RF Theory › Satellite Communications
Satellite Communications
A complete RF engineering guide to satellite communications — orbital mechanics, link budgets, EIRP, G/T, path loss, rain attenuation, modulation, Eb/N0, Shannon capacity and ground station design. Worked examples for Ku-band VSAT and LEO constellations.
01 — Orbital Mechanics
Orbit Types
The orbit determines everything in a satellite link — path loss, latency, coverage area, Doppler shift and ground terminal pointing requirements. RF engineers must understand the trade-offs before designing any ground segment.
Key Orbital Mechanics
T = 2π√(a³/μ) (orbital period, μ=3.986×10¹⁴ m³/s²)v = √(μ/a) (orbital velocity)
aGEO = 42,164 km from Earth centre (35,786 km altitude)
fD = fc·vr/c (Doppler — only radial velocity vr matters)
GEO: T=23h56m, v=3.07 km/s, fD=0 (geostationary) · LEO: T≈90min, v≈7.8 km/s, fD up to ±270 kHz at Ku
Orbit Comparison
| Orbit | Altitude | Latency | FSPL (Ku) | Coverage | Examples |
|---|---|---|---|---|---|
| GEO | 35,786 km | ~600 ms RTT | ~205 dB | Global (3 sats) | Intelsat, SES, Eutelsat, HTS |
| MEO | 2,000–35,786 km | ~125 ms RTT | ~192 dB | Mid-lat bands | O3b (SES), GPS, Galileo |
| LEO | 200–2,000 km | ~8–40 ms RTT | ~169–185 dB | Swath (needs constellation) | Starlink, OneWeb, Iridium |
| VLEO | 100–300 km | <5 ms RTT | ~165 dB | Very narrow | Capella (SAR), experimental |
| HEO | Elliptical | Variable | Variable | High latitudes | Molniya, SIRIUS XM |
GEO dominates broadcast: One GEO satellite covers 42% of Earth's surface — ideal for broadcast TV and HTS broadband. But 600 ms RTT kills interactive applications. LEO constellations sacrifice coverage per satellite (<0.1%) but achieve <30 ms latency — matching fibre performance for many applications.
02 — Free Space Path Loss
Free Space Path Loss
FSPL
FSPL = 20·log₁₀(4πd/λ) = 20·log₁₀(d) + 20·log₁₀(f) + 20·log₁₀(4π/c) dBFSPL (dB) ≈ 92.45 + 20·log₁₀(f[GHz]) + 20·log₁₀(d[km])
Doubles every 2× distance (+6 dB per octave of range)
FSPL at Key Satellite Bands
Example 1 — FSPL for GEO at 35,786 km and LEO at 550 km
| Band | Freq | GEO (35,786 km) | LEO (550 km) | LEO advantage |
|---|---|---|---|---|
| L-band | 1.5 GHz | 187.1 dB | 169.8 dB | 17.3 dB |
| S-band | 2.5 GHz | 191.5 dB | 174.2 dB | 17.3 dB |
| C-band | 6 GHz | 197.1 dB | 179.8 dB | 17.3 dB |
| Ku-band (DL) | 11.7 GHz | 205.3 dB | 188.0 dB | 17.3 dB |
| Ku-band (UL) | 14.0 GHz | 206.4 dB | 189.1 dB | 17.3 dB |
| Ka-band (DL) | 20.2 GHz | 209.6 dB | 192.3 dB | 17.3 dB |
| Ka-band (UL) | 30 GHz | 212.6 dB | 195.3 dB | 17.3 dB |
GEO Ku DL: 92.45+20·log₁₀(11.7)+20·log₁₀(35786)=92.45+21.37+91.1=204.9 dBm ≈205.3 dB (adding atmosphere). LEO advantage is always 20·log₁₀(35786/550)=17.3 dB regardless of frequency — the key number in LEO vs GEO link budgets.
03 — EIRP
Effective Isotropic Radiated Power
EIRP is the transmit power that an isotropic antenna would need to produce the same power density in the direction of the beam as the actual directional antenna. It combines HPA output power and antenna gain into a single figure for link budget calculations.
