Michael Lutzer

Three years coherent space to ground links: performance results and outlook for the optical ground station equipped with adaptive optics

Tesat is performing inter-satellite links (ISLs) for over 5 years now. Besides the successful demonstration of the suitability of coherent laser communication for high speed data transmission in space, Tesat has also conducted two major satellite to ground link (SGL) campaigns during the last 3 years. A transportable ground station has been developed to measure the impact of atmospheric turbulence to the coherent system. The SGLs have been performed between the Tesat optical ground station and the two LEO satellites TerraSAR-X and NFIRE, both equipped with a Tesat LCT. The capability of the LCTs of measuring the signal intensity on a direct detection sensor and on a coherent sensor simultaneously makes the system unique for investigating the atmospheric distortion impacts. In this paper the main results of the SGL campaigns are presented, including BER performance for the uplink and downlink. Measured scintillation profiles versus elevation angles at different weather conditions are illustrated. Finally preliminary results of an adaptive optics system are presented that has been developed to be used in the transportable adaptive optical ground station (T-AOGS) acting as the counter terminal for the LCT mounted on Alphasat, a geostationary satellite of the European Space Agency (ESA), in autumn 2013.

Commercial optical inter-satellite communication at high data rates

Laser communication terminals with data rates far above 1 Gbps have been in operation in orbit since January 2008, and the links established between two low Earth orbit (LEO) satellites have demonstrated error-free communication. Bit error rates better than 10−11 have been achieved without data encoding. Signal acquisition can be reproducibly achieved within a few seconds. After adaptation to larger link separation distances these laser communication terminals will be used in the low earth orbit-geosynchronous satellite (LEO-GEO) link of European data relay satellite (EDRS), the GEO European data relay system. LEO-to-ground and ground-to-LEO links have examined the impact of the atmosphere on such optical links. In the future, high data rate GEO-to-ground links will require ground stations equipped with adaptive optics, which are currently under development.

Optical inter-satellite communication operational

Optical inter-satellite communication based on TESAT Laser Communication Terminals (LCTs) is operational by now on LEO satellites for more than two years. The LCTs demonstrate their performance in LEO-LEO inter-satellite links (ISL) and they are used also for LEO-to-ground links to investigate beam propagation through the atmosphere. Based on homodyne BPSK, a highly robust and sun light immune modulation scheme, the LCTs offer a full duplex data rate of 5.625 Gbps at a bit error rate lower than 10−9 for ISL. The LCT operations in the dynamic LEO scenario has shown that spatial and frequency acquisition can reliably be achieved within a few seconds. Due to its demonstrated performance, TESAT LCTs were selected for the European Data Relay Satellite (EDRS) Program of the European Space Agency. Their performance features makes them well suited also for commercial GEO data relay applications and government system requiring high data rates for LEO-to-GEO and UAV-to-GEO links.

Laser communication terminals for the European Data Relay System

For the first time, laser communication terminals will be operational in a commercial service, EDRS, the European Data Relay System. Future earth observation satellites call for satellite-to-ground links with high availability to make their data immediately available to the user. High availability is only achieved by GEO relay links, i.e. communication links from LEO satellites to a geostationary relay satellite which transmits the data to the ground. While RF communication limits the GEO relays data rate to roughly 1 Gbps optical communication will extend its capacity into the 10 Gbps range. Today, laser communication terminals operated in LEO-LEO, LEO-to ground and ground-to-LEO links, all at a data rate of 5.625 Gbps, are applied in campaigns with scientific objectives and for technology demonstrating purposes. Adapted to the larger link distance of the LEO-to-GEO link the laser communication terminals will be applied in the GEO relay EDRS operated for the time being at 1.8 Gbps. Terminals will be delivered for the accommodation on Alphabus, EDRS-A and EDRS-C, European GEO-satellites, and on Sentinel-1a and Sentinel-2a, European LEO satellites.

