1. ROLE

3. GETTING STARTED

When you trigger this skill, I'll work with whatever you give me — but the more context, the better the output.

Minimum I need (pick one):

• "Design the X-band downlink for an Earth observation satellite at 525 km"
• "What antenna do I need to close a 150 Mbps Ka-band link from LEO?"
• "Calculate the link budget for a GEO TT&C uplink at S-band"

Helpful if you have it:

• Orbit altitude and inclination
• Required data rate (kbps, Mbps)
• Frequency band preference or regulatory constraint
• Transmit power available from the power subsystem
• Ground station antenna diameter and location (rain zone)
• Antenna pointing accuracy (affects pointing loss)
• Availability requirement (99.5%, 99.9%, 99.99%)

What I'll ask if you don't specify:

• "What orbit? LEO, GEO, deep space?" — slant range drives everything
• "Data rate requirement?" — determines bandwidth and ModCod
• "Link direction? Uplink, downlink, or both?" — asymmetric budgets are the norm

4. CONNECTORS

Shared Tools (in shared/tools/)

Shared Data (in shared/ — pack-level)

Cross-skill Connectors

5. TAXONOMY

5.1 Frequency Band Allocations

Rain fade values at 99.9% availability, 20 deg elevation, ITU rain zone K.

5.2 Antenna Types

Scan loss for phased array ~ 3-4 dB at 60 deg off-boresight.

5.3 Modulation Schemes

5.4 Forward Error Correction

Coding gain referenced against uncoded QPSK at BER = 10^-5.

6. PROCESS

Step 1: Link Definition

• Direction: uplink or downlink
• Orbit: altitude, inclination → worst-case slant range at minimum elevation
• Data rate: required user bit rate (Rb) after decoding
• Frequency: selected from 5.1 taxonomy based on data rate and licensing

IF orbit not specified → ASK. IF data rate not specified → provide parametric analysis for 1 kbps, 1 Mbps, 50 Mbps, 300 Mbps.

Step 2: Transmitter (EIRP)

EIRP (dBW) = P_tx (dBW) + G_tx (dBi) - L_tx (dB)

• P_tx = transmitter output power (after HPA, before feeder losses)
• G_tx = transmit antenna gain at boresight
• L_tx = cable/waveguide/combiner losses (typically 0.5-3 dB)

Step 3: Path Losses

FSPL (dB) = 20*log10(4*pi*d/lambda) = 92.45 + 20*log10(f_GHz) + 20*log10(d_km)

• d = slant range (km) — use worst case at min elevation
• Additional: atmospheric absorption (L_atm), rain attenuation (L_rain), scintillation, polarization mismatch

Step 4: Receiver Figure of Merit

G/T (dB/K) = G_rx (dBi) - 10*log10(T_sys) (K)

• T_sys = T_ant + T_LNA + T_feed (total system noise temperature)
• Typical ground: G/T = 20-45 dB/K; typical spacecraft: G/T = -10 to +10 dB/K

Step 5: Carrier-to-Noise

C/N0 (dBHz) = EIRP - FSPL - L_atm - L_rain - L_point + G/T - k

Where k = Boltzmann constant = -228.6 dBW/K/Hz.

C/N (dB) = C/N0 - 10*log10(B_noise)
Eb/N0 (dB) = C/N0 - 10*log10(Rb)

Step 6: Link Margin

Margin (dB) = Eb/N0_achieved - Eb/N0_required - Implementation_loss

• Required margin: >= 3 dB for LEO, >= 2 dB for GEO, >= 1 dB for deep space
• Implementation loss: 1-2 dB (modem, filter, timing imperfections)

WORKED EXAMPLE: X-band LEO-to-Ground Downlink

Scenario: Earth observation satellite, 525 km sun-synchronous orbit, 150 Mbps downlink to a 5.4 m ground antenna at 10 deg minimum elevation.

Frequency: 8.2 GHz (X-band space-to-Earth allocation) Lambda: c/f = 3e8 / 8.2e9 = 0.0366 m

Slant range at 10 deg elevation: d = sqrt((R_e + h)^2 - (R_e * cos(el))^2) - R_e * sin(el) d = sqrt((6371 + 525)^2 - (6371 * cos(10))^2) - 6371 * sin(10) d ~ 1,832 km

ModCod Selection: QPSK + LDPC 2/3

• Occupied bandwidth: 150 Mbps / (2 * 2/3) = 112.5 MHz (fits in 375 MHz X-band allocation)
• Eb/N0 required: 2.0 dB (LDPC 2/3 at BER = 10^-8, CCSDS 131.2)
• Implementation loss: 1.5 dB
• Link margin = 35.6 - 2.0 - 1.5 = 32.1 dB

Note: This 32 dB margin is extremely high — typical of X-band LEO downlinks with large ground antennas. In practice you would reduce transmit power to 0.5 W, shrink the spacecraft antenna to a patch array, or increase data rate to 500+ Mbps to use the available margin productively.

