Relay not working

[fabianovc]

Hi guys, I'm new to the game and I'm having this problem.

I'm trying to land a probe (using the first core, stayputnik?) on the mun but it is eventually occluded by the mun itself and I lose control over it. I thought the obvious: "let's place a satellite high on mun's orbit and relay the signal to control the landing probe".

That's what I did: my satellite is carrying the HG-5 relay antenna and is at a fairly high concentric orbit, around 2Mm.

Nice, now we get to the problem: when the landing probe (it is carrying no antenna) gets occluded by the mun, after losing contact with Kerbin it doesn't connect to the satellite to relay its signal. The satellite is on the same side of the mun as the probe and has full signal from Kerbin.

Any clues?

[voicey99]

The HG-5 is an extremely weedy relay antenna, and it can only communicate with a built-in antenna from up to 158km away. If you want to relay to the probe, give it a proper antenna. See below for a tool to calculate these things:

Edited December 1, 2017 by voicey99

[fabianovc]

But since the built in antenna can communicate all the way with Kerbin while not occluded by the mun, it should be able to communicate with the relay in mun's orbit, shouldn't it?

After all, it is MUCH closer.

[Rocket In My Pocket]

HG-5's can be combined, so just put more on it if you don't have access to better relays atm.

[fabianovc]

Yeah, I know, but it is in range. That's the weird part.

[Snark]

45 minutes ago, fabianovc said:

But since the built in antenna can communicate all the way with Kerbin while not occluded by the mun, it should be able to communicate with the relay in mun's orbit, shouldn't it?

After all, it is MUCH closer.

Yes, but it's also MUCH weaker. 

The HG-5 has a power of 2M.  Your tracking station back on Kerbin has a power of 2G, even if it's only level 1-- which is literally a thousand times more powerful.

So yes, it's "MUCH" closer... but it's not 1000 times closer.  The tracking station is so much more powerful than the HG-5 that it makes a huge difference.

28 minutes ago, fabianovc said:

Yeah, I know, but it is in range. That's the weird part.

But it's not in range.  Go back and look what @voicey99 said above, that's your problem right there:

The important thing to understand is that communication range depends on both the receiving and the transmitting antennas.  Your HG-5 has a nominal power of 2M.  But your probe has an abysmally crappy antenna:

On 11/28/2017 at 8:43 PM, fabianovc said:

the landing probe (it is carrying no antenna)

...Since you haven't put an antenna part on your probe, that means it only has the probe core's built-in antenna.  And that antenna is horribly awfully awful.  Its antenna power is a paltry 5K.

So, to figure out "at what range can an HG-5 talk to a probe core's built in antenna":  do the standard antenna-power calculation.  You multiply them together and take the square root.

So, in this case:

range = sqrt(<HG-5 power> * <probe core power>)
= sqrt(2000000 * 5000)
= 100000
= 100 kilometers

 

That's what your problem is, right there.  Your HG-5 can only talk to your probe within a range of 100 km.  Since it's orbiting higher than 100 km, it means it can't talk to the probe.

Fortunately, this is a very easily fixed problem.  Just put an antenna on your probe.  Doesn't have to be a powerful one-- even the basic Communotron-16 will work.  Why is that?  Because even the dinky little Communotron-16 has an antenna power of 500K, i.e. literally 100 times more powerful than the probe core's built-in antenna.

Your HG-5 relay can talk to a probe core only across 100 km.  But it can talk to a Communotron-16 across 1000 km.

On 11/28/2017 at 8:43 PM, fabianovc said:

at a fairly high concentric orbit, around 2Mm.

...that's still going to be a problem, because if you're orbiting 2000 km above the Mun, that's still too far even if you put a Communotron-16 on the probe.

So basically you have two options to fix this problem:

• Option 1:  Add a Communotron-16 to the probe.  Then lower your relay satellite's orbit to something comfortably lower than 1000km.  For example, 700 km would probably work well.
• Option 2:  Keep the probe as it is, but bring the relay satellite into a low orbit that's comfortably below 100 km.  For example, 50 km.  Of course, this means the relay can only "see" the probe over a very limited area of the Mun, so this would be a less robust solution than Option 1.

 

Note:  You could boost the range numbers a bit by stacking antennas.  But the fundamental takeaway point here is, you need to have an antenna on the probe, because you're currently far, far out of range.

[fabianovc]

Perfect!

I was not taking both powers into account! That's why! It can talk to Kerbin because KERBIN is very powerful!

I get it now! Thanks guys!