The AIOTS (Advanced In Orbit Test System) ESA Project generated the need to perform measurements with traffic. To cope with this goal it is necessary to implement spread spectrum techniques. Therefore, the goal to be achieved within this project is to demonstrate the feasibility of the Spread Spectrum In- Orbit Testing concept based on the following objectives:
- In-Orbit Test with Traffic: to have reliable and recurrent product which allows the performance of the in-orbit transponder measurements and monitoring without interfering on the traffic,
- Accuracy: precision will not be sacrificed by the possibility of making measurements with traffic,
- Test Duration: test time will not be significantly increased by the possibility of making measurements with traffic,
- Spin Off: additional applications for the final product can be ESVA testing as well as Traffic Monitoring,
- Drift Orbit: testing during drift orbit should also be possible using these techniques.
The most important transponder test parameters to be measured are gain/frequency response, group delay/frequency response and TWT operating point of the traffic system.
The key issue of the SS-IOT concept is the use of a spread spectrum instrument to be able to make measurements with traffic, without interfering or sacrificing the accuracy of the measurements.
Satellite in-orbit testing and periodic monitoring is a very necessary task in order to be able to predict possible malfunctioning of the transponders. The main benefit of SS-IOTS is that the communication satellite will be operational while measuring, following an identical test plan and maintaining the same accuracy level as the traditional IOT systems.
One solution to achieve the mentioned objectives is the use of a spread spectrum measurement system, which allows measuring with very low spectral density signals without interfering on the neighbouring satellite or on the transponder occupying payload.
The test signal will be a low-power spread spectrum signal that will have no measurable influence on the performance of the traffic signal. The most important feature is that the transponder performance is measured under real operational conditions and the periodic measurement enables early discovery of degradation tendencies. The spread spectrum instrument will be integrated into the AIOTS bench mentioned before, compounding the Spread Spectrum In Orbit Test System (SS-IOTS). The AIOTS has a classical IOT system architecture as shown in the following figure.
Figure 1: Classical IOT System Architecture
Four different parts can be differentiated in the SS device:
- Controller: responsible for the modem control and data interface with the rest of the AIOT system,
- Analogue Front-end: in charge of capturing the analogue signal and converting it into digital by sampling. Amplification and filtering is also necessary to accommodate analogue signals to desired levels,
- Hardware partitioning: together with the software partitioning they are the core parts of the instrument. The hardware will take care of the high speed processes computation,
- Software partitioning: the software will take care of the computation of complex algorithms and advance data processing at lower rate than the hardware.
The tasks within this project are performed in four different stages:
- The first stage is focused on the definition, specification and feasibility analysis covering the main aspects:
- Requirements definition,
- A feasibility study with simulations of the measurement algorithms,
- Test requirements definition.
The output of this stage is the production of the Basic Design Review documentation,
- The second stage is the design phase, which is intended to not only produce the design of the system (platform selection, Hw / Sw partitioning, controller module design, external interfaces, mechanical design and test tools) but also to iterate again with the feasibility study to perform additional simulations to consolidate the system requirements. The test plan and procedures definition (phases and facilities required), is also included in this phase. The output of this phase will be the consolidated system requirements, the architectural design, the interface control documents and the test plan,
- The third stage is the development both Hw and Sw, of each of the system components,
- Finally, the last stage will include the integration, test and demonstration which pursues the following plan:
- First, the spread spectrum instrument components integration and validation will be carried out. In this phase, the DUT simulator in a laboratory environment will be used to perform the subsystem validation tests,
- Then, the SS instrument will be integrated at system level with the AIOTS bench. The system validation will also be carried out in the laboratory environment,
- As a final step, the satellite tests in a real scenario together with the final acceptance test will be performed.
All the task of the project have been completed satisfactorily and the system developed demonstrated the accomplishment of the measurement requirements specified.
Intensive laboratory and real satellite tests in SES Astra premises were carried out during the validation phase of the project. Such campaigns demonstrated the ability to perform IOT campaigns without interrupting the service and not degrading the quality of service of the overall system, but being able to perform very accurate measurements under such traffic. This capacity represents a very attractive functionality for the satellite operators willing to minimize the effect of IOT campaigns in their satellite operations.
Moreover, the SS IOT System introduces a powerful user interface that automates most of the tasks, allows a fine tuning of the measurement and shows the results in a friendly format. The monitoring and control interface provided is based on an open TCP/IP interface that allows integrate the equipment in the customer networks and then carry out measurements remotely from any host of such network.
As a summary, the main benefits demonstrated by the system are listed below:
- Frequency response measurements are obtained with 0.1 dB accuracy in amplitude and 1 ns in group delay.
- It can measure at a level of at least 30 dB below the traffic carriers.
- It is able to keep the accuracy thanks to mechanism that allow to compensate propagation effects (Doppler, delay variation, and atmospheric attenuation) from the measurements. In turn, it can provide an additional functionality to track the Doppler effect produced by the satellite.
- The equipment computes with a high precision the overall frequency conversion drift.
Finally, it can carry out measurements in Open Loop Mode. That is, when transmitter and receiver are at different locations.