National Aeronautics and Space Administration

Glenn Research Center

System Description

The White Sands Site Test Interferometer (STI) monitors an unmodulated beacon signal at 20.2 GHz broadcast from geostationary satellite Anik F2. The unmodulated beacon signals at 20.2 GHz are received, amplified and down-converted to 70 MHz in the RF feed box enclosure. The 70 MHz signals are further down-converted to 455 kHz (double conversion) in the IF box enclosure. These 455 kHz signals are sent to the indoor facilities for analog-to-digital conversion (3.64 MHz sampling rate, with 524,288 samples) and further signal processing.

The system block diagram for the two-element interferometer is depicted in Figure 1 below.

System block diagram of the White Sands Site Test Interferometer

Figure 1. System block diagram of the White Sands Site Test Interferometer

The two 455 kHz IF signals are sampled at 3.64 MHz by the National Instruments (PCI-5124) data acquisition cards (see Figure 3) which are externally synchronized by the 1 pulse-per-second reference. The A/D card uses 524,288 samples to determine the 455 kHz signal spectrum by using a Fast Fourier Transform (FFT). From the spectrum, the DSP algorithm, which is implemented through LabVIEW, locates the carrier peak and determines its frequency domain in-phase (I) and its quadrature (Q) components. These values are stored at a sample rate of 1 Hz for further post processing.