Communication Pay Load

• Space and Ground segments

Architecture of a Satellite Comm System

Architecture of a Satellite Comm System• It comprises of a ground segment and a space segment • Space segment:

• Contains a satellite as well as terrestrial facilities for control and monitoring of Satellite

• It includes tracking, telemetry and command station(TT&C) together with the satellite control centre where all the operation associated with station keeping and checking the vital functions of the satellite are performed

• Uplink waves transmitted from earth station and received by satellite

• Downlink station transmitting to receiving earth station

• Link analysis Quality of radio link is specified by Carr-to-noise ratio. Quality of link from sta to sta is an important factor, discussed in detail in preceding lecs

• Multiple Access Satellite is a nodal point of network access to satellite or satellite transponder by several carriers implies the use of multiple access techs

• A satellite consist of payload and a platform• Payload consists of the Rx and Tx ants and all the

electronic equipment which supports Tx of carriers

Architecture of a Satellite Comm System

• Platform consists of all the sub systems which permit the payload to operate

• These include– Structure– Electric power supply– Temp control– Attitude and orbit control– Propulsion equipment– Tracking, telemetry and control (TT&C) equipment

Architecture of a Satellite Comm System

• The main role of Payload are:-– To amplify the received carriers for retransmission on

down link. Carrier power at the input of Satellite Rx is of the order of 100 pW to 1 nW. The carrier power at the out put of Tx Amp is 10 – 100 W. The power gain is of the order of 100 to 130 dB

– Change the freq to avoid re-injection in to receiver

Architecture of a Satellite Comm System

Payload functionality

• COLLECT microwave signals from given zone on earth• AMPLIFY radiofrequency carrier• CONVERT carrier frequency from uplink to downlink frequency• TRANSMIT microwave signals to given zone on earth

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• SATELLITE LINK MODEL– A Satellite System Basic Sections: Uplink, Satellite

Transponder, and Downlink• Transponder (Transmitter + Responder) Model

RF-to-RF Repeater

Tunnel Diode

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Contents

1 Introduction

2 Payload Function

3 Payload Constraints

4 Payload Specifications

5 Payload Configurations

6 Payload Equipment

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Communications Payload Function

Repeater

Uplink Downlink

Communications Payload = Antenna Sub-System + Repeater

Receive

Antenna

Transmit

Antenna

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Typical Repeater Functions• Receive and filter uplink signals• Provide minimum C/No degradation• Provide variable high gain amplification• Downconvert Frequency for re-transmission• Filter high power downlink signal and re-transmit• Provide high reliability in functionality• Beam-to-beam interconnectivity• Functional re-configurability• Beamforming

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Why High Reliability?• Everyone wants machines, tools, people, services to be reliable• What is special about Communications Satellites?• Inaccessibility of the orbits used

– LEO – Generally highly inclined– GEO – High altitude means: High potential energy AND High kinetic energy– Either way large high energy launch vehicles required

• Very expensive to launch in the first place• Inaccessible to astronauts or remote control vehicles• Repair by external intervention virtually impossible• The design must be tolerant of internal failures

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Pay Load Cosntraints• Accommodation

– Physical size, must fit on spacecraft platform, compatibility with launch vehicle fairing

• Thermal Dissipation– Limited ability of spacecraft to radiate heat, radiator area

• Mass– Impacts fuel, life, cost, functionality

• Power consumption– Impacts thermal design, mass of power sub-system

• Thermal Control– performance versus mass of thermal control hardware

• Received Noise– Thermal noise– Transmit ter Noise

• Includes: Passive Intermodulation Noise

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Quality of the Receive System – G/T

• The quality of the satellite receive system, in terms of its ability to receive a given signal with a high signal to noise ratio is usually expressed as: G/ T

• Where:G = Antenna Gain (Relative to that of an isotropic radiator and referenced

to an arbitrary interface at the utput of the antenna)T = The Noise Temperature of the complete System (Referenced to the

same interface at the output of the antenna)

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Noise Temperature

• Ts = Ta + T1 + T2 / G1 + T3 / (G1.G2) + T4 / (G1.G2.G3) ……...

