How do I calculate the effective isotropic radiated power of a satellite transponder?
Satellite EIRP Engineering
EIRP is the single most important parameter in the satellite downlink budget, directly determining the ground station antenna size and receiver quality required to close the link. Higher EIRP enables smaller, cheaper ground terminals.
| Parameter | GEO | MEO | LEO |
|---|---|---|---|
| Altitude | 35,786 km | 2,000-35,786 km | 200-2,000 km |
| Latency (one-way) | ~270 ms | 50-150 ms | 1-20 ms |
| Coverage per Sat | Full hemisphere | Regional | Local footprint |
| Handover | None | Periodic | Frequent |
| Path Loss (Ku-band) | ~206 dB | 190-206 dB | 170-190 dB |
Frequently Asked Questions
How much EIRP is needed for a VSAT link?
Depends on frequency band, data rate, and antenna size: Ku-band VSAT (1.2 m ground antenna, 2 Mbps data, DVB-S2 QPSK 2/3): required satellite EIRP ≈ 48-52 dBW at beam center. Ka-band VSAT (0.75 m antenna, 10 Mbps, DVB-S2X 16APSK): required EIRP ≈ 55-60 dBW (higher EIRP needed to compensate for smaller antenna and rain fade margin). For current HTS systems: ViaSat-3 provides >60 dBW EIRP per spot beam; SES-17 provides 55-58 dBW per beam. These high EIRPs enable consumer-grade terminals (0.5-1.0 m) at multi-Mbps data rates.
What limits the maximum satellite EIRP?
Three factors limit satellite EIRP: (1) DC power: the satellite solar arrays and batteries provide a fixed total DC power budget (typically 10-25 kW for a large GEO satellite). HPA DC-to-RF efficiency: 40-65% for TWTAs, 25-40% for SSPAs. A 15 kW satellite allocating 10 kW to the payload produces 4-6 kW of total RF output power. (2) Thermal dissipation: the 5-6 kW of waste heat from the HPAs must be radiated to space through thermal radiators. Radiator area limits the total dissipated power. (3) ITU power flux density limits: the satellite must not exceed specified PFD on the earth surface to protect terrestrial services and adjacent satellite systems. PFD limits effectively cap the EIRP per unit bandwidth per unit solid angle, constraining spot beam EIRP density.
How does EIRP relate to the satellite link budget?
EIRP is the starting point of the downlink budget: C/N_downlink = EIRP_dBW + G/T_ground_dB/K - FSPL_dB - k_dBW/K/Hz - BW_dBHz - L_misc_dB. For example: EIRP = 55 dBW, G/T = 20 dB/K, FSPL at 12 GHz and 36,000 km = 205.8 dB, k = -228.6 dBW/K/Hz, BW = 36 MHz (75.6 dBHz): C/N = 55 + 20 - 205.8 + 228.6 - 75.6 - 1.2 = 21.0 dB. This C/N supports up to 16-APSK 8/9 (spectral efficiency 3.6 bits/s/Hz). Each 3 dB increase in EIRP enables either: 3 dB reduction in ground antenna size (halving the antenna area), one additional modulation step (e.g., 16-APSK to 32-APSK, ~25% throughput increase), or 3 dB more rain fade margin.