Satellite Payloads

Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada

Slide Number 1Rev -, July 2001

Vol 2: Communication Satellites

12 ChannelInput

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Satellite PayloadsSection 3



2.3 Satellite Payloads

2.3.1 Introduction

2.3.2 Payload Types

2.3.3 Payload Units

2.3.4 Payload Testing


Satellite Payloads



2.3.1 Introduction

Satellite communication payloads form part of a wireless telecommunication system, not unlike terrestrial (ground-based) wireless telecommunication systems.

Satellite payloads function to receive, process and transmit radio frequency (RF) waves in the same way as terrestrial microwave relay towers.

One key difference lies in the fact that the payload hardware in a Geosynchronous Earth Orbit (GEO) cannot be serviced, repaired or replaced after launch, so reliability is paramount.

Sec 3: Satellite Payloads

Introduction




2.3.1 IntroductionSec 3: Satellite Payloads

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uplink beam

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payload

Introduction


Figure 2.3.1a Introduction



2.3.1 Introduction

All conventional communication satellite payloads perform the same basic functions:

• Receive signals from the Earth (uplink beam)

• Separate, amplify and recombine the signals

• Transmit the signals back to the Earth (downlink beam)

These basic functions resemble a “bent-pipe” in the sky, more appropriately named a “repeater”.

Some advanced payload functions include digital signal processing, also called “regenerative and non-regenerative on-board processors”.


Introduction




2.3.1 Introduction

Unlike ground based wireless systems that are limited to providing point-to-point, line-of-sight connectivity due to the curvature of the Earth, satellite systems can provide instantaneous wide-area network (WAN) connectivity of an entire hemisphere.

This means that satellite communication systems are capable of providing different types of connectivity to the end user.

The World Radiocommunication Conference (WRC) Service Categories assigned for satellite communications are:

• Fixed Satellite Services (FSS) where signals transmit to and from a limited number of fixed locations on the ground.

• Broadcast Satellite Services (BSS) or Direct Broadcast Service (DBS) where signals transmit directly to every subscriber.

• Mobile Satellite Services (MSS) where signals transmit to and from mobile terminals and/or fixed gateways.


Introduction





IntroductionThe use of FSS frequency bands has been expanding to include some BSS applications.

Because of the very wide coverage areas and the variety of communication services, interference from neighboring satellites can degrade the quality of service.

For this reason, the RF frequencies and power levels for each service type must be properly coordinated for operation from the chosen orbital slot.

Some frequency co-ordination activities need to be revisited frequently.




2.3.1 Introduction

The International Telecommunications Union (ITU) recommended frequency assignments for satellite communications developed at World Administrative Radio Conference’s WARC-85 are listed as follows:

Sub Band Designation Frequency Range

L Band 1.5 - 1.6 GHz

S Band 2.5 - 2.6 GHz

C Band 3.4 - 4.2, 5.9 - 6.7 GHz

Ku Band 10.7 - 14.5, 17.3 - 17.8 GHz

Ka Band 18.3 - 22.2, 27.0 - 31.0 GHz


Introduction




2.3.1 Introduction

Many C- and Ku-Band payloads occupy a total bandwidth of 500 MHz. Each payload consists of a number of channels, also called transponders. Operating bandwidth of each channel is typically:

• L - Band: 1.7 and 3.4 MHz

• C - Band: 36, 41 and 72 MHz

• Ku - Band: 24, 27, 36, 54, 72, 77 and 150 MHz

• Ka - Band: 250, 500 and 1000 MHz

Each channel can be used to carry 1 signal or many signals, each with a reduced bandwidth.


Introduction




2.3.1 Introduction

Because of operating frequency and bandwidth limitations, payloads typically employ frequency reuse schemes to maximize the system capacity.

Spatial frequency reuse is accomplished by using multiple uplink/downlink beams each dedicated to different coverage areas.

Spatial frequency reuse is typically used for MSS and intercontinental traffic and is very effective for providing dedicated or switchable inter-beam connectivity.


Introduction




2.3.1 Introduction

Within each beam/coverage area, frequency reuse is accomplished by using orthogonally polarized beams.

• Linear polarization schemes use vertical and horizontal electric field (e-field) beams.

• Circular polarization schemes use left and right hand circularly rotating e-field beams.

• The choice of polarization scheme affects the design and cost of the ground terminals, ease of ground installation, adjacent satellite interference and cross-polarization interference.


Introduction





Introduction


Figure 2.3.1b North American Up and Downlinks




Figure 2.3.1c North American Up and Downlinks

Introduction




2.3.1 Introduction

Coverage refers to the uplink and downlink beam patterns created on the Earth by the satellite receive and transmit antennas.

Coverage can be tailored to any predefined shape using conventional antenna reflector and feed technology.

Some examples of coverage beams include global, international, national and spot beams.

Multiple coverage area systems can provide dedicated or switchable inter-beam connectivity.


