The demands for high-sensitivity and high-resolution radiometry have recently led the science community to examine the use of large deployable mesh reflectors, which have heretofore only been used in telecommunications applications.
Traditionally, mesh emissivity is evaluated by radiometric measurement of a mesh flat plate, typically at an incidence angle of 45°. Emissivity is critically important for radiometric instrument performance. Low emissivity values, as low as 0.002 (0.2% in power) at L-band, are challenging specifications. The measurement at L-band is difficult, particularly to achieve the accuracy required. In addition, the test setup is costly.
This analytical model is developed to quantify emissivity of a mesh reflector. Wire mesh reflector material is simulated as an infinite array of unit cells describing the mesh geometry. The scattering matrix of the unit cell is determined using Ansoft’s High Frequency Structure Simulator (HFSS), and emissivity is subsequently computed from the scattering matrix elements.
This methodology represents a breakthrough in emissivity analysis, and it has recently been shown to provide high accuracy in predicting results of radiometric measurements performed on sample material. However, in contrast to the experimental method, the computer model can also accurately predict emissivity as a function of incidence angle, and it has been found that emissivity varies significantly vs. angle for both polarizations. By approximating the parabolic reflector with a series of flat plates, the emissivity at each point on the reflector is calculated using the appropriate angle of incidence. The emissivity of the mesh reflector is then computed for both polarizations. This new capability enables, for the first time, modeling of the emissivity of an actual parabolic reflector antenna.
Radiometers are critically important for many of JPL’s Earth missions. Mesh reflectors appear to offer a practical way to achieve high-resolution radiometric measurements. The Soil Moisture Active and Passive Mission (SMAP) in LEO (low Earth orbit) will be the first use of a mesh reflector, 6- meter diameter, for radiometry. Moreover, the general technique can be used for other mesh reflectors, and will be important for future high-performance JPL radiometers.