GPS Signal to Noise Ratio Data

A GPS receiver measures signal strength or signal to noise ratio (SNR). The main part of SNR is the direct signal. This is shown in black in Figure 1. It is low at the beginning of the arc, rising to ~55 db Hz at its peak, then slowly decaying again. The oscillations seen at the beginning and end of the satellite arc are caused by the interference of the direct and reflected GPS signals. The frequency of the interference tells you how far the antenna is above the reflecting surface. You can see in the example below (the Oklahoma SMAP in situ testbed) that the 6-meter antenna has higher frequency SNR data than the 2.5-meter antenna.

Figure 1.SNR data from two PBO H2O GPS sites in Oklahoma. One is 6 meters above the ground and the other is 2.5 meters above the ground. Both of these sites are used to produce daily soil moisture estimates.Top: SNR data. The black line represents the direct signal. Bottom: Top traces with a low-order polynomial removed. Now the data are plotted as a function of elevation angle, how high the satellite is with respect to the ground. In each plot, the SNR data traces have been offset vertically.

Figure 2.Both the 6 m (OKL1) and 2.5 m (OKL2) GPS antennas can be seen in this photo. You can also see several solar panels. These are used to power the GPS systems and other instruments.

Last modified: 2019-12-26  16:24:59  America/Denver  


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