Introduction




2.3.1 Introduction

National Coverage Beam


Figure 2.3.1d National Coverage Beam





International Coverage Beam

Figure 2.3.1e International Coverage Beam





Spot Coverage Beams

Figure 2.3.1f Spot Coverage Beams





Types of TrafficAnalog Signals (almost phased out in North America)

• Television

• Telephony (Asia)

Digital Signals• Television (QPSK) compressed

• VSAT (QPSK)

• High data rate (8-QPSK)

• Satellite news gathering (QPSK)

• Date for multiple user systems (TDMA)

2.3.1 Introduction




Certain functions in the payload are required to be controlled from the ground in order to optimize and maintain the service. Ground control of this nature is called “commanding”.

Likewise, certain indicators of performance are required to be monitored on a continual basis from the ground in order to optimize and maintain the service. This is called telemetry.

Fundamental telemetry parameters include:• Unit on/off status• Unit temperatures• Transponder channel gain setting status• Power amplifier health status parameters (i.e. helix or gate current, DC current and

anode voltage)• Antenna pointing position (if applicable)


Introduction

2.3.1 Introduction




2.3.2 Payload Types

FSS, BSS and Advanced Payloads

All types perform the same basic functions:

• Receive communication signals from the Earth (uplinks)

• Amplify the uplink signals and downconvert the frequency

• Separate the downconverted signals into channels

• Amplify the channelized signals

• Combine the amplified channels into a downlink signal

• Transmit the downlink signal to the Earth


Payload Types




To accomplish these functions, conventional payloads typically comprise the following major units:

• Receive and Transmit Antennas

• Input Filters

• Receivers

• Input Multiplexers

• Redundancy Switch Networks

• Transponder Amplifiers

• Output Multiplexers


Payload Types

2.3.2 Payload Types




2.3.2.1 FSS Payloads

FSS C-Band Payloads

• Arabsat 2A @ 26º E, Arab States

• Anik E1, 30:24 @ 11.5 W Canada and CONUS

• Anik F1, 32:24 @ 40 W North and South America

• Galaxy 10, 30:24 @ 40 W North America

• GE 4, 2X 16:12 @ 20 W US

Part 2: Payload Types

FSS Payloads

Figure 2.3.2.1a

Picture Courtesy of Telesat Canada

Vol 2: Communication Satellites, Sec 3: Satellite Payloads



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FSS C-Band Functional Block Diagram

Figure 2.3.2.1b FSS C-Band Functional Block Diagram

2.3.2.1 FSS PayloadsPart 2: Payload Types




FSS Ku-Band Payloads

• Arabsat 3A @ 26º E, Arab States

• Anik E1, 18:16 @ 50 W Canada and CONUS

• Anik F1, 58:48 @ 115 W NA and SA

• Galaxy 10, 30:24 @ 108 W NA

• GE 4, 2X 18:14 @ 110 W US

FSS Payloads

Figure 2.3.2.1c






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FSS Ku-Band Functional Block Diagram

Figure 2.3.2.1d FSS Ku-Band Functional Block Diagram





Receive (Rx) and Transmit (Tx) Antennas:The function of the Rx antenna assembly is to collect the signals in the uplink beam and direct them into the payload.

Likewise, the Tx antenna functions to send the signals from the payload down to the Earth in the downlink beam.

Each antenna assembly typically comprises a reflector and a feed horn as a minimum, although other types of antennas are also used.

FSS Payloads





In addition to a reflector and a feed horn:

• A dual polarization antenna assembly requires a device to separate and combine the two orthogonally polarized beams called an orthomode transducer (OMT) for linearly polarized beams and a polarizer for circularly polarized beams, and

• A combined Rx/Tx antenna assembly requires a device to separate the two frequency bands called a diplexer.

FSS Payloads





Input Filters:Input filters function to remove any unwanted signals from the uplink beam while permitting the wanted signals to pass into the receiver.

The receiver and the performance of the payload are sensitive to out-of-band signals so the input filters are typically comprised of:

• A bandpass filter to reject near band signals

• A lowpass filter to reject far out-of-band signals

FSS Payloads





ReceiversThe functions of the receiver are:

• To amplify the uplink signal while suppressing the noise

• To downconvert the uplink signals to the downlink frequency band (e.g. C Band from 6 to 4 GHz, Ku Band from 14 to 12 GHz)

Receivers typically provide approximately half of the total required transponder gain.

Receiver’s noise figure dominates the payload noise figure or G/T performance.

Receivers typically comprise:• A low-noise amplifier (LNA) and a downconversion mixer with a

local oscillator

FSS Payloads





Input Multiplexers (IMUXes)The function of IMUX is to separate the individual signals from the 250 - 500 MHz downconverted uplink beam into narrow band channels (e.g. 27, 36 or 54 MHz).

The key device in the IMUX is the high order bandpass filter.

Typical IMUX designs configure the filters in a non-contiguous (i.e. non frequency adjacent) arrangement using channel dropping circulators.

Basically, there are two types of IMUXes (i.e. waveguide or dielectric loaded).

