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Alphasat is a communications satellite. The Calibration Units developed in this programme are part of the spacecraft’s main payload; they are used to adjust the equipment in order to produce optimal beamforming.
Two different payload calibration units (PCUs) were developed; one that takes VHF RF inputs called the Return (RTN) unit and a second one that takes L-band RF inputs called the Forward (FWD) unit.
Due to their high measurement accuracy, very precise calibration can be achieved enabling sharp beam formation with minimum waste of RF energy, which results in better geographic coverage and control of handover areas. This translates into higher bitrates and revenue.
The objective is to design, manufacture, integrate, test and deliver two payload calibration units (FWD and RTN) for Alphasat. The units shall:
This project has allowed COM DEV Europe to expand and improve its capabilities in the following areas:
The PCUs produce a calibration signal (CW), also referred as calibration tone (CT), which is routed through the payload signal processing path to generate two tones, Test and Reference. These tones are routed back to the calibration unit; subsequently the magnitude ratio (MR) and phase difference (PD) between them can be measured and made available in digital form via a 1553B serial interface. Ultimately this is the information used in the payload calibration algorithm.
The information is extracted by amplifying the input CTs (Test & Ref); these are subsequently up or down converted to an intermediate frequency (IF) where they are digitised by analogue to digital converters (ADCs). Once in the digital domain a series of digital signal processing (DSP) techniques implemented in an FPGA are used to calculate the I & Q components and compute the MR & PD numbers.
The figure below shows a simplified PCU architecture. Side A is the nominal unit and side B the redundant.
Simplified PCU architecture
Both RTN and FWD units were successfully designed, manufactured, integrated, tested and delivered. Their performance meets all expectations and in particular exhibit a high level of MR and PD measurement accuracy even under noisy conditions.