Satellite communications payload and transponder testing

Satellite communications systems must cover a diverse set of user requirements in direct broadcast, wireless communications and remote sensing applications for both commercial and government systems. These systems must operate reliably around the clock with multi-decade system longevity. They must be thoroughly tested to ensure a quality of service over the life cycle of the satellite.

To deliver the next generation of satellite applications, operators are exploiting the on-board processing advantages of digital, wideband high-throughput payloads at higher frequencies. By moving to Ku, K, Ka, O, and V-band, larger bandwidths are available to deliver services such as real-time, ultra high-definition Earth observation, and low-latency internet.

Today’s satellite systems need to be future-ready to be fully compatible with both existing cellular networks and emerging wireless technologies. The technology evolution from bent-pipe or digital transparent payloads to digital regenerative payloads to increase system capacity and flexibility are increasing testing complexity. Regenerative transponders include additional functions such as digital signal demodulation, baseband signal processing and switching and signal modulation.

Satellite companies need to thoroughly test RF communications systems and components to ensure 24/7 operation with a high quality of service. On the other hand, satellite equipment manufactures need to reduce test time and cost while the total number of uplink and downlink beams is increasing. One digital stream of data on the uplink corresponds to multiple streams on the downlink.

Satellite Payload Architectures

A communication satellite typically consists of two main function blocks:
1. The spacecraft bus or service module
2. The communication payload module

With most commercial communication satellites the payload consists of two distinct parts with well-defined interfaces—the repeater and the antennas. Repeater designates the electronic equipment which performs a range of functions on the carriers from the receiving antenna before delivering them to the transmitting antenna. The repeater usually encompasses several channels (also called transponders) which are individually dedicated to sub-bands within the overall payload frequency band.

Bent-Pipe Transponders

In bent-pipe transponders  signals from the earth station are amplified in the satellite and the uplink frequency is converted to the downlink RF frequency and sent back to earth.

Regenerative Transponders

Alternatively, to the bent-pipe payloads, regenerative satellites use the on-board processing concept. The received signal in the satellite is demodulated, decoded, in some cases error correction schemes applied, re-encoded, modulated and converted to downlink carrier frequency before transmitting the signal back to earth. These kind of transponders are called regenerative satellite payload transponders. These systems have many advantages but are typically more complex than the bent-pipe transponders.

The main functions of the communications payload of a satellite are as follows:

• To capture the carriers transmitted, in a given frequency band and with a given polarisation,
  by the earth stations of the network (the stations are situated within a given region (service zone) on the surface of the earth and are seen from the satellite within an angle which determines the angular width of the satellite antenna beam. The intersection of the satellite antenna beam with the surface of the earth defines the receive coverage);
• To capture as little interference as possible (the interference is a carrier originating from a
  different region or not having the specified values of frequency or polarisation);
• To amplify the received carriers while limiting noise and distortion as much as possible (the level of the received carrier is of the order of a few tens of picowatts);
• To change the frequency of the carriers received on the uplinks to that on the downlinks (for
  example, from 14 to 11 GHz);
• To provide the power required in a given frequency band at the interface with the transmitting antenna (the power to be provided ranges from tens to hundreds of watts);
• To radiate the carriers in a given frequency band and with a given polarisation (which are
  characteristic of the downlink antenna beam) to a given region (service area) on the surface of the earth. The intersection of the transmit antenna beam with the surface of the earth defines the transmit coverage.

The signals can be FM analog video or quadrature phase shift keying (QPSK) modulated digital video broadcast-satellite (DVB-S) signals or any other kind of data used in point-to-multipoint applications

Characterization of the payload

The characteristic parameters of a communication satellite payload are as follows:

• The transmitting and receiving frequency bands and polarisations for the various repeater
  channels;
• The transmit and receive coverages;
• The effective isotropic radiated power (EIRP) or the power flux density achieved in a given
  region (satellite transmit coverage);
• The power flux density required at the satellite receiving antenna in order to produce the
  performance specified at the repeater channel output (this can depend on the channel or group of channels concerned);
• The figure of merit (G/T) of the receiving system in a given region (satellite receive coverage);
• The non-linear characteristics;
• The reliability after N years for a specified number (or percentage) of channels in working order.