FSS Payloads







FSS Payloads

Figure 2.3.2.1e C-Band Dielectric Resonator IMUX

Supplied courtesy of COM DEV Space (proprietary)





FSS Payloads

Figure 2.3.2.1f Ku-Band Dielectric Resonator IMUX




Redundancy Switch Networks

Electro-mechanical switches comprise an actuation mechanism to switch the RF transmission paths from port to port.

Typically, high power switches have waveguide RF paths and low power switches have coaxial RF paths.

There are various switch configurations used for both types including:

• Waveguide C (2 position) and R (3 or 4 position)

• Coaxial C (2 position) and T (3 position)

FSS Payloads





Transponder AmplifiersTransponder Amplifiers typically consist of two amplifier stages and a common Electric Power Conditioner (EPC):

The first stage is the Driver Amplifier (DA)• Typically, the DA is a high gain, low power, broadband, solid state

amplifier

• The DA provides the commandable gain control for the transponder

• Some DA units also have an automatic level control circuit that maintains the output signal level constant as the input signal level varies over a large range

FSS Payloads





The second stage is the Power Amplifier (PA)• Typically, the PA is a high gain, high power, broadband amplifier

• The PA provides the RF power required for the downlink EIRP

• Some PA units also have a linearizer that functions to optimize the phase and amplitude

• Depending on the output power level and frequency band, PAs fall into two different designs:

• Travelling Wave Tube Amplifier (TWTA)

• Solid State Power Amplifier (SSPA)

FSS Payloads





The power supply for both amplifier stages is provided by the EPC.

• The EPC provides the required voltages for the PA (5 V for SSPAs and up to 7 kV for TWTAs) from the bus.

• For TWTAs, the EPC typically has circuitry that protects the amplifiers from the effects of microdischarge events that occur in-orbit.

• If a large number of TWTAs are flown, it is common to have one EPC provide power to a pair of DAs and TWTAs and this is called a dual EPC configuration.

• For SSPA designs, it is common to house the DA and EPC with the PA all in one housing.

FSS Payloads





TWTAsMajor components in the TWT are:

• The electron gun, containing a cathode and an anode assembly, which produces a high density electron beam.

• The slow-wave or delay line circuit that supports a travelling wave of electromagnetic energy that interacts with the electron beam.

• The collector which collects the spent electron beam emerging from the slow-wave field.

• Packaging hardware. This provides a means of attaching the beam focusing structure and the cooling system for power dissipated within TWT.

FSS Payloads





• The slow-wave circuit usually employs a step velocity taper helix

• The collector employs a multi-stage (i.e. 3 or 4 stages) design with thermal conduction to a cooler outside surface

• The EPC supplies power to TWT, provides protection circuits and the command and telemetry data

• The key TWTA performance specifications are:• RF Output Power: 10-250 Watts

• Saturated Gain: 50-60 dB and Efficiency : 55-65 %

• Weight: ~ 2.5 - 3.5 Kilograms

FSS Payloads





SSPAsSSPAs have been available since late 1970’s and started in commercial satellite services in early 1980’s.

The SSPA capability depends on the performance of the output stage transistors and the efficiency of the combining techniques.

The types of transistor typically used are Gallium Arsenide (GaAs) Field Effect Transistors (FETs) or High Electron Mobility Transistors (HEMTs).

These devices can provide sufficient gain and power-added efficiency for high power modules. However, GaAsFET transistor output power is limited by the device’s gate-width, gate-length and breakdown voltages.

FSS Payloads





Typically, SSPAs have the EPC and DA units integrated directly into the same housing as the high power amplifier stages

Typical SSPA performance specifications are:• RF output power: 5 - 40 Watts

• Saturated Gain: 55-65 dB and Efficiency: 20-40 %

• More linear than TWTAs

• Weight: ~ 1.5 - 2.5 Kilograms

FSS Payloads





The Output Multiplexer (OMUX)The function of the OMUX is to combine the channelized, amplified signals and direct the signals to transmit antenna input port.

OMUXes typically comprise high power input isolators, lowpass or harmonic reject filters, high power, low order bandpass filters, a waveguide manifold and high power switches.

Some designs also employ a high power isolator and/or a high power receive band reject filter at the OMUX output.

FSS Payloads





OMUXes can be designed to provide contiguous (i.e. frequency-adjacent channels 1, 2, 3, 4, …) or non-contiguous (i.e. non-frequency-adjacent channels 1, 3, 5, …) channel performance.

Typically, OMUX channel filters are fabricated with a temperature-stable metal to minimize the filter’s sensitivity to temperature changes.

Some designs incorporate a temperature compensation mechanism to minimize the temperature effects and assist in the dissipation of heat generated in the loss mechanisms of the OMUX.