• Ta = Antenna Noise Temperature

1 2 3 4

l Concatenation of Noise Sourcesl Ts = Noise Temperature of the Complete System

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Payload Constraints• Spurious Products

– Mixing products: From Frequency Converters– Intermodulation products: Non linearity in active devices– Passive intermodulation products (PIMP): Transmit chain, post High

Power Amplification– In Band: Directly impacts C/N0

– Out of Band: Interference to other transponders or systems

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Payload Constraints – Spurious Products

– Linear devices can be characterised by:Sout = aSin

– Memoryless Non-linear devices can be approximated over a limited signal range by a polynomial relationship such as:Sout = a1Sin + a2Sin

2 + a3Sin3 + a4Sin

4 + …If 2 signals are applied such that:Sin = Asinω1t + Bsinω2tThen Sout is found to contain frequency components as follows:ω1, ω2, (ω1 - ω2), (ω1 + ω2), 2ω1, 2ω2, (2ω1 - ω2), (ω1 - 2ω2), 3ω1, 3ω2…

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Intermodulation Products (2)• Order of a product is m = n + k for frequency nf2 - kf1 for 2 carriers• For many closely spaced carriers, IMPs are distributed contiguously• 3rd order products most important in band• (C/I3) multi-carrier = (C/I3) 2carrier - 8 dB

f1 f2

5 th Order Products

5x(f2-f1)3x(f2-f1)

f1 f2

3rd Order Product

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Intermodulation Products (3)

Type of product Order Number of products of thetype

N=5 N=10

2F1 – F2 3 N(N-1) 20 90F1 + F2 – F3 0.5N(N-1)(N-2) 30 3603F1 – 2F2 5 N(N-1) 20 902F1 + F2 – 2F3 N(N-1)(N-2) 60 7203F1 – F2 – F3 0.5 N(N-1)(N-2) 30 3602F1 + F2 – F3 – F4 0.5 N(N-1)(N-2)(N-3) 60 2520F1 + F2 + F3 – 2F4 0.5 N(N-1)(N-2)(N-3) 60 2520F1 + F2 + F3 – F4 – F5 0.5 N(N-1)(N-2)(N-3)(N-4) 120 15120Total 400 21780

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Intermodulation Products (1)

-20 -15 -10 -5 0-20

-15

-10

-5

0

-35

-30

-25

-20

-15

Input Back Off (dB)

Output Back Off (dB) IMP Level (dB)

N=1

N=3

N=10

F1+F2-F3

2F1-F2

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Non Linearities• Transmit Characteristics

– Gain vs frequency• Gain slope• Gain ripple

– Group delay vs frequency• Group delay slope• Group delay ripple

– AM/PM conversion– AM/PM transfer

• AM modulation of one carrier transferred to PM modulation of another

Repeater Architecture

General: Organization of repeater is based on mission and technological specs, large power gain and low noise temp over a wide bandwidth are desired in addition to frequency conversion• Low Noise Amplification and Frequency

Conversion: Mixer or freq converter has higher NF and requires amplification of the signal, LNA serves this purpose,

Characterization of Non Linarites• AM/AM Conversion Coefficient: slope of the Characteristic curve is called

AM/AM conversion, In the linear region it is =1, it decreases close to Sat, where its value is less than1

• Power Gain: ratio of Po to Pi, it is constant in the linear region and called small signal gain, gain decreases as saturation is approached, at sat, Gsat

• Point of Compression to I dB: The out put power obtained when the actual characteristic deviates by 1 dB from an extension of the linear region, this point corresponds to a reduction of 1 dB gain from Gsat

• AM/PM Conversion factor: The effect of non linearity also appears in phase, Kp = Δφ/ΔP1

i • Input / Output Power Back Off , already discussed• Transfer Coefficient Kt: In multicarrier operation non linear phase effects also

cause transfer of AM of one carrier into phase mod of other carriers, Kt is defined by the slope of the AM/PM curve

• Capture Effect: When power of one carrier at the input is lower than other by Δpi, and at output difference is Δpo w.r.t. other carrier, then capture effect Δ = Δpo /Δpi , it is always greater than 1.

