National Aeronautics and Space Administration

Glenn Research Center

System Description

The Goldstone Site Test Interferometer (STI) is configured on a 256 m east-west baseline and tracks the 20.2 GHz beacon on the geostationary communication satellite, ANIK-F2, at an elevation angle of 48.5 degrees. Each element of the interferometer consists of a 1.2 m offset-fed parabolic reflector equipped with un-cooled low-noise amplifiers (LNA). The antenna half-power beamwidth is 0.7 degrees. This is large in comparison to the satellite’s diurnal motion in the sky (0.1 degrees), and hence the reflectors do not require tracking.

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 (via 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 post-detection bandwidth can be electronically adjusted between 7Hz to 222Hz to improve signal-to-noise ratio (SNR) if required.

The 1.2 m diameter, offset-fed antennas are molded from glass fiber reinforced polyester with an embedded metallic mesh. The outdoor units for the interferometer are shown in Figure 1. The system block diagram for the two-element interferometer is depicted in Figure 2.

Antenna 1 located at Venus Site in Goldstone, CA

Antenna 1 located at Venus Site in Goldstone, CA

The 10 MHz reference oscillator in Figure 2 is distributed to the two antennas via 0.5 inch diameter Heliax® cable. This cable has a foam polyethylene dielectric and has been thermally cycled to stabilize its phase change with temperature. The cable is placed in an above ground conduit that is partially protected from direct sunlight for minimizing both the total diurnal temperature variation and the rate of change of temperature. A separate set of Heliax® cables are used to distribute all other Local Oscillator (LO) frequencies and IF signals both to the IF box and from the IF box to the Analog-to-Digital (A/D) converters in the Digital Signal Processing (DSP) receivers.

The two 455 kHz IF signals are sampled at 3.64 MHz by the National Instruments (PCI-5124) data acquisition cards which are externally synchronized by a 1 pulse-per-second timing 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 quadrature (Q) components. These values are stored at a sample rate of 1 Hz for further post processing.

Goldstone STI Block Diagram

Block Diagram of the Goldstone STI (as implemented in first year of data collection)