Experimental Study on GNSS-R to Detect Effective Reflector Height
Authors: Altuntas Cemali , Tunalioglu Nursu 
Published: March 2020

&lt;p&gt;GNSS provides high accurate point positioning for several geodetic facilities. To achieve that, data obtained from the GNSS should be eliminated from the major error sources through modeling, filtering and differential techniques. Multipath, which is one of the major error sources, is caused by reflecting surfaces around the receiver. In multipath theory, a signal transmitted from a satellite uses more than one path to arrive at the GNSS antenna. Here, the signal may be reflected from the surface around where the receiver is set. The reflected and direct signals, which interfere with each other at the phase center of the GNSS antenna, are recorded simultaneously. The ground reflected signal has an additional path, which enables to estimate the vertical distance between reflected surface of the signal and phase center of the GNSS antenna. Several studies have been conducted to estimate the reflector height to extract environmental variables, such as snow depth, vegetation height etc.&lt;/p&gt;&lt;p&gt;This study aims to control the quality of the signal-to-noise ratio (SNR) data to retrieve the effective reflector height and accuracy assessment of it. A field test was performed on top of a building, where surface of the roof is quite flat at Yildiz Technical University Campus in 2019. The dimensions of roof&amp;#8217;s building are approximately 12 m x 75 m. About seven hours GNSS observations during six days (DoY: 211, 212, 213, 214, 215, 217) were collected with a GNSS receiver (CHC i80) considering GPS-only observations for L&lt;sub&gt;1&lt;/sub&gt; frequency at a sampling rate of 1-Hz, mounted on a tripod. The height of the tripod from horizontal ground surface to receiver antenna reference point was arranged to 1.700 m and a decrement of 10 cm in every two days was implemented. The daily in-situ measurements were used to validate the estimated reflector heights. The powers of the frequencies on SNR series have been investigated under different elevation mask variations to obtain the effect of the noisy frequencies that can affect the estimations. &amp;#160;Accordingly, the first-Fresnel zones of signals are determined to identify the border and location of the potential candidate reflected signals. Here, the correlation coefficient calculated from estimated reflector heights and in-situ measurements is computed as 0.9346 for a totally 46 estimations. The daily mean values of estimated reflector heights and root mean square errors with respect to DoYs given are determined as 1.727 m &amp;#177; 3.0 cm, 1.738 m &amp;#177; 4.0 cm, 1.586 m &amp;#177; 3.3 cm, 1.582 m &amp;#177; 3.6 cm, 1.498 m &amp;#177; 4.1 cm and 1.489 m &amp;#177; 4.8 cm. Moreover, the model efficiency, computed from the differences between the 6-day estimated reflector heights and in-situ measurements was estimated to figure out how the model fits the field observation. Then, the accuracy of the model was determined as 2.2 cm. Finally, it can be concluded that the outcomes of this experimental study show that the studies related to the SNR data evaluations may be used for depth/height estimations with high correlation results.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Keywords:&lt;/strong&gt; GNSS, SNR, multipath, Fresnel zones, effective reflector height, elevation mask&lt;/p&gt;