EIRP
EIRP = PTX·GTX (linear) · EIRP(dBW) = PTX(dBW) + GTX(dBi)PTX = HPA output − waveguide/feed losses
GTX = 10·log₁₀(η·(πD/λ)²) for parabolic dish (η≈0.55–0.65)
Satellite EIRP: 40–65 dBW (spot beam) · VSAT ground terminal: 35–50 dBW
Starlink Ku UL: ~40 dBW (flat-panel phased array, 50W SSPA, ~30 dBi)
Example 2 — VSAT ground terminal EIRP (1.2 m Ku-band dish)
1
Dish gain: G = 10·log₁₀(0.6·(π×1.2/0.021)²) = 10·log₁₀(0.6×32288) = 42.9 dBi (f=14 GHz, λ=21 mm)
2
HPA output: PTX=5 W=7.0 dBW, feed loss=0.5 dB → effective PTX=6.5 dBW
3
EIRP = 6.5+42.9 = 49.4 dBW
✓ 1.2 m VSAT EIRP = 49.4 dBW. Increasing dish to 1.8 m: G = +3.6 dBi → EIRP = 53.0 dBW. 50% larger dish = 3.6 dB more EIRP.
04 — G/T Figure of Merit
G/T — Figure of Merit
G/T (gain-over-temperature) is the receive quality metric for a satellite terminal. Higher G/T means better receive performance. It accounts for both antenna gain and the total system noise temperature — including the LNA, feed losses, sky noise and antenna spillover.
G/T Calculation
G/T = GRX(dBi) − 10·log₁₀(Tsys) (dB/K)Tsys = Tant + Tfeed + TLNA·Lfeed + … (noise temperature cascade, Kelvin)
Tant ≈ 20–35 K (Ku clear sky) → 40–80 K (heavy rain) → 290 K (terrestrial hot spot)
Tfeed = 290·(Lfeed−1) where Lfeed is feed loss as linear ratio
Typical VSAT G/T: 14–20 dB/K · GEO spacecraft G/T: −4 to +5 dB/K (large spot beams)
Example 3 — G/T for 1.2 m Ku-band VSAT terminal
1
GRX=10·log₁₀(0.6·(π×1.2/0.026)²)=10·log₁₀(49930)=46.98 dBi (f=11.7 GHz, λ=25.6 mm)
2
Tant=25 K (clear sky Ku) · Feed loss Lf=0.5 dB=1.122 linear · Tfeed=290×0.122=35.4 K
3
LNA NF=0.8 dB=1.202 linear → TLNA=290×0.202=58.6 K · Contribution=58.6×1.122=65.7 K
4
Tsys=25+35.4+65.7=126.1 K
5
G/T=46.98−10·log₁₀(126.1)=46.98−21.01=25.97 dB/K
✓ G/T = 26.0 dB/K — excellent VSAT terminal. Downgrade to LNA NF=2 dB: TLNA=170 K → Tsys=250 K → G/T=22.97 dB/K — 3 dB degradation. LNA is critical.
05 — Satellite Link Budget
Satellite Link Budget
C/N₀ and C/N
C/N₀ = EIRPTX(dBW) − FSPL(dB) − Lmisc(dB) + G/TRX(dB/K) − 10·log₁₀(k)k = Boltzmann = 1.381×10⁻²³ J/K → 10·log₁₀(k) = −228.6 dBW/Hz/K
C/N = C/N₀ − 10·log₁₀(B) · Eb/N₀ = C/N₀ − 10·log₁₀(Rb)
Required C/N for QPSK (DVB-S2): ~5 dB · 8PSK: ~9 dB · 16APSK: ~12 dB · 32APSK: ~15 dB
Complete Ku-Band VSAT Link Budget
Example 4 — GEO Ku-band downlink: 11.7 GHz, EIRP=52 dBW, 1.2 m terminal
1
Satellite EIRP = 52.0 dBW
2
FSPL = 92.45+20·log₁₀(11.7)+20·log₁₀(35786) = 205.3 dB
3
Atmospheric absorption = 0.3 dB · Pointing loss = 0.5 dB · Polarisation loss = 0.1 dB → Lmisc = 0.9 dB
4
G/TRX (from Example 3) = 26.0 dB/K
5
C/N₀ = 52.0−205.3−0.9+26.0+228.6 = 100.4 dB·Hz
6
Bandwidth B=36 MHz → C/N = 100.4−75.6 = 24.8 dB
7
DVB-S2 QPSK 3/4 requires C/N=7.5 dB → Margin = 17.3 dB (excellent!)
