The Miras Payload

MIRAS is the single-instrument payload of SMOS. The mechanical layout of the antenna is Y-shaped with a central support structure (McMullan). With its arms extended, the instrument-weighing 360 kg-has a wingspan of 8 m. Sixty-nine Lightweight Cost Effective (LICEF) receivers distributed uniformly along the three antenna arms and within the centre section constitute the main elements of the 2-D synthetic aperture interferometric design.

Three dual LICEF-sets at the centre double as Noise Injection Radiometers (NIR). Each arm is composed of three segments inter-connected by hinges. The arms are folded by the sides of the central structure during launch. Deployment of the arm segments in orbit is spring activated and is controlled by a synchronisation

system consisting of steel cables and pulleys. The deployment speed is controlled by a speed regulator based on an escape (clockwork) mechanism. A pyrotechnic hold-down system maintains the arms in stowed configuration during launch. A photograph of the deployed flight payload is provided in Fig. 7.

The key element of MIRAS is the LICEF. Figure 8 shows a simplified block diagram of a single receiver and the noise injection network which is used for internal

Fig. 8 Schematic layout of a LICEF receiver with antenna (upper) and Noise Injection Radiometer (lower) both connected to the Noise Distribution Network used for internal calibration (Corbella et al. 2004)

calibration purposes. The four-way input switch selects one of the two antenna outputs (H or V), uncorrelated noise from a resistor at ambient temperature (U) or correlated noise from the noise distribution network (C). The RF part includes a filter to select the band 1404-1423 MHz within the protected radio astronomy range which the mixer down-converts to 8-27 MHz using a local oscillator common to all receivers. Two output signals, the in-phase (I) and quadrature (Q) components, are produced. One of them is sent to the Power Measurement Subsystem (PMS) consisting of a diode detector and integrator acting as a total power radiometer. Simultaneously, both output signals are clipped using zero-voltage comparators to produce 1-bit digital signals which are sent to a centralized matrix of 1-bit 2-level correlators. Each individual correlator cell is an exclusive NOR gate and the correlation is measured by accumulating its output during an integration time at a rate given by the clock frequency fs = 55.84 MHz. Five different kinds of correlation products are available:

• Between I channels of different receivers

• Between Q and I channels of different receivers

• Between Q and I channels of the same receivers

The correlator counts and all PMS outputs constitute the raw data sent to ground. These are used to generate the MIRAS visibility function, the inverse Fourier transform of which gives brightness temperature maps.

In the case of the Noise Injection Radiometer (NIR) depicted in Fig. 8, two LICEFs are permanently connected to the antenna, one each to ports H and V. The NIR is used to measure the full polarimetric antenna noise temperature and the amplitude of the noise injected by the noise distribution network.

Another important element within the MIRAS payload that supports the LICEF imaging mission is a central computer containing the payload Correlator and Control Unit (CCU) with distributed Control and Monitoring Nodes (CMN), one per antenna segment. MIRAS data containing correlator counts, instrument mode information, PMS values and LICEF temperatures, are formatted into source packets and stored in a (redundant) 20 Gbits Mass Memory Unit (MMU) until they are transmitted to ground by the on-board software. The transmission of the accumulated MMU data is via a dedicated X-Band transponder that is fully controlled by the payload.

A distributed Local Oscillator (LO) design is implemented in MIRAS and features separate microwave oscillator modules integrated in each CMN and synchronised to a common reference clock.

Data and reference-clock interfaces between the LICEFs, CMNs and CCU are via an optical fibre network immune to electrical interference and purposely developed and qualified for SMOS.

In addition to its standard instrument control and data management functions, the CCU software also implements a thermal control system that minimises the temperature gradient across the MIRAS arms. For this purpose, 12 thermal control-loops are implemented and operate in parallel to ensure a thermal gradient of less than 1°C across any arm segment and 6°C maximum gradient between any pair of LICEF receivers in line with the capability of the thermal control concept implemented for MIRAS. The MIRAS on-board software is fully re-programmable from ground.

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