CommNet is a relay and control communication system implemented first in KSP 1.2 pre-release. Probes now have limited attitude and thrust control if they are not linked back to Kerbin or a control point via a relay antenna.  Antennae have limited range from which control and transmission of science data is possible.
|Tracking Station Level||5k (Probes / Manned)||500k (Communotron 16)||5M (HG-5 High Gain)||2G (DTS-M1 / RA-5)||15G (HG-55 / RA-50)||100G (Communotron 88-88 / RA-100)|
||3.1Mm (halfway to Mun)||31.6Mm (Minmus)||100Mm||2Gm||5.5Gm||14.1Gm|
||~15.8Mm (Mun)||~158Mm (Deep Space)||500Mm||10Gm (Duna if close)||27.4Gm (Duna/Dres always, Jool if close)||70.7Gm|
||35.4Mm (halfway to Minmus)||354Mm||1.1Gm||22.4Gm (Dres if close)||61.2Gm||158Gm (Jool always, Eeloo if close)|
Each antenna has a "power rating" measured in meters. It ranges from 5k (5000m) for integrated default antennas available in every spacecraft, to 100G (100 billion meters) for the largest dishes. A connection can be established between two stations (craft to ground, or craft to relay) when the square root of the product of the strength ratings is greater than the distance between the two objects. When a craft has multiple, combinable antennas, the antenna's power ratings stack in a way that provides a diminished return using the following formula:
- Vessel Antenna Power = Strongest Antenna Power * (Sum of Antenna's Powers/Strongest Antenna Power)^(Average of Combinability Exponents for All Antennas)
The range can then be calculated as stated above by taking the square root of the product of the two Antenna Strength ratings
- Range = SQRT ( Antenna Strength 1 * Antenna Strength 2 )
To obtain your signal strength for a link at any given distance between the two points it is mostly the relative distance between the two points. The true calculation requires plotting the relative distance on a Bezier curve with they keys '0 0 0 0' and '1 1 0 0' to obtain the true Signal Strength. This can be calculated as follows:
- 1 - ( Distance away from 'Link Point' / Max Range of Antenna Link ) = 0 < x < 1 (as a fractional percentage)
- Substitute the result above into the Bezier Curve (roughly) equation of y = -2x^3 + 3x^2 - 3E-14x - 9E-15
This will give you the Signal Strength between the link at your specified distance.
Calculating the Signal Strength between a Level 3 DSN Tracking Center and a probe around Jool at a distance of 73.4Gm from Kerbin with two Communotron 88-88 antennas.
Max Power of Lvl 3 DSN connection: 250,000,000,000
Max Power of Probe: 100,000,000,000 * (200,000,000,000 / 100,000,000,000)^0.75 = 168,179,283,051
Max Range of Connection: SQRT( 2.5E11 * 1.68E11 ) = 205,048,337,625m
Relative distance between Link Points: 1 - ( 73.4 / 205 ) = 0.64
Signal Strength: y = -2x^3 + 3x^2 - 3E-14x - 9E-15 = 2(0.64)^3 + 3(0.64)^2 - 3E-14(0.64) - 9E15 = 0.71 = 71%
Therefore the Signal Strength for this connection would be 71%.
Each upgrade of the tracking center improves the strength of Kerbin's terrestrial-based Deep Space Network (DSN). Based on the provided figures, the Level 1 tracking station has an antenna strength of 2G, Level 2 is 50G, and Level 3 is 250G. This is equal to the small, medium and large relay antenna ranges respectively.
The same document from the KSP developers states that science returned via a relay satellite will have a bonus applied (some fraction of the difference between the recovery and transmission science potential). The exact amount of the bonus depends upon signal strength.
Example orbits for setting up a planetary network can be found at: Tutorial:Ideal Orbits for Communication Satellites
These figures may change with the final release of 1.2.