Background
Fifty years ago, the last two moonwalkers carried out a novel experiment to investigate the internal structure of the Moon to depths of up to a few kilometers. The Apollo 17 Surface Electrical Properties (SEP) experiment was a radiofrequency (RF) interferometer and a progenitor of modern Ground-Penetrating Radar (GPR). The SEP comprised of two orthogonal 70-meter dipole antennas which transmitted six frequencies spanning 1-32 MHz.
With NASA’s Artemis program scheduled to make crewed landings no earlier than 2025, we intend to develop a modern RF sounder called the Synthetic Pulse Artemis Radar for Crustal Imaging (SPARCI, pronounced “sparky”) that can probe to depths of several kilometers beneath the landing site near the south pole and answer outstanding fundamental questions about how the outermost crust of the Moon was shaped and evolved. For SPARCI, astronauts will deploy two orthogonal asymmetric dipole-transmitting antennas that are ~180 meters tip-to-tip to radiate energy over a bandwidth of 100 kHz – 100 MHz. SPARCI’s receiving antennas consist of an array of four monopoles, which are used to find the x, y, z components of the electric field. They will be located on a rover or a future Lunar Roving Vehicle. Thus, SPARCI will use large stationary antennas to radiate low-frequency radar energy into the subsurface. As moving such a large antenna is impractical, we use an electric field sensor mounted on a rover, as was pioneered by SEP. Using advanced signal processing, we will create a high-resolution ultra-wideband synthetic pulse. Advances in electrical components and data recording allow SPARCI to operate as a GPR compared to the interferometer of SEP.
Approach
We built a transmitter (electronics and antenna) that can inject electromagnetic (EM) energy over a very broad range of frequencies from ~100 kHz to 100 MHz. We used an asymmetric antenna design that significantly reduces null frequencies of the emitted EM bandwidth. For the electronics box, we used higher quality parts to obtain and increase our original goal of a 64 MHz upper bandwidth limit. This allows for increased resolution to map the depth to the base of the regolith.
We then built the receiver electronics and antenna that allows the measurement of electric field in three-components over the same bandwidth of the transmitter. Note: the antenna is composed of four monopole antennas to get the three components. The electronics were shielded in a Faraday cage (metal box) to reduce any noise generated by the instrument (Figure 1).
We then field tested the fully deployed transmitter and receiver system of SPARCI at Big Dune near Amargosa Valley, Nevada. The electrical properties of the Earth’s subsurface are significantly more lossy than compared to the Moon. Thus, we did not expect to penetrate nearly as deep, but our goal was to demonstrate the ability to receive a high-bandwidth signal.
Accomplishments
We successfully designed, built, and tested SPARCI to demonstrate Technical Readiness Level (TRL) 4: “Component and/or breadboard validation in laboratory environment.” The latter was accomplished during a field test where we were able to detect the direct path and a reflection off the bottom of the sand dune. This TRL was needed for our team to submit two multi-million-dollar proposals to NASA’s Development and Advancement of Lunar Instrumentation (DALI) program and be included on a Payloads and Research Investigations on the Surface of the Moon (PRISM) mission. Unfortunately, the latter proposal was not selected, but we expect to resubmit an improved PRISM mission proposal, with SPARCI, at the next opportunity; expected in less than a year.
Figure 1: Receiver in the field.