Coherent inter-satellite and satellite-ground laser links

Since February 2008 coherent laser communication links are operational in-orbit. Transmitting data at a rate of 5.625 Gbps they verify the capability of laser communication exemplarily in LEO-LEO and Ground-LEO constellations. The LEO-LEO links run with a bit error rate of 10-11. Acquisition typically is closed within seconds. On the basis of these results laser communication terminals are adapted to LEO-GEO links with a still scalable user data rate of 1.8 Gbps. The terminals will be applied in the European data relay system.

Alphasat-Sentinel-1A optical inter-satellite links: run-up for the European data relay satellite system

Laser Communication Links in Orbit have become routine for Alphasat TDP1 GEO data relay and Sentinel-1A LEO satellite. Since November 2014, an extensive campaign has demonstrated stable and bit-error free links over distances of up to 45.000km with 1.1W optical transmit power and data rates of up to 1.8Gbps. Link acquisition is achieved reliably within less than 55s. Links with low grazing altitude investigate the impact of atmosphere to link performance. The optical links between Sentinel-1A and Alphasat are in collaboration of ESA, DLR, and TESAT Spacecom. Alphasat TDP1 is the precursor for European Data Relay Satellite System EDRS [1].

Alphasat and sentinel 1A, the first 100 links

The paper details the approach, sequence, and results of the in-orbit campaign for E/O data relay using the Tesat Laser Communication Terminals from a LEO spacecraft - Sentinel 1A - a part of the EU Copernicus program, to the TDP1 payload of Alphasat, Europes largest telecommunication satellite. After configuring the LCT́s on both spacecrafts to a standard operating mode in the first 2 month of the campaign, the next few months have been used for testing the capabilities of the systems. Links with transmit powers as low as 100mW over 42000km and links probing the upper atmosphere are examples of this test campaign. In addition, far field laser beam profile and tracking performance measurements have been performed, using the unique capability of the LCT on board of Alphasat to deliver 25kHz sampled data for all internal sensors (tracking and acquisition).

Roadmap to wide band optical GEO relay networks

Optical inter-satellite communication has been verified in-orbit. Between two LEO satellites bit error rates better than 10-11 at a data rate of 5.625 Gbps are achieved. Optical LEO-to-ground and ground-to-LEO links have been demonstrated. For the first time ESA will apply optical communication for operational services. The LEO-to-GEO link of a GEO relay, EDRS, the European data relay system, shall be established by optical communication at a data rate of 1.8 Gbps with a bit error rate of 10-8. For the time being the GEO-to-ground link still is based on radio frequency communication limiting the overall GEO relay data rate to the value of about 2 Gbps. Significantly higher data rates will be achieved by optical space-to-ground links with ground stations equipped with adaptive optics as has been demonstrated in a free-field measurement campaign. Optical LEO-to-ground and GEO-to-ground links shall be verified in the near future. Optical Air-to-GEO links are under preparation.

Laser Com in space, the operational concept

Laser communication in space is an attractive and available alternative to classical RF communication. Optical communication has been successfully demonstrated and verified in the recent past, and is today used in commercial system. The use of Lasers offer intriguing properties and advantages in particular for point-to-point communications in a variety of applications. Optical data links between satellites are used for data relay, to link LEO Earth observation mission data to ground via GEO-stationary spacecraft. To achieve the best system performance, an adequate operational concept was developed, taking into account the technical and organisational requirements of customers, control centers, and spacecrafts. The concept is based on experience gained during more than six years of in-orbit link experiments, and has been taylored towards commercial application, suiting the needs of European Data Relay Satellite System EDRS.

Progressing towards an operational optical data relay service

The European Data Relay System, EDRS [1], will provide quasi real time access to earth observation data created by low earth orbiting spacecrafts using Gbit laser communication links. Currently five EDRS compatible Laser Communication Terminals (LCT) are in orbit, three of them on earth observation spacecrafts (Sentinel 1A, Sentinel 2A, Sentinel 1B) and two geostationary systems on Alphasat and Eutelsat 9B, the host of the first EDRS data relay payload (EDRS-A). The paper will report on the recent progress on the in-orbit commissioning campaigns for the individual units.