UP and Down C/N

Downlink carrier to noise (C/Ndown) ratio refers to the result of the link budget calculation for the path from the satellite to the receive earth station.

Uplink carrier to noise (C/Nup) ratio refers to the result of the link budget calculation for the path from the earth station to the satellite.

In each case the normal link budget formula is used:
C/N = eirp - path loss + G/T - bandwidth + 228.6
e.g.
C/N = 40 - 199 + 3 - 10log(200,000) + 228.6

To get overall C/N you need to add up all the C/Ns in the system including C/N interference (say C/Nint=25dB) and C/N intermodulation (say C/Nim=21 dB).

C/N total=sum of all the C/Ns=C/Nup+C/Ninterference up+C/Nsatellite intermod+C/Ndown+C/N interference down.

Overall C/N is what matters, and it is the receive signal quality at the receiver demodulator.

For educational purposes, use this satellite link budget calculator.  Try many times with different powers, earth station sizes, bandwidths, satellite characteristics etc and discover what happens.

See also Eb/No calculator

Before doing any link budget calculation you must find out: satellite G/T and PFDsat on the uplink beam coverage contour and downlink eirp on the downlink contour. PFDsat can be adjusted on the satellite, so ask.

Best regards, Eric.