Communication

Question 1

How is communication in space possible?

Answer 1

Via electromagnetic waves in a frequency range below the visible light spectrum.

note:
• Preferred range for space communication is the low MHz to medium GHz
• Communication is essential to fulfill the mission and to command the spacecraft (TT&C)!!!

Question 2

Explain the Modulation of a Signal:

Answer 2

1) We create EM waves with an alternating electric current in an electric conductor designed to radiate the EM waves (antenna) -> transmitter
2) A second antenna picks up the EM wave and transforms it back into electric current -> receiver
3) Signals are imposed on carrier waves (much higher frequencies) -> modulation/demodulation

Note:
• Transmitter and receiver need to be in the line of sight in order to communicate

Question 3

Types of Modulation (3):

Answer 3

1) Amplitude (AM):
• Low sound quality
• Simpler technology
• Long range transmission

2) Frequency (FM):
• Good sound quality
• Less sensitive to interference
• Higher sensitivity to physical barriers

3) Phase

Question 4

What is noise and what is its main source?

Answer 4

• > Can be defined as unwanted distortion of the signal

- > Main source is thermal noise - motion of charge carriers has power

Question 5

Mention different ways to reduce the impact of noise (4):

Answer 5

1) Power Level
2) Choosing right frequency
3) Temperature
4) Digital Signal

Question 6

How is the Radio Frequency (RF) spectrum determined in terms of Satellite Design? (3)

Answer 6

Determined by:

1) Power System
2) Antenna Size
3) Transmitter Power

Question 7

Difference between vs Analog and Digital

Answer 7

• Analogue ‐> noise always degrades signal

* Digital signal -> reduced noise sensitivity and error rate and can be easily regenerated (relay station)

Question 8

Data Transmission Steps: (3)

Answer 8

Step1: Sampling -> Nyquist criterion*
Sampling the analog signal with a frequency about times 2.2 higher than the highest frequency in the original analog signal.

Step 2: Quantization:
Converting the sample (analog) into a digital signal with a given number of bits.

Step 3: Encoding:
Combine the (binary) numbers and include extra digits for synchronization and checks.

Question 9

What is Data Rate?

Answer 9

Define as the number of samples per second x bits per sample

Question 10

Characteristics of an Antenna: (5)

Answer 10

• An antenna should send or receive a signal without causing losses.
• No antenna is fully isentropic -> alignment matters
• Same principals apply for transmission and reception
• Do not radiate in all directions equally
• One direction preferred, especially for transmission

Question 11

Types of Antennas: (5)

Answer 11

1) Yagi: Isentropic Antenna
2) Monopole Antenna
3) Dipole Antenna
4) Parabolic: Directional antenna
5) Horn Antenna

Question 12

Type of Communication losses:

Answer 12

1) Interference with other EM sources (external and internal)
2) Attenuation due to distance (~1/r^2)
3) Thermal noise (proportional to the system temperature)
4) Atmospheric losses
5) Antenna losses
6) System losses (cable losses etc.)

Question 13

Define the term “Antenna Gain”

Answer 13

Describes how much power is transmitted in the direction of peak radiation compared to an isotropic source

Question 14

What does the term “Communication Architecture” includes? (6)

Answer 14

1) Payload
2) Electric Power System
3) Thermal Control System
4) OBC -> Encryption, modulator, amplifier, TX RX,
5) Data Storage
6) Antenna

Question 15

Communication Architecture - Ground/Satellite Alternatives: (4)

Answer 15

1) Store & Forward
2) Geostationary
3) GEO Crosslink
4) LEO Crosslink

Question 16

Ground/Satellite Communication: Store & Forward [pros, cons]

Answer 16

+ Low Cost
+ Low transmitter power
+ Possible Polar Coverage

• Potentially rare comm. windows
• Short comm. windows
• Low data rate

Question 17

Ground/Satellite Communication: Geostationary [pros, cons]

Answer 17

+ Potentially no relay sat needed
+ No need for High Accuracy Antenna

• High Cost
• No Polar Coverage
• Propagation Delay

Question 18

Ground/Satellite Communication: GEO Crosslink [pros, cons]

Answer 18

+ Coverage of large area
+ Reduced propagation delay
+ Flexibility in GS location

• Increased system complexity
• No polar coverage

Question 19

Ground/Satellite Communication: LEO Crosslink [pros, cons]

Answer 19

+ Connection redundancy
+ Low transmitter power
+ Possible Polar Coverage

• Complex architecture
• A large number of launches needed

Question 20

Communication Architecture - Selection Criteria (7)

Answer 20

1) Nature of Mission & Orbit
2) Cost
3) RF spectrum
4) Data Rate:
5) Link availability: How often the GS is available
6) Link access time: time it takes to link with sat
7) Threat: Military use & radiation, space debris