The VLEO Link Budget — Clear Math, Big Consequences
- Bridge Connect
- Sep 22
- 4 min read
Introduction: Boards Need to Understand the Decibels
Link budgets may seem like “RF engineering territory,” but when your organisation is evaluating satellite-to-phone service, rural coverage restoration, or capex for resilient backhaul, understanding the dB math is a fiduciary duty.
This blog demystifies the key equations and shows why very-low-Earth orbit (VLEO) is a game-changer. We compare GEO, LEO, and VLEO scenarios, highlight frequency-dependent losses, and translate this into strategic implications for boardrooms.
1. The Anatomy of a Link Budget
At its core, a link budget is a power accounting exercise — a dB ledger from transmit power to receiver sensitivity. The main terms:
EIRP: Effective Isotropic Radiated Power of the transmitter.
FSPL: Free-Space Path Loss (function of range & frequency).
Rx G/T: Receive antenna gain over system noise temperature.
L_total: All additional losses (polarisation, pointing, rain fade).
SNR_margin: Required signal-to-noise ratio for target QoS.
The received carrier power, in dBW, is:
C = EIRP + G/T - FSPL - L_total
If C/N meets the demodulator’s Eb/N0 requirement with margin, the link closes. If not — no service.
2. FSPL: Where VLEO Wins
FSPL Equation
FSPL(dB) = 20 log10(d_km) + 20 log10(f_GHz) + 92.45
This is where orbit altitude matters. Slant range d shrinks dramatically at VLEO altitudes.
Comparison Table
At 2 GHz, dropping from 550 km to 300 km yields a 3.6 dB gain — equivalent to doubling transmit power, or using a larger antenna, without actually doing either.
At 10 GHz (Ku-band), the gain is similar, but atmospheric effects start to matter more (see below).
3. Beyond FSPL: The Other Losses
Boards often get shown just FSPL, but a robust business case needs to account for real-world impairments:
Polarisation mismatch: ~0.5–3 dB.
Pointing loss: Especially relevant for electronically steered arrays — 1–2 dB margin advisable.
Atmospheric absorption: Negligible at S-band, grows above 10 GHz.
Rain fade: At Ku/Ka-band, fade margins can exceed 10 dB in heavy rain zones.
Ionospheric scintillation: 0.5–1 dB (tropical regions worst).
These terms must be included to avoid optimistic business cases.
4. Direct-to-Device (D2D) Feasibility in VLEO
Why It Works Better at Lower Orbits
Handset antenna gain is fixed (~0 dBi) — so every dB matters.
VLEO adds 3–4 dB margin vs mid-LEO, enough to close LTE/NB-IoT links in narrow 1.4–5 MHz channels at low SNR.
SpaceX, Lynk, AST SpaceMobile and others are betting that 3GPP Rel-17/18 NTN standards will allow seamless roaming onto satellites. VLEO makes this more spectrally efficient, with lower satellite transmit power and smaller payload antennas.
5. Spectrum Implications: MSS & Coexistence
The link budget improvement means operators can use lower power-flux densities (PFDs) and still close the link. This helps meet strict FCC & ITU limits on interference into terrestrial services — critical for:
AWS-4 (2000–2020 / 2180–2200 MHz)
H-Block (1915–1920 / 1995–2000 MHz)
Better link margins also allow dynamic power control to reduce interference footprint and comply with OOBE masks.
6. Illustrative Link Budget (Worked Example)
Here is a simplified downlink budget for a VLEO satellite at 300 km, 2 GHz:
This closes comfortably with 2–3 dB margin — but would not close at GEO without huge dishes and hundreds of watts of EIRP.
7. Latency: The Other Benefit
Boards often underestimate latency’s commercial impact.
VLEO RTT: 40–60 ms round-trip — suitable for video calling and even low-latency financial applications.
GEO RTT: 480–600 ms — too slow for real-time interactivity.
Low latency means space links can carry revenue-generating services previously impossible over satellite.
8. Trade-offs & Risks
VLEO isn’t free lunch:
Propulsion Duty Cycle: Must counter drag continuously — affects power system design and capex refresh rate.
Satellite Lifetime: Shorter (2–5 years typical) → more replenishment launches needed.
Capex/Opex Balance: Boards must budget for “evergreen” constellation replenishment.
Space Traffic Management: 15,000+ satellites → collision avoidance burden and regulatory scrutiny.
9. Competitive Landscape
Expect VLEO filings to surge following SpaceX’s September 2025 request. Amazon Kuiper, Telesat, and sovereign players (EU IRIS², China Guowang) may follow. Competitive dynamics could turn VLEO shells into spectrum land grabs.
10. Board-Level Checklist
Demand a Full Link Budget: Require FSPL, losses, margins, and G/T assumptions in any business case.
Pilot in Representative Climates: Rain fade and scintillation vary — test in worst-case zones.
Align Spectrum + Orbit Choices: Link budgets should drive MSS/PCS band strategy.
Stress-Test Capex Models: Include propulsion power draw, solar array sizing, and refresh rate.
Govern Space Risk: Track conjunction alerts, debris risk, and compliance obligations.
Conclusion: Link Budgets = Business Budgets
VLEO gives boards a new lever: improve user experience, meet ESG goals (self-cleaning orbit), and tap underused spectrum — all with a physics-driven dB gain.
But the benefits only flow if boardrooms understand the math and hold engineering teams accountable for realistic margins and lifetime costs.
Your competitors are already running link budgets — the question is whether yours are robust enough to inform strategic investment decisions in 2026–2028.