Equipment Characteristics• Receiver: LNA at fu, followed by Frequency converter are accommodated in the same housing • at RF conductors are micro strip circuits produced by Photolithography, Aluminium is used as

substrate and covered by gold using vacuum thin film technology• at IF hybrid circuits are employed and un encapsulated active components are used,• size is generally limited to30x20x10 cm• power consumption is 5 to 15W• LNA at input is major contributor to G/T, initially Tunnel diodes were used, then parametric

amplifiers, FETs using GaAs and high mobility electron technology(HMET), for higher frequencies(30 GHz and above) for their low noise contribution

• Frequency Converter: frequency of LO = Fu- Fd, standardized for C band at 2.2 GHz, KU band at 1.5,2.58, or3.6 GHz, conversion loss= input power level/ output power level at the converter, gen 5-10 dB, Freq stability LO= +-1to+-5x10-6 at specific temp, frequency is obtained by multiplier circuits or by direct synthesis using VCO locked to Quartz reference freq

• Amplification after freq conversion, gen multistage amplifiers are used and a variable attenuator (PIN diode ) is used to control gain through tele command , over all receiver gain is 60-70 dB, this gain should be constant over the entire BW, ripple if any should not exceed 0.5 dB, requires meticulous matching- achieved through isolators/circulators- that dissipate waves reflected at the interface

Input Multiplexers(IMUX)• MUX is a passive device used to combine signals at different freqs from different sources onto a single output or to

route signals from a single source to different outputs. MUX are configured as interconnected high selectivity BPFs, performance depends on tech used, insertion loss, channel spacing and isolation etc

• IMUX: divides the total BW into sub bands, BPFs define the BW of various channels/ transponders, to ensure more channel spacing, separate batteries of odd and even Channels are arranged as shown in IMUX fig. Loss of MUX depends on number of time a signal passes through the isolator@0.1 dB , losses thus differ from channel to channel. The farthest transponder suffers max loss, this loss is compensated by the HPA

• OMUX: Recombines the transponder out puts after PA. Losses at OMUX are critical as these affect the EIRP directly. Coupling in this case is used by coupling the output filters to a common wave guide, short circuited at one end, thus characteristics of each filter influence the output of the entire system. Design and optimization of OMUX is more critical when guard bands are narrow. For certain application (back up sat) MUX with tuneable (tele command) BPFs are used.

• Major features of BPFs: Amplitude and group delays should be minimum, ripples in pass band to be min, sharp roll off, ripples cause spurious AM/PM and this degrades performance of demodulators at ES. Chebashev or Elliptic filters with several poles(4-8) are commonly used. Group delay equalizers are also used to achieve the desired performance.

• WG Cavity filters have high Q factor, Bi,tri and quadric mode filters have also been developed• Coupling of TE &TM mode filters is in progress, will offer new possibilities• In order to limit drift f0 below2.5x10-4 over the life time, dimentional variations of the cavity resonators must be

avoided, Al (coefficient of thermal expansion 22x10-6) is being replaced with C (1.6x10-6) • MUX using Surface Acoustic Wave(SAW) tech has small size, sharp cut off but group delay is nearly 25ns

Channel Amplifier(HPA)• Due to loss at the out put of IMUX/power splitter power is insufficient to drive the channel’s out

put stage, Channel Amplifier /driver gain (20-30 dB) compensates through linear behaviour. Gen Bipolar or FETs are used for this purpose.

