The SMOS Payload Module (PLM), in order to calibrate some of its parameters, will routinely perform certain calibration operations. These will include injection of noise to the LICEF by the internal calibration system (internal calibration) and looking at specific known (or that can be accurately estimated) targets that lie outside the PLM (external calibration). In both cases the PLM output is processed on ground to retrieve the calibration parameters, in order to track and compensate for PLM drifts and temperature effects.
Internal calibration is performed by injecting PLM-generated noise signals, in order to characterize certain properties of the PLM system. The following parameters are characterized:
- Power Measurement System (PMS) gain and offset
- Fringe-washing function at the origin FWF(0)
- Shape of the Fringe-washing function FWF
- Visibility Offsets
In addition, the sensitivity to physical temperature of PMS gain and offset is also calculated from the PMS gain and offset calibration data.
Internal calibration should be performed at physical temperatures as close as possible to those of the nominal measurements. Thus, it will be performed under nominal measurement attitude. When the data is needed for external calibration processing, internal calibration will be performed while in external attitude.
External calibration is performed by looking at known targets, or targets whose brightness temperature can be estimated accurately. Though in general external calibration is expected to be performed by aiming the PLM to a given direction of the sky, it is possible that for some external verification events a different type of target (known Earth location, a combination of Earth and sky) could be used. Note that this is similar to vicarious calibration/validation, which is different from external calibration, since external calibration is performed routinely in order to update the PLM model and processing parameters throughout the mission. For the current baseline, the external calibration will be performed using the sky as the only external target.
The external calibration process is added to the PLM operations due the existence of a set of requirements and constraints from the calibration process. It is therefore important to identify them explicitly. External calibration is needed to calibrate the following:
- Noise Injection Radiometer (NIR) model parameters
- Visibility errors due to antenna patterns (the Flat Target Response)
- The Parametric G-matrix
Once the calibration requirements have been set, the operations team shall be able to produce a calibration timeline that is compliant with all requirements and that maximizes quality and quantity of scientific data. Nevertheless, a baseline strategy is proposed here, to ensure that all requirements can be met with margin. It is up to the operations team to use this baseline proposed here or to develop another one based on information available during the mission, taking into account the operational constraints that are active at the moment.
One should remember that this calibration strategy can be subjected to changes depending on the ground work results, as well as ground tests results and commissioning phase results.
Nonetheless, the calibration strategy is based in three types of calibration events:
- long calibration: a series of internal calibration events that take place about once per month, and that last one entire orbit
- short calibration: a series of internal calibration events that take place often (with short intercalibration period), and that have short duration.
- external calibration: a series of calibration events in which the instrument is calibrated by looking at external targets. It takes place twice per month.
Since operations are going to be defined on a weekly fashion, the time devoted to calibration will be calculated for 4 weeks, and monthly events will be baselined to be performed every 4 weeks.
- Long Calibration: all parameters that can be measured by internal calibration are calibrated. They are the following:
- PMS Gain and Offset
- PMS Gain and Offset Sensitivity with physical temperature
- Fringe-Washing Function at the origin FWF(0)
- Shape of FWF
- Visibility Offset
They are calibrated during a full orbit (6000 s), which is subdivided into 20 segments of 250 epochs each (one epoch = 1.2 s which mean 250 epochs = 300 s). In each segment, a fixed sequence of PLM modes is activated, which allows the retrieval of all the parameters. The long calibration orbit is performed once every four weeks.
- Short Calibration: a subset of the long calibration parameters are calibrated once every 1.25 orbits (7500 s). They are the following:
- PMS Gain and Offset
- Fringe-Washing Function at the origin FWF(0)
They are calibrated at changing points along the orbit, in order to obtain data at different parts of the orbital period. The duration of each short calibration segment is of 19 epochs (22.8 s, taking into account eh corrupted epochs), in which a sequence of 12 steps is performed. Short calibration segments are interspersed with science measurements, except during other calibration events. The average time spent on short calibration is 6129 epochs (7355 s) every four weeks.
- External Calibration: part of an orbit is devoted every two weeks to observe the sky to calibrate parts of the PLM that cannot be properly calibrated by internal calibration. They are the following:
- NIR units
- Deviations in the antenna pattern (Flat Target Response: FTR)
- Parametric G-matrix
In a month there will be two external calibration events. One will be devoted to NIR and FTR calibration, and the other one to NIR and Parametric G-matrix calibration.
Calibration is performed by observing an external target selected in the sky in inertial attitude for part of the orbit. The inertial attitude is attained by an in-plane tilt maneuver, and the return to nominal attitude is achieved by performing a turn in the same sense as the first one (that is, performing a backflip). The duration of external calibration observation (External Target Observation: ETO) is of 33 minutes, with special events of internal calibration placed right before and after ETO. The target and timing of the ETO are selected to minimize perturbations, by having the Sun and Moon in the backlobes and avoiding equatorial targets. The total duration of the external calibration event is 8419 seconds, including maneuver and settling time.
- Time Allocated for PLM Calibration: the total time taken up by the calibration events is the following (measured per 4-week period):
- Long Calibration: 6000 s
- Short Calibration: 7355 s
- External Calibration: 8419 s
That adds up to a total of 21774 s, which represents 0.90% of the total time, well within the 1% requirement. Overall Calibration Scheme: the following diagram shows the overall calibration scheme.