Satellite testing

Mission success in any satellite campaign greatly depends on test and measurement. At every stage of production, starting from the component design and eventually leading up to the final integration and launching phase, uncountable amount of tests are performed in order to ensure intended performance in all mission-critical aspects. Satellite test campaigns hence have a reputation of being time-consuming

When developing satellite electronics, testing occurs throughout all stages of spacecraft development, from characterising the performance of analogue parts, digital logic, SpaceWire/SpaceFibre interfaces, RF circuits, and antennas during the initial system architecture, to verifying the functionality of hardware demonstrators and validating proof-of-concepts at the prototyping (EM) phase.

This is followed by measuring the performance of complete payload sub-systems and then entire spacecraft validation in a representative environment using thermal-vacuum chambers during the qualification (EQM) stage. Following component and sub-system test, the Payload and Satellite Integration and Test (PSIT) includes the stages Near Field Range, Compact Range, Payload Integration, Integrated Payload Test, Payload Thermal Vacuum, Integrated Spacecraft Test, Vibe Testing & Spacecraft Thermal Vacuum Test (SCTV or “Shake & Bake”).

Prior to lift-off, final integration checks are typically performed at the launch site and throughout operation, regular in-orbit checks of the transmission links are made to monitor and confirm quality of service (QoS). Final Integrated Spacecraft Test (FIST), and Final Pre-Launch Test of the satellite after mating with the Launch Vehicle

Transponder testing

In order to evaluate the integrity of a satellite transponder, it is necessary to measure its characteristics (frequency response, for example) and the characteristics of its high power amplifier (HPA), such as sensitivity and saturated output power. After the launch, these properties have to be measured in a beginning-of-life test, and since they are subject to aging, they have to be monitored regularly throughout the operation of the system to guarantee that they still meet the initial specifications.

The 30/20 GHz transponder consists of three major elements, the low noise receiver, IF matrlx swltch, and high power amplifier.

The Ka-band communication payload of Communication, Ocean, and Meteorological
Satellite (COMS) provides 100MHz wide four channels for fixed-satellite service. Three channels for onboard switching for multi-beam connection and one channel for bent pipe connection.

Functional testing of satellite transponder

The transponder performances are validated on ground by special testing system called Electrical
Ground Support Equipment (EGSE). The EGSE for communication payload can configure
transponder configuration and also performs RF performance testing of the transponder.

The main task of EGSE is to check out satellite systems, at system or subsystem level, during
integration and validation phases of their life-cycle. Through a combination of hardware and software elements, EGSE supports manual, semiautomatic and fully automated testing. Automation is achieved by offering users simple, yet powerful means to write their own test application programs (test sequences) in high-level, test-oriented language and to run them in a strict realtime environment. The core of this environment is a user-configurable real-time database, containing all the information needed to calibrate acquired.

EGSE comprises two parts, namely 1. CTS: Communication Test system; and 2. PCTS: Power Command & Telemetry System.  CTS performs the function for accurate automatic testing of transponder RF performance. stimulus & monitor instrumentation module of CTS consists of a signal generator and signal measurement device, which generates various signals required for transponder performance measurement, and analyzes the transponder output signals.

RF switch matrix routes and transfers the signal to the transponder generated from RF
stimulus & monitor instrumentation suitable to each measurement item for automatic
performance measurement, and simultaneously perform the function for routing the output signal to the measurement instrumentation.

Transponder performance measurement can be executed accurately as well as fast by using this RF switch matrix. CTS controller has built-in program required for measurement, and appropriately sets the parameters for various instruments according to each measurement, and provides the function for automatic measurement by controlling the various switch operations of switch matrix. CTS controller stores the measurement results in the DB, and compares the results with the specification, and also printout the results.

PCTS controls the parameters of various transponder equipment as well as performs the function for monitoring each equipment operation status. BEIS receives the transponder control command generated from the PCTS controller and transfers it to the transponder after transforming to the suitable command format.

