At the beginning of the second decade in the second millennium a wind of change came up in the field of payload data transmission. In March 2012 the first and current issue of CCSDS 131.2-B-1 Flexible Advanced Coding and Modulation Scheme for High Rate Telemetry Applications was adopted and one year later in March 2013 the first and current issue of CCSDS 131.3-B-1 CCSDS Space Link Protocols over ETSI DVB-S2 Standard followed. Both Blue-Books offered quite similar performance improvements for payload data downlinks with respect to flexibility and efficiency and both opened the door to dynamic adaptation to varying channel conditions (VCM/ACM). In 2013 STT-SystemTechnik GmbH joined the ongoing PPP-project ‘E-SAIL’ in Phase B1. The public/private partnership was supported by ESA in the framework of ARTES 21-2B, with LuxSpace acting as prime contractor for the micro-satellites and with exactEarth, Canada planning to operate and using these satellites for their global AIS services. With their contribution to Phase B1 STT could convince all partners to adopt DVB-S2 (CCSDS 131.3-B-1) instead of the projected traditional legacy transmission schemes for the Payload Data Downlink (PDD). The DVB-S2 approach survived critical questionings during preparation and at the beginning of Phase B2. STT started with the conceptual design of the PDD late 2014. Mid 2018 the flight model (TRL-8) of the E-SAIL PDD was delivered to LuxSpace and the launch of the satellite is expected to be accomplished in Q3/2019.

The DVB-S2 based Transmitter for E-SAIL operates with a modulator symbol rate of 50 MBd, with a DAC sample frequency of 300 MHz and supports MODCOD 1 to 23, i.e. QPSK, 8PSK and 16APSK with all code-rates prescribed in the standard to be combined with these modulation schemes. With MODCOD 23 (16APSK-9/10) a data rate of ca. 180 Mbit/s is achieved. The complete DVB-S2 functionality has been implemented in the novel, radiation-tolerant RTG4 FPGA from Microsemi. For providing the comprehensive DVB-S2 functionality STT uses an IP-core, which has proven its standard-conformity and excellent performance in many applications around the world, including payload data transmission from micro-satellites used for earth observation. The modem generates a modulated output signal in the intermediate frequency of 70 MHz. The implementation of a signal reconstruction FIR filter in the digital domain, optimized for the DAC alias output at 370 MHz, allows the exploitation of this higher intermediate frequency for a one-step up-conversion to C-Band or X-Band. The avoidance of direct modulation in the analog domain with its inherent temperature-dependent signal distortions results in very good signal quality over the full operating temperature range.

The demand on output power up to 10 W at the power amplifier output with the need for high signal quality to support 16APSK on the one hand and the limited primary power provided by a micro-satellite on the other hand calls for a thorough optimization of power efficiency versus transmit power and signal quality. STT uses novel GaN-FETs for the power amplifier design to accomplish this task. The Error Vector Magnitude (EVM) remains below 8% and Power Added Efficiency (PAD) remains above 35% over the full operating temperature range.

For E-SAIL a downlink frequency in C-Band had to be used, which unfortunately is burdened by a low level of permitted power flux density on the Earth surface. To comply with these requirements, the transmit power has to be accurately balanced between maximum achievable data rate and maximum power level permission. Transmit power of STT’s PDD-Transmitter can be set remotely (by OBDH) in very small steps over a wide dynamic range. STT optimized a digital transmit power control loop with the result that power setting can be kept in a range of ±0.2 dB over the full operating temperature range.

In the ARTES-21 program OHB-LuxSpace and STT-SystemTechnik GmbH also joined ESA’s endeavors to promote penetration of CAN-Bus technology into the LEO market. CAN-Bus advantages in mass, volume and power savings are especially important for micro-satellites. The CAN-Bus IP-cores for CAN-Bus controller and CANopen protocol suite, made available by ESA, have successfully been integrated into the Payload Data Transmitter, passed intensive testing with professional CAN-Bus stimulation and analyses tools at STT and during system level tests at spacecraft integration at LuxSpace.

Supported by an ARTES34 contract, STT is now enhancing the achievable data rate based on the RTG4 FPGA and a high-speed DAC. The goal is a triplication of modem symbol rate, to provide the capability of more than 1.2 Gbit/s downlink rate when using two of these transmitters in parallel with polarization diversity.

