Innovative Approaches to Outer Planetary Exploration 2001–2020 4034.pdf

KUIPER BELT MAPPING RADAR. A. Freeman and E. Nilsen, Jet Propulsion Laboratory. California Institute of Tech- nology, Mail Stop 301-170S, 4800 Oak Grove Drive, Pasadena, CA 91109. E-mail: [email protected]

Introduction: Since their initial discovery in 1992, to period of sustained observation precludes a ground-based date only a relatively small number of Kuiper Belt Objects radar solution.] After the radar transmits one pulse, the (KBO’s) have been discovered [1]. Current detection tech- expected round-trip delay for returns from the far edges of niques rely on frame-to-frame comparisons of images col- the Kuiper Belt would be as long as one earth day. Returns lected by optical telescopes such as Hubble, to detect would be coherently processed for integration times of up KBO’s as they move against the background stellar field. to 12 hours. The limits of performance of one design for Another technique involving studies of KBO’s through the proposed space-based radar system are given in Table occultation of known stars has been proposed [2]. Such I, where it is shown that a 25 km diameter object would be techniques are serendipitous, not systematic, and may lead detectable at ranges up to 100 AU. to an inadequate understanding of the size, range and dis- tribution of KBO’s. Parameter Value In this paper, a future Kuiper Belt Mapping Radar is Range < 100 AU proposed as a solution to the problem of mapping the size Object diameter 25 km distribution, extent and range of KBO’s. This approach Object σo - 10 dB can also be used to recover radar albedo and object rota- Radar wavelength 10 cm tion rates. Antenna diameter 1 km Background: Radar mapping of bodies in the solar Antenna beamwidth 0.1 mrad system has allowed scientists to study the Moon, the inner Peak Transmit power 10 MW planets, the Galilean satellites and, more recently Near Avg.Transmit power 12.5MW Earth Asteroids [3]. Radar astronomy measurements have Pulse repetition freq. 1 Hz been made at several facilities, notably the 70 m Deep Pulse Bandwidth 1 kHz Space Network (DSN) antenna at Goldstone and the Are- Integration time 0.5 days cibo radio telescope. Planetary radar measurements have Single-pulse S/N -73 dB not been targeted at bodies in the outr solar system because Integrated S/N 11 dB of the prohibitively large distances involved in studying more remote objects. Large distances to the objects under Table I: Parameters for the Kuiper Belt observation lead to severely attenuated signals due to the Mapping Radar R-to-the-fourth-power law for two-way propagation of electromagnetic waves, and the wide angular beam of the Challenges: The concept described here offers a radar antenna. number of implementation challenges. Not the least of Synthetic Aperture Radar (SAR) is a mature technique these is the need for a lightweight reflector antenna with 1 used in imaging radars for mapping [4]. The relative Dop- km diameter and surface control to +/- 1 cm. The high pler between the SAR antenna and the observed features is power levels required and the narrow bandwidths would filtered in order to construct a ‘synthetic aperture’ image of drive the development of new RF technologies. [The cur- the object with dramatically sharper resolution than the rent state-of-the-art for ground-based radars is Arecibo, angular beamwidth of the illuminating antenna provides. which has a 305 m antenna and a transmit power of 1 Inverse Synthetic Aperture Radar (ISAR) operates under MW.] Another challenge would be signal processing, the same principle [5], except that in ISAR the radar is given unknown motion characteristics (rotation, transla- stationary and the observed feature moves, e.g. through tion) of the KBO’s which can be resolved by applying rotation or translation. The signal processing involved in high-speed computing capability. either case achieves two objectives: i) significantly en- References: [1] Jewitt, D. C. (1999). Kuiper Belt Ob- hanced angular resolution along the arc of relative motion; jects. Annual Review of Earth and Planetary Sciences, 27, and ii) significant increase in Signal-to-Noise (S/N) for 287-312& Planet. Sci., 32, A74. [2] TAOS Project, isolated point-like targets within the radar’s field-of-view. http://taos.asiaa.sinica.edu.tw/. [3] Ostro, S. J. (1993). Re- Both are achieved by coherent integration of radar returns views of Modern Physics 65, 1235-1279. [4] Wiley, Carl collected over a given time interval. A. (1965), Pulsed Doppler Radar Methods and Apparatus, Measurement Concept: The proposed Kuiper Belt U.S. Patent 3,196,436. [5] Ausherman, D. A., et al (1984), Mapping radar would be positioned in an Earth-trailing Developments in Radar Imaging, IEEE Trans. AES-20, No. 4, orbit or at L2. This would allow it to be targeted at succes- pp. 363-400,. sive patches of sky for long observation intervals without interference from the gravitational pull of the Earth-Moon The operations scenario for the proposed system would be to scan a segment of the sky for periods up to 2 days. [This