutes ago, DerekL1963 said:

The trick isn't matching Pe and Ap - it's matching SMA.   Use KER or MJ to see that value.    And it takes a little practice, and a very low t/w ratio, to get them positioned properly.

If you match up Pe and Ap your SMA will match up as well.

[DerekL1963]

25 minutes ago, Rodhern said:

If you match up Pe and Ap your SMA will match up as well.

However your SMA, and therefore your period, can match without precisely matching Ap and Pe.  Some people find working with a single number to be easier.

[Rodhern]

24 minutes ago, DerekL1963 said:

However your SMA, and therefore your period, can match without precisely matching Ap and Pe.  Some people find working with a single number to be easier.

Yups, I agree. That is a good point.

[RoverDude]

Signal strength is calculated as follows.

1.  Take the square root of the product of both power levels.  So a 5M antenna pointing at a 5K antenna has a MAX range of about 158km

SQRT(5,000,000 * 5,000) = 158,113

2.  Look at the actual distance and figure out what percentage this is of our max range.  So if you were at 100Km, this percentage would be about 36%

1 - (100,000 / 158,113) = 0.367

3.  Run that number (0.36) through the float curve in your antenna config.  The default float curve a simple 0-1 curve, so your signal strength in this case would be 36% if this was a direct shot.

If it bounces via relays, gather up all of the signal strengths and multiply them out.  So if you had a 36% hop then a 50% hop, your final signal strength would be 18%.

 

 

[Kelderek]

Thanks for this explanation RoverDude.  In my case I am only using 100 Gm antennas apart from the tracking station itself.  It was confusing to me that I could move up to 5.9 Gm apart before I saw it drop from 100 to 99% signal strength - if it works like you had said I would have expected roughly 94% strength at that distance.  I'm probably not counting the tracking station which is a huge number like 250 Gm.

At any rate it looks like if you really care about having a high signal strength (for science!) then you need a lot of these relay antennas.  The 100 Gm ones say they are combinable so I should look into making my outer solar system relays use 4 or more antennas each.  It works on a square rule right?  So I would need 4 antennas to get double the strength?  I'm just screwing around in sandbox right now, so the science gain is a non-issue, but I hope to apply what I'm learning of this new system to a future career game.