American Institute of Aeronautics and Astronautics

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Satellite Payloads for Optical Telecommunications

Valeria Catalano*, Lamberto Zuliani† Agenzia Spaziale Italiana

Viale Liegi 26, Roma, 00198, Italy

The increasing demand of bandwidth for satellite communication applications forces the scientific community to explore higher and higher frequency ranges. Whereas frequency range availability is being solved from a regulatory point of view, new technologies are required to enable broadband services and applications in high frequency bands.

Nomenclature Eb/N0 = energy per bit to spectral noise density ratio G/T = antenna gain-to-noise-temperature

I. Introduction

Starting from the heritage acquired through the SIRIO program in S-band(2), ITALSAT F1 and F2 missions in Ka and Q/V bands(3), and the DAVID (Data and Video Interactive Distribution) scientific mission in the W-band(4), the Italian Space Agency (ASI) identified three advanced technology projects for broadband satellite telecommunications respectively at Q/V, W and Optical bands. This paper describes the main technological developments required to perform the identified Optical Telecommunication missions.

II. Optical Telecommunication Optical Telecommunications (OT) technologies will play a fundamental role in future applications, as shown by

many international programs in this field. ASI started to make investments in optical research since many years, and a telescope, located at its Operative Center in Matera – Italy, is already used for Lunar laser ranging. In 2004 ASI financed a feasibility study for an Optical Band Telecommunication Payload.

The study examined different missions in which the optical payload could be used, i.e. LEO to Ground, LEO to LEO, LEO to UAV, MEO to LEO, GEO to LEO, GEO to GEO, GEO to Ground, and Interplanetary mission. All the missions have been analyzed in order to meet a set of major spacecraft requirements, reported in figure 1, like the transmitting wavelength (1550 nm), the uplink and downlink data rates (up to 2.5Gbits), and the Bit Error Rate (10-9).

During the link budget evaluations a particular emphasis has been given on several phenomena that affect the light propagation through the atmosphere. A laser beam propagating through the atmosphere can quickly lose useful energy due to molecular scattering, molecular absorption, and particulate scattering. Refractive turbulence may also contribute to energy loss, however, mainly degrading the beam quality, both by distorting the phase front and by randomly modulating the signal power. The presence of opaque clouds may occlude the signal completely rendering the time-of-sight communication link useless. The problems described above are quite distinct from each other, and the difficulties presented by each of these obstacles were examined independently, in order to determine the minimum antenna diameter necessary to obtain 2.5 Gbps of minimum data rate and assuming a BER=10-9. The results are summarized in Fig. 2.

* Telecommunication and Navigation Technology Department, Agenzia Spaziale Italiana † Head of the Telecommunication and Navigation Technology Department, Agenzia Spaziale Italiana

SpaceOps 2006 Conference AIAA 2006-5949

Copyright © 2006 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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Spacecraft requirements:

The attitude accuracy in roll, pitch and yaw: ±0.15° (±2.6 mrad) to ±0.2° (±3.5 mrad) each TBC

The uncertainty of the angular velocity: ±0.0057° (±100 µrad/s) to ±0.02°/s (±350 µrad/s) each TBC

PAT requirements

Point Ahead Angle (LEO missions) ~ 50 µrad

Point Ahead Angle (GEO missions) ~ 72 µrad

Point Ahead Angle (GEO to GEO missions) ~ 0.5 µrad

Orbit Determination (LEO missions) ~ 2.0 mrad

Orbit Determination (GEO missions) ~ 48 µrad

Optical requirements:

Telescope Terminal single telescope terminal shared between Tx and Rx sections

Telescope wavelength (Tx and Rx section): 1550 nm

Beam divergence Tx: 100 µrad

Tx mode coupling: single mode coupling

Rx mode coupling: single/multi mode

Optical Transceiver:

Power Rx demodulator: Optical Transceiver Rx demodulator always ON

Power Tx modulator: Optical Transceiver Tx Modulator ON/OFF capability

Tx uplink data rate up to 2.5 Gbps

Rx downlink data rate: up to 2.5 Gbps

modulation type optical NRZ-L

Receiver section technology: APD or PIN detector

BER 10-9 with a minimum sensistivity of –35 dBm

OBDH requirements:

