Calculator Inputs
Use the responsive grid below. Large screens show three columns, medium screens show two, and phones show one. Enter 99 dB for negligible interference.
Example Data Table
| Scenario | Uplink Freq | Downlink Freq | Bit Rate | Uplink C/N0 | Downlink C/N0 | Net Eb/N0 | Margin |
|---|---|---|---|---|---|---|---|
| Reference GEO VSAT Link | 14.0 GHz | 12.0 GHz | 8.0 Mbps | 82.4 dBHz | 85.1 dBHz | 7.8 dB | 1.3 dB |
| Rain-Stressed Link | 14.0 GHz | 12.0 GHz | 8.0 Mbps | 79.8 dBHz | 82.2 dBHz | 5.2 dB | -1.3 dB |
| Improved Receive G/T | 14.0 GHz | 12.0 GHz | 8.0 Mbps | 82.4 dBHz | 87.4 dBHz | 9.1 dB | 2.6 dB |
Formula Used
The calculator follows standard satellite link budget relationships and combines uplink, downlink, and optional interference contributions.
EIRP_uplink = Tx Power + Tx Antenna Gain - Tx Feeder Loss
FSPL = 92.45 + 20 log10(Frequency in GHz) + 20 log10(Range in km)
Remote G/T = Effective Receive Gain - 10 log10(System Noise Temperature in K)
C/N0 = EIRP + G/T - Total Path Loss + 228.6
1 / (C/N0_total) = 1 / (C/N0_uplink) + 1 / (C/N0_downlink)
Symbol Rate = Information Bit Rate / (Bits per Symbol × FEC Rate)
Noise Bandwidth = Symbol Rate × (1 + Roll-Off)
C/N = C/N0 - 10 log10(Noise Bandwidth)
Eb/N0 = C/N0_total - 10 log10(Information Bit Rate)
Net Eb/N0 = Available Eb/N0 - Implementation Loss
Link Margin = Net Eb/N0 - Required Eb/N0
If C/I values are supplied, the tool combines them with thermal C/N using inverse-power addition. This gives a more realistic overall carrier-to-noise result when interference is not negligible.
How to Use This Calculator
- Enter uplink and downlink frequencies plus slant ranges.
- Add earth station transmit power, antenna gain, and feeder loss.
- Enter satellite receive G/T and satellite downlink EIRP.
- Provide receive antenna gain, feeder loss, and system noise temperature.
- Include atmospheric, rain, and miscellaneous losses for both directions.
- Set information bit rate, modulation bits per symbol, FEC rate, and roll-off.
- Enter required Eb/N0 and implementation loss for your modem target.
- Optionally add uplink and downlink C/I values. Use 99 dB when interference is negligible.
- Press the calculate button. The results appear above the form under the header.
- Use the CSV or PDF buttons to export the calculated output table.
FAQs
1. What does a VSAT link budget measure?
A VSAT link budget estimates signal strength and quality from transmitter to receiver. It tracks gains, losses, noise, Eb/N0, and final margin for a planned satellite connection.
2. Why are uplink and downlink calculated separately?
Each direction uses different frequencies, losses, equipment, and G/T values. Separate calculations let you see whether the weak point is the earth-to-satellite leg or the satellite-to-earth leg.
3. What is G/T in satellite communications?
G/T is gain-to-noise-temperature ratio. It shows receive sensitivity. Higher G/T usually improves C/N0 because the receiving system collects more useful signal relative to thermal noise.
4. How does rain affect the result?
Rain adds attenuation, especially at higher microwave bands. As rain fade increases, received carrier levels and C/N0 drop, which can quickly reduce net Eb/N0 and eliminate margin.
5. Why does roll-off matter?
Roll-off increases occupied noise bandwidth beyond the raw symbol rate. A wider bandwidth lowers C/N for the same C/N0, which directly influences available Eb/N0 and link margin.
6. What is a good link margin target?
That depends on service class, fade goals, and availability needs. Many designs aim for at least a small positive margin, while rain-sensitive links often require more reserve.
7. Should I use range or satellite altitude?
Use slant range, not satellite altitude. Slant range reflects the actual signal path length between the earth station and satellite and gives the correct free-space path loss.
8. Can higher EIRP alone fix a bad link?
Not always. Poor receive G/T, heavy rain, large feeder losses, or high required Eb/N0 can still limit performance. Good design balances power, antennas, noise, bandwidth, and fading.