  We describe the design of a radio interferometer composed of a Global
Navigation Satellite Systems (GNSS) antenna and a Very Long Baseline
Interferometry (VLBI) radio telescope. Our eventual goal is to use this
interferometer for geodetic applications including local tie measurements. The
GNSS element of the interferometer uses a unique software-defined receiving
system and modified commercial geodetic-quality GNSS antenna. We ran three
observing sessions in 2022 between a 25 m radio telescope in Fort Davis, Texas
(FD-VLBA), a transportable GNSS antenna placed within 100 meters, and a GNSS
antenna placed at a distance of about 9 km. We have detected a strong
interferometric response with a Signal-to-Noise Ratio (SNR) of over 1000 from
Global Positioning System (GPS) and Galileo satellites. We also observed
natural radio sources including Galactic supernova remnants and Active Galactic
Nuclei (AGN) located as far as one gigaparsec, thus extending the range of
sources that can be referenced to a GNSS antenna by 18 orders of magnitude.
These detections represent the first observations made with a GNSS antenna to
radio telescope interferometer. We have developed a novel technique based on a
Precise Point Positioning (PPP) solution of the recorded GNSS signal that
allows us to extend integration time at 1.5 GHz to at least 20 minutes without
any noticeable SNR degradation when a rubidium frequency standard is used.
