GEO · Uplink / Downlink · dB Budget

Satellite Link Budget Calculator

Computes the end-to-end dB budget of a GEO satellite link: earth-station EIRP, free space path loss, G/T, carrier-to-noise ratios and link margin. Everything updates live as you change the inputs. The geometry block derives elevation, azimuth and slant range from latitude/longitude.

Geometry GEO

Earth stn. latitude + north°

Earth stn. longitude + east°

Satellite longitude GEO, + east°

Elevation angle–°

Azimuth (from north)–°

Slant range–km

Carrier & Modem DVB-S2

Symbol rate (R_s)MBd

Roll-off (α)

Standard margin on Eₛ/N₀

ModCod mod · code rate

Spectral eff. (S_eff) info bit/sym

Required E_s/N₀dB

Add interference (C/I)dB

Uplink (earth → sat) ↑

Uplink frequencyGHz

HPA output powerW

Feeder/branching lossdB

TX antenna dia. (D_tx)m

TX antenna eff. (η_tx)%

Satellite G/TdB/K

Atmosphere + rain + otherdB

Earth stn. EIRP–dBW

Required SFD sat. saturation fluxdBW/m²

PFD at satellite–dBW/m²

SFD margin + saturation, − back-off–dB

Downlink (sat → earth) ↓

Downlink frequencyGHz

Satellite EIRP per carrierdBW

RX antenna dia. (D_rx)m

RX antenna eff. (η_rx)%

Earth stn. system temp. (T_sys)K

Atmosphere + rain + otherdB

Earth stn. RX gain–dBi

Earth stn. G/T–dB/K

Signal path

Results

C/N₀ uplink

– dBHz

C/N₀ downlink

– dBHz

C/N₀ total

– dBHz

Occupied BW

– MHz

C/N uplink

– dB

C/N downlink

– dB

C/N total

– dB

Info rate (R_b)

– Mbps

Eₛ/N₀ achieved

– dB

Eb/N₀ achieved

– dB

Link margin required – · achieved –

– dB

dB breakdown (waterfall)

Term	Uplink	Downlink

Positive rows add to the budget, negative rows are losses. C/N₀ = EIRP − L_fs − L_atm + G/T − k.

Formulas and assumptions

Geometry (spherical Earth, GEO). Local east-north-up (ENU) components are derived from the earth-station and satellite ECEF vectors; elevation, azimuth and slant range follow from them. R_e = 6378.137 km, GEO orbit radius = 42164 km. Station altitude is neglected.

Antenna gain.

G(dBi) = 10·log₁₀( η · (π·D·f / c)² )

D = diameter (m), f = frequency (Hz), η = efficiency, c = 2.998×10⁸ m/s. The same dish gives different gains at the uplink and downlink frequencies.

EIRP.

EIRP = P_HPA(dBW) − L_feeder + G_tx

Power flux density (PFD) at the satellite and SFD margin. Spreading loss = 10·log₁₀(4π·d²), with d in metres.

PFD = EIRP − 10·log₁₀(4π·d²) − L_atm · SFD margin = PFD − SFD_req

SFD_req is the flux density needed to drive the transponder into saturation. A positive margin means the satellite is pushed toward saturation; a negative one means that many dB of input back-off.

Free space path loss (FSPL).

L_fs(dB) = 92.45 + 20·log₁₀(d_km) + 20·log₁₀(f_GHz)

Carrier-to-noise density. Boltzmann constant k = −228.6 dBW/(K·Hz).

C/N₀(dBHz) = EIRP − L_fs − L_atm + (G/T) − k

Earth-station G/T = G_rx − 10·log₁₀(T_sys). Satellite G/T is entered directly.

Combining the links. Uplink, downlink and (if enabled) interference are summed in the linear domain:

(C/N₀)_tot = −10·log₁₀( 10^(−C/N₀,up/10) + 10^(−C/N₀,dn/10) + 10^(−C/I₀/10) )

For interference C/I₀ = C/I + 10·log₁₀(B). When interference is off this term is dropped.

Modem metrics. Symbol rate R_s, info bit rate R_b = R_s·S_eff, occupied bandwidth B = R_s·(1+α).

Eₛ/N₀ = C/N₀ − 10·log₁₀(R_s) · Eb/N₀ = C/N₀ − 10·log₁₀(R_b) · C/N = C/N₀ − 10·log₁₀(B) Link margin = (achieved threshold) − (required threshold)

Noise bandwidth is taken as ≈ R_s (Nyquist). The threshold depends on the standard and the values are embedded in the code:

DVB-S (QPSK + Viterbi inner code + RS(204,188) outer code): the target is E_b/N₀ (BER = 2×10⁻⁴ after Viterbi, QEF after RS). Net spectral efficiency S_eff = 2·r_inner·(188/204). Margin is computed on E_b/N₀.

DVB-S2 (ETSI EN 302 307, FECFRAME = 64800): the target is E_s/N₀ (ideal AWGN values). S_eff comes straight from the ModCod table; E_b/N₀ = E_s/N₀ − 10·log₁₀(S_eff). Margin is computed on E_s/N₀.

Manual mode lets you enter S_eff and the threshold yourself, and choose whether the margin is based on E_s/N₀ or E_b/N₀.