What is the adjacent satellite interference analysis for a VSAT terminal?
VSAT Adjacent Satellite Interference Analysis
ASI analysis is a mandatory regulatory requirement for every VSAT network. The analysis ensures that the VSAT system operates without causing harmful interference to neighboring satellite systems, which is essential for the orderly sharing of the GEO arc.
| Parameter | GEO | MEO | LEO |
|---|---|---|---|
| Altitude | 35,786 km | 2,000-35,786 km | 200-2,000 km |
| Latency (one-way) | ~270 ms | 50-150 ms | 1-20 ms |
| Coverage per Sat | Full hemisphere | Regional | Local footprint |
| Handover | None | Periodic | Frequent |
| Path Loss (Ku-band) | ~206 dB | 190-206 dB | 170-190 dB |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Frequently Asked Questions
What antenna size do I need to comply with ASI limits?
The minimum antenna size is driven by the off-axis EIRP density requirement. For Ka-band (30 GHz transmit): 0.75 m antenna is the minimum for 3-degree satellite spacing (marginal compliance). 1.2 m antenna provides comfortable margin for 2-degree spacing. 1.8-2.4 m antenna is needed for high-power uplinks or band-edge channels where interference is most critical. The FCC and ITU specify minimum antenna diameter requirements for different bands: Ka-band typically requires > 0.75 m to qualify for blanket license (routine earth station) operation.
How does spread spectrum affect ASI?
Spreading the transmit signal over a wider bandwidth reduces the EIRP density linearly: spreading by a factor of 10 (10 dB) reduces the EIRP density by 10 dB. This is the basis of the CDMA-based VSAT systems (e.g., Hughes DirecWay) that can use smaller antennas while complying with ASI limits. The total EIRP is the same, but the spectral density is lower. This makes spread-spectrum VSAT systems attractive for small antenna terminals.
What happens if my VSAT fails the ASI analysis?
If the VSAT exceeds the off-axis EIRP density limits: the terminal cannot operate without a specific coordination agreement with each affected adjacent satellite operator. Coordination is time-consuming (typically 3-12 months) and may require power or bandwidth restrictions. Alternative solutions: use a larger antenna (the most common fix), reduce the maximum transmit power, use spread-spectrum modulation, or operate with automatic transmit power control (ATPC) that reduces power below the limit during clear sky.