# Portable L-Band Radiometer (PoLRa): Design and Characterization
## 4 Discussion  
The design and characterization of the Portable L-band Radiometer (PoLRa) was outlined. Detailed technical discussion is presented to demonstrate that the radiometer hardware is functioning as expected, and to provide an estimate of its noise temperature measurement uncertainty.  
While using a similar architecture common with other radiometers, PoLRa is unique in its antenna design, simple electronics, low power consumption, cost-effectiveness, and no required active temperature control. The radiometer presented here requires no temperature stability, due to the novel active cold source (ACS) characterization approach. Modeled cold-sky brightness temperature was used to characterize the response of ACS noise temperature to its varying physical temperature over the range of expected operational temperatures. This initial characterization allows full internal calibration of the radiometer afterwards, without need for further sky measurements.  
The uncertainty of measured physical temperatures of the internal calibration noise sources (RS and ACS) are one of the primary contributors to the total uncertainty of noise temperatures measured at PoLRa's input ports. The accuracy of the radiometer could likely be improved by improving the quality of the temperature sensor, but this would also necessitate investigation of second-order uncertainty terms, such as the non-linearity and mismatch. Total uncertainty values ranging between 0.6 K and 1.4 K, for the range of noise temperatures expected for measurements over natural footprints, is still low compared to satellite-based passive L-band measurements. For instance, SMOS has an uncertainty of 3 K or greater [36; 37], and the NASA SMAP radiometer has a comparable uncertainty of 1.3 K [3].  
The PolRa, with total mass less than 4 kg, including all mounting hardware, can be mounted on an unmanned aerial vehicle (UAV) such as a multi-copter drone, or can be used as a ground-based instrument on a tower or a simple pole. The radiometer could also be mounted on other vehicles, such as agricultural tractors, cars, or used on an aircraft. The low power consumption of the system  
Figure 8: Calculated systematic and total noise temperature uncertainty as a function of measured noise temperature for two different integration times \(\tau\).  
allows operation with a compact battery, or with a small solar panel and battery system for off-grid ground-based use. The cost-effective design allows for production of a large number of such radiometers, which would allow for use in widespread networks valuable for satellite ground-validation purposes, or mass production of the hardware for use in agriculture and civil engineering.  
Applications in agriculture could be for drone-based soil moisture and vegetation water content mapping. Soil moisture information could be used to inform smart irrigation systems, saving water, reducing crop stress, and increasing crop yield. Vegetation water content retrievals could be used for assessing crop health, and ripeness of crops, such as wheat and cereals, for optimal harvest timing.  
Uses of the drone-based PoLRa in civil engineering would include finding leaks in levees and dams, and assessing soil moisture for surveys and building planning. Other potential uses of PoLRa in the future include landslide risk prediction and mitigation, and mitigating avalanche risk by spatial mapping of snow wetness and density.  
This publication has introduced the hardware design, characterization, calibration, and uncertainty analysis of the PoLRa radiometer. We have only included free-space measurements of cold-sky for characterization of the active cold source (ACS) calibration reference. Other measurements introduced here have been performed in the laboratory. Future publications will introduce both ground- and drone-based measurements using the PoLRa, and associated retrievals of environmental parameters, including soil moisture and vegetation optical depth, for instance.