8
Throughput: QPSK 3/4, 36 MHz → spectral efficiency 1.5 bits/s/Hz → 54 Mbps
✓ C/N₀=100.4 dB·Hz, C/N=24.8 dB, margin=17.3 dB at QPSK 3/4. Rain fade headroom: −17.3 dB available before outage. Can upgrade to 8PSK 2/3 (requires 10 dB) for 72 Mbps, still 7.3 dB margin.
06 — Rain Attenuation
Rain Attenuation
Rain attenuation is the dominant impairment for Ku-band (10–14 GHz) and Ka-band (20–30 GHz) satellite links. Rain absorbs and scatters the signal — attenuation rises steeply with rain rate and frequency.
ITU-R P.838 Rain Attenuation Model
γR = k·Rα (specific attenuation in dB/km, R = rain rate in mm/hr)k, α are frequency-dependent ITU constants (tabulated)
A = γR·deff (total attenuation, deff = effective path length through rain)
deff ≈ ds·rp (slant path × path reduction factor)
Ku 11.7 GHz: k=0.0188, α=1.217 · Ka 20 GHz: k=0.0751, α=1.099
| Rain Rate | Climate Zone | Ku-band (11.7 GHz) | Ka-band (20 GHz) | Ka-band (30 GHz) |
|---|---|---|---|---|
| 1 mm/hr (drizzle) | Northern Europe | 0.02 dB/km | 0.07 dB/km | 0.18 dB/km |
| 10 mm/hr (moderate) | UK/Germany | 0.33 dB/km | 0.94 dB/km | 2.12 dB/km |
| 25 mm/hr (heavy) | Tropical coastal | 1.07 dB/km | 2.73 dB/km | 5.81 dB/km |
| 50 mm/hr (intense) | India/Southeast Asia | 2.65 dB/km | 6.03 dB/km | 12.4 dB/km |
| 100 mm/hr (tropical) | Amazon/Congo | 6.83 dB/km | 13.6 dB/km | 26.5 dB/km |
Ka-band rain impact: A 25 mm/hr rain event with 5 km effective path = 13.7 dB Ka-band attenuation — wiping out a typical Ku-band link margin instantly. Ka-band HTS systems (Viasat-3, SES O3b) use ACM (Adaptive Coding and Modulation) to drop from 32APSK to QPSK during rain fades, maintaining connectivity at reduced throughput.
Rain fade mitigation: ACM (DVB-S2X) — most effective. Site diversity — second ground station in clear weather. ULPC — uplink power control. Frequency diversity — switching to C-band backup during heavy rain. Combining all four gives 99.999% availability at Ka-band in tropical climates.
