Antenna Planning Guide
The antenna planner exists to answer the following question: "What transmission rates can I expect between antenna A (primary) and antenna B (peer) at distances X and Y"? The planner does not configure your antennas for you, so you will need to know what factors improve or degrade connections to make the best use of the planner. Therefore, there are several important basic RealAntennas concepts to understand before you start using the planner. Make sure you go over the topics covered in the Basic Advice and Antennas 101 pages listed in the sidebar, then you can start asking questions with the planner.
To access the antenna planning GUI find and click the antenna planning button under the RealAntennas tab of the part action window (the right click menu, hereafter PAW). Note that you can make an additional planning window by pressing the antenna planning button again. If you have installed Kerbalism and would like to use the Kerbalism planner for electric consumption, then you should set the Active Transmission Time slider to an appropriate percentage of time that you believe your craft will spend actively transmitting. If you have not installed Kerbalism, there will be no Active Transmission Time slider.
Before jumping into the specific examples using the antenna planner below lets go over the main parts of the antenna planner GUI.
- Ground Station TechLevel This shows you the TechLevel of your existing ground stations, as well as the TechLevel of the ground station you are planning against. You can adjust the level of your ground station for planning purposes by changing the slider position and pressing Apply. Use this to plan future missions that will have more sensitive ground stations. Note that the ground station level counts up from starting at zero and not one.
- Antenna Selection You can cycle each column between displaying vessel and ground station antennas by pressing the cycle button immediately below the GUI text Primary and Peer. You are free to select any antenna on an existing craft, or any antenna on an open craft file in the VAB/SPH, or any GroundStation antenna. Note that the default peer antenna will be the most sensitive ground station that matches the band of the antenna you used to open the planning GUI. The most sensitive ground station matching your primary antenna will always be at the top of the list of ground station antennas with the orange text [Best Station]. While antennas on the active vessel in either the VAB/SPH or flight scene will be at the top of the list of Vessel antennas.
- Remote Body Presets These radio buttons are a convenient option to pick from if you wish to plan connections to a specific celestial body. Simply pick a button corresponding to a celestial body and the planner will automatically set the min and max planning distances to the min and max distance between the home body and the selected celestial body. If you do not want to set the planning distance to a celestial body then there is no need to make a selection here as you can manually set the distance in the Parameters section.
- Parameters This section displays the question the planner is answering and the result, so it is the most important part of the GUI. The primary and peer antenna are listed along with their configuration [band/gain (dBi)/Tx power (dBm)/TL/encoder type/bandwidth]. Below the primary and peer are the minimum and maximum distances between the two antennas. You can manually set the distances by entering a number and selecting a radio button for the desired unit of length (kilometer, megameter, etc). It will also show you the planning result at the given distances as transmit (Tx) and receive (Rx) data rates in bits per second(bps). If either Tx or Rx is reported as zero you will not have a connection at the given distance. Note that manual changes to the distances will not automatically be reflected in the Tx/Rx rates. If you have manually changed these values you will want to press Plan! below afterwards.
- Plan! Press this button when you want to know the result of attempting a connection between the two selected antennas at the given distances. The adjacent toggle for Show Details will open the rather detailed debug GUI in a new window if enabled when you press Plan!
In this first example, the antenna will be representative of an early lunar probe in Real Solar System. The planning peer is set to the default, so the calculations will be in reference to the most sensitive ground station. The planning altitude is set using the Remote Body Preset radio button for the Moon which sets the maximum distance (400 Mm) and the minimum distance (356 Mm) between the Moon and the Earth. Looking at the rates for transmit/receive, we see there will not be a connection at either the minimum or maximum distance. Since the transmit rate at both distances is 0 bps, the planner result is no connection. If we want to establish a connection with our ground station at these distances we will have to either increase the transmit power on our primary antenna, use a primary antenna with more gain, or wait for a higher TL.
If you want to plan for interplanetary distances instead of Earth to Moon distances, then you simply need to select the appropriate radio button under the Remote Body Preset section. Or if you have a specific encounter in mind, you can manually set an intermediate distance between those set by the presets.
