Investigation of Propagation Effects for SIGNAL (SAR FOR ICE, GLACIER AND GLOBAL DYNAMICS)

Kurzfassung

SIGNAL is an innovative Earth explorer mission proposal with the main objective to accurately quantify, study, and characterise topographic changes in polar regions (ice masses) and fast flowing glaciers in mountainous areas like the Alps and Himalayan regions. In contrast to Earth observation using sensors in the visible- or infrared regime of the electromagnetic spectrum, the proposed system uses microwaves which are often considered to possess the advantages of both day/night and all weather operational capabilities. Whereas the first argument is true since we are dealing with an active sensor; the second does not hold in cases for which the operating frequencies are above ~3 GHz. Indeed, the SAR performance can be significantly affected by atmospheric effects (losses), especially at unfavourable weather conditions. The principal reason for the restriction on the use of higher frequencies can be found in clear air losses (water vapour and oxygen), cloud attenuation and attenuation due to precipitation, primarily rain. The specific attenuation through rain depends on a number of parameters like the frequency, the polarization, the dropsize distribution (DSD), and rain rate. Attenuation through rain increases with frequency and for the intended frequency of 35 GHz (Ka-band) values of 5 dB/km under rain conditions of 20 mm/hr are prevalent. Depending on the location on Earth, the probability for precipitation events differs to a large extent. At higher latitudes and especially at the polar region the probability is extremely low. Thus, it is valid to conclude that only gaseous attenuation and cloud/fog attenuation will contribute to the atmospheric attenuation budget. For non-polar regions such as Europe, the influence of attenuation due to rain has to be taken into account for a certain amount of data which is far less than 10 %. In order to show how frequently measurements will be effected, pertaining statistical information is provided. The annual path attenuation versus the probability is shown for 3 different climatic regions; the polar regions, Europe and tropical regions, for comparison. As expected, the polar regions are effected the least, whereas European and Tropical regions suffer from higher yearly attenuation percentages. From an interferometric point of view, signals traversing from Earth orbiting satellites to ground are subject to delays caused by the fact that the refractive index in the atmosphere differs from one. The tropospheric delay depends on parameters like the temperature, humidity, air pressure and varies with the height of the surface relative to the platform. The main advantage of a single pass interferometric configuration considered for SIGNAL is that the time between the two acquisitions is far less than the atmospheric decorrelation time. This means that the propagation path does not significantly change between two acquisitions like in dual-pass measurement scenarios. The atmospheric budget of SIGNAL is investigated in detail and conclusions for the proposed mission are drawn.