Alphasat TDP#2: Optical Deep Space Link (O-DSL) Technology Demonstrator

Status date

The objective of this phase A study has been to confirm the feasibility of the Alphasat TDP#2 technology demonstrator to achieve both the objectives defined by CSAG for the validation of this technology, together with the programmatic objectives as defined by ESA for the Alphasat programme.

The technical objectives for the TDP#2 are to demonstrate the in-flight performance and flight worthiness of deep space optical telemetry technologies. The in-flight demonstration has the following objectives:

  • Autonomous optical tracking and beam steering based upon detection of a pulsed beacon being transmitted from the ESA optical ground station (OGS). Pulsed detection is to be achieved using a 4 quadrant avalanche photodiode (4-QD APD).

  • Master oscillator power amplifier (MOPA) based transmitter modulated using a 16-PPM modulation format for the transmission of telemetry data to the ESA OGS thereby verifying deep space communications principles through the Earth's atmosphere.

  • The data to be transmitted over this link could be telemetry data generated on-board Alphasat.

The O-DSL TDP#2 has not been selected for the Alphasat mission.


The expected main benefits for the project are:

  • Definition of an agreed engineering baseline for the O-DSL TDP#2 such that the objectives of the technology demonstration using the Alphasat opportunity can be met,

  • Definition of a detailed programme plan for phase B / C / D needed to achieve the development in due time of the TDP#2 for Alphasat,

  • Consolidated industrial team for the O-DSL TDP#2.

Following the Task 5 synergy workshop it has been agreed that the Alphasat TDP#2 will be combined with the TDP#1 laser communication terminal (LCT) coming from Tesat such that two TDPs share a common optical head (to provide the pointing capability and required optical telescope gain to maintain an optical link to the ESA OGS). The common optical head is to be provided by TDP#1 with a simple interface to the TDP#2 using an optical fibre for the optical telemetry channel and electrical harness between the two terminal control processors and between the TDP#2 pulsed detector (4-QD APD) that will be integrated into the TDP#1 LCT.

A simplified block diagram for the combined TDP#1 and TDP#2 is provided below:

 click for larger image

The interfaces for the TDP#2 have been defined and are fully in-line with the Alphasat requirements defined during the course of the study. A CAD image of the TDP#2 is shown below. The mass has been estimated to be 7.3 kg and power consumption of 38.5 W.

click for larger image

A second agreement reached at the Task 5 synergy workshop is to implement a downlink capability for the Alphasat TDP telemetry and scientific data using the TDP#2 as an additional telemetry downlink to the main Alphabus TM/TC. The data from the TDP?s could be collected and multiplexed by the TDP#8 which will then interface to TDP#2 for transmission of the data to the ESA OGS. If this option is not followed then the data would be a PN sequence generated within the TDP#2 electronics.


The Alphasat TDP#2 Phase A study has been completed and the results confirmed the technical and programmatic feasibility of the optical deep space telemetry demonstrator for AlphaSat. However, the TDP#2 has not been selected for the Alphasat mission.

Current status

The Alphasat TDP#2 Phase A study has been completed and a technical baseline, programme plan and industrial team for the development of the TDP#2 payload element defined. The Final presentation has been delivered on September 29th 2006 at ESTEC.