3.1

DESIGN OF THE OPTICAL COMMUNICATION TERMINAL ON AIM (OPTEL-D)

OPTEL is the name of a family of optical communication terminals designed over the last 15 years by RUAG Space. OPTEL-D, where D stands for deep space, is based on this design heritage and on components that are available to technology readiness level up to TRL6. OPTEL-D’s single mirror coarse pointing assembly (CPA) can move in azimuth and (to a limited extend) in elevation direction and is used to sequentially perform communication links with Earth and laser ranging of the asteroid’s surface. Figure 2 shows the CPA mounted onto an interface plate connected via three vibration-damping legs (yellow) to a satellite panel. A 135 mm aperture diameter telescope and all optical and electronical components are located on the opposite side of the interface plate covered by a housing and thus inside the spacecraft. The use of an interface plate for the optical head ensures a parallel integration flow of the spacecraft. The alignment of the optical head can be finalized at the OPTEL-D payload integrator without interference with the satellite platform integration.

Figure 2.

OPTEL-D laser communication terminal schematically mounted on a satellite panel (base plate adapter) with the coarse pointing assembly protruding out of the S/C and the optical bench inside the S/C

Figure 3 shows the schematic design of OPTEL-D. The light from an Earth Beacon is received via the coarse pointing assembly (CPA) and an off-axis telescope assembly (TLA) with a free optical diameter of 135 mm.

Figure 3.

Schematic design of OPTEL-D

The technology readiness level of the TLA is TRL 9 (used in all laser communication terminals of the European Data Relay System, EDRS). The Earth Beacon light may be superimposed with light from an Inertial Pseudo Star Reference Unit (IPSRU). This can be used to cancel out platform vibrations, improving the back-to-earth pointing capabilities of the terminal. The implementation of an IPSRU has not been decided yet. This is a trade-off between payload and platform complexity.

The Earth Beacon is also routed via two Dichroic Beam Splitters (DBS) onto Mercury Cadmium Tellurite (MCT) Beacon Camera and TX Monitor. It provides the pointing, acquisition and tracking functionality and enables calibration of the transmitter and receiver via a Point Ahead Assembly (PAA), a corner-cube retro-reflector (Retro) and the DBS.

The laser data transmitter is routed in the opposite direction via a Point Ahead Assembly (PAA).

A laser ranging functionality (spacecraft to Didymos) will be implemented into OPTEL-D. The currently preferred concept is the use of a circulator inserted into the single-mode output fiber of the power laser transmitter. With the second output of the circulator connected to an InGaAs detector OPTEL-D will be able to simultaneously transmit and receive over link distances below 100 km. A pseudo-random bit sequence will be transmitted towards the target and upon reflection cross-correlated for range determination. Scanning the target with the FPA (or PAA) will produce a three dimensional surface map to support the scientific research objectives.

3.2

ESA OPTICAL GROUND STATION (OGS)

The European Space Agency’s Optical Ground Station (OGS) is located at the Observatorio del Teide (OT) in the Canary Islands (Tenerife, Spain) shown in Figure 4. Its main purpose is to check out and commission laser communication terminals onboard orbiting spacecraft, i.e. EDRS. In addition the OGS is used for space debris, asteroid and near Earth object detection and for standard astronomical observations.

Figure 4.

The Observatorio del Teide in Tenerife, Spain (left) and OGS building, which open dome shutter (right).

The Canary island archipelago also offers the opportunity to perform inter-island experiments. From the island of La Palma, at a distance of 142 km, laser and quantum communications experiments are being performed.

The OGS building houses a 1 m aperture diameter Zeiss telescope. Current development status of the AIM communication terminal confirms that all objectives for OPTEL-D can be met with the existing 1 m telescope. For the AIM mission the availability of larger ground based telescopes is being investigated and negotiations are ongoing with the Rohzen National Astronomical Observatory in Bulgaria. Rohzen could make a 2 m Zeiss telescope available and serve as a backup baseline.

The 1 m sized OGS will also validate the use of larger telescopes for future deep-space optical communication missions. From a link budget point of view the ground telescope diameter scales linearly with link distance and a successful link with the 1 meter diameter OGS telescope over 0.5 AU would therefore validate a 5 m diameter telescope over 2.5 AU (longest distance to Mars) and a 12 m telescope over 6 AU (longest distance to Jupiter).

Figure 5 (left) shows the equatorial mount of the OGS telescope. It is also called an English mount, because the hour axis (which is parallel to the Earth axis) is supported on both sides, by a (higher) North pillar and a South pillar.

Figure 5.

