A zeppelin experiment to study airborne altimetry using specular Global Navigation Satellite System reflections

Abstract

[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.

Empirical Results of a Surface-Level GNSS-R Experiment in a Wave Channel

“…Coherent GNSS reflected signals have been tracked and reported in the past [18][19][20][21]. In 2012, the PYCARO GNSS-R instrument flying along the Catalan coast tracked the coherent reflected GPS signals under moderate-to-high wind conditions (>11 m/s), and high elevation angles (θ e > 30°) [13].…”

Sea surface topography retrieved from GNSS reflectometry phase data of the GEOHALO flight mission

“…This model is used to reduce phase observations for a facilitated retrieval. Following Semmling et al [] filtered observations γ flt are constructed from I , Q samples of the GORS receiver. The reduction of γ flt using the model p ( A , G ) provides a residual γ 0 .…”

A Phase-Altimetric Simulator: Studying the Sensitivity of Earth-Reflected GNSS Signals to Ocean Topography

“…The Master-Slave concept has been demonstrated airborne [8], [23] to sample direct (Master channel) and reflected (Slave channel) signals for ΔH of 500 and 3500 m, respectively. The Master sampling follows conventional GNSS receiver tracking loops to acquire code-based path delay P d , Doppler shift F d , and carrier phase φ d of the direct signal.…”

“…Corresponding delays P and Doppler shifts F have reduced magnitudes and are preferred for the receiver tracking (cf. the GPS open-loop differential real-time receiver (GOLD-RTR) [22], Master-Slave sampling described in [23], or a dedicated spaceborne tracking concept [24]). …”