MMU at least 100 Gbits

MMU transfer data rate up to 2.5 Gbps

FEC: RS+Viterbi hardware implementation at 2.5 Gbps

Figure 1. Optical payload general requirements

Mission Transceiver Telescope Ground segment LEO (all missions) multi mode Rx

single mode Tx (0.1 W) multi mode Equivalent optical Telescope 40-90 mm

LEO (all missions) single mode Rx single mode Tx (0.1 W)

single mode Equivalent optical Telescope 40-90 mm

GEO (all missions) single mode Rx + Opt Ampl single mode Tx + booster (1 W)

single mode Equivalent optical Telescope 180-360 mm

Figure 2. Summary of the optical link budget results The functional architecture of the optical payload is sketched in Fig. 3. It is subdivided in two sub-blocks: the Remote Unit, which is the electronic part of control and modulation/demodulation of the optical signals, and the Optical Head mainly composed by the telescope, the Optical bench, and the Pointing Attitude and Tracking (PAT) System. The Telescope, the beacon and its relative optical bench constitute the main optical equipment mounted inside the payload. The main goal is to focus the optical source during the transmission toward the external optical terminal and, vice versa, to focus the incoming external optical source into the receiving area sensor. The PAT system keeps the optical head aligned with the line of sight between the satellite and the external optical terminal by monitoring the received beacon signal. The actuation can be performed at different stages: a coarse pointing system (e.g., by means of gimbals), a fine pointing system, (e.g., by means of a fast steering mirror), a fiber alignment system (e.g., by fast fiber nutation). The Optical Transceiver provides the capability for acquiring telemetry from the spacecraft and translates it optically. It acts as the electrical-optical interface inside the optical payload. It includes all elements for reception, demodulation, modulation and transmission. Its fundamental functions shall be receiving and demodulating the uplink signals in the Optical Band for telecommand purposes, and modulating and transmitting the downlink

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LOW-END TERMINAL

OPTICAL HEAD

Receiverelectronics

Amplifier for reception

Transmitter electronics

Amplifier for transmitter

Optical bench

Telescope and beacon

PATsensor

PATmechanism

Thermal control HW

On-Board Processing Unit

Power Supply

REMOTE UNIT

Receiver transceiver

Transmitter transceiver

LOW-END TERMINAL

OPTICAL HEAD

Receiverelectronics

Amplifier for reception

Transmitter electronics

Amplifier for transmitter

Optical bench

Telescope and beacon

PATsensor

PATmechanism

Thermal control HW

On-Board Processing Unit

Power Supply

REMOTE UNIT

Receiver transceiver

Transmitter transceiver

Figure 3 - Optical payload functional architecture

telemetry data signals in the optical band. The On Board Processing Unit of the Optical Payload supervises electronically the Optical Payload and manages the control communication with the Spacecraft. In particular the On

Board Data Handling (OBDH) is related to the mechanism and sensor control electronics and implements the PAT control electronic law. Finally, the decoding and encoding of receiving and transmitting data is performed internally the On Board Processing Unit. During the encoding and decoding phase it is possible, in accordance with the mission requirements, to adopt particular error recovery coding to improve the overall reliability of the optical transmission.

On the basis of the requirements and the results of the link budgets reported in Fig. 1 and Fig. 2, different payload configurations were considered (Fig. 4). It is straightforward to see that different mission require similar payload configurations thus reducing the payload analysis only to three different mission typologies (with increasing stringent requirements): (a) LEO to Ground (missions 1–4), (b) GEO to Ground (missions 5–9), (c) Deep Space to GEO (mission 10) and consequently GEO to Ground.

The Pointing Attitude and Tracking (PAT) System requires the implementation of a Coarse Pointing Assembly (CPA), a Pointing Ahead Angle (PAA), Fine Pointing Assembly (FPA), and an Acquisition Sensor (AS) for the (a) mission typology. A Tracking Sensor (TS) for faster tracking within a small area is required for mission typologies (b) and (c). The Rx optical transceiver also presents different requirements depending on the mission typology. While for mission typology (a) single and multi mode communications can be foreseen by means of an avalanche photodiode detector (APD), single mode was investigated for long distance communications – mission typologies (b) and (c) – by means of both APD and p-type-intrinsic-n-type (PIN) detectors. Erbium doped fiber amplifiers (EDFA) need to be employed for mission typology (c) as well as in the Tx optical transceiver when an excessive gain loss forces to boost the intensity of optical signals. The Mass Memory Unit (MMU) is another critical part of the payload. The high data rate achievable with optical communications (up to 2.5 Gbps) needs a storage unit of large capacity whose reliability is increasingly affected by radiations with respect to larger capacities. Finally Forward Error Correction (FEC) algorithms should be hardware-implemented in order to provide fast responses with respect to the considered bit rate.

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N. MISSIONS PAT Configurations OPTICAL TRANSCEIVER OPTICAL TRANSCEIVER MMU FEC

