Satellite Link Budget

 

 Understanding Satellite Link Budget: A Comparison Between GEO and LEO Systems

In the world of satellite communications, a link budget is a fundamental calculation that determines whether a signal transmitted from one point (such as a satellite or a ground station) can be received with sufficient strength and quality. It accounts for all gains and losses from the transmitter, through the medium (space), to the receiver.

Link budgets are essential for designing satellite networks, ensuring reliable service for applications like broadband internet, maritime connectivity, remote sensing, and military communications.

This post will compare the link budget design for GEO (Geostationary Earth Orbit) and LEO (Low Earth Orbit) satellite systems — considering frequency band, antenna types and sizes, modulation, access techniques (TDMA, IP, or dedicated), and elevation angles.

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🛰️ Orbit Basics and Link Considerations

Feature	GEO	LEO
Orbit Altitude	~35,786 km	500–1,200 km
Looking Angle	Low (10–20°) at poles, 90° at equator	Steeper (60–90°) and rapidly changing
Services	Dedicated, TDMA/IP for enterprise, TV	Broadband IP, mobile backhaul, IoT
Antenna Size	1.2 – 2.4 m (ground station)	0.3 – 0.9 m (user terminal)
Modulation	QPSK, 8PSK, 16APSK	QPSK, 16QAM, OFDMA
Frequency Band	Ku, Ka, sometimes C	Ka, Ku, increasingly V-band

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 What is a Link Budget?

A link budget calculates the received signal power at the end of a communication link. The basic formula is:

Received Power (dB) = Transmit Power + Gains - Losses

Key Components:

• Transmit Power (from satellite or Earth station)

• Antenna Gains (transmit and receive)

• Free Space Path Loss (FSPL)

• Atmospheric Losses

• Polarization and Rain Fade Losses

• System Noise Temperature and SNR (for service quality)

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 Link Budget Calculation: LEO vs. GEO

Let’s consider Ku-band (~14 GHz uplink, ~12 GHz downlink) as our reference.

 1. GEO Satellite Link Budget (TDMA IP service)

Assumptions:

• Frequency: 14 GHz (Uplink), 12 GHz (Downlink)

• Distance: ~38,000 km (one-way)

• Transmit Power: 10 W (40 dBm)

• Antenna Gain: Tx (Earth station): 45 dBi, Rx (Satellite): 30 dBi

• Free Space Loss: Calculated

• Modulation: 8PSK

• Antenna Size: 1.8m (Earth Station)

• Service: TDMA/IP (e.g., enterprise VSAT)

• Elevation Angle: 25° (worst-case)

• Rain loss: 2 dB

Free Space Path Loss (FSPL):

$$FSPL(dB) = 20 \log_{10}(d) + 20 \log_{10}(f) + 92.45$$ $$= 20 \log_{10}(38000) + 20 \log_{10}(14) + 92.45 \approx 191.04 \text{ dB}$$

Link Budget GEO Uplink (simplified):

• Tx Power: 40 dBm

• Tx Antenna Gain: 45 dBi

• Path Loss: 191 dB

• Rain Loss: 2 dB

• Rx Antenna Gain: 30 dBi

Received Power:

$$P_{rx} = 40 + 45 - 191 - 2 + 30 = -78 \text{ dBm}$$

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 2. LEO Satellite Link Budget (Dedicated IP Broadband)

Assumptions:

• Frequency: 14 GHz (Uplink), 12 GHz (Downlink)

• Distance: ~1,200 km (worst-case)

• Transmit Power: 5 W (37 dBm)

• Antenna Gain: Tx (User Terminal): 35 dBi, Rx (Satellite): 25 dBi

• Antenna Size: 75 cm user terminal

• Modulation: QPSK or 16QAM with adaptive coding

• Service: IP Broadband

• Elevation Angle: ~60°

• Rain Loss: 1 dB

Free Space Path Loss (FSPL):

$$FSPL = 20 \log_{10}(1200) + 20 \log_{10}(14) + 92.45 \approx 161.65 \text{ dB}$$

Link Budget LEO Uplink (simplified):

• Tx Power: 37 dBm

• Tx Antenna Gain: 35 dBi

• Path Loss: 161.65 dB

• Rain Loss: 1 dB

• Rx Antenna Gain: 25 dBi

Received Power:

$$P_{rx} = 37 + 35 - 161.65 - 1 + 25 = -65.65 \text{ dBm}$$

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Link Budget Calculation Excel Sheet 

https://satcom-services.com/VSAT/linkbudget.html

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 Summary Table

Feature	GEO Link	LEO Link
Altitude	~36,000 km	~1,200 km
Looking Angle	10–30°	60–90°
FSPL	~191 dB	~162 dB
Tx Power	10W (40 dBm)	5W (37 dBm)
Rx Power (est.)	~-78 dBm	~-65.6 dBm
Services	TDMA/IP, broadcast, dedicated	IP broadband, IoT, mobile backhaul
Modulation	QPSK, 8PSK, 16APSK	QPSK, 16QAM, OFDMA
Antenna Size	1.8 m (earth)	0.75 m (user)

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 References

1. ITU-R Recommendation P.618-13 – Propagation data and prediction methods

2. ESA – Link Budget Calculator and Design Guidelines

3. Starlink Technical Specs – SpaceX

4. HughesNet and Viasat System Design Whitepapers

5. "Satellite Communications" by Dennis Roddy

6. https://www.satsig.net/link-budgets/link-budget.htm

7. https://www.itu.int/en/ITU-R/

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