BEIS also receives the operation status as telemetry, and transfers this message to the
PCTS controller by transforming to TCP/IP format. PCTS controller also provides operator GUI
environment, and interfaces with BEIS for command generation and monitoring of transponder. PCTS also supplies DC power required for the operation of transponder equipments CTS controller and PCTS controller are connected by TCP/IP, for sharing various information required for test, control and monitor of transponder. The PCTS controller also has TCP/IP interface with spacecraft EGSE for the transponders testing on spacecraft level.

Before start satellite transponder testing, EGSE verification was need to be performed with
transponder simulator for the EGSE functional verification. After EGSE verification with simulator testing, EGSE will performs satellite transponder RF performance testing.

The parameters of the satellite transponder RF performance testing by EGSE are as following,

Input, output VSWR,  Noise figure,  Transfer response,  Saturation output power,  Phase shift, AM/PM conversion coefficient,  Channel amplifier gain control functions,  LO frequency stability,  In band frequency response & group delay,  Out of band frequency response,  Inter-modulation characteristics,  Spurious output.

Transfer response

The center of each channel will be power swept using the VNA from small signal at~20dB
nominal back off to 3dB nominal overdrive. When the channel amplifier is operated in linear mode, transponder maximum power is defined as the point in the knee where maximum output power is achieved.

After the response is swept, the uplink signal will be 30% AM modulated with a 1 KHz sine wave. With the Spectrum Analyzer set up to measure the Am Modulated Carrier, The downlink signal will be connected to the spectrum analyzer and the uplink drive will be adjusted to achieve a
null on the SA. The input power to determine this null will be the input level to achieve saturation.
Measurement results will be graphed with markers indicating the saturation point and the SFD
required producing saturation.

Phase shift, AM/PM conversion

The downlink signal is evaluated for excess PM created by the non-linearity of the system. The input drive is set to the AM Null Input drive with the AM turned ON. The uplink is switched to
the spectrum analyzer, and the AM Input into the system is measured in a spectral analysis. An
average of the upper side band and the lower side band is used to achieve the best results. The side band measured in dBc is then converted to AM in dB.

The Downlink is then switched to the spectrum analyzer, and the output PM is measured in a spectral analysis. Because the transponder is at the AM Null point, the contributing AM is removed as a contributing spectral component. Again, and average of the upper side band and the lower side band is used to achieve the best results. The sideband measured in dBc is then converted to PM in Degrees using the standard equation the PM(Deg)/AM(dB). The results are tabulated and annotated.

Frequency response & group delay

To characterize the quality of a transmission path in satellite communications, phase distortions are determined using group delay measurements. By using a two-tone stimulus signal, the vector network analyzer can measure the phase difference between two signals, at the input and then at the output of the DUT. Comparable to the classic S-parameter technique, the group delay is calculated from the phase difference and the frequency offset. The frequency offset Δf between the two signals is the aperture.

The uplink signal from the VNA can be sampled at Sensor 2A and adjusted using the calibration data so that the payload will receive the desired input signal level. The input drive will be maintained at the input power for saturation (at channel center) as measured during transfer
response. This leveling technique will prevent the TWTA from being pulled out of saturation if
substantial loss fluctuations over frequency are present in the uplink circuit. Port 2 will measure
the output power of the payload. About 1,202 measurements will be performed to characterize
each channel across frequency by alternately sampling uplink power then downlink power and
incrementing the frequency.

Inter modulation

Two tone inter-modulation distortion measurements will be input carrier balanced. Each carrier will be adjusted independently for the required ISL. Third order products will be measured and plotted across the input signal range. The carrier separation is 20MHz, 10MHz on either side of the channel frequency. The integrated carrier power is referenced to the saturation point determined during transfer response. IMD will be measured at the following input back-off values: -3dB, -10dB, -17dB. IMD will be calculated the worst case method the ratio of the highest third order product to the lowest carier.

Spurious output

Searching for spurious emissions with spectrum analyzers is an essential measurement when
designing, verifying and manufacturing RF and microwave devices. Transmitter and receiver devices in satellite applications have to fulfill very stringent spurious emissions limits. This
implies searching for very low level spurs over a wide frequency range. In general, a narrow resolution bandwidth (RBW) is required to make the measurements with high sensitivity. The tradeoff is a much longer measurement time. Even when working with fast spectrum analyzers
with FFT filters, a spur search may take several hours or even day.