7. OUTPUT TEMPLATE

# [Mission Name] — Link Budget

## Link Parameters
| Parameter | Value |
|-----------|-------|
| Direction | [uplink/downlink] |
| Frequency | [X.XX] GHz ([band]-band) |
| Orbit | [altitude] km, [type] |
| Data Rate | [X] Mbps |
| Availability | [XX.X]% |

## Transmitter
| Parameter | Value | Unit |
|-----------|-------|------|
| Tx Power | [X.X] | dBW |
| Tx Antenna Gain | [X.X] | dBi |
| Tx Losses | [X.X] | dB |
| **EIRP** | **[X.X]** | **dBW** |

## Path
| Loss Component | Value (dB) |
|----------------|-----------|
| Free-Space Path Loss | [X.XX] |
| Atmospheric Absorption | [X.X] |
| Rain Attenuation | [X.X] |
| Pointing Loss | [X.X] |
| Polarization Mismatch | [X.X] |
| **Total Path Loss** | **[X.XX]** |

## Receiver
| Parameter | Value | Unit |
|-----------|-------|------|
| Rx Antenna Gain | [X.X] | dBi |
| System Noise Temp | [X] | K |
| **G/T** | **[X.X]** | **dB/K** |

## Link Performance
| Parameter | Value | Unit |
|-----------|-------|------|
| C/N0 | [X.XX] | dBHz |
| Eb/N0 (achieved) | [X.X] | dB |
| Eb/N0 (required) | [X.X] | dB |
| Implementation Loss | [X.X] | dB |
| **Link Margin** | **[X.X]** | **dB** |

## ModCod Selection
| Parameter | Value |
|-----------|-------|
| Modulation | [scheme] |
| Coding | [type], rate [X/X] |
| Spectral Efficiency | [X.X] bps/Hz |
| Required Bandwidth | [X.X] MHz |

## Recommendation
[Architecture summary, margin assessment, next steps]

8. CLASSIFICATION

9. VARIATIONS

• A: LEO TT&C — S-band, omnidirectional patch, QPSK + conv 1/2, 32-256 kbps, no rain fade concern, margin > 6 dB for safe commanding
• B: LEO High-Rate Downlink — X-band or Ka-band, 0.3-0.7 m dish, QPSK + LDPC 2/3, 150-500 Mbps, short contact windows (8-12 min), onboard storage sized to data volume per orbit
• C: GEO Broadcast / HTS — Ku/Ka-band, shaped reflector or MBA, 16/32APSK + LDPC, high rain margin (6-12 dB), transponder power budget, interference coordination (ITU filing)
• D: Deep Space — X-band or Ka-band, 1-5 m HGA, BPSK + turbo 1/6, data rates 0.01-10 Mbps, DSN 34/70 m ground antennas, one-way light time delay, Doppler pre-compensation
• E: Inter-Satellite Link (ISL) — Ka-band RF or 1550 nm optical, no atmospheric loss, line-of-sight geometry, Doppler from relative velocity, 1-10 Gbps laser crosslinks (Starlink, EDRS)

10. ERRORS & PITFALLS

• E1: Using altitude as slant range (at 10 deg elevation, slant range ~ 3.5x altitude for 500 km LEO)
• E2: Forgetting rain fade at Ka-band (20 dB fade at 99.99% availability in tropical zones kills the link)
• E3: Quoting antenna gain at boresight without pointing loss (0.5 deg error on a 1 deg beam = 3 dB loss)
• E4: Ignoring system noise temperature (T_sys = T_ant + T_LNA; a 30 K LNA behind a 200 K antenna = 230 K, not 30 K)
• E5: Confusing C/N with C/N0 (off by 10*log10(bandwidth) — typically 50-90 dB difference)
• E6: Using occupied bandwidth instead of noise bandwidth for C/N calculation (roll-off factor matters)
• E7: Neglecting Doppler shift in LEO (up to +-200 kHz at X-band for 525 km — receiver must track it)
• E8: Assuming clear-sky margin is "free" margin (rain, scintillation, aging, and misalignment consume it)

11. TIPS

• T1: Start from required data rate and orbit → work backwards to EIRP and antenna size
• T2: Budget 3 dB margin minimum for LEO, 6 dB for Ka-band, 1-2 dB for deep space (DSN link is precious)
• T3: Sanity check FSPL: LEO X-band ~ 170-180 dB, GEO Ku-band ~ 205-207 dB, Mars X-band ~ 260-280 dB
• T4: For LEO contact time: T_contact ~ (2/omega) * arccos(cos(el_min) / cos(nadir_angle)) — about 10 min at 500 km, 10 deg
• T5: Data volume per pass = data rate * contact time — size onboard storage to at least 2 orbits of payload data
• T6: Ground antenna cost scales roughly as D^2.7 — a 7.3 m dish costs ~4x a 5.4 m, not 1.8x
• T7: Calibrate against known systems: Landsat (X-band, 384 Mbps, 0.7 m dish, 525 km), Starlink (Ka, phased array, 550 km)
• T8: When margin is excessive, trade it: increase data rate, reduce Tx power (saves watts), shrink antenna (saves mass), or increase coding rate (saves bandwidth)

12. RELATED SKILLS