Well, there are several terms you need to know as a satcoms engineer. Here are just a few:

EIRP, PFD, Antenna Gain, HPA, Forward Power, Reverse Power, Decibels, C/No, Eb/No
and the type of modulation - QPSK, CDMA, TDMA.

EIRP:

This is the Effective Isotropic Radiated Power of an antenna. Now an Isotropic reference antenna radiates, or transmits equal amounts of power or signal in all directions. This is of little use and is a theoretical antenna. It has unity gain so what you put in you get out.

So the EIRP is a comparative measurement which is used to compare every antenna to one single reference, the Isotropic antenna. The measurement is calculated by adding the antenna gain and forward power minus any loss.

PFD : Power Flux Density

Normally expressed as PFD for saturation, that is the power flux density measured in watts per sq. m at the input to the satellite's antenna which produces maxiumum power output. The power flux density from an earth station is its EIRP divided by (4 x pi x distance squared). In dBs, PFD is expressed as dBW/sq.m. A typical value is -80 dBW/sq.m for a commercial Ku band transponder. Having said all this it's very difficult to extract the actual figures from satellite operators, they will tell you transponder EIRP readily but not PFD sat.

Antenna Gain

The ratio of the power required at the input of a loss-free isotropic reference antenna to the power supplied to the input of the given antenna to produce, in a given direction, the same field strength at the same distance.

HPA

High Power Amplifiers are used to amplify the relatively small signals big enough to reach a satellite. They can be either Solid State (SSPA or SSHPA) or they can be Travelling Wave Tubes (TWTA). The latter is more like a valve in it's characteristic and uses a helix tube and a cathode in it's make up. Solid State amps are more stable and reliable and are increasingly used to replace TWT's but the high power 3kW+ amplifiers are still TWTA's. Klystron Amplifiers are amongst the most common of the really big TWT amplifiers. Here is or was a very good explanation of Klystron TWTA's.

Forward Power:

This is the power we put into the dish, the power from the HPA less any losses in the system, measured at the point of entry to the feed and the antenna. For a 50W HPA the forward power is 10 Log (50) which is 17 dBW or 47 dBm.

Reverse Power:

Reverse Power is the opposite of Forward Power, not suprisingly, and is used to determine how well matched a feed system is to a HPA. An antenna that is not very efficient will refelect some of the power back into the RF system. The amount of power reflected should be very low in a well designed system.

Decibels:

As engineers we like to use the good old Decibel (dB), this is because it makes life so much easier. Scientists tend to do things the hard way but for this example we will be engineers and take the easy path. (Actually scientists also use dB because its so much easier.)

Antenna gain is usually measured in dBi, the 'i' refers to the reference used to compare the antenna against, the Isotropic antenna mentioned earlier.

Forward power is usually measured in dBm.
(Scientists seem to insist on using dBW which is just dBm - 30)

To calculate the EIRP then we just add the two numbers together.

EIRP (dBm) = Forward Power (dBm) + Antenna Gain (dBi)

Why is dBW just dBm - 30? Well because when calculating dB's we take a power which is in Watts and find the LOG of it. LOGs are Logarithmic functions common in everyday mathematics. Then we multiply it by 10 to get decibels.

So 1 Watt = 10 x LOG(1) in dBW

Since milli Watts are 1000 times smaller than a Watt and 10 x the LOG of 1000 is 30, we just add 30 to the dBW value to get dBm.

Decibels allow engineers to work with both large and small values easily.

100 Watts = 10 Log 100 = 20 dBW or 50 dBm

10 Watts = 10 Log 10 = 10 dBW or 40 dBm

C/No or (C/Kt) :

In satellite communications, the ratio of the received carrier power, 'C', to the receiver noise power density, 'No', where 'C' is the received carrier power in watts, and 'No' is dervied from 'k' which is Boltzmann's constant in joules per kelvin, and 'T' which is the receiver system noise temperature in kelvins. 'kT' or 'No' is the receiver noise power in 1 hertz.

Eb/No:

This is classically defined as the ratio of Energy per Bit (Eb) to the Spectral Noise Density (No). It is a measure of signal to noise ratio for a digital communication system. It is measured at the input to the receiver and is used as the basic measure of how strong the signal is.

QPSK:

Phase-shift keying (PSK) involves shifting the phase of the carrier to represent digits. When encoding bits, the phase shift could be 0° for encoding a "0," and 180° for encoding a "1," or the phase shift could be -90 for "0" and +90° for a "1," thus making the representations for "0" and "1" a total of 180° apart. Quadrature phase-shift keying (QPSK) is PSK in which four different phase angles are used. The four angles are usually out of phase by 90°.

CDMA:

Code-Division Multiple Access (CDMA): A coding scheme, used as a modulation technique, in which multiple channels are independently coded for transmission over a single wideband channel. On reception, each carrier can be distinguished from the others by means of a specific modulation code, thereby allowing for the reception of signals that were originally overlapping in frequency and time.

TDMA:

Time-Division Multiple Access (TDMA): A communications technique that uses a common channel (multipoint or broadcast) for communications among multiple users by allocating unique time slots to different users. A multiple access technique whereby users share a transmission medium by being assigned and using (one at a time) for a limited number of time division multiplexed channels.