Since 2013, CNES activities in the domain of High Data Rate Telemetry (HDRT) involved the development of a new generation of payload data downlink equipment. Pushed by the perspective of new Earth Observation projects, the development of a complete X-band chain with DVB-S2 transmitter associated to an Antenna Pointing System based on an Antenna Pointing Mechanism was pursued during the late years.

In 2016, a first publication for TTC conference described the overall architecture for each chain component and some preliminaries results. Since then, the final X-band modulator prototype was delivered in CNES and a commissioning phase has started. After several months, a functional validation phase could start.

The CNES HDRT laboratory was equipped with a test environment representative of realistic transmission conditions in order to provide reliable results. Thus, a power amplifier rack was developed from a TWTA spare, providing a low power output distorted by the non-linear effects of the amplifier. At the output, a radiofrequency filter is added to emulate the effect of an OMUX filter. A cross-polarization interference generator provides the static or dynamic interference related to imperfections of the transmitting and receiving antennae (axial ratio). The X-band signal is then translated to an intermediate frequency to be demodulate by the high data rate receiver. The overall chain can be automatically piloted in order to optimize test duration and exhaustiveness.

The architecture involves a dual polarization transmission with one channel of 300 MBauds per polarization. The modulator provides the DVB-S2 signal in X-band which can be digitally predistorted to reduce the degradation induced by the non-linearity.

Several configurations were tested in order to validate the performances step by step. A major interest of the developed modulator is its capacity of handling DVB-S2 Variable Coding and Modulation (VCM). This feature is particularly important for the next generation of payload data downlink equipment as it allows a better exploitation of the channel capacity with minor added complexity.
The VCM capacity of the modulator was extensively tested to ensure a continuous data transmission for different system scenarios (nominal downlink, downlink with interruption, continuous downlink on several ground stations in a row).

The paper will present first the chain validation performances obtained on a representative propagation channel. The effects of amplification, filtering and cross-polarization interference will be detailed as long as the deployed mechanisms in the receiver to mitigate them. The benefits of digital predistorsion will also be discussed.
In a second part, system performance analysis will be provided, exploring the capacity of the overall HDRT to be mounted on an agile satellite taking pictures while downloading the data.

In parallel to this development, research on new Solid State Power Amplifier (SSPA) based on GaN technology has been done leading to the possibility of coupling the DVB-S2 X-band modulator to a 10W RF GaN SSPA creating an attractive product with reduced cost, mass and volume compared to TWTA technology. The perspective of an in-flight demonstration will be discussed in the last part of the paper.

