Lecture_2_Sattelite_ Space-- segment.pdf

engineering

1. Satellite Communication System
Dr. Rashed Md. Murad Hasan
2025
1

2. Outline
•Space segment
2

3. 3
Satellite Uplink & Downlink

4. 4
Satellite Uplink Model

5. 5
Satellite Transponder Model

6. 6
Satellite Downlink Model

7. 7
Satellite System Elements
❑ Systemcomposition
▪Space segment: including satellite and measurement and control;
▪Ground segment: earth station

8. 8
Space segment
❑ Space segment
▪Satellites (platform and payload),
▪Satellite Control Center (SCC),
▪Tracking Telemetry and Command Station (TT&C).
❑Several concepts of space segments
▪Uplink: Radio waves from the ground segment to the satellite.
▪Downlink: Radio waves from the satellite to the ground segment.
▪Link quality: Τ????????????
0 determining the bit error rate (BER)
▪Multiple access technology: FDMA, TDMA, CDMA, SDMA
▪Interstellar link: Communication link between satellites
▪Payload and platform: Power supply, structure, solar panels, etc.
▪Satellite role: amplify carrier, frequency conversion.
▪Repeater: transparent and regenerative repeater.
▪Satellite redundancy, lifetime, reliability

9. ▪Satellite bus is a platform that supports the payload
operation reliably throughout the mission life
9
Satellite bus and Payload
Communication
payload
Satellite
bus
▪Remote sensing
▪Metrology
▪Scientific
▪Communication

10. ▪To maintain the position and orientation of the satellite
towards service area (e.g., Earth)
1. Attitude and orbit control system (AOCS)
▪To provide proper thrust to the satellite to maintain its
attitude and orbit
2. propulsion system
10
Functions of spacecraft Bus

11. ▪To provide the status and health of the subsystems to
the ground monitoring station
▪To accept the command from the ground control and
▪To execute them in order to meet the performance
requirements of the subsystems
▪To support the ground system to track the satellite
3. Telemetry, Tracking and Command (TTC) system
11
Functions of spacecraft Bus

12. ▪To provide primary and secondary DC power to the
subsystem electronics.
4. Power system
▪To maintain the temperature of the different systems in
the bus and the payload within their operating range.
5. Thermal system
▪To provide mechanical and structural support to the
satellite during the orbit raising and normal operational
period.
6. Structure system
12
Functions of spacecraft Bus

13. ▪Satellite bus is a platform that supports the payload
operation reliably throughout the mission life
13
Satellite bus and Payload
Payload
Microwave repeater, antenna
Satellite Bus
AOCS, TTC, Power, Propulsion, Thermal, Structure

14. Attitude control needs to sense the spacecraft orientation
towards earth and then control the orientation in case of error
14
Attitude and Orbit Control System

15. Orientation of the satellite changes with time,
It needs to be monitored and corrected
15
Attitude and Orbit Control System
Earth sensors (edges): Accuracy of this IR sensors are 0.05 deg. [IR sensing]
RF sensors: It has accuracy of 0.01 deg. [from earth]
Sun sensors: Yaw axis error adjustment
Star sensors: It gives accuracy of 0.001 degree

16. 16
Attitude Control
Required torque is generated by two separate units:
▪Reaction wheel
▪Thrusters
RW generates a gyroscopic stiffness in the axis spin.
The disturbing torque is estimated from the sensor output
and by changing the wheel speed the axis is made stable.

17. 17
Change Over
✓When the maximum speed of the reaction wheel is reached and
still more torque is required then the thrusters are used.
✓This change over is done by gradually reducing the wheel speed
and increasing thrust by thrusters.
✓Once wheel speed is reduced to zero and thrusters have enough
torque to cancel the disturbing torque, thrusters are switched off.
Reaction wheel can now take over for further correction in future.
This is called Momentum dumping in space.

18. 18
Thrusters
It produces force by expelling materials through nozzles.

