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Radio Propagation Modelling for Coordination of Lunar Micro-Rovers

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Radio Propagation Modelling for Coordination of Lunar Micro-Rovers i-SAIRAS2020-Papers (2020) 5047.pdf RADIO PROPAGATION MODELLING FOR COORDINATION OF LUNAR MICRO-ROVERS Virtual Conference 19-23 October 2020 Shreya Santra1, Leonard Bryan Paet1, Emanuel Staudinger2 and Kazuya Yoshida1 1Department of Aerospace Engineering, Tohoku University, Aoba 6-6-01, Aramaki, Aoba-ku, Sendai, Miyagi 980-0879, Japan, Email: [email protected], [email protected], [email protected] 2Institute of Communications and Navigation, German Aerospace Center (DLR) e.V., Oberpfaffenhofen, Muenchener Str 20, Wessling 82234, Germany, Email: [email protected] ABSTRACT link quality prediction is a crucial requirement for evaluating the design of wireless communication This paper focuses on radio propagation modelling channels and mission planning. A robust and reliable towards a coordinated path planning solution for communication architecture between the multiple micro-rovers on the lunar surface. Radio agents will enable better coordination and result in an communication scenarios on the Moon greatly differ effective collective behavior [1]. from those on Earth. We account for these differences Radio propagation modelling is primarily concerned by reviewing several existing terrestrial around the way radio waves travel between systems at communication models to devise a site-specific radio both ends of a wireless communications link. The goal propagation model that accommodates the lunar is to determine whether a signal from a transmitting conditions, particularly for micro-rovers with low- device can be successfully received by another device height antennas. Using the devised model and the in a different location. Large-scale propagation Digital Elevation Model (DEM) of Apollo 15 Mission modelling, which typically covers distances from landing site, simulations were performed for three hundreds to thousands of meters, predicts the micro-rovers to predict the viable communication combined path losses affecting the radio signal based range and link quality. Received signal strength on available knowledge about the prevailing indicator (RSSI) maps and path loss graphs were propagation environment [2]. generated taking into account the lunar regolith Various propagation phenomena occur during radio properties, terrain geometry, operating signal transmissions that determine the path loss. For frequency, and antenna properties. instance, the receiver may receive a direct attenuated The results presented in this paper enable reliable signal (also called as line-of-sight (LOS) signal) from point-to-point communication and maintain the transmitter and indirect signals (or non-line-of- continuous connectivity between the moving micro- sight (NLOS) signal) due to the physical and rovers exploring the lunar surface. environmental effects like reflection, refraction, diffraction, and scattering. Observations show that the Keywords: Radio Propagation, Lunar surface, Swarm propagation characteristics are significantly affected Robotics, Path Planning, Communication by the Fresnel zones, terrain geometry, surface materials, antenna properties, and operating signal 1. INTRODUCTION frequency [3], [4]. Therefore, knowledge of the operational environment Lunar exploration has recently gained momentum, and is necessary to accurately model the radio propagation. several lander missions are proposed for the coming Various propagation models exist to determine the decade. Multi-robot systems are now being signal strength and predict link quality for terrestrial extensively studied for surface exploration missions as scenarios. However, the conditions on the Moon are they allow flexibility and provide a cost-effective different compared to that on Earth, and these models solution offering a promising alternative to single need to be reviewed carefully for applicability at lunar large rovers for wide-area lunar operations, such as sites. [5]. resource prospecting for deposits of water ice and This paper presents site-specific radio propagation volatiles. Precise coordination between these rovers is modelling and path loss characterization for the a challenge that involves inter-agent communication traverse path of the Apollo 15 landing site. The to enable the exchange of information while objective is to enable information driven path- navigating through unknown environments. Radio planning and coordinated surface exploration. i-SAIRAS2020-Papers (2020) 5047.pdf 2. RADIO PROPAGATION BASICS 2.2 Free-Space Path Loss Model is used to model the LOS path loss incurred in a free-space environment, This section discusses selected basics of radio without any absorption, diffraction, reflections, or propagation and reviews the models related to the other characteristic-altering phenomena. The equation work described in the later sections of this paper. for the combined path loss in the absence of any obstructions in the Fresnel zones is given by Eq. 2 2.1 Fresnel zones are ellipsoid-shaped regions of 휆 space between two radio antennas that determine the 퐹푆푃퐿[푑퐵] = 20푙표푔 ( ) (2) ‘radio LOS’ between them. Fig. 1 shows an example 10 4휋푑 communications link with the relevant Fresnel zones. where 휆 is the wavelength of the radio signal and d is the separation distance between transmitter and receiver [6]. 