Review of atmospheric effects on remote sensing by MMW radar and radiometer systems

Abstract

For high-resolution active or passive remote sensing systems a main challenge is the availability of appropriate broad frequency ranges without or at least minor interference or jamming. The disturbing impact may occur in both directions, either the own system is interfering other systems, or the own system is degraded by signals from other sensors. High resolution for active or high sensitivity for passive MW and MMW remote sensing systems is mostly achieved by large system bandwidths of few to even several ten GHz depending on center frequency. At the lower MW region (< 30 GHz) such continuous broad frequency range without any interference impact is usually not available due to the frequency assignment and regulations by national (e.g. BNA – BundesNetzAgentur - in Germany) or international (e.g. ITU – International Telecommunication Union) bodies. Hence, for some suitable applications, a possible way to overcome the bandwidth problem is to shift the center frequency to the MMW region (30 GHz < f < 300 GHz). Here, at least for some bands, the frequency assignment allows broadband applications, and several atmospheric windows provide sufficient transparence due to reduced absorption by water and oxygen molecules (e.g regions 30-50, 75-110, 125-150, 200-250 GHz). However, at these frequencies the influence of atmospheric effects such as attenuation and refraction become more pronounced compared to the lower MW regime. Hence, the correction or inclusion of those impacts can be much more difficult, considering that these means require mostly appropriate modelling. Although meanwhile many models for most impacts exist, they are minor or even not valid for each situation and region across the Earth. For that reason investigations on true atmospheric contributions are required. In this paper a discussion of simulations using existing models, some radiometric measurements of atmospheric attenuation at different frequencies and different weather conditions are illustrated and compared with the simulations.