• HPA Provides for power output for each channelNominal o/p power is defined at saturation, operating point is adjusted to control IM products, IBO is adjusted for max value of (C/No)T

• An imp parameter is its efficiency, is ratio of output RF power to electric power consumed by HPA

• Types of HPA commonly employed • TWTA, impart Kinetic energy to the EM wave by an electron beam, suitably accelerated, finally

the electrons are collected by the collector, an electric power conditioner (EPC) generates assorted voltages required for the operation(upto 4000 V),its efficiency is 80 % but overall efficiency is 40 %, total mass is 2.2 Kg(TWT o.7 & EPC 1.5 Kg),

• Main features, power at saturation=8-50W, Efficiency=40-50%, (C/N)IM at saturation =10-12 dB, AM/PM conversion coefficient=4.50/dB

• SSA, FETs used, operating freqs and power delivered constantly improving, initially used for C , now Ku band, Power output= 10 W at 4 GHz, efficiency 20-30%, Gsat 50 dB, (C/N)sat 14-18dB, AM/PM conversion coefficient= 2o /dB power supply provides bias voltages, efficiency is 85-90%, mass1to2 Kg

• SSAs show more linear behaviour, more efficiency, lower power levels so far

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Payload Configurations - Channelisation

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Payload Configurations - Redundancy

Sw

itch Netw

ork

Sw

itch Netw

ork

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Payload Configurations - Eutelsat 2

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Payload Configurations – Inmarsat 3

C-BAND

Rx HORN

LHCP

RHCP

C-BAND

RECEIVER

LHCP

RHCP

FORWARD

I.F.

PROCESSOR

L-BAND Tx

ANTENNA

BEAM

FORMEROUTPUT

NETW ORK

22 OFF

SSPAs

L-BAND TRANSMIT SECTION

L-BAND Rx

ANTENNA

22-OFF

LOW NOISE

AMPLIFIERS

RETURN

COMBINER

RETURN

I.F.

PROCESSOR

LHCP

RHCP

C-BAND

SSPAs OMUXLHCP

RHCP

C-BAND

Tx HORN

TT & C

~~~~~~

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Payload Configurations – TrendsMobile SS

MARECS INMARSAT 2 INMARSAT 3 INMARSAT 4

Payload Mass (Kg) 100 130 208 932

Payload Power (W) 500 660 1725-2138 9000

Design Lifetime (Years)

7 10 13 13

Launch Periods 1981-84 1990-92 1996-97 2004

No of S/C in Series 3 4 5 2 + 1

FSS/DBS ECS EUTELSAT 2 HOTBIRD W3A

Payload Mass (Kg) 117 208 268 507

Payload Power (W) 638 2090 4188 6900

No Of Channels 12/14 16 20/22 50

Design Lifetime (Years)

7 8-10 12-15 12+

Launch Periods 1983-88 1990-95 1996-98 2004

No of S/C in Series 5 6 6 1

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On-board Processing – Why?l Beamformingl Beam-to-beam interconnectivityl Improved link performancel More flexibilityl Improved immunity to interferencel Multi-rate communicationsl Reduced complexity of earth stations

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On-board Processing – Why Not?l Power dissipationl Massl Thermal dissipationl Packagingl Radiation hardnessl Reliabilityl Difficult to make “Future Proof”l Should not do processing onboard which could be done on the

ground by reconfiguring the overall system

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l Transparent- Channel to beam routing flexibility in multi-beam coverage- Uplink to Downlink frequency mapping flexibility- Channel Bandwidth flexibility

l Regenerative- Independent optimisation of uplink and downlink access,

modulation and coding- Link advantage through isolation of uplink and downlink noise

and interference effects- Data rate conversion and signal reformatting- Packet level switching- Security features

Transparent Or Regenerative

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• SATELLITE LINK MODEL– A Satellite System Basic Sections: Uplink, Satellite

Transponder, and Downlink• Downlink Model

Architecture of a Satellite Comm System• Ground / Earth Station:

• Contains earth Sta’s , End user eqpt • Vary in size, 30m dish (INTELSAT Network) to 0.6m

dish (Dir television receiving station) • It contains Major sub subsystem of Ground Station :-

a) High Power Amplifier (HPA)b) Solid State Power Amp (SSPA)c) Modem Sub Systemsd) Antenna Sub Systemse) Power Sub Systems

Earth Station

Earth Station Architecture