For a High Data Rate 8-PSK Modulator two main applications can be targeted.
The first application for the proposed 8 PSK modulator is the direct downlink of data from LEO satellites, mainly earth observation satellites, to the ground station during fly over of the satellite.
The second application is on board a data relay satellite. A possible scenario for such a GEO communication satellite is to receive data via an inter-satellite link from a LEO satellite and transmit them from GEO to ground. Data relay systems provide the capability to transmit very high data volumes within a short time interval between the generation of the data on the LEO satellite and their reception on ground. The LEO satellite is in most cases an earth observation satellite, e.g. with the generation of huge data volumes by taking as many as possible high-resolution photographs. Beneath the described scenario a data relay satellite in GEO orbit can also be used for ground to ground connections used for transmit huge volumes of data over long distances.
For future flight applications it is essential to demonstrate the maturity of a design by building up an Engineering Qualification Model (EQM) and to perform a complete qualification test campaign, this is also true for the High Data Rate 8-PSK Modulator and can be considered as the main objective of this project.
Nevertheless, during the activity, special attention will also be paid on the conscious redesign of the predecessor modulator EM, developed in frame of a former ESA GSTP contract. In particular the redesign activities will address the following topics:
- Implementation of a redundant Data Input Interface, which can be used if required by the customer,
- Implementation of additional commands for switching between nominal and redundant data interface,
- Implementation of a test mode offering the possibility to deliver an unmodulated carrier to the modulators output,
- Further design optimizations will be carried out in order to optimize the modulators overall electrical performance.
There can be two key issues envisaged for this project, the first is clearly to design a high performance 8-PSK modulator working in X-Band offering among others the following features:
- Modulation directly performed at the carrier frequency, which leads to less complexity, lower mass and lower power consumption,
- Forward error correction encoding according to CCSDS,
- 4D TCM coding according to ECSS-E-ST-50-05C
- Baseband pulse shaping,
- Max. Channel Symbol Rate of 200 Msysmbols/s,
- In-Orbit switchable carrier frequency (two frequencies predefined and selectable via High Level Command).
The second key issue is the EQM test campaign, which will include the following test:
- Several electrical tests at ambient, cold and hot temperature,
- Physical Measurement,
- Sine and Random Vibration,
- Shock Test,
- Thermal Vacuum Test,
- Electromagnetic Compatibility (EMC) Test.
The following main benefits are expected to be achieved at the end of the project:
- A modular equipment architecture that will give the opportunity to cope with changing customer requirements without major redesigns, especially in terms of interfaces.
- A good manufacturability that will allow Tesat to offer the modulator schedule and price competitive.
- An electrical performance that will be able to cope with the future customer need of increasing data rates.
For the modulator a modular concept was chosen which can be easily adapted to specific customer and system requirements with regard to output power and system configuration.
The whole modulator consists of four sections, the INTERFACE SECTION contains the TMTC interface, the nominal and redundant data / clock input interface and is responsible for Reed-Solomon encoding, interleaving and scrambling.
The second section, called DIGITAL SECTION includes the 4D-TCM encoding and the 8-PSK constellation symbol mapper. After filtering the digital baseband signal is converted to a differential analog signal for I and Q path.
These signals are forwarded to the MODULATOR SECTION. The analog I and Q signals control the analog X-band modulator. After amplifying the modulated carrier is available at the RF output.
The fourth section is the DC/DC CONVERTER SECTION which supplies all sections with power.
The Modulator hardware concept is rather modular and flexible. If for example, the input data interface has to be changed only the INTERFACE SECTION would be affected. A further advantage of the concept is the isolation of RF and digital functions which leads to best RF performance and lowest EMC sensitivity.
The four sections of the modulator are mounted in three cases. The DIGITAL UNIT contains the board of the INTERFACE SECTION and the board of the DIGITAL SECTION. The MODULATOR SECTION and the DC SECTION have their own housings. All three cases are bolt together and build up the modulator assembly. Under a cap on top of the housing the supply lines are placed. For interconnection between the INTERFACE and the DIGITAL board a flexible PCB with 30 lines is used. A further flexible PCB with 20 lines is applied to connect the DIGITAL SECTION with the MODULATOR board.
The project High Data Rate 8-PSK Modulator can be divided into three main tasks. At first the 8-PSK Modulator EM, previously developed within an ESA GSTP project, will be redesigned in order to optimize the modulators performance and overall architecture. After redesign completion an Engineering Qualification Model (EQM) will be manufactured applying flight proven processes, materials and standards. The last and main task of the activity will be a rough qualification test program the modulator will undergo in order to prove the equipments flight worthiness.
With the 8-PSK X-Band Modulator Tesat-Spacecom can complete its product portfolio related to the satellite communications payload and is well prepared for future customer requirements for high data rate X-Band downlink transmission subsystems.
The project is completed, the Final Review or rather Qualification Review was successfully held on 28 Nov, 2012. The qualification of the 8PSK Modulator has been declared as successful.
In the course of this project two contracts for the delivery of flight models could be acquired. The first flight models have been delivered in December 2012. This project can therefore be considered as a complete success with respect to the objectives of the ARTES 3/4 program line.