FSS Payloads





C-Band OMUX



Figure 2.3.2.1i C-Band Dielectric Resonator OMUX




Ku-Band OMUX



Figure 2.3.2.1j Ku-Band Temperature Compensated Dielectric Resonator OMUX




2.3.2.2 BSS Payloads

• NIMIQ, 44:32 @ 120 W or 22:16 Combined @ 230 W NA

• Echostar 4, same as NIMIQ

• USDBS 3, 32 Channels @ 120 W US

• Koreasat 3, 9:6 @ 120 W

BSS Payloads

Figure 2.3.2.2a NIMIQ, A Broadcast Satellite






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Figure 2.3.2.2b BSS Ku-Band Functional Block Diagram

2.3.2.2 BSS PayloadsPart 2: Payload Types




Basically, BSS payloads are the same as FSS except that they either employ higher power PAs or output power combining circuits that function to “boost” the output power by pairing PAs.

This is done to provide a higher EIRP which translates to a higher PFD at the Earth, so smaller (i.e. 45 cm) dishes can be used by each subscriber.

Typically, BSS payloads comprise FSS powered TWTAs with additional power combining hardware (the boost assemblies) that effectively doubles the EIRP for up to half of the channels.

BSS Payloads





The power combining hardware typically comprises:

• Input boost assemblies (IBAs) that split the DA output in order to equally drive a pair of TWTAs

• Phase adjusters that can be used to phase match the split DA signals such that the TWTA output signals have a known phase relationship

• Output boost assemblies (OBAs) that phase combine the TWTA output signals to, effectively, double the EIRP

• Due to practical limitations, the power usually only increase by 2.7 dB or a factor of 1.86

BSS Payloads





2.3.2.3 FG vs. ALC Mode

Fixed Gain (FG) and Automatic Level Control (ALC) Modes refer to the two different gain control methods for the DA.

In the FG mode, the DA is commanded to a specific gain setting, regardless of the uplink PFD.

In this state, the DA drives the PA proportionally to the the input signal received by the DA.

The FG mode is effective for multi-carrier operation from multiple uplink sites.

But, uplink PFD fluctuation and any gain frequency variation in the receive common input section causes the operating point of the PA to change, even if the PFD is ideally set for saturation of the PA (i.e. SFD).

FG vs. ALC Mode





In the ALC—also referred to as Automatic Gain Control (AGC)—mode, the DA is commanded to a specific output power level setting, regardless of the uplink PFD.

In this state, the DA samples its own output power and automatically changes the gain in order to achieve the commanded output power level.

Thus, the power amplifier is driven with at a constant operating point because any PFD fluctuation and gain frequency variation is removed by automatically controlling the gain.

The ALC mode is very effective for single carrier, steady state operation at PA saturation.

FG vs. ALC Mode

2.3.2.3 FG vs. ALC ModePart 2: Payload Types




Certain frequency bands (Ku and Ka) are typically very susceptible to atmospheric attenuation fluctuations (e.g. rain fade) while others (L, S and C) are less susceptible.

Thus, ALC mode is a typical requirement for Ku- and Ka-Band payloads, especially if steady state operation at saturation is a required application.

Sufficient commandable and dynamic range of operation is required in FG and ALC mode to account for the uplink dynamic range settability, thermal noise contributions and PA aging effects.

FG vs. ALC Mode

2.3.2.3 FG vs. ALC ModePart 2: Payload Types




2.3.2.4 Advanced Payloads

Advanced payloads have been designed for 3rd generation of MSS and for multimedia FSS.

The lack of spectrum at L-Band, coupled with the desire to provide service directly to small, hand-held user terminals, requires the use of multiple beams (i.e. up to and sometimes over 100 beams) and digital processors to interconnect them.

The large throughput requirement for multimedia and small user terminals also requires multiple beams and digital processing.

Advanced Payloads






Main Design Features• Complex multiple beam antenna system with analog or digital

beam forming.

• Large number of receivers and amplifiers in comparison to conventional payloads.

• Additional level of downconversion to baseband and upconversion to Ku- or Ka-Band.

• Use of regenerative or non-regenerative on-board processor to interconnect the user’s terminals to gateways and gateways to user terminals (some systems also provide user-to-user connection).

Advanced Payloads






• Regenerative processors are used in a technique that reconstructs the original digital signal before transmission back to Earth.

• As with regeneration in terrestrial links, noise and distortion imparted on the uplink will not be present in the downlink.

• Non-regenerative processors do not reconstruct the original digital signals (i.e. uplink noise and distortion will be present in the downlink). Thus non-regenerative payloads are said to have “bent-pipe” architecture.

Advanced Payloads






RCVR

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A/D Demux

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Horizontal Polarization Horizontal Polarization

Vertical Polarization

TDMA


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120 MHzRCVR A/D Demod/Decod

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Figure 2.3.2.4a Regenerative Digital Processing






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17 Transponders 17 Transponders

Figure 2.3.2.4b Non-Regenerative Digital Processing






MSSs were first able to use an advanced payload because of their relatively low data rates (i.e. 2.4 to 64 kbps).

Application Specific Integrated Circuit (ASIC) power consumption and size reduction during the past few years has made higher rate, multimedia payloads possible.

