APSS - Active Pixel Sun Sensor

Status date

The objectives of the project are the design, development and qualification of an EQM unit of the Active Pixel Sun Sensor APSS, together with the definition of flight opportunities for the sensor.

The design is based on the existing Low Cost Digital Sun Sensor (LCDSS) and includes trade-off results on new features and applications. The main design goals are accuracy, weight and low cost with the following improvements:

  • Improve commercial attractiveness for different missions (TLC, EO and SC including interplanetary ones),

  • Harsh radiation environment tolerance,

  • Interface architecture (data and power) consolidation,

  • Use or ITAR free components.

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The main characteristics goals are (with DC/DC converter):

  • Mass: < 380gr

  • Qualification Temp: -35ºC - +70ºC

  • Operating Temp: -25ºC - +60ºC

  • Power Consumption: < 1W

  • Field of view:

    • Physical: ±64º

    • Virtual: min. ±3.8º

  • Accuracy

    • Bias: 0.01º (3Ã)

    • NEA: 0.01º (3Ã)

    • Alignment: 0.06º (3Ã, including temperature effect)

  • Power Interface:

    • 16V up to 40V unregulated

    • 24V to 50V unregulated

    • +5.5V±5% (2Vrms noise)

  • Data Interface: DS48 with RS422 electrical levels

The APSS development is mainly addressed at a low mass power and cost digital sun sensor. The achievement of this goal has been reached using an APS detector and a simple optical path, based on a pin hole.

The unit architecture has been conceived to work in different configurations suitable for different space missions, including usage in spinned satellites.

Due to its design, the APSS is attractive for the space market in terms of cost and technical characteristics, and is suitable for GEO TLC satellites, Earth Observation and Scientific missions.

An ITAR free configuration is also available, increasing the Sensor exportability.


The APSS is an high accuracy/large FOV digital sun sensor based on APS detector, to be used on board telecommunication satellites, as well as for scientific missions.

It is based on the barycentre detection of the incident energy of the Sun, in the visible light spectrum, allowing an accurate attitude measurement of the Sun disk center, with integrated capabilities of direct pixel addressing, analog to digital conversion and anti blooming circuit.

All the logical functionality of the sensor is managed by a dedicated ASIC, and the APS is provided of a latch-up protection circuit.

The APSS is based on:

  • An attenuation filter with a photoengraved pin hole on its back side,

  • A focal plane assembly with a 1024x1024 APS detector,

  • A processing (implemented in ASIC) and data interface electronics (proximity electronics) including the power section,

  • DC/DC converter in the range between 16V to 40V with upgrading to 50V and optional secondary voltage supply configuration.
The Sensor is completely autonomous switching from sun acquisition to sun tracking modalities.

The APSS is able to manage two different configurations:

  • Default: missions in Earth orbit (up to 1AU) and in Venus orbit (0.7AU),

  • Interplanetary: mission from 1AU up to 50AU (towards Pluto) and from 0.3AU to 1AU (towards Mercury).
For each configuration the sensor can work in:

  • Slow mode: for stabilized satellite

  • Fast mode: for spinned satellite (up to 100rpm)

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The project is divided into the following main phases:

  • Sensor re-design and development,

  • EQM model manufacture,

  • New design validation by analysis and tests with a complete qualification campaign on the EQM,

  • APSS flight opportunities definition.
The first phases of the project were aimed to consolidate the existing design into a commercially attractive product. Sensor requirement specifications were defined, as well as ASIC full design and development. In the subsequent phases an EQM unit was manufactured and underwent a qualification campaign.

The following scheduled key milestones have been completed:

  • Kick Off: April, 9th 2003

  • PDR: July, 7th 2003

  • CDR: December, 16th 2003

  • TRR: November, 27th 2004

  • FR: November 2005
The contract ended in February 2006
Current status

The qualification campaign of the APSS EQM unit (in the Default Slow Mode configuration) has been completed.

The unit has been qualified at the following environmental conditions:

  • Vibrations: sine (30g) and random (X-Y 23.5grms; Z 26.8grms),

  • Shock: up to 2000g from 1000Hz to 10000Hz,

  • Thermal tests: -35C to +70C,

  • EMC Tests: compatible with typical TLC EMC requirements.
During the contract two flight opportunities became concrete and three FM units have been manufactured, tested and delivered in July 2005:

  • GOCE: Gravity Field and Steady State Ocean Circulation Explorer Mission. Scheduled in launch in 2006, 2 sensors delivered (1PFM + 1FM),

  • CPD: The APSS is mounted on the Coarse Pointing Device (CPD) for sun tracking in observation and biological payload, 1 sensor delivered (FM).
A flight qualification is foreseen with the GOCE mission whose launch date is scheduled within mid 2007.

The APSS sensor has been chosen as Digital Sun Sensor on board LISA Pathfinder and SICRAL-1B TLC satellite.