In this example, we'll look at craft to craft communications at relatively short distances. The only major difference between this example and the previous example is that we have to select the peer antenna and specify some reasonable distances. To set the peer we have changed the cycle button for Peer from GroundStation to Vessel and then selected the appropriate relay antenna from the list.
In this scenario, we're planning a relay network of four satellites to reduce communication drop outs with craft in low orbits. This network will consist of four evenly spaced satellites around the Earth in circular orbits about 3400 km above the surface. Since we are not concerned with building a high data rate network in this example and are only using omni antennas with a fixed gain, we select the VHF band to minimize path losses. In this case our primary antenna is the built in omni antenna in a procedural avionics part, and the peer antenna is the fixed Communotron 16-S omni antenna on the Relay I vessel.
The distances have been set to the distance between our relay and a hypothetical NewVessel. The maximum distance corresponds to NewVessel sitting above the limb of the Earth (phase angle = +/- 90 degrees) as viewed from our relay. The minimum distance corresponds to the hypothetical NewVessel crossing directly below our relay (phase angle = 0 degrees). If our relays are in circular orbits 3400 km above the surface and our spacecraft are in 200 km altitude circular parking orbits some basic geometry will tell you these distances are approximately 11.8 Mm and 3.2 Mm respectively.
The planning result is that our relay network will work at both the minimum and maximum distances. At maximum separation we have a tenuous 98 bps connection, but as the distance narrows it becomes a much more respectable 1.6 Kbps. It is not shown in this example, but this relay will also easily be able to talk with ground stations and other relays positioned 90 degrees ahead and behind in their orbits. If you are building your own relay network, you should make sure your relays can talk to the ground stations.
Sometimes you may find yourself in a situation where you don't quite understand why you are getting a specific result. In those cases you may want to enable Show Details button at the bottom of the planning GUI and look at the new debug GUI it opens. This GUI will show you the specific values being used by the planner to produce the results it reports. You can also open the debug GUI for any antenna on a loaded vessel. This will enable you to debug the connection between that antenna and any possible peer for that antenna.
If you do not have a connection but the planner indicates that you should have a connection at the given distance then there are a few things you can look at here. Remember that there are blocks of debug information for both the primary receiving the peer and the peer receiving the primary. You may need to scan through more than one block to find the problem.
- If the ground station receiving elevation is very low (<15 degrees) then connections with no margin may start to fail due to increased atmospheric path losses. Very apparent at X-band and above in zero margin missions. Will manifest as periodic dropouts that come and go up to 3 times a day as the spacecraft rises to a higher elevation at each DSS station.
- If you see that the Body Noise component of TotalNoise is large (6000 K), then you probably are experiencing interference from the Sun. You will have to wait for a more favorable celestial alignment, or do some advanced antenna targeting if you have enough margin.
- If you see that a connection has a pointing loss of 200 dB, then you have antennas which are pointed in entirely the wrong direction. If you see a more tame pointing loss (<20 dB), then the antennas are simply pointed in kindof the wrong direction. To resolve these cases you should fix the antenna targeting.
Here's some tidbits of wisdom from the most important bits of this.
- The Min Link EB/N0 tells you how close to the edge of your connection range you are. If this number is >0 then we have signal to noise ratio to spare and can go further with the connection.
- The valid rates are powers of two multiplied by the minimum rate which is the product of encoder rate (0.87) with bandwidth (4 Hz). Our achieved rate of 860 is coming from the five bandwidth halvings we need at this distance (Steps: 5). The number of bandwidth halvings remaining is 2*TL+3-Steps.
- Here we have a Min Link EB/N0 of 24.6 and the achieved rate over the minimum rate is 256=2^8. That means we have eight more bandwidth halvings to go before everything falls apart.
- Every time we double the distance the received signal will drop by 6 dB. Each bandwidth halving will buy 3 dB back. The eight bandwidth halvings remaining is equivalent to doubling the distance four times (2^4). So we know we'll have connection out to a little past 2^4 * 5.38 Mm. But not very far past that because those four distance doublings we bought with eight bandwidth halvings leaves us with a Min Link Eb/N0 of 0.6.
- Remember that higher TL antennas will get you more bandwidth halvings.