The European Data Relay System, High Speed Laser

Based Data Links

Frank Heine, Gerd Mühlnikel, Herwig Zech,

TESAT-Spacecom GmbH &Co.KG

TE/LP

71522 Backnang, Germany

Frank.heine@tesat.de

Sabine Philipp-May, Rolf Meyer

DLR Space Administration

53227, Bonn – Oberkassel, Germany

Abstract— The European Data Relay System (EDRS) is

currently implemented as the first operational laser based data

relay service, creating a virtual ground station for low earth orbit

earth observation missions. It is based on the Tesat Laser

Communication Terminal (LCT), which was developed under

contract from DLR for continuous communication service in

LEO and GEO orbits. The paper details the structure,

development and verification of the LCT, and describe the

developments at Tesat for single photon heterodyne detection,

that can be used as receiver for deep space communication links

to enable communications even under severe background noise

levels like near to sun communication links. Keywords—Inter

Satellite Laser Communication, Coherent Detection

I. LASER COMMUNICATION IN SPACE

Laser Communication can serve as a deep space high throughput data transfer communication system. Recently a NASA mission (LADEE) [1] demonstrated a moon to ground link with >600 Mbit / s data rate by using pulse position modulation (PPM). This the modulation method of choice in “photon starving” communication links, were less than 1 photon / bit is the design link budget [2].

The European Data Relay System is currently established to represent a near earth data relay service giving a 1.8 GBit/s data download capability, especially for Low Earth Orbit space crafts and high flying aircrafts like high altitude long endurance (HALE) platforms. Tesat Spacecom has been contracted by the German space agency to develop, qualify, and deliver the laser communication terminals (LCT) for a number of LEO and GEO spacecrafts.

II. HOMODYNE DETECTION

The communication principle of the EDRS Lasercom [3] is based on homodyne binary phase shift keying (BPSK) at 1064nm wavelength that possesses the lowest photon / bit ratio for „photon rich“ optical links, were ~10 Photons / bit are available, enabling a power efficient communication at distances from 1000km to 100000 km range. The optical communication principle is always bidirectional, even in the cases, were the forward channel (from ground to LEO Spacecraft) is not used, as in the case for the Sentinel earth observation spacecrafts. BPSK ensures communication even with sun in the field of view of the receiver. In the cases were

a bidirectional communication is not required, the link from the receiver terminal to the transmitter is used as beacon for the transmitter.

III. THE TESAT LASER COMMUNICATION TERMINAL (LCT)

The LCT is a single frame unit with approximately 60*60*60 cm. Fig. 1 shows the principle structure and functional blocks. Weight is around 50 kg, average power consumption 160W.

Fig. 1: Block diagram of the LCT

The LCT uses a common transmit (TX) and receive path (RX), both are separated by polarization and laser frequency offsets in order to eliminate self blinding. The 135 mm aperture telescope is a 4 mirror off-axis design from Ruag Space (Swiss), combining high wave front accuracy with high throughput and minimum back scatter. The coarse pointing assembly (CPA) is from Synopta (Swiss), having a high absolute pointing accuracy over the complete hemispherical field of regard. The CPA is equipped with optical (sub µrad resolution) encoders, developed by Tesat in cooperation with Renishaw [4] based on their patented detection principle. The LCT is qualified for 15 years GEO operation under extreme conditions (exposed to space without structural shielding for radiation and temperatures).

Tesat is currently contracted for seven identical LCTs embarked on 3 GEO and 4 LEO missions (Alphasat, EDRS-A,

2014 7th Advanced Satellite Multimedia Systems Conference and the 13th Signal Processing for Space CommunicationsWorkshop (ASMS/SPSC)

978-1-4799-5893-1/14/$31.00 ©2014 IEEE 284

EDRS-C (all GEO missions), Sentinel 1A/B and Sentinel 2A/B (LEO earth observation missions) in the ESA Copernicus program).

Fig. 2: The Sentinel 1 A S/C separating from the Fregat upper stage of the

launch vehicle, LCT is located inside the launch adapter, 03.April 2014.[5]

Fig. 3: EDRS-A LCT shortly before delivery to Airbus Space and Defence -

Toulouse 27.May 2014

IV. LCT VERFICATION TEST CAMPAIGN

The LCT is tested as a complete subsystem at Tesat premises.

All subsystem (lasers, electronics, mechanism) are acceptance

tested prior to integration of the units into the LCT frame

structure. Performance measurements are done in a thermal

vacuum chamber having high quality optical windows and

realistic thermal conditions (shroud temperatures can vary

from -160°C to + 70°C). A piezo shaker is installed for

validation of the acquisition and communication performance

under in-orbit micro vibrations. The optical performance of

the LCT is evaluated by wave front sensors and

interferometers, located outside the TV chamber. Typically the

LCT is subjected to protoflight environmental conditions

during the shake and bake tests. The installation of the LCT

onto the space craft is routinely performed in 3-4 days. The

mechanical integration is done in a few hours.