2.3 Reflections Due to Irregular Terrain starts to become significant when a portion of the terrain, mutually visible to both transmitter and receiver antennas, breaches the 2nd Fresnel zone boundary, as illustrated in Fig. 2. Figure 1: Fresnel Zone Ellipsoids and Clearances The radius of the n-th Fresnel zone boundary at any point between a transmitter and receiver can be approximated using Eq. 1: 푑 푑 퐹 ≈ √푛휆 1 2 (1) 푛 푑 where 휆 is the wavelength of the radio signal, and d1 and d2 are the distances of the transmitter and receiver respectively from the point where the Fresnel zone Figure 2: Reflection due to irregular terrain radius is being calculated [6]. The volume enclosed by the boundary defined by F1 is the ‘1st Fresnel zone’, When this happens, the first challenge is to ascertain if while the intermediate volume between the boundaries specular reflection is possible. This is done by finding of F1 and F2 is the ‘2nd Fresnel zone’. In theory, there potential specular reflection point/s along the mutually is an infinite number of Fresnel zones between visible terrain. If a candidate point is not found, then transmitters and receivers. In practice, however, the specular reflection is not possible and the excess path first two zones are sufficient for determining the loss due to reflections is set to 0 [7]. If a specular reflection and diffraction phenomena occurring in reflection point is found, then the excess path loss due long-distance communication links [6]. to reflections (in dB) can be calculated using Eq. 3: When the 1st and 2nd Fresnel zones are clear of obstacles (i.e., when the path clearance hc at any point 2 in the terrain profile in Fig. 1 is below the 2nd Fresnel 퐿푅 [푑퐵] = −10푙표푔10|1 + 훤푒푓푓 푐표푠(훥휑)| (3) zone boundary), then the propagation environment can be treated as ‘free-space.’ The combined propagation where 훥휑is the phase difference between the LOS path loss is equal to free-space path loss (FSPL) as signal and the reflected wave, given by Eq. 4: described in Sec. 2.2. Otherwise, the effects of reflection or diffraction become significant. The path 2휋 훥휑 = [(푑 + 푑 )− 푑 ] (4) loss in excess of FSPL due to these phenomena can be 휆 푖푛 푟푒 퐿푂푆 calculated using the methods outlined in Secs. 2.3 - 2.4. i-SAIRAS2020-Papers (2020) 5047.pdf and 훤푒푓푓is the effective reflection coefficient, loss predictions. The advantage of such an approach is presented in Eq. 5: that it considers actual lunar terrain profiles to predict the communication link between agents. This is 훤 = (휌 퐷) ∗ 훤 (5) particularly important when multiple micro-rovers are 푒푓푓 푠 following given trajectories. The limitations of the existing models are considered and adapted to achieve where the reflection coefficient (훤) at the specular a valid communication network design at the chosen reflection point modified by a surface roughness location on Moon. parameter (휌푠) and a divergence factor due to curvature of the surface (D). The reflection coefficient 3.1 Lunar Terrain Modelling (훤) can be computed using standard equations based on the surface electrical properties, antenna Apollo 15 was one of the few lunar landing missions polarization, and grazing angle at the specular which conducted several important scientific activities reflection point [7]. The surface roughness parameter on the surface of the Moon. The Apollo 15 landing site (휌푆) and divergence factor (D) are computed based on was located at 26.132°N, 3.634°E at the foot of the standard equations found in [7]. Apennine mountain range, with the objectives of sampling material from the rim of the Imbrium basin 2.4 Diffractions due to Irregular Terrain becomes a and studying the volcanic processes that produced significant cause of path loss when any part of the Hadley Rille. The Hadley-Apennine region has rock terrain covers 55% or more of the 1st Fresnel boundary fragments, rounded hill formations, and expansive [8]. Given a defined terrain profile between transmitter craters [13]. Fig. 3 shows the traverse paths for the and receiver antennas, we used methods outlined in astronauts during which they covered a total of 27.9 Section 4.5 of Recommendation ITU-R P.526-15 [8] km. to calculate the excess path loss due to diffraction (LD, in dB). 2.5 Irregular Terrain Model (ITM), also known as the Longley-Rice Model [9], is a semi-empirical model that takes into account the irregular terrain profile and electromagnetic properties to predict the signal distribution in a given area for frequencies between 20MHz and 10GHz [9]. ITM with necessary modifications has the highest potential for application to planetary surface communication, particularly for moving rovers utilizing the path-specified 3D geometry [10]. 3. LUNAR RADIO PROPAGATION MODELLING Figure 3: Apollo 15 map with traverse path. Credits: Lunar terrain features and conditions are very different United States Geological Survey (USGS) from those of Earth.
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