Satlinker, Link-Budget Calculator

This tutorial does not describe the principles and basic formulas of a link-budget calculation, you can learn these things in various other places e.g. here. Here we discuss how to get the right data and how to use this link-budget calculator.
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At program start you will be asked to enable macros and update links. You must enable macros, but there is an option to disable this question. If you are connected to the internet, press yes to update links and after a short time the newest satellite data are downloaded. The program seems to freeze while download is in progress, just wait some seconds. There are some station data and approved sample link-budgets included. You can take them as a guideline or delete them if you don't need it.

Program handling

If you want to insert columns, copy an existing column, so all formulas in the cells will also be duplicated, then modify the copied data. I know this is not what you expect from a "real" program, but so you can get most flexibility and have lots of calculations on one sheet. So if you want to insert a new station or make a new link-budget, copy the first example column, insert it into an empty column and modify all but red cells. The red cells contain formulas and will be calculated automatically.

Some data in a table is related to data in another table e.g. if you type the name of a location in the link-budget table, the program gets the coordinates for this location from the Loc table. So location-, station- and IRD-names are linked with data in their corresponding tables. In the first column there is a tooltip help in cells with a red dot in the upper right corner.

The Point tab is a database for satellite related data. Satellites are imported automatically from N2YO. The table shows the azimuth, elevation and other angles to each satellite from the location that you can specify in the gray cell in first row. The difference angle to a reference satellite is also displayed, that is a pointing help for uplinkers. The names of the location and the reference satellite must exist in the Loc and Point tables.

The Loc tab contains location data, available at Heavens Above or at Earth Info. Write the data of your location(s) or the next town near you.

The IRD tab describes the quality of the IRD with a minimum needed Eb/N0 in dependence of FEC. ETS300421 specifies a minimum Eb/N0 for a BER of 2×10-4 after inner decoding (post Viterbi). That is Quasi Error Free (QEF ~ 10-11) after outer decoding (post Reed Solomon). However modern IRDs are better, I included the data of modern decoders, that slightly depends on the symbol rate. The quality of the inner decoder depends on the number of errors that it can correct and not only detect.

The Station tab must be filled only once for your specific antenna.

The Link-budget tab contains a link in each column. Satellite related data must be filled in the green cells, earth stations related data in the blue cells. Results will be calculated by software in the red cells.

The program calculates the needed usable HPA power for the uplink station from the EIRP that you entered. You need to know the maximum radiated "usable" HPA power (flange power) e.g. from a power meter measured behind the HPA output, it's not always equal to the HPA power specification! The HPA must be operated with a back off and cannot be driven into saturation. That is also the reason why TWTAs cannot be compared directly with SSPAs, only usable power or EIRP are unambiguous (contact me for further details to actual HPAs). If the TWTA is operated in the topmost 3dB of it's power specification (nominal power) it might produce a lot of distortion. So you should not use this range, you might disturb your neighbors on the transponder. Some TWTAs deliver more power than specified, because they loose power again when getting older.

For all who didn't recognize: the transistor has been invented. In the lower power range (up to about 150W) a SSPA has a better efficiency and handling. Especially big dishes can profit from a SSPA because there is no need to keep a tube warm in case of a power breakdown. New gallium nitride (GaN) transistors deliver up to 250W in the KU-Band and are not bigger than a shoe box.

To get the minimum needed power for the link, increase the EIRP of the uplink station as long as the "Link margin to Eb/N0min" becomes zero. If you increase EIRP as long as IBO reaches nominal IBO, you can see if your earth station has a satisfying power margin. If the satellites' transponder is full and gets saturated, adjust IBO-OBO like explained below and correct EIRP once again.

Satellite related data

Your greatest problem will be to get the satellites' data from the sat operators. Please fire questions at them to write the correct values on their homepages or on the channel bookings (often they don't know it off-hand). To show you how an accurate datasheet should look like, see the Hellas satellite. You need these things:

  • G/T (Gain/Temperature) of transponder in satellite beam center and location disadvantage from G/T-footprint (satellite receive coverage).
  • EIRP (Equivalent Isotropically Radiated Power) of transponder in satellite beam center and location disadvantage from EIRP-footprint (satellite transmit coverage).
  • SFD (Saturation Flux Density) of transponder = Saturation IPFD (Input Power Flux Density).
  • Transponder Operating Input Back Off ("nominal" IBO).
  • IBO/OBO relation of the transponders' HPA (Input Back Off vs. Output Back Off).

The SFD needs some explanation. Often there is an SFD range in the datasheets, e.g. from -(92 + x) to -(77 + x), with x is the G/T location loss at the reception location. Each transponders' amplification can be remotely adjusted from the ground control room. Dependant on the actual amplification (also called gainsteps), you need another IPFD at the satellite dish to drive the transponders' HPA into saturation. The actual value must be obtained from the sat operator. For a worst case calculation take the most positive value, in our example -(77 + x). Cause of the location dependancy of SFD, write the value for beam center (-77dB), the program does the rest for you.

The nominal input back off (IBO) is the minimal headroom to HPA saturation. If the transponders' HPA is driven up to saturation, it produces maximum output power but also distortion and a phase shift, especially TWTAs. To avoid these effects, the operator stops the feeds some dBs below. Each transponder in a satellite may have another IBO and therefor OBO. You will not find the nominal IBO in the datasheet, but it's not critical because the program only needs it to warn you if your EIRP is to high. IBO and OBO are both positive values in here because they are considered as headrooms, this can be different in literature.

The very important IBO vs. OBO relation depends on the HPA type in the transponder, that can be a TWTA (Traveling Wave Tube Amplifier), a linearized TWTA or a transistorized SSPA (Solid State Power Amplifier). It is slightly different for multicarrier and singlecarrier operation. Try to get the transponders' IBO vs. OBO curve or table from the sat operator and adjust "IBO-OBO" in the link-budget table according to the calculated IBO. As long as the amplifier is operated in the linear region, the curve is almost constant, so IBO minus OBO is constant too. Most amplifiers differ, so we have no generic formula for all. A typical approximation for a TWTA with digital multicarrier operation looks like that:
for IBO > 13dB: OBO = IBO - 7
for IBO ≤ 13dB: OBO = 1.7 + 0.0313 × IBO2
If the nominal IBO is > 13dB in this case, then the HPA is operated in the linear region and we have a constant IBO - OBO = 7dB.
If IBO is < 13dB, you have to correct IBO - OBO and calculate again until you approach the result:
e.g. IBO = 6dB, OBO = 1.7 + 0.0313 × 36 = 2.83dB, IBO - OBO = 3.17dB
For further information look here in the TWTA tutorial.