• The required speed of the ASIC is directly impacted by the bandwidth that requires processing (i.e. the larger the bandwidth, the more ASICs are required).

Advanced Payloads






The power consumption and mass of digital processors are directly proportional to the overall throughput of the payload.

Main differences between MSS and Multimedia:• Data rate of Multimedia is much higher (i.e. 2.4 Mbps)

• Traffic is asymmetric in multimedia, with higher traffic from gateway to user

• Multimedia will require data rate conversion on-board, in addition to format conversion, so it requires regenerative processors to demodulate, process, and remodulate for transmission

• For MSS, the uplink signal is identical to the downlink with no need for a format conversion or data rate conversion, so it utilizes non-regenerative processors

Advanced Payloads






Overall Assessment• Advanced payload capability is much greater than that of

conventional payloads

• Advanced payloads are more complex, larger, heavier and require higher power

• Advanced payloads represent deployment of new technology, thus presenting high risk

• Advanced payloads are more expensive than conventional payloads

Advanced Payloads





2.3.3 Payload Units

Payloads consist of three different types of “units” or “devices” that introduce different levels of risk for in-orbit operation:

Passive RF units:• Do not require the application of DC power to operate

• Cause the RF signal passing through to lose power

• This loss of RF power produces heat. This is called RF heating

• Do not typically exhibit wear-out or life-limiting features, so redundant units are not typically provided


Payload Units




2.3.3 Payload Units

Active RF units• Require the application of DC power to operate

• Cause the RF signal to either lose or gain power

• RF losses generate RF heating as does the consumption of DC power

• Typically exhibit wear-out or life-limiting features, so redundant units are usually provided

On-Board Processors• Can be analog active intermediate frequency (IF), RF

processors, or digital processors


Payload Units




2.3.3.1 Passive Low Power Units

These units typically have the lowest operating power levels in the payload, and the most benign environmental effect on the spacecraft.

Because of this, these units typically present the lowest risk for in-orbit operation.

These units include:• Input filter assemblies (IFAs), hybrid couplers, circulators and

isolators, input multiplexer (IMUX) assemblies, attenuators and phase adjusters, switches and input switch networks (ISNs), low-level beam-forming networks (BFNs), interconnecting waveguide and coaxial cable

Passive Low Power Units

Part 3: Payload Units




2.3.3.2 Active Low Power Units

These units typically have moderate operating power levels. They typically comprise components such as transistors, capacitors, Monolithic Microwave Integrated Circuits (MMICs) and hybrids that present a risk of failure in-orbit.

These units provide most of the required signal amplification in the satellite and perform all of the frequency down conversion and analog signal processing functions, so they typically present low to medium risk for in-orbit operation.

Active Low Power Units

These units include:• low noise amplifiers (LNAs), down converters, driver amplifiers (DAs)

with commandable gain controls, limiters (LIMs) and linearizers (LINs), ferrite and solid state switches and switch matrices, analog on-board processors including Surface Acoustic Wave (SAW), IF and RF signal processors





2.3.3.3 Passive High Power Units

These units typically have the most stringent operating power levels and environmental conditions in the spacecraft.

This is because the higher the RF power, the higher the RF heating, and the higher the operating temperature.

Also, RF heating can increase dramatically as the signal frequency drifts away from band-centre toward the band-edge (this is known as a “band-edge carrier”).

Furthermore, units that pass multiple channels will exhibit a proportional increase in the RF heating (i.e. if one channel causes 10 W of RF heating, then 8 channels would cause an average of 80 W of RF heating).

Passive High Power Units






In units that pass multiple channels, the signals can superimpose upon each other in a manner in which their total RF power briefly reaches peak levels that are much higher than the average.

In these cases, the increase is proportional to the square of the number of channels (i.e. from the earlier example of 8 channels, the increase is 82 = 64 times).

This peak power level is not typically sustained long enough to increase the RF heating, but it can lead to a vacuum breakdown phenomenon known as multipaction that can cause a temporary interference to the signal or even permanent damage to the unit.







Passive high power units are subjected to several potentially damaging operating conditions that must be precluded by:

• Proper design

• Proper fabrication by special materials and processes

• Proper testing

• Proper in-orbit operation


Since the industry trend toward higher downlink EIRP directly translates into higher RF power in these devices, the technology is continuously being driven to its limit.






Because of these stringent operating and environment conditions, and the industry trend towards higher RF power, these units present low to medium risk for in-orbit operation.

These units include:• output receive reject filters, harmonic filters, power dividers and

combiners, circulators with remote loads or isolators, output multiplexer (OMUX) assemblies, output switch networks (OSNs), high-level beam-forming networks (BFNs), coaxial connectors, receive/transmit diplexers, antenna feed horns, orthomode transducers (OMTs), polarizers and interconnecting waveguide






2.3.3.4 Active High Power Units

These units have the most stringent operating power levels and environmental conditions in the spacecraft. They require a large amount of DC power to operate.

These units and their EPCs are susceptible to performance degradation and/or wear-out over the life of the satellite.

