# Climate-Quality Calibration for Low Earth-Orbit Microwave Radiometry +
## 2 Radiometer Design  
Rather than prescribing a detailed instrument design here, we recommend some approaches to achieve accuracy, linearity, stability, and SI-traceability. The following design features [10; 11] would be desirable in a microwave reference radiometer:  
* A phase-locked local oscillator (LO) prevents frequency drift.
* The system response of the amplifiers and detectors should be as nearly as possible linear with input power. An internal noise diode will allow correction of residual non-linearity. Firing the noise diode during part of the viewing time of high and low calibration temperatures yields four calibration points, which can be solved for three coefficients of the instrument transfer function (e.g., a quadratic form) and also the excess noise temperature of the diode [12]. (The noise temperature of the diode is not assumed to be constant.) However, nonlinearity in common amplifier stages can introduce crosstalk between channels because of gain compression [13], which this method will not correct. Therefore, linearity is extremely important for those amplifiers. Digital processing may be useful in this context, given that the non-linearity inherent in binary representation of signals can be corrected analytically [14].
* Instead of total power detection, sensitivity to short-term gain fluctuations within the calibration cycle should be reduced by Dicke-switching, by digital correlation, or by some other means.
* A low-sidelobe horn antenna without a reflector eliminates spillover of energy at the edge of the reflector, making the directivity pattern accurately calculable. The radiometer section would rotate along with the antenna, to scan the earth and also view cold space and an on-board black-body target, as sketched in Figure 1. A monostatic return loss of 55 dB from 31 to 84 GHz has been measured on a folded inverted-cone target [15]. The antenna should be closely coupled to the target when viewing it.
* The scan should include the earth limbs for comparison of limb radiometric measurements to GNSS radio-occultation (RO) measurements, as discussed below in Section 3. Comparison with radiometric measurements at the limb instead of closer to nadir avoids the difficulties associated with different viewing geometries (e.g., see [16]).
* Dual-polarization measurements, using two radiometers coupled to orthogonal modes of the antenna, allow exact transfer of calibration to rotating-polarization instruments like ATMS, for channels sensitive to surface polarization. For frequencies not sensitive to the surface, the two polarization measurements would be averaged together.
* Control of instrument temperature, including the calibration target, contributes to radiometer stability and also simplifies requirements for pre-launch testing.  
In addition to monitoring climate trends, an orbiting microwave reference radiometer designed in this way could transfer its calibration accuracy to weather-satellite instruments, whose data are assimilated into forecast models. Several footprints of the other instrument can be combined in that comparison, so the reference-radiometer's antenna beamwidth can be as large as several degrees. To serve as a calibration reference for sounders like ATMS or SSMIS, the reference instrument may need to duplicate only a few channels for each target on the other radiometer (assuming that target emissivity is a smooth function of frequency). The MSU channel frequencies would also be among those with a high priority, but for the purpose of providing long-term continuity of climate measurements.  
In the 50-60 GHz band, the instrument characteristics listed in Table 1 should be achievable with currently available technology. The first four are k = 1 uncertainties, while the others should be interpreted as upper limits. Radiometer noise for a 400 MHz passband is 0.05 K at 1 s integration, but the actual integration time \(\tau_{r}\) will depend on the scan rate.