RX TX RS+Viterbi

1 LEO to Ground CPA PAA FPA TS AS 1) multi and APD detector single @ 100 mW 10-100 Gbits Sw

2)single and APD detector single @ 100 mW 10-100 Gbits Hw

2 LEO to Ground CPA PAA FPA TS AS 1) multi and APD detector single @ 100 mW 10-100 Gbits Hw

2)single and APD detector single @ 100 mW 10-100 Gbits Hw

3 LEO to LEO CPA PAA FPA TS AS 1) multi and APD detector single @ 100 mW 10-100 Gbits Hw

2)single and APD detector single @ 100 mW 10-100 Gbits Hw

4 LEO to UAV CPA PAA FPA TS AS 1) multi and APD detector single @ 100 mW 10-100 Gbits Hw

2)single and APD detector single @ 100 mW 100 Gbits Hw

5 MEO to LEO CPA PAA FPA TS AS TS 1) single APD single @ EDFA 1 W >100 Gbits Hw

2) single PIN single @ EDFA 1 W >100 Gbits Hw

6 GEO to LEO CPA PAA FPA TS AS TS 1) single APD single @ EDFA 1 W >100 Gbits Hw

2) single PIN single @ EDFA 1 W >100 Gbits Hw

7 GEO to GEO CPA n/a FPA TS AS TS 1) single APD single @ EDFA 1 W >100 Gbits Hw

2) single PIN single @ EDFA 1 W >100 Gbits Hw

8 GEO to Ground CPA n/a FPA TS AS TS 1) single APD single @ EDFA 1 W >100 Gbits Hw

2) single PIN single @ EDFA 1 W >100 Gbits Hw

9 ISL CPA n/a FPA TS AS TS single PIN single @ EDFA 1 W >100 Gbits Hw

10 Interpl Mission CPA PAA FPA TS AS TS single PIN + EDFA single @ EDFA 30 W >100 Gbits Hw

Figure 4. Optical Payload possible configurations

The differences in the three mission typologies inspire the evolution plan that ASI is currently pursuing. The

LEO to Ground mission typology represents a prototype mission that can be based on a Commercial Optical Transceiver. Since the PAT and the coupling between telescope and optical transceiver (single or multi mode) appear as the most technical demanding points, it should be necessary to realize a simplified ground prototype to validate the PAT control law and to test the coupling between the telescope and the optical transceiver. The same architecture (single mode coupling) used during the LEO mission, but improved with the EDFA booster in transmission, can be updated and enhanced to be used in the GEO to Ground mission in a subsequent phase. The last and future step will be the updating of the optical payload –used and tested during the GEO to Ground mission – for an Interplanetary mission.

In the last months of 2005 ASI financed a phase A2 study for the analysis of optical links from a Stratospheric Aircraft to Ground and from a Stratospheric Aircraft to LEO, including the International Space Station. The first step of phase A2 will be to consolidate the mission scenarios and the analysis of Ground Station. The possibility of utilizing Matera ground station will be validated before the definition of the stratospheric mission. The second step will be to perform the preliminary design for the Optical Telecommunication Payload. In this phase, the accommodation on the M55 stratospheric aircraft, in particular the interface M55 aircraft – OTP, will be also designed.

The ISS – stratospheric aircraft link will be also analyzed and all the opportunities on board (the Columbus External Payload Facility, EXPRESS Pallet or Japanese Experiment Module Exposed Facility) will be evaluated. The interior accommodation in Nadir Research Window will be also considered. During the preliminary design phases of the stratospheric mission it is foreseen to perform the final selection of the critical technology to be verified during the prototyping phase. The prototype will be designed, manufactured, integrated and tested. It will include the predevelopment of the following parts:

• Optical Transceiver: it will generate the optical signal starting from and electrical signal; • Telescope and relative pointing mechanism: the telescope transmits light generated by the optical

transceiver and/or collects light coming from the other terminal and focalizes the laser beam into the optical fibers. It will be installed on a pointing mechanism able to mechanically sustain it and to move it in order to reach the correct pointing;

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• Pointing mechanism drivers and control logic: the pointing mechanism is controlled by a motor driver and control SW that will implement the acquisition and pointing algorithm.

After being integrated, the prototype will be tested in an indoor and open field campaign. The objective of this indoor demonstrator is to test the complete system integration and to perform the link experiment without atmospheric effect. An indoor optical free space link of approximately 200 meters length will be realized by using the sub-system supplied from the project partners. In the experimental indoor demonstration the parameters measured are: Optical transmitted and received power, Optical transmitted and received power spectrum, Bit Error Rate measurements versus received optical power and power budget measurements without atmospheric effect. The previous parameters will be measured in case of perfect telescope alignment and not, in order to simulate a little misalignment error. The objective of this open field demonstrator is to test the complete system integration and to perform the link experiment with atmospheric effect in order to emulate the Ground to aircraft link. The link length will be 10 – 20 Km; the final distance value will be evaluated on the basis of the results of the optical link analysis and on the site availability. The same parameters analyzed in the indoor campaign will be measured in the open field. In this way it will be possible to correlate the obtained measurements in indoor and outdoor situations in order to have an experimental characterization of the atmospheric signal degradation.

III. Conclusion We presented one of the projects that ASI is coordinating on advanced technologies for satellite broadband

telecommunications. We described the general results obtained during the definition of an optical payload for which ASI is also planning international collaborations.

Acknowledgments The authors would like to thank the Industries and Research Institutions for their precious contributions in the

presented feasibility studies. Special thanks go to: Alcatel Alenia Space Italy, Prime Contractor of the Optical Band feasibility studies.

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