Equatorially mounted 1 m Zeiss telescope of the OGS (left) and Coudé optical path schematics (right)

Figure 5 (right) shows the Coudé optical path routing via an evacuated feed-through, which prevents turbulent air exchange and therefore scintillation effects between the telescope floor and the Coudé floor. However, for the AIM mission the Coudé path will be altered by removing the first mirror at the lower right end of the hour axis, such that the Coudé focus will move to the telescope floor inclined by the latitude of the OGS, namely by 28.3 degrees. In this way all refractive elements in the Coudé path are avoided and powerful uplink beacon lasers can be launched from there.

The OGS laser uplink beacon will be intensity modulated at half the camera frame rate on AIM. By subtraction of subsequent image frames the beacon laser can be distinguished from the Earth background and other sources.

Laser downlink communication from the AIM spacecraft will be performed from the stationary Coudé focus.

The receiver design is based on the development for the Lunar Laser Communication Demonstration (LLCD), where a ½-rate serially concatenated pulse-position modulation (SCPPM) turbo like code was implemented. The modulation format is 16-PPM and the user data rate was 38.55 Mbps. For the AIM mission the modulation format will stay 16-PPM, but the user data rate is reduced to 195 kbps at the longest link distance of 75 Mio km.

3.3

LINK BUDGET CALCULATIONS

Table 2 shows the downlink budget from the AIM spacecraft to the OGS and Table 3 the uplink budget. Both tables indicate the budgets for the maximum and the minimum link distances of 75 Mio km and 15 Mio km respectively.

Table 2

Downlink Budget from AIM to OGS

Table 3

Uplink Budget from OGS to AIM per detector pixel

For the downlink a 3 Watt average power limited amplifier is assumed and the modulation format is 16-PPM. To keep the energy density low, the pulse duration is set to 512 ns. Adding 4 guard slots to the 16-PPM modulation the symbol duration is 10.24 µs and the pulse repetition rate 97.7 kHz. This results in a raw data rate of 391 kbps. With a code rate of 0.5 the effective user data rate becomes 195 kbps from 75 Mio km link distance. During its mission the AIM spacecraft will approach earth further resulting in a reduced free space loss. This gain in received photos per laser pulse will be used to increase the data-rate to the Mbps regime. With constant transmitted average power, one can increase the pulse repetition rate and the data rate.

The effective duty cycle of the 16-PPM modulation with guard slots is 0.05 resulting in a modest laser peak power of 100 Watts, which can be handled by space qualified fiber amplifiers.

The detector diameter at the receiving OGS is 200 µm, covering a field of 3.1 arcsec with a focal length of 13.3 m. The downlink budget is put in relation with the blue-sky brightness (at daytime), however all communication downlinks from Didymos in 2022 will be performed with the OGS in darkness and the blue-sky brightness can be ignored.

With the optical power of 1.8 pW arriving at the detector surface, we expect to achieve data-rates close to the channel capacity by making use of SCPPM coding. According to first simulations based on a Poisson noise dominated, PPM communication channel, we need to ensure a detector noise below l_d < 10⁶ with a detector quantum efficiency of QE > 0.5 (s. Figure 6).

Figure 6.

Channel capacity dependence on detector dark noise (left) and quantum efficiency.

In Table 3 the uplink budget is calculated for four 600 W laser beacon sources transmitted from the OGS at wavelengths of 1064 nm. The transmit apertures are selected as 250 mm. This leads a beam divergence of the laser beacon (35.3 µrad at 1/e²), which needs to be aligned to the receive path and to calibration stars with an accuracy at least five times better.

A 2.4 kW beacon needs to be launched from multiple sub-apertures (e.g. four) of the OGS telescope in order to reduce the power density of the optical surfaces. This can be done by using four individual lasers or with a single laser split into four beams. The four laser solution has the advantage of operational redundancy and of a more homogenous far-field illumination pattern caused by the superposition of four laser beams. A single laser solution would generate stronger interference effects (fades and surges) in the far field caused by beam interference due to the coherence length of the laser.

The uplink budget for the beacon laser is presented in Table 3. At longest link distance, the earth background outshines the received optical power by almost 10 dB. Therefore, a slow modulation of the uplink laser is used to distinguish between background and signal photons.

3.4

OGS RECEIVER/TRANSMITTER DESIGN AND ALIGNMENT

The transmit laser is coupled into the telescope’s Coudé path via a dichroic beam splitter (a glass plate that reflects the wavelength spectrum above 1.2 µm and transmits the wavelength spectrum below 1.2 µm), whose reflection is optimized for 1.06 µm and transmission for 1.55 µm.

For alignment of the optical axes of transmitter and receiver, a high precision (<1 arcsec) corner-cube retro-reflector (CCRR) with a diameter of 120 mm is mounted at the 1 m aperture of the OGS telescope. A weak 1550 nm pilot laser beam, co-aligned to and emitted from the transmit laser, is then retro-reflected through the telescope and the beam splitter towards the four quadrant detector.