These units are susceptible to RF and DC power consumption heating effects and peak power effects.

Active High Power Units

Moreover, the performance and reliability of these units significantly depends on the RF power operating points that are used.

With higher amplifier RF powers being used, operation above the well defined and safe operating point can introduce life-limiting damage to these units.






Because of these stringent operating requirements and their susceptibility to damage and wear-out, these units present medium to high risk for in-orbit operation.

These units are the power amplifiers, which can be:

• Travelling wave tube amplifier assemblies, or

• Solid state power amplifiers







TWTA risk issues:• In-orbit experience (has the chosen model seen orbit, and if

so, how did it perform?)• Heritage and design changes from heritage units (is the

chosen model based on an earlier model, and if so, how has the new model been changed?)

• Qualification and life-testing (was is thoroughly tested?)• In-orbit anomalies and corrective actions taken (if it

experienced problems in orbit, were these correctable?)• Overdrive and ESD susceptibility (how “delicate” is the

chosen model with respect to typical forms of misuse and damage?)

• Output power, and telemetry (helix current, DC current, anode voltage) anomalies (is this unit capable of reporting on itself reliably?)







SSPA risk issues:

• In-orbit experience

• Heritage and design changes from heritage units

• Qualification and life-testing

• In-orbit anomalies and corrective actions taken

• Power stage design (GaAs FET vs. BJT)—strengths and weaknesses in each design

• Overdrive, multi-carrier traffic and ESD susceptibility

• Burn-in and screening is paramount






2.3.3.5 Overview of Digital Processors

Two Types of Digital Processors:

• Regenerative: Where the original information is recovered on-board the spacecraft by demultiplexing and demodulating the signal

• Non-regenerative: The signal is not demodulated on-board, only demultiplexed for switching circuit by circuit

Non-regenerative processors are ideal when uplink and downlink data rates are identical and the same format is used.

Overview of Digital Processors






Main Functions• Interconnect large number of inputs to a number of outputs

according to ground commands or according to information located within the signal (regenerative)

• Performs data rate conversion

• Performs format conversion

• Power level measurement for uplink power control at Ka-Band

• Synchronization of TDMA networks







CircuitSwitch

OverheadsProcessor

Command& SignalingProcessor

PCRInsertion

A/D DecodeDemod

DVB-S Demodulator - 33 MHz

IFto

Baseband

A/D Demux DecodeDemodIFto

Baseband


Baseband


Baseband


Baseband

1

2

3

18

DVB-SModulator

ForwardData

HandlerD/A

Basebandto

Ka-Band

DVB-SModulator

ForwardData

HandlerD/A

Basebandto

Ka-Band

DVB-SModulator

ForwardData

HandlerD/A

Basebandto

Ka-Band

DVB-SModulator

ForwardData

HandlerD/A

Basebandto

Ka-Band

Figure 2.3.3.5 Regenerative On-Board Digital Processor






Main Components• Analog-to-Digital (A/D) Converters

• Application Specific Integrated Circuit (ASIC)

• Random Access Memories (RAM)

• On-Board Processor (OBP) Controller, Command Controller

• Internal or external power supply unit







EvolutionFirst commercial non-regenerative processor deployed was Skyplex on-board Hotbird-4 and 5. No switching was involved, only a multiplexing function.

The Asian Cellular Satellite (ACeS) was the first non-regenerative processor with only digital components with the exception of A/D converters, called hybrids.

Power consumption of ACeS ASICs is approximately 0.5 micro watt per MHz per gate. Federal System is offering ASIC with 0.02 micro watt and Honeywell is offering 0.06, a reduction by 10 to 20 times in 4 years.







Design Requirements• ASICs are vulnerable to Single Event Upset (SEU) so they

must be radiation-hardened

• ESD protection is required

• Clock distribution and timing disruption could lead to serious problems

• Processor needs to meet performance specification in addition to functional requirements







Digital Processor Units Performance specifications, such as implementation losses, can be measured during integration.

Functional requirements require a much more elaborate test set-up:

• Terminals

• Command Link

• Gateways

• Extensive test equipment such as signal/ATM cell generators






2.3.4 Payload Testing

Beyond design integrity, proper material and process selection and well controlled fabrication techniques, the integration and testing (I&T) of payload equipment provides the last opportunity for payload risk mitigation prior to launch.

I&T typically comprises very detailed and procedural operations at 3 distinctive levels throughout spacecraft construction:• Unit level I&T

• Payload subsystem level I&T (i.e. prior to bus mate)

• Spacecraft level Testing (i.e. post bus mate)


Payload Testing




2.3.4.1 Unit Level I&T

Payload units must be tested extensively in order to mitigate in-orbit insurance risk.

For brand new unit designs with no flight heritage, typically several models are fabricated and subjected to various levels of environmental and operational tests prior to fabricating the units that will actually be flown.

