2. Sat Systems and Fundamentals

Question 1

What are the three segments of a basic sat communication system?

Answer 1

1. Control segment
2. Ground segment
3. Space segment

Question 2

What are the two different types of payload?

Answer 2

Transparent -
Regenerative -

Question 3

For a communication link, what are the two important metrics defining the performance of the uplink and dowlink?

Answer 3

uplink - EIRP
downlink - G/T - Receiver gain over system noise temperature

Question 4

What are the four components of the system noise temperature?

Answer 4

T_ant + T_feed + T_LNA + T_rec

Question 5

Why is frequency downconverting used at the input of the communication system?

Answer 5

To increase isolation between the uplink and downlink - onboard signal processing and associated signal leaks wont impact link performance

Question 6

What is Power Flux Density (PFD)?

Answer 6

The power per square meter at a distance from the transmitter

PFD = EIRP – 10log(4πd²) – link losses

Question 7

What is EIRP (Effective Isotropic Radiated Power)?

Answer 7

The power radiated by a hypothetical isotropic antenna with the same power as the actual antenna.

EIRP = Transmit Power (dBW) + Antenna Gain (dBi)

Question 8

What is Effective Aperture (AeR)?

Answer 8

The effective area of the receiving antenna used to capture power from an incoming wave.

Question 9

What is the received carrier power (C) equation?

Answer 9

C = EIRP – FSL + G

Question 10

What is the Carrier-to-Noise Ratio (C/N) equation?

Answer 10

C/N = EIRP – FSL + G – 10log(KTB)

Question 11

What is the Carrier-to-Noise Density Ratio equation (C/No)?

Answer 11

C/No = EIRP – FSL + G/T – 10log(K)

Question 12

What is Free Space Loss (FSL) and the equation?

Answer 12

The loss in signal strength over distance in free space.

FSL = 20log(d) + 20log(f) + 92.45 (for d in km, f in GHz)

Question 13

How is rain attenuation (AR) estimated?

Answer 13

AR (dB) = γ × Le, where:

• γ = specific attenuation (dB/km)
• Le = effective path length (km)

This attenuation increases with frequency and is a major issue at Ka-band (30/20 GHz), especially in tropical regions.

Question 14

What is αR in rain attenuation?

Answer 14

Fraction of the beam affected by rain.

Question 15

How does rain affect noise temperature (Ts’)?

Answer 15

Ts’ = α(αR * Tsky + (1 – αR) * TR) + (1 – α)Tf + TLNA + …
Where TR ≈ 275K

Question 16

What is depolarization in satellite links?

Answer 16

A distortion in signal polarization due to rain/ice particles.

Question 17

What is G/T and an associated equation?

Answer 17

Gain-to-system noise temperature ratio.

G/T = Antenna Gain (dBi) – 10log(System Noise Temperature)

Question 18

How to calculate system noise temperature (Ts)?

Answer 18

Ts = Ta + (1 – α)Tf + TLNA + TIPA + TD/L

Question 19

What causes intermodulation noise?

Answer 19

Nonlinear amplification in earth station or satellite leading to mixing of carriers.

Question 20

What is the total carrier-to-noise ratio (C/No)T?

Answer 20

(C/No)T⁻¹ = (C/No)U⁻¹ + (C/No)D⁻¹ + (C/No)IM⁻¹ + (C/I)U/D⁻¹

Question 21

What is system availability (Asys)?

Answer 21

Asys = (Required time – Down time) / Required time

Question 22

What is the impact of rain on link availability?

Answer 22

Reduces C/No by attenuating the signal and increasing noise, potentially causing outages.

Question 23

How does geometry affect satellite communication?

Answer 23

Sub-satellite point (ΦS) and Earth station longitude (ΦE) define pointing angle (ΦES = ΦE – ΦS)

Elevation angle (E) affects atmospheric path length

Antenna misalignment reduces gain significantly

Question 24

What does the ITU-R P.618 model provide for satellite link design?

Answer 24

It models rain attenuation based on:

• Rain rate maps for % of time
• Specific attenuation (γ) based on frequency and rain characteristics
• Effective path length (Le) through rain
• Output: Attenuation for specified % of time, typically used for 0.01% worst-case scenarios

Question 25

What causes depolarisation in satellite communications, and how is it mitigated?

Answer 25

Caused by asymmetric raindrops or ice crystals changing the polarization of signals. - Impacts circular polarization more than linear. - Modeled using Cross Polar Discrimination (XPD) - Mitigation: better antenna design, diversity techniques, linearizers.

Question 26

What mobile channel models are used in satellite communications?

Answer 26

Narrowband models: - Time-series power vs. time - Rician distribution (accounts for dominant direct path) - Markov models: Good/bad state ratios, e.g., 2–4 states - ERS (Empirical Roadside Shadowing) Wideband models: - Use Tapped Delay Line (TDL) - Each tap models a delay path with amplitude and phase - Power Delay Profile shows multipath fading and echo delays

Question 27

What are the key sources of noise in an Earth Station?

Answer 27

Antenna noise (Ta) from ground, sky, cosmic sources Feeder loss (Tf) LNA/IPA (TLNA, TIPA) Downconverter (TD/C)

Question 28

How does rain affect system noise temperature in an Earth Station?

Answer 28

Rain increases antenna temperature by adding thermal noise from raindrops New Ts’ includes weighted contribution from rain temp (TR ≈ 275 K) Increased Ts leads to a decrease in G/T Example: Rain-induced antenna temp increase = (1 – αR) × TR If αR = 0.75 → Increase ≈ 206K

Question 29

What is intermodulation noise, and when does it occur?

Answer 29

Occurs when multiple signals mix in nonlinear components (e.g., amplifiers), producing unwanted frequencies. Significant in FDMA systems Noise behaves like filtered white noise 3rd-order products are especially damaging at Ku-band Mitigation: linearizers, proper back-off

Question 30

What is system availability impacted by?

Answer 30

Rain fading Hardware failures Depointing Implementation losses

Question 31

What is the cost trade-off in designing for high system availability?

Answer 31

More transmit power (EIRP) Higher G/T Better redundancy → Results in significantly higher cost Design tip: Avoid overdesign unless strictly necessary