Communication

Question 1

Draw a schematic block diagram of a generalised satellite communication system and label
each constituent block. Explain the purpose of the following subsystems: (i) Space segment,

Answer 1

The space segment contains one or several active and spare satellites organised into a constellation.

Question 2

Explain the purpose of the following subsystems:
(ii) Ground segment,

Answer 2

The ground segment consists of all the traffic earth stations. Depending on the type of service considered, these stations can be of different size, from a few centimetres to tens of metres.

service user interface

Question 3

Explain the purpose of the following subsystems: (iii) Control segment operating in space.

Answer 3

The control segment consists of all ground facilities for the control and monitoring of the satellites, also named TTC (tracking, telemetry and command) stations, and for the management of the traffic and the associated resources on-board the satellite.

Question 4

Describe functions of the Telemetry, Tracing and Command (TTC) block.

Answer 4

Transmission (downlink) telemetry (TM) and data

Reception (uplink) commands (TC).

Gathering and processing: data ready to transmission

Processing and routing: commands from ground station.

Transponder for ranging: tracking satellite’s orbit parameters and position

Support services for payload: control, data handling, data storage, data processing.

Question 5

In a satellite communication system describe what is meant by the terms: (i)
downlink and uplink transmission;

Answer 5

A downlink is the link from a satellite down to one or more ground stations or receivers, and an uplink is the link from a ground station up to a satellite

Question 6

In a satellite communication system describe what is meant by the terms: (ii) the effective isotropic radiated power of an antenna;

Answer 6

the power that an omnidirectional antenna would need to radiate to give the same radiated power P as a directional antenna of gain G

Question 7

In a satellite communication system describe what is meant by the terms:
(iii) the free space propagation path loss.

Answer 7

Free-space path loss, 𝐿𝐹𝑆, is the loss in signal strength of an EM wave that would result
from a line-of-sight path through free space, with no obstacles nearby to cause
reflection or diffraction.

Question 8

Explain the term “Avionics” for a spacecraft or satellite. Indicate its two meanings used in space industry.

Answer 8

1. The equipment and systems used to control and monitor a spacecraft or satellite. This includes hardware such as computers, sensors, communication systems, and propulsion systems.
2. The software used to program and control the hardware and systems. This includes software for navigation, communication, guidance, and control.

Question 9

List all on-board data handling (OBDH) functions.

Answer 9

Ø Receiving and distribution commands

Ø Enabling flow of housekeeping and science data
Ø Providing data storage
Ø Performing data compressions

Ø Time distribution around the spacecraft
Ø Executing command and schedules
Ø Controlling payload and subsystems
Ø Monitoring spacecraft health
Ø Making autonomous decisions
Ø Performing telemetry and telecommand protocols

Question 10

Describe the basic forms of on-board data and how they are handling and prepared
for transmission

Answer 10

Data: 3 basic forms:
(i) analogous
Analogue data to condition to the standard scale: 0 .. 5.12 V. Filtering. Sampling: converting into digital form

(ii) digital bi-level
Digital bi-level are grouped into 8 or 16-bit words

(iii) digital serial: binary words
Digital data: acquired in a serial form
Prepare to be transmitted

Question 11

Why does the telemetry signal have to be modulated for transmission to the earth?

Answer 11

Signals to be transmitted by a communication system normally consist of a band of frequencies (up to tens kHz for speech and up to tens MHz for TV) which are too low for direct transmission as radio waves.

Therefore, the signal is imposed on a carrier wave of much higher frequency suitable for transmission.

Question 12

What are main types of modulations?

Answer 12

basic types of modulation: amplitude (AM), frequency (FM) and phase (PM).

Question 13

Describe all types of modulation of a digital signal.

Answer 13

ASK: amplitude shift keying
FSK: frequncy shift keying
PRK: phase reverse keying (shift by +-180)
QPSK: quadrature phase shift keying (shift by +-90)

Question 14

Describe the process of converting of an analogue signal to the digital form.

Answer 14

1.Sampling
2.Quantization
3.Encoding

Question 15

State the Nyquist–Shannon sampling theorem. How it is applied for manipulation with a signal in satellite communication?

Answer 15

The Nyquist–Shannon sampling theorem states that a continuous-time signal can be perfectly reconstructed from its samples if the sampling rate is greater than twice the maximum frequency of the signal.

This ensures that the signal is accurately reconstructed when it is received at the other end, allowing for accurate manipulation and processing of the signal.

Question 16

How the bandwidth of a signal can be defined?

Answer 16

The bandwidth of a signal is defined as the range of frequencies that it contains.

It is typically measured in Hertz (Hz) and is usually represented as the difference between the highest and lowest frequency components of the signal.

Question 17

What are the advantages of digital form of a signal compared to analogue one?

Answer 17

-The signal can be compressed. As a result bandwidth of the compressed signal can be lower than that of the analogue signal.

-Can be transmitting, copying, processing without loss of information (albeit not always)

-The information can be encrypted

Question 18

Explain the term: transmitting bit error (BER).

Why the uplink BER should be essentially smaller than the downlink BER?

Answer 18

(i) In telecommunication, the transmitting bit error is the share of corrupted bits in a digital bitwise signal. Typical allowed uplink BER at present is one false bit from 105. In general, error-control encoding is a less intensive processing task than decoding → More complex error control task is in the ground segment.

(ii) A small error in telemetry is not important but could be disastrous in a mission critical command! That is why special care should be taken to avoid error in commands, i.e. decrease the uplink BER to minimal possible values. Command links use an automatic retransmission mechanism delivery (protocol COP-1: command operation protocol) to guarantee by detecting errors and retransmitting the failed frames.

Question 19

describe the main forms of telemetry data

Answer 19

(i)analogous (ii)digital bi-level (iii)digital serial: binary words

Question 20

telemetry data stages of its handling: encoding, preparing for transmission, transmission

Answer 20

(1) Encoding
Analogue signal
* Scaling to a common full scale range (0..5.12 V)
* Filtering: frequencies ≤ (1/2) f sampling
* Analogue switches: sampling, quantization, encoding
Bi-level data: from relay contact closes: state “Off” = 0V; “On” = 5.12 V. Individual bits are
grouped together into 8- or 16-bits parallel-to-serial converter

(2) Grouping all signals: all the data are located into a shift register: (used as a memory).
Clocked out at the telemetry bit-rate as a continuous PCM (pulse code modulation) bit stream.
Unique synchronization and identification codes are inserted in the bit stream: the data can be identified when reaches the ground

(3) Modulation: imposing of a low frequency signal onto a high frequency carrier suitable for
transmission.

(4) Transmitting

Question 21

describe methods for tracking satellite distance

Answer 21

1. Using GPS or similar navigation system or their satellite .navigation or similar systems
2. Using TC/TM sub-carrier