19. 19
Thrusters
▪Thrusters have capability to generate torque of few
N to tens of N.
▪Small thrust of fraction of N are generated by
variable duty cycles of on/off pulsing the thrusters.
▪With 1 meter length and Force of 10
-4
to 10
-1
N,
Torque of 10
-4
to 10
-1
Nm can be realized.

20. 20
Thrusters
▪Fuel is Hydrazine, N
2H
4
▪ Catalyst is iridium
▪Oxidizer is nitrogen tetroxide, N
2O
4

21. 21
Orbit Control
▪The satellite body in pitch axis is spun to
get gyroscopic stiffness
▪For sufficient torque 300 to 1000 rpm
may be required
▪But then the communication antenna
needs to be low again
▪Cylindrical portion with solar panels are
spun and the rest of the body is de-spun
using bearing like slip ring
This is called spin stabilized satellite

22. 22
Spin Stabilized Satellite
▪Spin rate decay with time and period
corrections are necessary.
▪NS station keeping by the thrusters
parallel to spin axis
▪EW station keeping by thrusters
perpendicular to spin axis
▪ Spin stabilize satellite has advantage of
simplicity in satellite structure
▪But the solar panel size is more and half
the portion does not see the sun

23. 23
Three axis body Stabilized Satellite
▪To generate the stiffness, inside the satellite body three
momentum wheels are used.
▪Rest of the structure of the satellite is fixed with the three axes of
the momentum wheel.
▪Advantage is easy orientation of antenna and solar panel could
be made of optimal size.
This is called 3 axis body stabilized system

24. 24
Telemetry Tracking and Command (TTC)
Functions of TTC:
▪To monitor the subsystem health and status parameters (Telemetry)
▪To support detecting the orbital parameters (Tracking)
▪To provide a source of earth station to track satellite (Tracking)
▪To receive and execute commands to perform required function (Command)

25. 25
Telemetry
▪Primarily telemetry subsystem monitors health and status of satellite
Bus and payload.
▪ For Remote sensing satellites, telemetry is also used for transmitting
observed data collected by sensors from earth/planet/star to ground
system.

26. 26
Tracking
To track the satellite from ground, look angle and range need
to be found.
Look angle for tracking is found with the help of separate
beacon signals transmitted by satellite system or from the
telemetry carrier.
Ground station uses tracking to determine look angle
precisely and thus to find the angular position of satellite.

27. 27
Tracking
Range or distance to satellite from earth station is found by
measuring round trip delay of a signal (tone or code).
Tone sent from ground station is received by satellite TTC
subsystem and retransmitted back to ground station.
On ground, phase delay difference of transmit and receive
signal is two times the range, as the signal travels the path
two times.

28. 28
Power
Primary energy source: Solar
Secondary source: Battery
Average Solar radiation = 1370 W/m
2
near earth.
Earth orbital eccentricity varies this value by about 89%.
Solar sells are commonly made of Si or GaAs.

29. 29
Power

30. 30
Power

31. 31
Power

32. 32
Power

33. 33
Battery Capacity

34. 34
Battery Capacity

35. 35
Communication Payload
Communication payload is a microwave repeater. Its
main function is to receive, amplify and transmit the
signal to earth.

36. 36
Communication Payload
Signal at the input of satellite is extremely weak due to
free space loss over long distance
What is the typical signal level received at satellite from
bank ATM VSAT?

37. 37
Communication Payload

38. 38
Communication Payload
Overall amplification required is more than 130 dB.
With this high gain, there is a high possibility of the
instability in the repeater.
Sufficient isolation is needed between receiver and
transmitter.

39. 39
Communication Payload
Nominal figure of this isolation is 40 dB more than the
gain 130 + 40 = 170 dB.
This isolation is achieved through:
-Frequency separation between transmitter and
receiver.
-Filtering
-RF shielding
-Orthogonal polarization between Tx and Rx.

40. 40
Spectrum allotted by ITU
1. Why up link and down link frequencies are widely separated?
2. Why down link frequency is lower than uplink frequency?

