NLSI Lunar Science Conference (2008)
DESIGN OF A LUNAR ARRAY PRECURSOR STATION. D. L. Jones1, R. J. MacDowall2, and T. J. W. Lazio3, 1 Jet Propulsion Laboratory, California Institute of Technology (M/S 138-308, 4800 Oak Grove Drive, Pasadena, CA 91109, Dayton.Jones@jpl.nasa.gov), 2Goddard Space Flight Center (8800 Greenbelt Road, Greenbelt, MD 20771), 3 Naval Research Laboratory (4555 Overlook Ave., SW, Washington, DC 20375).
Introduction: The Lunar Array Precursor Station (LAPS) will answer a fundamental question about the lunar environment: What is the density and variability of the lunar ionosphere? The lunar ionosphere is a major, and likely the most dynamic, component of the lunar atmosphere/exosphere. • The experiment proposed here is a simple, direct measurement of electron density during both lunar days and nights. LAPS will provide immediate science results on both the short and long term evolution of the lunar ionosphere. • Our lack of knowledge about the lunar ionosphere is a limiting factor in plans to design and build lunar-based low frequency radio arrays to image and track solar radio bursts as they approach Earth. • Variations in absorption of low frequency radio waves will be measured with a small, low power Relative Ionosphere Opacity Meter (riometer), a widely used technique for studying Earth’s ionosphere. Riometry is also applicable to other solar system bodies. There is considerable uncertainly about the density and geometry of a lunar ionosphere. Some dual-frequency radio occultation experiments imply electron densities >2000/cm3 at heights of several km; the corresponding plasma frequency is about 2 MHz. Other data show a much lower electron density. The differences may be explained by limited spatial and temporal sampling of an intrinsically dynamic environment. What is needed is long-term monitoring from a fixed surface location. • Riometry: The riometer concept was described by , and these instruments have been used to monitor changes in the electron density of Earth’s ionosphere for decades. The basic idea is simple: Use the cosmic background as a source of constant noise and measure, with a stable receiver, changes in the received noise level caused by variable ionospheric attenuation. Electromagnetic waves cannot propagate at frequencies below the plasma frequency of the ionosphere, which in turn is proportional to the square root of the electron density. At higher frequencies waves can propagate but are attenuated. At frequencies well above the plasma frequency the absorption is negligible. Basic design: The requirements for a lunar-based riometer are 1) a receiver covering at least 0.1-10 MHz with high dynamic range, 2) digitization of the re-
ceived total power, 3) a means for detecting changes in received power, 4) minimal electrical power load to allow operation during the lunar night with a small battery mass, and 5) a combination of wide temperature range electronics and thermal design to allow operation over the more than 300C range of lunar surface temperatures. High dynamic range receiver. The power received from a dipole antenna can vary by up to 70 dB between lunar night and periods of strong solar radio bursts. We plan to use two variable-gain amplitiers in series to provide an automatic gain control (AGC) range of 80 dB. Digitization. Because the AGC circuit maintains a nearly constant output voltage from the receiver, we need only enough bits/sample to accommodate narrow, strong solar bursts and terrestrial interference. We plan to use a micro-power, 100 Msample/second, 8-bit analog-to-digital converter. Calibration. Absolute power level calibration has traditionally used a stable broad-band noise diode at the receiver input . We plan to avoid the need for a noise diode by adding a low power 1024-point FFT chip to produce spectra of our bandpass. Ionospheric absorption has a predictable frequency dependence, which can be detected in the absence of absolute power level information. The simplifies the thermal design of the riometer and reduces power consumption. Electrical power. We plan to use a Li-ion battery, charged by a small (~10 x 10 cm) solar array, to supply power during the lunar night. A low duty cycle will be used during the night except near sunrise and set. Thermal design. We plan to use SiGe electronics as much as possible to take advantage of the wide operating temperature range. This minimizes the need for thermal insulation or heaters, except probably for the battery. A small mass of aerogel can provide good temperature protection for a small battery. Summary: The design developed during this concept study has two advantages over riometers that monitor power in one a few fixed, narrow frequency channels. These are the use of real-time FFTs for solar burst and RFI excision, and spectral fitting over a wide frequency range to reduce the calibration requirements. References:  Little C. G. and Linbach H. (1959) Proc IRE, 47, 315.  Fry C. D. et al. (2000) Radio Science, 35, 263.