The specifications for spurious response will be specified in terms of dBc in a 4KHz BW. The
search will be performed with the spectrum analyzer set to a resolution BW that satisfies both
of the two conditions first, RBW<4kHz second, The analyzer noise floor is >6dB below the search
specifications. The noise bandwidth for each point in the trace will be assumed to be identical to the spectrum analyzer RBW. Each point in the trace will then be normalized to a 4kHz bandwidth and compared to the specification.

Noise power ratio (NPR)

Noise power ratio is an important standard RF payload test. NPR measurements are used to test
the linearity of a RF transponder and simulate a Gaussian noise-like distribution of a multi-channel communications payload.

Signal quality measurements

Modulation accuracy and bit error rate (BER) measurements are necessary to verify the quality of
satellite links during satellite integration as well as during in-orbit operation. Modulation accuracy and BER measurements often involve analyzing the power and digital demodulation
characteristics of carriers transmitted from an in-orbit satellite or a ground station at different frequencies. The error vector magnitude (EVM) is a key parameter in determining the quality of the regenerated signal inside the transponder.

Automatic test equipment (ATE)

Automatic test equipment (ATE), or automatic testing equipment, is computerized machinery that uses test instruments to carry out and evaluate the results of functionality, performance, quality, and stress tests performed on electronic devices and systems. As its name implies, ATE automates traditionally manual electronic test equipment and processes, and requires minimal human interaction. The device whose attributes are assessed by the ATE is usually referred to as the device under test (DUT), unit under test (UUT), or equipment under test (EUT).

A typical automated test solution consists of five main components: hardware, software, test instruments, signal sources, and test probes or handlers.

• Hardware, including standard 19-inch rack mount servers and workstations, power supplies, PCIe backplanes and related PXI modules, interface modules, embedded controllers, analog inputs and outputs, digital input/output, and AC/DC outlets
• Software, for test development and management of data collection, storage, reporting, and analysis
• Test instruments, such as a digital storage oscilloscope (DSO), digital multimeter, or inductance, capacitance, and resistance (LCR) meter
• Signal sources, such as an arbitrary waveform generator (AWG), function generator, pulse generator, or radio frequency (RF) generator
• Test probes or handlers, which establish a connection between a test instrument and a DUT, UUT, or EUT

Environment testing

Most of the satellite characterization testing is initially performed at the component and
sub-system level. As a hard requirement for ensuring mission success, a satellite is required to go through a series of different environmental stress testing. These environmental testing also known as test-like-you-fly, and a serves a crucial part of the satellite integration and assembly process.

The whole spacecraft, including the launch vehicle and the payload, has to undergo thermal stress testing. The satellite is placed inside the Thermal Vacuum Chamber (TVAC) in one part of test cycle and tested for multiple weeks in emulated extreme conditions. This helps to demonstrate the activation and performance of the satellite when in space.

Prior to launch, the Compensated Compact Range (CCR) or Compact Antenna Test Range (CATR) testing is carried out. The CCR/CATR testing is the primary reference or benchmark for the end-to-end link margin test. Obviously, the importance of the other environmental testing (i.e. acoustic testing, vibration testing and shock testing) cannot be stressed enough.

Compact Range Testing

The test range consists of a main reflector, a sub reflector, a transmit (Tx) range feed antenna and a receive (Rx) range antenna. Generation of stimulus signals and analysis of received signals is generally performed outside the chamber. Depending on individual test requirements, a VNA, or a combination of VSG and Signal Analyzer may be used.

Tests performed are Antenna Patterns, EIRP (Equivalent Isotropic Radiated Power),
PIM (Passive Intermodulation, to verify the waveguide connection to the antenna), Gain
Transfer, NPR (Noise Power Ratio), Carrier to Noise or Noise Temperature, Amplitude
Frequency Response, Group Delay.

EIRP MEASUREMENTS

INPUT POWER FLUX DENSITY

Software Defined Radio (SDR)

Immense testing of the communication systems onboard a satellite and the associated ground station is required before deployment in order to ensure proper reliable operation in adverse conditions. These tests can be performed using software-defined radios (SDRs) as they provide a platform to build reconfigurable, lightweight communication systems that can be quickly modified to include new features and compatibility with emerging protocol standards.