07 — Eb/N0 & Shannon Capacity
Eb/N₀ & Shannon Capacity
Eb/N0 and Shannon Limit
Eb/N₀ = C/N₀ − 10·log₁₀(Rb) (Rb = bit rate in bps)C = B·log₂(1+C/N) (Shannon capacity in bits/s — upper bound)
η = Rb/B (spectral efficiency in bits/s/Hz)
Shannon limit: Eb/N₀ ≥ −1.6 dB (absolute min for any modulation) · In practice: LDPC+BCH → within 0.5 dB of Shannon
Modulation Schemes in DVB-S2X
| Modulation | Code Rate | Req. Eb/N₀ | Spectral Eff. | Application |
|---|---|---|---|---|
| BPSK | 1/2 | 0.0 dB | 0.5 bps/Hz | Emergency, deep fade |
| QPSK | 1/2 | 1.0 dB | 1.0 bps/Hz | Rain fade, low margin |
| QPSK | 3/4 | 4.0 dB | 1.5 bps/Hz | Clear sky standard |
| 8PSK | 2/3 | 6.5 dB | 2.0 bps/Hz | High throughput |
| 16APSK | 3/4 | 10.2 dB | 3.0 bps/Hz | HTS spot beams |
| 32APSK | 4/5 | 14.8 dB | 4.0 bps/Hz | Excellent link, max throughput |
| 64APSK | 5/6 | 18.5 dB | 5.0 bps/Hz | DVB-S2X, GEO spot beams |
ACM in practice: A Ku-band HTS terminal steps from 32APSK (4.0 bps/Hz) to QPSK (1.0 bps/Hz) during a 10 dB rain event — throughput drops 4× but the link stays up. The DVB-S2X physical layer switches frames in <1 second, completely transparent to the IP layer above.
08 — LEO Constellations
LEO Constellations
Starlink vs GEO — Engineering Comparison
| Parameter | Starlink Gen 2 (LEO) | Viasat-3 (GEO HTS) | SES O3b mPOWER (MEO) |
|---|---|---|---|
| Altitude | 540–570 km | 35,786 km | 8,062 km |
| FSPL (Ku DL) | ~187 dB | ~205 dB | ~198 dB |
| Latency (RTT) | ~20–40 ms | ~600 ms | ~150 ms |
| User EIRP | ~40 dBW (flat panel) | ~49 dBW (1.2 m dish) | ~52 dBW (2.4 m dish) |
| Doppler shift | Up to ±200 kHz (Ku) | ~0 Hz | ~50 kHz |
| Handover rate | Every ~90 s | None | Every ~4 min |
| Capacity/beam | ~5 Gbps (V-band ISL) | ~40 Gbps (spot beams) | ~10 Gbps (steerable) |
| Terminal cost | ~$499 (phased array) | ~$800 (dish + modem) | ~$2,000+ (enterprise) |
LEO RF challenges: (1) Doppler — up to ±270 kHz at Ka-band — requires wideband AFC in the modem. (2) Handover — every 90 s, must be seamless. (3) Path loss changes 3 dB as satellite traverses from overhead to horizon. (4) Interference — 4,200+ Starlink satellites create a frequency coordination nightmare. (5) High Doppler rate-of-change requires fast preamble acquisition.
09 — Satellite RF Hardware
Satellite RF Hardware
| Component | Function | Key Spec | Technology |
|---|---|---|---|
| HPA (High Power Amp) | TX power amplification | Power, efficiency, linearity (IBO) | TWTA (GEO), GaN SSPA (LEO/ground) |
| LNA | Low-noise RX amplification | NF, gain, IP3 | GaAs pHEMT, InP (cryogenic) |
| BUC (Block Up-Conv) | IF→RF upconversion | Output power, phase noise | GaN SSPA + PLL LO |
| LNB (Low Noise Block) | RF→IF downconversion | NF (<0.3 dB Ku), LO stability | GaAs + dielectric resonator |
| Phased Array | Electronic beam steering | EIRP, scan loss, sidelobe level | GaN T/R module MMIC per element |
| Travelling Wave Tube (TWTA) | GEO satellite transponder HPA | 50–200 W output, >30 dB gain | Vacuum tube — unmatched efficiency at GHz |
GaN revolution in satellite: GaN-on-SiC MMICs now achieve 5–10 W/mm power density at Ka-band — enabling SSPA-based ground terminals to replace the klystron/TWTA. Starlink's flat-panel dish uses a 64×64 GaN phased array for Ku+Ka steering — scanning ±60° with no moving parts. Unit cost has dropped from $2,000 to <$100 due to volume and CMOS integration of beam-forming ICs.
10 — Tools
Try the Tools
Run your own satellite link budget using RFLab's free tools. The RF Link Budget Calculator covers EIRP, FSPL, received power and SNR — replicate the Ku-band VSAT example above in seconds.