41. 41
Repeater
Functions of the repeater now are:
✓Receive signal from service area (Receive antenna)
✓Amplify only the required receive band (Filter & LNA)
✓Convert to a down link band (Mixture, LO, Filters, Amplifiers)
✓Amplify and remove the spurious (Power amplifier, Filters)
✓Transmit it to the service area (Transmit antenna)

42. 42
Input Bandpass Filter
▪It rejects the out of band signals that includes the
transmission from
-Same satellite
-Adjacent satellite
-Any out of band signal received from service area
▪Typical bandwidth is 500 MHz.

43. 43
LNA
▪The first amplifier provides about 20 dB gain to the very weak
signal and adds low noise that is why it is called LNA.
▪ More gain of the order of 30 dB or more are provided in the
subsequent amplifying stages to the input requirement of down
converter.
▪ Important specifications of RF amplifiers are:
-Noise figure
-Receiver sensitivity or minimum detectable signal level
-Gain
-Dynamic range

44. 44
Down converter and LO
▪It is non-linear device which mixes input signal with locally
generated signal to produce required down link frequencies.
▪To reduce unwanted harmonics, a BPF is put after the mixer
▪In some cases down conversion to IF at lower frequency is also
done and then signal is up converted. This is called double
conversion type repeater.

45. 45
Local Oscillator
▪Local oscillator base frequency is of the order of 10 to 100 MHz.
▪This is multiplied and amplified to generate the required LO
frequency for mixing. Stability of LO is typically 1ppm.
▪There is a band for a satellite. The uplink band is 5.625 GHz to
6.425 GHz. Downlink band is 3.4 GHz to 4.2 GHz. What will be
the LO frequency?
▪Important parameters for LO are:
-LO stability
-Oscillator phase noise
(short term random fluctuation in frequency or phase)

46. 46
Need for Frequency Segmentation
▪Generally power amplifiers cover complete band but gain
provided is relatively low. Also it is very non-linear.
▪ For large number of carriers input carriers, each carrier gets
only a small share from the power amplifier and large number of
inter modulation products are generated that increases the
noise in the wanted signal.

47. 47
Segmentation
▪To avoid these two constraints, the band is divided into several
sub bands called channel.
▪Each is provided with a separate power amplifier and then, all
signals are combined.
▪This part of sub band generation and signal handling is called
Transponder.

48. 48
Segmentation
▪Input sub-band formation is done through a set of BPF called
input (de)Multiplexer.
▪These filters should have high adjacent channel rejection and
low amplitude and phase ripple over the pass band.

49. 49
Power Amplifier
▪When power requirement of more than 20 watt, TWTA is used as HPA.
▪TWTA introduces nonlinearity.
▪Linearizers are used but that increases the complexity, weight and cost.
▪SSPA are used when lower than 20 W is required.
▪SSPA is less efficient compared to TWTA
▪But it needs less space, weight and lower voltage of operation.

50. 50
Power Amplifier
Critical specification for the power amplifier are:
▪Input power for output saturation
▪Gain variation
▪1 dB compression point
▪Third order intercept point
▪AM to PM conversion
▪DC to RF efficiency

51. 51
Output MUX
▪All HPA outputs are combined through another bank of BPF. This
is called output multiplexer.
▪This is high power and low loss filter
▪Output of OMUX is connected to transmit antenna.
Transponder
▪Consist of input MUX, Power amplifier and output MUX.
▪The overall specification of transponder is
-channel bandwidth
-adjacent channel rejection

52. 52
Communication Payload

53. 53
Regenerative Transponder
➢When there is requirement of base band data routing the
demodulation switching and remodulation is done after IF stage.
➢Uplink noise is not directly amplified and transmitted in the
down link
➢In a symmetrical or balanced link regenerative transponder has
certain 3 dB advantage over the bent pipe transponder.

54. 54
Regenerative Transponder

55. 55
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