Link budget calculator
Downlink margin for any transmitter, path and ground station. The presets reproduce the Grizu-263A budget of the review (Table 7) and the reference 1P link of the articles.
Show the data as a table
| Elevation (°) | Margin |
|---|---|
| 0 | −3.6 |
| 5 | −1.7 |
| 10 | 0.1 |
| 15 | 1.7 |
| 20 | 3.1 |
| 30 | 5.5 |
| 45 | 8.0 |
| 60 | 9.5 |
| 90 | 10.7 |
| Spacecraft transmitter | ||
|---|---|---|
| Transmit power | −3.0 | dBW |
| Line and connector loss | −0.5 | dB |
| Antenna gain | 0.0 | dBi |
| EIRP | −3.5 | dBW |
| Geometry and losses | ||
| Slant range | 1,695 | km |
| Free-space path loss | −149.8 | dB |
| Polarization loss | −3.0 | dB |
| Atmospheric and ionospheric loss | −1.0 | dB |
| Pointing and pattern allowance | −3.0 | dB |
| Ground station | ||
| G/T | −13.0 | dB/K |
| Boltzmann constant | −228.6 | dBW/(K·Hz) |
| Signal | ||
| C/N₀ | 55.3 | dB-Hz |
| Data rate | 39.8 | dB-Hz |
| Eb/N₀ | 15.4 | dB |
| Required Eb/N₀ | 13.4 | dB |
| Implementation loss | 2.0 | dB |
| Link margin | +0.1 | dB |
Free-space propagation over a spherical Earth, uncoded modulation, Earth rotation neglected in the pass estimate. Real links add fading from tumbling, ionospheric scintillation and interference; keep a margin.
DETERMINISTIC PHYSICS & DATA METHODOLOGY
How This Calculator Works
Source modules: lib/physics/link-budget.ts, lib/physics/orbit.ts
Evaluates spherical-Earth slant range, free-space path loss, Friis receiver noise-temperature figure of merit G/T, carrier-to-noise-density C/N₀, and available versus theoretical Eb/N₀ margin across elevation angles from 0° to 90°.
Governing Equations & Physical Model
d = √((R_E + h)² − (R_E·cos ε)²) − R_E·sin ε, FSPL = 20·log₁₀(4π·d·f / c)Computes exact line-of-sight distance d [m] for spherical Earth radius R_E = 6,371 km at elevation angle ε.
C/N₀ = EIRP − FSPL − L_misc + (G_rx − 10·log₁₀ T_sys) − k_B, Eb/N₀ = C/N₀ − 10·log₁₀ R_bUses Boltzmann constant k_B = −228.6 dBW/(K·Hz) and sums polarization, pointing, and atmospheric losses into L_misc.
P_b,BPSK = Q(√(2·Eb/N₀)), P_b,FSK = ½·exp(−Eb / (2·N₀))Solves required Eb/N₀ by bisection for target BER (10⁻³ to 10⁻⁶) plus hardware implementation loss.
Inputs, Parameters & Units
| Parameter | Unit | Physical definition |
|---|---|---|
| f, h, ε | MHz, km, ° | Carrier frequency, circular orbit altitude, and ground-station elevation angle. |
| P_tx, L_tx, G_tx | W, dB, dBi | Transmitter RF output power, feed-line loss, and spacecraft antenna gain. |
| G_rx, T_sys | dBi, K | Ground-station receive antenna gain and cascaded system noise temperature. |
| R_b, BER, L_impl | bps, —, dB | Channel bit rate, target uncoded bit-error probability, and modem implementation loss. |
Assumptions & Validity Limits
- Evaluates uncoded theoretical BPSK and non-coherent 2-FSK; convolutional, Reed–Solomon, or LDPC coding gains must be deducted separately or modelled via effective Eb/N₀.
- Atmospheric attenuation is treated as a user-set scalar loss (0.5–1.0 dB typical at 437 MHz UHF; rain fade dominates X-band at low elevations).
- Usable pass throughput integrates overhead pass geometry (ε_max = 90°) for all intervals where link margin ≥ 3 dB.
Worked Default Reference Case
At 437.2 MHz, 500 km, 10° elevation, 0.5 W RF, and 9600 bps non-coherent FSK into a 14 dBi / 500 K Yagi, available Eb/N₀ = 15.43 dB barely clears the 15.35 dB requirement (+0.08 dB); Grizu-263A (100 mW, 4800 bps at 30° elevation into 13 dBi / 350 K) closes with +4.67 dB margin.
- Reference slant range (10°)
- 1,695 km (FSPL 149.8 dB)
- Reference margin (9600 bps, 10°)
- +0.08 dB
- Grizu-263A slant range (30°)
- 951 km (FSPL 144.8 dB)
- Grizu-263A margin (4800 bps, 30°)
- +4.67 dB