Nowadays the number of Earth Observation missions based on nano and micro satellites is exponentially increasing. Each satellite may embark payloads which produce a big amount of data, i.e., starting from a few Kb/s to Gb/s. CCSDS 131.2-B-1 standard is a possible solution for bandwidth efficient, high data-rate data downlink. Its powerful Serially Concatenated Convolutional Codes (SCCC) with modulations ranging from QPSK to 8PSK and 16-, 32- and 64-APSK, provides high degree of flexibility in terms of coding rate and bandwidth efficiency. Such flexibility allows to maximize the link capability by exploiting different modulation and encoding schemes (ModCods), and also optimize downlink efficiency with techniques such as Variable Coding and Modulation (VCM), and Adaptive Coding and Modulation (ACM). In this paper, the SCCC SW EGSE is presented, a software tool which allows to simulate and test CCSDS 131.2-B-1 compliant transmitters during development phase. SCCC SW EGSE is also equipped with hardware-in-the-loop capability, and therefore represents an affordable and complete solution supporting the development of CCSDS 131.2-B-1 transmitters, from the algorithm modelling phase down to the hardware implementation.
The SCCC SW EGSE consists of a software transmitter module, a software channel module and a software receiver module. It is highly configurable via a configuration file that allows to set different parameters to control the software behaviour.
The software transmitter module performs the SCCC encoding and modulation of the information starting from a Transfer Frame stream or a Channel Data Access Unit (CADU) stream, to the Physical Layer Frames. The software transmitter also includes a configurable baseband Square-Root Raised Cosine (SRRC) filter module. The input data can be read from an input file or it can be generated randomly by the software itself.
The software channel module can simulate different effects: AWGN, doppler effect, phase noise (Jitter), non-linearity of power amplifier, and I/Q imbalance. Both phase noise and Power Amplifier (PA) non-linearity rely on external files that describe the phase noise mask to be used, and the AM/AM and AM/PM curves of the PA. A static symbol pre-distortion algorithm is implemented within the transmitter module to mitigate PA distortion effects.
The software receiver module performs matched filtering, soft-demodulation and soft-decoding of the received I/Q signals, and produces the decoded Transfer Frames information. It also implements a frequency and phase recovery algorithm to mitigate Doppler and jitter effects. The decoding phase is based on BCJR algorithm. The software can run in two different modes: “debug” and “performance”. The “debug” mode allows for step by step investigation of the data work flow by saving data after each transformation. The “performance” mode allows to obtain both uncoded and coded Bit Error Rate (BER), Codeword Error Rate (CER), and Frame Error Rate (FER) statistics over a configurable Eb/N0 range. The SCCC SW EGSE can perform both “TX/RX” or “only RX” simulations. In the former case both the transmitter and receiver modules are active, while in the latter case only the receiver software module is enabled, so this configuration is particularly useful to analyse data produced by a real hardware transmitter. In case a hardware transmitter is included in the loop, input data to be transmitted are sent to the hardware transmitter by the application software, then the transmitter output data are fed back to the software, passed to the software channel module and finally the data processing chain is completed with the software receiver module. The communication between the SCCC SW EGSE, and the hardware transmitter is obtained through a specific Application Programming Interface (API); this allows to include any third party CCSDS 131.2-B-1 compliant hardware transmitter in the data processing chain of the SCCC SW EGSE, to test its performance in a full chain scenario including data generation, channel module and receiver module. As case study the SCCC SW EGSE was tested with IngeniArs CCSDS Telemetry Transmitter IP Core. The hardware setup consists of a PC running the SCCC SW EGSE, a SpaceART link analyser and the Transmitter IP Core. The SpaceART device acts as bridge between the PCIe port of the PC, and the SpaceFibre port used by the Transmitter IP Core.
In conclusion, the SCCC SW EGSE aims to be a flexible and effective tool in helping telecom and hardware engineers in developing CCSDS 131.2-B-1 transmitters and systems, both in early algorithm modelling/development phase, and in final validation phase. Its ease of use allows for fast integration with any hardware transmitter compliant with the CCSDS 131.2-B-1 standard, and therefore a full evaluation of the DUT capabilities.

In the last years the trend to use very high bit rate downlinks in the Earth observation satellites has become a fact, with several on-going or planed missions today selecting the K-Band as the downlink band due to its high capability to allocate signals operating at hundreds of megabits per second.

In the frame of the expansion of the Copernicus program, several missions are currently under study, some of them requiring to manage very high data rates between 4 to 8 Gbps while others with rates around 1 Gbps could be allocated either in X-Band or K-Band. Then, it will be needed to identify modular architectures both in X and K band allowing to cover the required rates.

In the frame of commercial programs, Thales Alenia Space has developed and qualified an advanced K-Band data transmission solution implementing the novel CCSDS 131.2-B-1 standard where flexible advanced coding and modulation schemes are used, allowing the system to operate in different ACM formats from 8PSK up to 32APSK. The use of this variable coding and modulation capability allows to optimize the link with the ground station configuring lower ACM formats for low elevations where the attenuation due to atmospheric conditions is very high. Then, the variable coding and modulation solution presented is a way to maximize the data download for the future missions requiring very high data rates. In the X-Band, the experience on previous Copernicus missions like Sentinel and in commercial observation programs are stating points to define architectures for the future Copernicus missions allowing also to maximize the use of the X-Band

In this paper an overview of different architectures for X and K bands based on experience in previous programs is presented, showing the main advantages for each case and presenting developments required to cover the needs of future Copernicus missions.

For K-band architectures, data downlink capabilities including results based on system level end to end tests with ground station representative hardware are presented and discussed on this paper. The main advantages of using SCCC encoding vs other classical encoding solutions is also discussed together with the expected performances on the ground segment. Finally the limits on the use of the K-Band for some missions and potential solutions are put on the table.

For X-Band architectures a trade-off of encoding solutions is presented with the impacts on the different units (modulators, amplifiers, antenna). The use of new encoding solutions allows to implement compact architectures in terms of hardware. Also the replacement of X-Band TWTAs by SSPA technology either based on GaAs for low to medium rates or the more efficient new generation of SSPAs based on GaN for high rates up to 1 Gbps is discussed.