These tests inlude:• Engineering Breadboard Models (EBB or EM)

• Engineering Qualification Models (EQM)

• Life Test Models (LTM)

Unit Level I&T

Part 4: Payload Testing





Environmental and operational testing performed at these levels typically exposes the units to levels beyond the expected exposure levels in-orbit.

With the exception of LTMs that may continue to be tested even after launch, successful completion of these tests is typically required prior to fabricating the units that will be launched.

Unit Level I&T

The units to be launched are called Flight Models (FMs) and are typically tested to environmental and operational levels that are less severe than EQMs and LTMs, yet marginally more severe than the predicted in-orbit requirements.

Sometimes, the first FM in a batch of units is tested to environmental levels that are intermediate to FMs and EQMs. These units are called Protoflight Models (PFMs).






Environmental Testing includes:• Vibration, shock and acoustic tests

• Thermal and thermal vacuum tests

• Electromagnetic compatibility (EMC) tests

Operational testing includes:• Unit burn-in, RF power overdrive, under/over voltage, average and

peak RF power tests

• Performance testing is conducted at hot, cold and ambient temperatures

Unit Level I&T





2.3.4.2 Payload Subsystem I&T

Payload Subsystem I&T typically comprises:

• Payload sections I&T

• Payload Subsystem I&T

The Payload can be divided functionally into four subsections during integration:

(1) The Common Input Section comprises:

• receivers, input test coupler, input isolator, input filter assembly, RF switches, output hybrid, interconnecting waveguide and coaxial cables and select-in-test (SIT) attenuators

Payload Subsystem I&T






Payload Subsystem I&TCommon Input Section I&T comprises:

• Gain alignment for the primary and redundant receiver paths by choosing the correct SIT attenuator values

• RF path switching functional check

• Receiver DC current drain measurements once the unit is turned on

(2) The Second Section comprises:

• IMUXes, input redundancy switch networks, interconnecting coaxial cables and SIT attenuators






Payload Subsystem I&TThe Second Section I&T comprises:

• Equalizing the channel path losses by choosing the correct SIT attenuator values

• RF path switching functional check

• Input Group Delay and Frequency Response measurements

(3) The Transponder Amplifier Section comprises:

• DAs, TWTAs or SSPAs, interconnecting waveguide and coaxial cable, SIT attenuators and for BSS payloads, IBAs, OBAs and phase adjusters





Payload Subsystem I&TThe Transponder Amplifier Section I&T comprises:

• Gain alignment for the primary and redundant transponder amplifier paths by choosing the correct SIT attenuator values

• Phase adjuster alignment for TWTA pairing (BSS only)

• DA and PA DC current drain measurements after the units are turned on

• Command and telemetry functional checks

• Gain transfer measurements

2.3.4.2 Payload Subsystem I&TPart 4: Payload Testing




(4) The High Power Output Section comprises:

• OMUXes, harmonic/lowpass filters, output redundancy switch networks, output test coupler, high power output isolators and receive rejection filters and interconnecting waveguide

The High Power Output Section I&T comprises:

• A functional check of the RF switches






Once all 4 sections are integrated and tested, the Payload Subsystem is then subjected to its first end-to-end testing

Payload Subsystem End-to-End I&T typically comprises:• Command and telemetry function

• RF Leakage and Susceptibility (“Sniff and Spray”)

• DC power drain

• Receiver frequency translation

• Gain Transfer

• Input power to saturate

• Saturated output power






• Overall Inband and Out-of-band Frequency response

• Overall Group delay response

• Linearity

• Gain Control

• Inband and Out-of-band Spurious

• TWTA Helix current/SSPA Gate current telemetry

• TWTA Anode voltage telemetry

• RF continuity verification for all possible RF paths in the payload






Anik E I&T

Figure 2.3.4.2 Anik E I&T






2.3.4.3 Spacecraft Level Testing

After the Payload Subsystem is mated with the Bus Subsystem, the next phase is referred to as Spacecraft-Level Testing and it comprises several distinct test phases:

• Initial Spacecraft Test

• Spacecraft Vibration Test

• Spacecraft Thermal Vacuum Test

• Final Spacecraft Test

• Antenna Range and EMC Test

Spacecraft Level Testing





Anik F Just Prior to Spacecraft Level Testing

Phot

o Co

urte

sy o

f Tel

esat

Can

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Phot

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Figure 2.3.4.3 Anik F Prior to Spacecraft Level Testing

2.3.4.3 Spacecraft Level TestingPart 4: Payload Testing




2.3.4.3.1 Initial Spacecraft TestThe purpose of the Initial Spacecraft Test, also called the Initial Performance Test (IPT), is to establish a performance reference prior to Spacecraft environmental tests (vibration and thermal vacuum).

