MIRAS has two types of in-orbit calibration: external, in which the instrument makes a maneuver to point to the cold sky, and internal, in which noise is injected to the receivers. This one in turn has two modes, one of short duration carried out periodically interspersed with scene measurements, and other lasting a full orbit and used to obtain the more stable parameters and the sensitivity of all parameters with temperature.
Table 1.2-1Overview of SMOS MIRAS In-orbit Calibration Modes
Internal calibration: This is based on periodically injecting noise to all receivers via dedicated input switches. Due to the difficulty in making a large noise distribution network, a distributed noise approach is used for large baselines. Internal calibration also includes a so-called self-calibration mode that is used for automatically correcting samplers offset and quadrature error. Various on-board means are provided to measure the accuracy and stability of the payload data. These are:
In all calibration activities, the antenna proper is not involved; this represents the "unknown" factors. Therefore, provisions have been made to perform "external" calibrations, by pointing the entire spacecraft to specific targets: either to deep space, the moon, the sun, or known ground targets.
External calibration: Part of an orbit is devoted to observe the sky to complete the in-orbit calibration, in particular those parameters that cannot be properly calibrated by internal calibration. They are the following: NIR units, Deviations in the antenna patterns and Parametric G-matrix.
The LICEF (Lightweight Cost-Effective Front-end) receiver parameters of MIRAS are sensitive to temperature and ageing. Hence, they need to be calibrated in-flight to ensure that the accuracy requirements of the mission can be met. For this purpose MIRAS has been provided with:
The NIR (Noise Injection Radiometer) subsystem, which consists of three NIR units, work as part of the MIRAS instrument. The main objectives of NIR are:
To measure the noise temperature level of the internal active calibration source for individual receiver calibration CAS (Calibration Subsystem).
The CAS device of MIRAS includes a total of ten Noise Source (NS) units, twelve two-to-six Power Divider units (PD), and interconnecting RF cables. CAS forms a noise distribution network that generates and distributes correlated noise into all the receivers of MIRAS at two different noise levels (hot and warm).
The NIR subsystem has several operational modes for measuring the antenna and CAS noise temperature and for calibration. The antenna temperature is measured using a noise injection mode called as the NIR-A mode, and the CAS noise level is measured using a noise injection mode called as the NIR-R mode. Also, total power modes are used, for example, to solve the receiver noise temperature.
Data and Processors