Alphasat - Payload Calibration Units

Status date

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:

  • Provide a calibration tone on a frequency channel selected by the payload. The tone can be enabled or disabled.
  • Measure the relative amplitude (MR) and phase (PD) between the test and reference inputs of the equipment, including when the calibration tones are submerged in noise. The accuracy of these measurements needs to be in the order of tenths of a dB (MR) and in the order of a degree (PD).
  • Provide full redundancy.
  • Provide telemetry and support telecommands via the 1553B interface, including making measured data available through this interface.
  • Operate in the presence of Inmarsat payload traffic occupying adjacent channels; this is seen as noise by the calibration units.
  • Stringent mass and power consumption requirements.
  • Provide calibration over wide temperature range (-5°C to +55°C); this is achieved by having a self-calibration feature that compensates for analogue performance changes over temperature. Most calibration units operate over a narrow thermal range.
  • Non-operational survivability from -40°C to 75°C.
  • Have a 15 year lifetime.
  • The original specification had very challenging requirements that in some cases were extremely hard to realise. COM DEV Europe worked closely with the customer to understand their needs and optimise the PCU specification.
  • Due to the high complexity and measurement accuracy required from the units, producing a test system that was able to reliably measure such a high number of variables and level of performance was a challenge.
  • A sophisticated software-driven test system was developed. A highly skilled team of engineers and test technicians was deployed to work around the clock executing lengthy test campaigns.



This project has allowed COM DEV Europe to expand and improve its capabilities in the following areas:

  • Develop high-reliability mixed signal, analogue & digital space electronics using advanced digital signal processing techniques for radio demodulation.
  • Produce, test and qualify high-complexity, high-accuracy hardware that meets very demanding specifications and pushes the boundaries of technology.
  • Manage high-complexity programmes that require multidisciplinary teams of designers, assemblers and test technicians to work in a unified and synchronised way during the development, manufacturing, integration and test campaign phases.

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

click for larger image



Current status

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.