Typically, the test requirements resemble the Payload Subsystem End-to-End testing, and IPT is performed by:

• Terminating the payload output ports with either high power loads or by high power RF absorber boxes

• Calibrating the uplink and downlink test interface before performance testing





2.3.4.3.2 Spacecraft Vibration TestThe purpose of Spacecraft Vibration Testing is to verify the integrity of the Spacecraft mechanical structure after integration by subjecting it to a simulated launch environment

Typically:

• The antenna assemblies and solar panels are installed prior to the test

• The command and telemetry functional test is conducted before and after the vibration

• Payload performance testing is not done during this phase





2.3.4.3.3 Spacecraft Thermal Vacuum TestThe purpose of Spacecraft thermal vacuum test (SCTV) is to gather performance test data in a simulated space environment, to thermally exercise the recently integrated payload and to perform tests that mitigate the risk associated with the quality workmanship.

The SCTV phase typically consists of:

Performance Tests:• Ambient environment testing (also called open door testing)

similar to IPT

• Vacuum hot and cold plateau performance testing similar to IPT





2.3.4.3.3 Spacecraft Thermal Vacuum TestWorkmanship Tests:

• Thermal vacuum cycling

• Thermal balance to ensure that the payload units operate at their predicted temperatures

• Transponder small-signal gain monitoring through temperature transitions (i.e. hot to cold and cold to hot)

• Hot, high power soak monitoring in which several transponders are operated at the designed operating point simultaneously until thermal stability is achieved





Unit failure identified, unit was replaced andretest confirmed nominal operation

2.3.4.3.3 Spacecraft Thermal Vacuum Test

Figure 2.3.4.3.3a Small Signal Gain Monitoring Test





Latent workmanship problem found (sudden drop in the RF power). Contamination was found inside one of the RF connectors. The unit was replaced and retest confirmed stable hot operation.

Figure 2.3.4.3.3b Hot, High Power Soak Test Data



2.3.4.3.3 Spacecraft Thermal Vacuum Test



2.3.4.3.4 Final Spacecraft TestThe purpose of the Final Spacecraft test is to demonstrate that payload performance did not degraded after exposure to environmental tests and to establish a reference for Antenna Range and Launch-site Spacecraft tests.

• Test requirements are typically identical to IPT

• Testing is typically conducted through the input and output test coupler ports because the Antenna assembly is typically integrated prior to testing





Figure 2.3.4.3.4. NIMIQ: Just Prior to Final Spacecraft Test




2.3.4.3.4 Final Spacecraft Test



2.3.4.3.5 Antenna Range TestAntenna Range test is typically performed in a near-field range or a compact antenna test range (CATR) facility with the Spacecraft in full flight configuration (i.e. all thermal blankets and sun shields installed).

Tests typically comprise:

• Receive antenna airlink pattern measurements

• Transmit antenna airlink pattern measurements

• EMC and Passive intermodulation measurements





Figure 2.3.4.3.5 MSAT Just Prior to Antenna Range Test




2.3.4.3.5 Antenna Range Test



The purpose of Launch-site Spacecraft testing is to verify the integrity of the payload after spacecraft delivery to the launch-site.

Typical tests include:

• Command and telemetry functional check

• Sometimes, a transponder noise mound test is conducted for each channel

• Results are then compared with data that was measured at Final Spacecraft testing

Launch-Site Spacecraft Test

2.3.4.4 Launch-Site Spacecraft TestPart 4: Payload Testing




The purpose of performance trending is:• To track the performance of key payload parameters across all

Spacecraft test phases in order to identify any anomalies prior to launch

• To provide a baseline for predictions of performance during in-orbit testing

The following charts show examples of:• Saturated gain vs. test phase

• SFD vs test phase and vs. frequency (3D plot)

• Receiver conversion frequency vs. test phase

Performance Trending

2.3.4.5 Performance TrendingPart 4: Payload Testing




Unit failure identified, unit was replaced

Transponder realigned for in-family gainPerformance Trending

Figure 2.3.4.5a Performance Trending Test (Transponder Gain at Saturation)



2.3.4.5 Performance Trending



Pronounced sensitivity at cold identified and correlated to unit level data

Figure 2.3.4.5b Performance Trending Test (Sensitivity at cold)







Receiver vacuum sensitivity highlighted, unit was retuned for vacuum operation

Figure 2.3.4.5c Performance Trending Test (Vacuum Sensitivity)







The purpose of In-orbit testing (IOT) is to demonstrate that the Spacecraft performance has not degraded after launch and drift orbit environmental exposure.

The payload IOT typically commences after the fully deployed Spacecraft has arrived at its IOT orbital slot.

The major payload tests typically include:• Antenna patterns• Gain transfer, EIRP and SFD• Gain-to-noise temperature ratio, G/T • In-band frequency response• Transponder gain control• Frequency conversion

Test results are then compared to the trend of test results taken during ground testing and to the in-orbit predicted performance.

In-Orbit Testing



2.3.4.6 In-Orbit Testing



The purpose of in-orbit monitoring is to continue the performance trending of the key payload parameters that indicate the “health” of the payload

Typically, this is done by monitoring the following telemetry parameters:

TWTA helix and DC current or SSPA gate current

TWTA anode voltage

In-Orbit Monitoring



2.3.4.6 In-Orbit Testing

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