An ambient light sensing module for wireless sensor networks for planetary exploration

Abstract

A sensitive ambient light sensing module with wide dynamic range has been designed for planetary wireless sensor networks. Its performance has been evaluated under different illumination conditions. Variations in ambient light intensities from <1 to 1.2×10⁵ lx could be detected from dawn to dusk, including cloud obstructions during monsoon. Moon light variations during the waning phase and total lunar eclipse have been monitored to demonstrate detection of finer variations. The module is capable of registering the fine variations in ambient light expected on different planetary surfaces. This module along with other sensors integrated into a Wireless Sensor Network (WSN) would be a suitable prospective tool for many future planetary exploration tasks.

Introduction

Information about the illumination conditions of the surroundings is an important aspect of in situ exploration of a planet. Ambient light refers to the light available in the surroundings and can come from numerous sources. It basically includes the visible part of the spectrum from 400–700 nm and provides information about the optical environment in the vicinity. For planetary bodies, the principal source of ambient light is solar illumination—direct, reflected or scattered. During in situ operations on a planetary surface, local optical environment and photometric properties of the surroundings largely affect the imagery of the planetary surface and surroundings (Lemmon et al., 2004, Shepard and Campbell, 1998, Teti et al., 2005). The spectrum of illumination is necessary for interpreting these images of the planetary surfaces and rocks. A large uncertainty exists in understanding these illumination conditions which are different for different planetary bodies. While glare dominates the images taken on an airless planetary body like the Moon (Taylor, 1966), Martian images suffer from diffused illumination conditions (Levin and Levin, 2004). Information on in situ illumination conditions is necessary in these cases. A photometric function is generally considered to account for these variations which are mostly theoretical and global in nature (Hapke, 1966, Kreslavasky et al., 2000). But, illumination condition of a local target strongly depends upon the local topography and incident sun angles (Thomas et al., 1999) and a general photometric function cannot be easily obtained under these conditions. Therefore, simultaneous measurement of ambient illumination is required in this case for color calibration and image interpretation (Levin and Levin, 2004).

Secondly, information about ambient light helps in inferring day–night transitions useful for navigation, path-planning and maintaining electrical operations of the rover (Rekleitis et al., 2007; Trebi-Ollennu et al., 2001). Sun-synchronous navigation that keeps the rover always in sunlight requires the state of illumination (Wettergreen et al., 2001). The amount of light available in the vicinity dictates the operation of the rover as the amount of power generated by a solar panel directly depends upon the local illumination conditions (Kan, 2003). This information will also help in switching the rover between operation and standby/hibernation modes. Precise measurement of day–night transition also provides key auxiliary information for de-convolving the science data.

Furthermore, ambient light conditions on a planetary surface help in understanding various atmospheric parameters and processes occurring on the planet (Sagdeev et al., 1986). For example, on Mars, presence of dust in the atmosphere significantly affects ambient illumination because dust absorbs sunlight between 400 and 600 nm (Lemmon et al., 2004). A large scale light sensing network deployed on a Martian surface will help in assessing the concentration of the dust and its movement in the atmosphere (Durga Prasad et al., 2012). Using such a wireless network for studying certain physical parameters such as surface/subsurface temperature, pressure etc. as a function of dust dynamics aid in deriving meteorological parameters such as local winds and thermal inertia of the surface (Bell et al., 2008, Ringrose et al., 2007). These types of light sensing nodes when deployed using descent balloons can help in estimating the vertical distribution of solar illumination in the thick venusian atmosphere. This will also help in estimating the height of cloud layers and also in understanding the nature of upward and downward solar fluxes of the atmosphere of Venus (Lacis and Hansen, 1974). These miniature wireless light sensing nodes can find numerous other applications with respect to planetary exploration that one can think of.

Most of the above mentioned tasks require simultaneous measurement of physical properties of surface and atmosphere along with ambient light levels. While navigation application requires multiple light sensors mounted on rovers/robotic arm, image interpretation and local meteorology demand simultaneous long-term measurements distributed on the planetary surface. Such measurements cannot be accomplished even by using rover or lander platforms. A WSN can be thought of a prospective tool for such spatio-temporal measurement tasks. In the present work, we report the design and evaluation of an ambient light sensing module for planetary WSN.