High altitude GPS radio occultation measurements have established knowledge on the characteristics of atmosphere turbulence in tropical regions. Moist air turbulence measurements from Haleakala, Hawaii, using directive antennas pointed towards the horizon, give signal recordings down to the lowest layers of the atmosphere. The main atmospheric modulation of the GPS signals in low-elevation measurements is attenuation and frequency shift due to ray bending, whereas the presence of turbulence causes a spectral broadening of the signal. Displaying the power spectrum as function of frequency difference from the main signal peak reveals the characteristic domains of the spectrum. Up to 10 Hz, the spectrum is approximately sloping as the inverse of the frequency squared. While for higher frequencies, in the range 10 - 500 Hz, the spectrum flattens. The latter part of the spectrum originates from thermal noise, while the first sloping part is characteristic for the rubidium frequency reference used in the receiver. Analysis of the trend of the mean slope in the spectra for different frequency domains showed an increased slope as function of the elevation of the received signal above the horizon, indicating turbulence and eddies in the beam direction. Analysis of the trends of the mean slope in the spectra for different frequency domains will be discussed in relation to the characteristics of turbulence. The open-loop data sampling rate of 1000 Hz enables detection and investigation of the characteristics of the noise and the multi-path signal error sources through the determination of the refractive index structure constant Cn2 of the atmosphere turbulence. Based on knowledge of the geometry of the experiment and models for the turbulence, it has been possible to determine scale sizes and structure constant changes from the measured spectral variances.
|Publication status||Published - 2008|