A ground station consists of an antenna (parabolic dish), feed horn, waveguide, power amplifiers, and most often these days, an SDR transceiver. Some antennas are covered with a radome or a giant protective dome, and the application determines the ground station antenna size.

SDR is a powerful platform for testing and prototyping modulation and demodulation schemes for satellites and ground stations since development can be done on low-latency field programmable gate arrays (FPGAs). Through the use of APIs, SDRs allow for a means to test beamforming in emulation environments, prototyping pulse shaping, as well as testing host system requirements needed for processing, passing data to a subsystem, and various other tasks. This enables the development teams to explore different algorithm options, evaluate design trade-offs, and optimize system parameters.

Compared to older ground stations, it’s now becoming common to use satellite ground systems in networked systems due to RF to IP technology, where the digitized data is now used for cloud processing, optimizing operations, maximizing network performance, and more. Ground stations are now composed of smaller and more efficient components, modems, and transponders, which enable communication with multiple satellites and have increased efficiency and power. Furthermore, other technologies are investigating ground-stations-as-a-service (GSaaS), where satellite operators may lease or rent time on a specific ground station that provides LOS connectivity. Thanks to Earth service stations, customers and partners can now downlink data from their satellites on a pay-as-you-go basis without installing or operating their own satellite earth station equipment, which can have prohibitive licensing and capital costs

SDR is also a relatively new technology increasingly being incorporated into ground stations. SDR lets ground stations accommodate variable demand by beam-hopping (such as that incorporated into the new DVB-S2X standard), adjusting coverage, and targeting high-capacity regions. Moreover, SDRs are capable of processing the much greater downlink/uplink data requirements due to their RF to IP communications, which use 10/40/100 Gbps Ethernet connections to packetize data over to a host system or network. The amount of data will further increase as developments in modem technology pushes what’s possible in terms of satellite-based internet, radio, TV, and various other services. This will require the flexibility of SDRs to ensure operation in a congested spectrum. Having satellites that can reliably communicate with ground stations and process higher throughput is imperative to ensuring functionality and success.

Testing in thermal vacuum chambers

Satellite payload qualification requires performance testing using a thermal vacuum chamber (TVAC) to simulate the environment and temperature extremes of space. Satellites in outer space and spacecraft with payloads that are travelling to space must survive in that environment. This determines the qualification and verification process for components and subsystems used in the satellite. In-space conditions (open space and sun exposure) can be re-created inside a Thermal Vacuum Chamber (TVAC). The chambers are designed to simulate customized atmospheric conditions for altitude (temperature and pressure) testing.

Satellites undergo continual temperature variations because of their spin and path in
earth’s orbit. Temperature variation of satellite components in space between approx. –
170°C to +123°C per temperature cycle has been observed in the Low Earth Orbit (LEO). Thermal cycling tests performed in the TVAC are important to verify choice of materials and processes.

Thermal cycling tests (“test like you fly”) are performed to mimic the real conditions a
payload will be facing during its launch, cruise, and mission. Temperature is not altered
continuously. Typically, discrete steps are used; the dwell times are in the range of
several hours. A total of 100 cycles are not uncommon. This means a DUT may spend
several weeks within a TVAC to guarantee optimal performance

Commercial off-the-shelf test and measurement equipment is not designed to work under TVAC conditions. Spectrum analyzers, vector network analyzers, signal generators, etc. have to remain outside the TVAC and require long cables to interface to the DUT inside the chamber.

Cables, adapters and switches in the test setup will drift as the temperature changes in the TVAC. The calibration performed outside the chamber at a different temperature is no longer valid. After moving cables and changing temperatures, regular recalibration is necessary to ensure accurate measurements. The calibration procedure inside the TVAC is especially challenging because the operator cannot access inside the chamber when the vacuum is applied.

References and Resources also include:

https://www.koreascience.or.kr/article/JAKO200748647947491.pdf

https://www.rfglobalnet.com/doc/sdr-for-prototyping-satellite-ground-stations-0001

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