European Space Agency

Alphasat TDP#1: Broadband Data Relay


The performance of a 2 Gbps optical LEO-to-GEO link will be verified in-orbit as part of an experimental broad-band data relay transmitting a 300 Mbps user data stream from Tandem-X, a German earth observation LEO satellite, to ground.

Besides optical LEO-to-GEO links, the laser communication terminal on Alphabus will also be used for:

  • Optical bi-directional GEO-Alphabus links,

  • Optical aircraft-to-GEO links as well as

  • Optical Alphabus-to-ground links.

Possible applications of the TDP#1 payload

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Homodyne binary phase shift keying (BPSK) is based on coherent detection, i.e. the signal to be detected is superposed to a local oscillator laser running on the same frequency as the signal's carrier.

The optical phases of both, signal and local oscillator need to be locked posing strong requirements on the laser?s frequency stability, since the phase noise must be sufficiently low. For this application Tesat developed a reliable space qualified CW Nd:YAG laser shown in the figure below. The laser is unique on the space applications market. Flight models have already been developed by Tesat for scientific ESA and NASA programs .

Space qualified CW Nd:YAG laser for homodyne BPSK modulation
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The receiver front end demodulates data signals up to 10 Gbps and generates all control signals needed for spatial acquisition, frequency acquisition, phase locking, and heterodyne as well as homodyne tracking. The receiver front end is rugged and small since the optical bench typically used in conventional communication terminals has been miniaturised to an optical monolith with outer dimensions of only 20 x 20 x 10 mm3. The receiver front end can simply be adjusted to its mount as can be seen by the mechanical interface in the figure.

Receiver front end of a homodyne BPSK based laser communication terminal

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For the first time an optical LEO-to-GEO link with a high data rate - the major advantage laser communication terminals have compared to conventional RF payloads - will be verified in-orbit.

Since the data relay payload, especially its laser communication terminal, will also be used for later bi-directional optical links to other GEO satellites, to ground and to aircrafts, it will be possible to verify the performance of every optical link within a complete communication chain from e.g. an aircraft via a network of GEO satellites to ground.


The broadband data relay payload consists of:

  • The laser communication terminal (LCT) receiving data from the TanDEM-X satellite, and

  • The RF-payload transmitting the user data to ground.
The LCT is based on homodyne binary phase shift keying (BPSK), the most sensitive optical modulation scheme and the only one that is immune against any kind of false light, e.g. sunlight. The LCT is designed for a bit error rate of 10-8, at a data rate of 2 Gbps and 45.000 km link distance. The RF down-link, however, is the bottle-neck of the data relay limiting the user data rate to 300 Mbps.

On the basis of a trade-off between Ka-Band and an upgraded Q-Band payload, TDP#5, the Ka-Band has been selected as baseline for the RF-down-link.


The laser communication terminal (LCT) is a further development of already existing flight hardware: the LCT accommodated on the TerraSAR-X LEO satellite used for both, optical TerraSAR-X-to-ground links and optical links between TerraSAR-X and the US LEO satellite NFIRE.

LEO LCT fight model mounted on the TerraSAR-X satellite for integrated system tests

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The TDP#1 payload is a contribution in kind by DLR.

Current status

The TDP#1 mission has been defined. Requirement specifications have been derived. Specifications of both, the laser communication terminal and the RF payload have been established. The development of the LCT has been kicked-off.

Status date

Tuesday, October 28, 2008 - 13:34