September 30, 2016

Satellite

Germany's Lidar payload will be mounted on a French Myriade Evolutions platform for small satellites. The Myriade satellite series was initiated in 1998. It provided for a consistent range of mission objectives and offered scientists a versatile tool for testing instruments weighing about 60 kg and requiring a power supply of some 60 W. The series was designed for short, low-cost missions with a rapid development schedule.

The Myriade Evolutions programme is an enhancement of the original concept, offering the payload more allocated power (around 150 W) and a greater mass (around 140 kg). The main improvements are the new structure compatible with a satellite mass up to 400 kg; an increased solar array capacity and power supply; a more effective propulsion system; an improved Attitude and Orbital Control System (AOCS); an increased capacity for payload data storage; improved reliability figures; obsolescence handling and finally compatibility with low orbits (addressing atomic oxygen issues).

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Merlin platform based on Myriade Evolutions

The previous figure shows a layout of the Merlin platform. The tank is situated in the middle of the bottom floor panel, the four thrusters being under this same panel. Two S-band antennas and two GNSS antennas can be seen on the outer part of the side panels. The three magnetorquers could be accommodated inside or outside the platform. The following equipment units are fitted inside the platform: a power control and distribution unit, a battery, a central computer with its interface unit, a mass memory, four reaction wheels, two magnetometers, two S-band transmitters and receivers, an X-band transmitter, two GNSS receivers and a star tracker.

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Merlin satellite

The previous figure show the general aspect of the Merlin satellite. The satellite is presented in its operational configuration with the solar arrays deployed. The Lidar payload is housed on the upper panel of the platform. Some platform units are located close to the instrument: two star tracker optical heads with their baffle, two solar arrays, one X-band antenna and a Sun sensor notably to take into account a Sun avoidance mechanism.

Artist view of the Merlin satellite in orbit
© CNES/Illustration D. Ducros

The satellite integration is foreseen in the Airbus Defence and Space France facilities in Toulouse, beginning with the platform activities, continuing with the payload accommodation on the "upper" platform panel, after the delivery of the payload by Germany to France.

Satellite integration
© Airbus Defence and Space

The main components of Merlin lidar are the instrument control unit (a), laser system (b), a frequency reference unit (c), an energy calibration unit (d), detection unit (e), receiving telescope (f) and transmitting telescope (g).

Merlin will be the first in-orbit IPDA Lidar system. Because of the development cost and qualification issues, it is not possible for all sub-systems to use state-of-the-art technologies that are not yet space qualified. The choice of sub-systems must also take into account:

  • Power consumption: limited due to the Myriade Evolutions capabilities,
  • Mass and volume: limited due to platform and launcher constraints,
  • Thermal behaviour: the system must be thermally regulated with high precision despite the harsh conditions in orbit,
  • Resistance to radiation: the system has to work under space conditions and no degradation due to radiation is allowed.

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Basic instrument setup

(a) instrument control unit (ICU), (b) laser, (c) frequency reference unit, (d) energy calibration unit, (e) receiver unit, (f) receiver telescope, (g) transmitter optics

The Instrument Control Unit (ICU) is the main instrument control element. It enables the internal instrument timing, creates the necessary control and bias voltages, and reads the housekeeping data. The other part of the ICU is the high-speed ADC read-out.

The choice of an appropriate laser wavelength for an IPDA instrument depends mainly on the following factors: methane absorption bands, detector efficiency and eye safety considerations. In the short-wave infrared where eye safety is less critical, methane lines are abundant, but the presence of water vapour and carbon dioxide lines drastically constrains the choice. Two atmospheric water vapour transmission windows around 1.6 and 2.3 µm allow methane measurements. Detector performance is significantly better in the 1.6 µm region where low-noise InGaAs avalanche photodiodes are available. Furthermore, the scientific interest in methane measurements focuses on the tropospheric methane volume mixing ratio. A space Lidar system has to be small, lightweight, reliable and very efficient while using little power. The laser used for the Merlin mission is not a completely new development since it builds on work carried out for ESA's FULAS laser (FUture LAser System) electrical and qualification model, currently under construction. Therefore the Merlin mission will completely benefit from this FULAS heritage. Laser parameters are listed in the table below:

onoffOutput pulse energyPulse repetition rate
(for double pulses)
nmnmmJHz
1645.5521645.846920