Fig. 4: LCT in thermo vacuum (TV) chamber

V. DATA RELAY VERSUS DIRECT DOWN LOADS FOR NEAR

EARTH APPLICATIONS

The advantages of a near earth data relay service in

comparison to conventional data dumps is that the LEO

spacecraft can download the data whenever a GEO

counterpart is in the field of regard. Fig. 5 illustrates the use

of the GEO communication node as virtual ground station.

Fig. 5: Left side, typical LEO data dump scenario, Xband radio frequency

(RF) downlink (300-600 MBit/s) over a polar RF ground station. Right side;

Data relay service performed over an 1.8 GBit optical link from LEO to GEO.

Increased data volume / second and contact times allow more data / orbit to be

transferred. Bidirectional links can be used for rescheduling the LEO´s

observation program

2014 7th Advanced Satellite Multimedia Systems Conference and the 13th Signal Processing for Space CommunicationsWorkshop (ASMS/SPSC)

978-1-4799-5893-1/14/$31.00 ©2014 IEEE 285

The link planning of the LEO to GEO communications can be

done a few days in advance, depending on the orbit forecast

accuracy and space craft autonomy requirements.

VI. SINGLE PHOTON DETECTION IN PRESENCE OF

BACKGROUND RADIATION

The intrinsic advantage of homodyne or heterodyne detection

is the high spectral selectivity of the receiver even without

additional narrow band optical filters. The sun penalty for a

1.8 GBit homodyne BPSK transmission (transmitter is in front

of the sun disk) is measured to be within 0.5 dB. The

heterodyne detection principle can be utilized in deep space

communication links were the sun is close to or directly in the

field of view (L1 missions or mission to inner planets). For

link budget reasons the receiver apertures have to be

significant. 7 m in space or 10 m on ground are reasonable

sizes. Currently the on ground solution is clearly preferred.

For heterodyne PPM detection, in contrast to direct detection

system, the transmitter is a single frequency laser source, were

the pulse spectral contend is transform limited. The receiver is

basically a photo diode were a continuous wave (cw) single

frequency local oscillator (LO) with mW power level is co-

aligned to the received light. This enables a shot noise limited

receive performance. The frequency offset of the LO to the

RX light is chosen to be in the range of an electronically

matched filter, additionally rejecting unwanted background

light interference and electronic noise.

Fig. 6: Tesat single photon receiver, local oscillator delivery fiber is on lower

right

A. Ground segment modifications for coherent PPM detection

For coherent detection a direct detection system has to be

modified: The RX telescope has to be able to tolerate direct

sun input into the system. Furthermore, a single frequency

laser, the coherent receiver, and a slow Doppler frequency

compensation loop have to be installed. The necessary phase

front reconstruction can be performed for example by the sun

itself, a system that is already introduced for sun observation

telescopes by standard adaptive optics systems. This is

especially useful for L1 missions were the sun is present in the

field of view of the RX telescope.

B. Performance of a coherent single photon receiver.

Tesat has build and flight tested a receiver optimized for

pulsed coherent single photon detection, see Fig. 6. The

receiver sensitivity was tested down to -80dBm received

power, equivalent to a 1 photon/ 20 ns equivalent pulse, see

Fig. 7. The dynamic range of the receiver is more than 40 dB.

Fig. 7: Sensitivity of a heterodyne pulse detection receiver. -80dBm Beat

frequency is 125 MHz.

The receiver itself is a double quad cell, the signal response on

the combined four segments can be used for tracking

purposes.

References

[1] Space Technology Innovation: Enabling Exploration Above—Improving

Life Below; conference Jan 14.2014

[2] The Mars Laser Communication Demonstration* Stephen A. Townes et al. IEEE Aerospace Conference, Big Sky, MT, March 6-13, 2004.

[3] Status of the European Data Relay Satellite System, Michael Wittig et al. Proc. International Conference on Space Optical Systems and Applications (ICSOS) 2012, 5-1, Ajaccio, Corsica, France, October 9-12 (2012)

[4] High Precision Encoders for GEO Space Applications, Martin Reinhardt et al.,Proc. International Conference on Space Optical Systems and Applications (ICSOS) 2012, 8-1, Ajaccio, Corsica, France, October 9-12 (2012).

[5] Separation video is available at the Esa home page.

2014 7th Advanced Satellite Multimedia Systems Conference and the 13th Signal Processing for Space CommunicationsWorkshop (ASMS/SPSC)

978-1-4799-5893-1/14/$31.00 ©2014 IEEE 286

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