Beginners Guide To Link Budgets

So what is a link budget?

A link budget is actually not as complicated as it sounds. Put simply, it is the sum of all the losses between your transmitter and the satellite and back down again to a receiver. These losses are reduced by any gain you have at the transmitter, satellite or receiver. So in order to see if your signal is still going to be big enough to use after it has been sent to a receiver via satellite, the gains and losses are effectively added together and the result will be the net gain or loss. A loss means your signal has got smaller, and a gain means it has got bigger.

This is a very simplified explanation, but it gives you an idea of what the link budget is trying to calculate. The following more in depth explanations talk you through each major parameter. The maths behind all of this, and there is a lot of it, is not looked at here. Words are much easier to understand than equations i think.

These parameters may seem scary or alien to you. Fear not, learning what they are is all that is important at this stage. See also our satcom terms page.

The transmitter:

The transmit frequency is the RF radio frequency at which this carrier wave is transmitted.

The EIRP is a measure of the power which is required to transmit the carrier wave so that it reaches the satellite.

The G/T is a measure of the performance of the transmitter and is based on the gain of the transmitter (the amplifier, other parts of the uplink equipment chain and the antenna) and the noise of the equipment in the uplink chain.

Just as in an audio system, a noisy amplifier is not as good as a quiet amplifier.

The uplink chain is the series of stages the signal goes through before leaving the antenna on its way to the satellite.

The Lat and Long is the location of the transmit terminal on the earth. Measured in degrees the latitude and longitude is a global position reference used by the GPS system amongst others.

The elevation is the angle up from horizontal (0 degrees) that the antenna must point at to see the satellite in conjunction with azimuth.

The azimuth is the compass angle from true north that the antenna must be pointed at to see the satellite in conjunction with elevation.

Path loss is the attenuation of the signal due to the inverse square law and the earths atmosphere which reduces the size of your signal on its way to the satellite. Inverse square law is explained here.

Losses are the attenuating factors within the transmitter system such as RF radio frequency cable connectors and different types of RF cable.

Lastly the margin is used to allow for extra atmospheric attenuation due to localised rain or snow at the transmitter location.

The Satellite:

Starting with the translation frequency, this is used by the satellite to convert the transmitted signal to a new frequency so that the satellite doesn't retransmit the signal at the same frequency as the transmitter on earth. If it did the two signals would interfere with each other and the result would be unusable. Instead, the signal is moved, usually down in frequency to a 'Receive Band'.

The translation frequency is the amount by which the transmitted signal is moved in MHz.

The EIRP is again the same for the satellite as it is for the transmit terminal.

The G/T is also the same for the satellite as it is for the transmit terminal.

The C/No Sat is the carrier signal level to noise level ratio of the transmitted signal when it reaches the satellite. This is a measure of the signal reaching the satellite after travelling through the atmosphere.

The Long is the longitude of the satellite. The satellite is located above the equator at a latitude of 0 degrees. Thus, only the longitude is required to identify the satellites location.

Ant Gain is the gain of the receive antenna on the satellite.

Transp Gain is the gain of the transponder on the satellite, this is in effect one channel of many that are arranged in bands of frequency on the satellite. They can be independantly controlled to increase or decrease the gain. They are sometimes even organised so that they cover different areas of the earth through the use of different antenna systems.

Transp Gain is the gain of the satellite transponder.

Req'd EIRP is the amount of power the satellite has to use to get your signal back to earth.

% EIRP is the percentage of total satellite power available for all of the signals using it, that it must devote to your signal.

Pwr @ Sat is the actual power of your signal transmitted from the satellite.

The Modem:

Data Rate is described at the beginning of this article and is the amount of data you wish to transmit per second. This is measured in bits.

Eb/No Req'd is the energy per bit to noise level ratio that is required to provide error free data. This is usually specified by the modem manufacturer.

Status is an indication of whether the receivers modem would be happy with your link budget. If it locks, the chances are that the link would be succesful and error free.

Link margin is the overall amount of attenuation in any part of the satellite link that can be tolerated by the modems before they loose lock. Losing lock means losing the signal and thus the satellite link is lost, no data received.

The Receiver:

The Rx Freq is the receive frequency determined by the satellite translation freqency. the receiver must be tuned to this frequency to pick up the signal from the satellite.

C/No is the carrier power to noise level ratio which is a measure of how much signal is visible above the noise. In audio terms this would be exactly the same as music and hiss. Less hiss more music.

G/T is the same as the transmitter G/T except it is now for the receive terminal.

Lat is the same as the transmitter Latitude except it is now for the receive terminal.

Long is the same as the transmitter Longitude except it is now for the receive terminal.

Elevation is the same as the transmitter Elevation except it is now for the receive terminal.

Azimuth is the same as the transmitter Azimuth except it is now for the receive terminal.

Path Loss is the same as the transmitter Path Loss except it is now for the receive terminal.

Losses is the same as the transmitter Losses except it is now for the receive terminal.

Margin is the same as the transmitter Margin except it is now for the receive terminal taking into account any weather at the location of the receive terminal.

So Calculate:

With all this data and a lot of maths, the calculations can be made and the Link Margin obtained, if the Link Margin is too small, extra losses may occur due to atmospheric conditions which cause the link to fail.

Balancing the budget is to end up with either 0 dB if no extra margin is required, or about 3 dB if some protection is needed. The maths is all taken care of behind the scenes on our Online link budget calculator, all you need to know are the parameters for the satellite link.

Armed with all of this knowledge, you might now understand how to send text using our Online Satellite Broadcasting Simulator.