The purpose of the Methane Frequency Reference Unit (MFRU) sub-system is to provide laser signals at both the on-line and off-line frequencies to be injected into the respective pulsed Optical Parametric Oscillator (OPO). The laser's stability and knowledge of the output signal frequency have a strong influence on measurement quality. They are also the main contributors to the Relative Systematic Error, which is why special care has to be taken when designing this unit. The current design is based on a wavelength meter using a Fizeau etalon. The wavelength meter will be periodically calibrated for an absolute wavelength using a reference diode whose wavelength is locked through a methane gas cell and the associated control loop.

The Merlin IPDA Lidar is a remote sensing system which relies solely on the relative intensity measurement of signals. The intensity of the recorded return signal is a direct function of the emitted pulse energy. Even for very good lasers, there is a variation of some 2 to 4%. It is therefore necessary to accurately monitor the energy of each emitted laser pulse and to correlate this with the intensity of the received signals. This makes it possible to compensate for pulse energy variations throughout the IPDA measurement on a pulse by pulse basis. In order to generate these internal calibration signals, a small fraction of the emitted laser pulses is extracted and directly fed to the instrument detector, which also records the signal received from the target. Using the same detector to measure the return and calibration signals ensures that there is no difference in the response of the processing of the signal, so both signals can be directly compared. The challenge is also due to the very weak return signal, the reference signal has to be attenuated by a factor of a significant magnitude. This attenuation has to be stable over the whole period of operation so that the signal remains within the same range. The current baseline design for the energy calibration unit uses an integrating sphere and optical fibres for attenuating, or even removing, local hot spots.

The transmitter optics are designed to adapt the natural angular divergence of the laser beam such that the illuminated ground spot has the required diameter. The spot on the ground must be large enough to be consistent with the total error budget when considering the effect of correlations between heterogeneous surface reflectance and laser pointing jitter in both wavelengths. On the other hand, it has to be small enough to limit surface height variations within the footprint to reduce the probability of cloud contamination, enabling observation in broken cloud conditions.

The area of the receiving telescope has to be as large as possible. The current design for the primary mirror is a lightweight structure of about 0.69 m in diameter built as an afocal Cassegrain-Mersenne telescope. The choice of material influences not only the weight but also the thermal behaviour, which is critical for space borne applications. A further challenge when designing the receiving telescope is the size of the satellite platform. This already limits the diameter, while the launcher fairing adds height constraints. Yet the field of view (FOV) has to be large enough to contain all of the laser ground spot (including a margin for satellite and laser pointing). Finally, another limiting factor for telescope design is the detector size. With avalanche photodiodes (APDs), the noise increases drastically with detector area, which limits the choice to a detector with a small active area.

Due to the limited laser pulse energy and receiving telescope size, both the noise caused by the receiver electronics and the detector is critical when attempting to meet the required measurement accuracy. Therefore, special attention should be paid to the choice of detector. The detector and subsequent amplifier stages have to generate as little noise as possible. In the 1.65 µm region, InGaAs avalanche photodiodes seem to be the most promising candidates. They can be operated either in linear or Geiger mode. Geiger mode is used to detect single photons. This sounds like an interesting alternative to linear mode, but Geiger mode operation was not considered for Merlin due to the APD's inevitable down time after detecting a photon, which may be of the order of a microsecond. A Merlin laser pulse will last less than 30 ns. Consequently, a maximum of one photon per laser pulse could be detected. Mercury Cadmium Telluride (MCT) detectors are also of interest. They offer some interesting features for detection in low light conditions around 1.65 µm due to their low excess noise. Unfortunately, they are not yet available as a commercial product and cannot therefore be considered for Merlin's baseline design.

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Merlin payload