Scenario & signal
nm
Gbps
ph/bit
Transmitter & pointing
W
dB
µrad
cm
λ rms
µrad
µrad
Path, atmosphere & receiver
km
°
m
km
–
km
m⁻²ᐟ³
m/s
cm
% of D
dB
Summary
Geometry & turbulence channel
Link budget table
What this means
dB waterfall — gains up, losses down
Margin sweep — elevation / range
Scintillation fade margin vs availability
Method: the DLR link-budget recipe of Giggenbach, Knopp & Fuchs (IJSCN 41, 460, 2023):
Gaussian TX gain gTx = 10·log₁₀((4√ln2/θ_FWHM)²) or diffraction-limited (πD/λ)² with Strehl
exp(−(2πσ)²); Friis free-space loss (λ/4πL)²; spherical-Earth slant range (Eq 14); mean jitter loss from the
β-model (Eqs 11–12); clear-air attenuation T_z^(1/sin ε) (flat-Earth airmass, valid ε ≳ 5°, no clouds); Kim
visibility model for horizontal paths. Turbulence: HV5/7 Cn²(h) profile integrated numerically for
plane-wave Fried r₀, isoplanatic θ₀ and downlink Rytov σ²_R (Andrews & Phillips 2005), aperture averaging
A = [1+1.062·kD²/4Leff]^(−7/6) with Leff = 12 km/sin ε, and the lognormal fade margin of
Giggenbach & Henniger (Opt. Eng. 47, 046001, 2008) at the availability target. Sensitivities are
photons-per-bit anchors from Caplan (JOFCR 4, 225, 2007). Limits to respect: weak-fluctuation theory only
(σ²_R ≲ 1 — the tool flags saturation), mean pointing loss (not percentile pointing fades), downlink
scintillation statistics (uplink beam wander and anisoplanatism are not modeled), ideal antenna gains.
Why does the scintillation fade margin shrink with a bigger receive telescope?
Turbulent scintillation arrives as a speckle pattern whose cells are roughly the
Fresnel-zone size √(λL) — some tens of cm for a LEO downlink at 1550 nm. A photodiode-sized receiver rides
one speckle and sees the full lognormal power swing, but an aperture spanning several speckles averages
independent bright and dark patches: the power scintillation index drops by A ≈ (1.062·kD²/4L)^(−7/6),
often 5–10× for a 40–60 cm ground telescope. That converts directly into dB of fade margin you don't have
to buy with laser power — one of the quiet reasons optical ground stations use bigger apertures than the
mean-power budget alone would justify.