[1] This paper describes an altimetric method based on data recorded during experimental zeppelin flights over Lake Constance. Interferometric observations for this method are obtained using a Master-Slave receiver configuration. These observations contain the relative phasing of direct and reflected signals and are used for altimetry. Separated antennas are attached to the receiver to record direct and reflected signals at slant elevation angles. Filtering is required to remove direct contributions in this slant geometry. Filtered observations are corrected using an altimetric model, and thus Doppler residuals are retrieved. This correction reduces the width of the spectral reflection peak from 3 mHz to less than 10 mHz. Doppler residuals are sensitive to surface height. Lake level is estimated inversely for the residuals at different trial heights. A case study of reflection events is presented. Lake level is estimated using data from antennas with right-handed and left-handed circular polarization. Reference level is determined from tide gauge data for stations around the lake. Mean deviation of estimates from reference level is 50 cm. Doppler shifts of different model corrections are compared. The altimetric correction is the most important, with mean Doppler shifts between 316 and 560 mHz. Mean Doppler shifts are much smaller for baseline correction (less than 0.2 mHz) and water-vapor correction (0.1-1.0 mHz). In addition, the geoid undulation effect (up to 25 cm amplitude) is predicted with mean Doppler shifts between 0.1 and 0.9 mHz. Precision of Doppler residuals (0.5-0.6 mHz) is insufficient to resolve the geoid undulation effect. The resolution from phase residuals is better. The effect of geoid undulation, however, is not dominant in phase residuals.

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The Global Navigation Satellite System (GNSS) signals are always available, globally, and the signal structures are well known, except for those dedicated to military use. They also have some distinctive characteristics, including the use of L-band frequencies, which are particularly suited for remote sensing purposes. The idea of using GNSS signals for remote sensing -the atmosphere, oceans or Earth surface -was first proposed more than two decades ago. Since then, GNSS remote sensing has been intensively investigated in terms of proof of concept studies, signal processing methodologies, theory and algorithm development, and various satellite-borne, airborne and ground-based experiments. It has been demonstrated that GNSS remote sensing can be used as an alternative passive remote sensing technology. Space agencies such as NASA, NOAA, EUMETSAT and ESA have already funded, or will fund in the future, a number of projects/missions which focus on a variety of GNSS remote sensing applications. It is envisaged that GNSS remote sensing can be either exploited to perform remote sensing tasks on an independent basis or combined with other techniques to address more complex applications. This paper provides an overview of the state of the art of this relatively new and, in some respects, underutilised remote sensing technique. Also addressed are relevant challenging issues associated with GNSS remote sensing services and the performance enhancement of GNSS remote sensing to accurately and reliably retrieve a range of geophysical parameters. Review

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This paper introduces the GPS based attitude determination system (GENIUS) onboard the university small satellite Flying Laptop. The attitude determination algorithm which is based on a Kalman Filter and processes single differences of the C/A-code and carrier phase measurements is shortly described. The algorithm uses the LAMBDA-method to resolve the integer ambiguities of the double differences of the carrier phase measurements. These resolved ambiguities are then used to fix the single difference ambiguities in the filter. The results of ground based tests and numerical simulations are introduced and the accuracy of the attitude determination algorithm is assessed.