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Authors: Shaw, Jason
Royal Military College of Canada / Collège militaire royal du Canada
Bedard, Donald
Gregg, Wade
Keywords: Multicolour broadband photometry
geostationary satellite
GEO satellite
light curves
colour indices
space situational awareness
Abstract: Multicolour broadband photometry has been used extensively to characterize artificial Earth-orbiting satellites, but relatively few studies have been dedicated to measuring the e ffects of the space environment on the spectral reflectance of satellite materials. The studies that have been undertaken mainly focused on inactive unstabilized satellites which resulted in an unknown, and constantly changing, illumination and observational geometry. As a consequence, it was very diffi cult to determine which satellite surfaces and materials were reflecting sunlight to the sensor when a measurement was taken. To address this shortcoming, an observational experiment was conducted using active geostationary (GEO) satellites with a fi xed attitude, and thus known illumination and observational geometries. The aim of this experiment was to assess whether multicolour broadband photometry could be used to measure the eff ects of the space environment on the spectral reflectance of these satellites. Multicolour photometric measurements were taken, during 16 nights between late June and late October 2015, of five similar active GEO satellites with di fferent launch dates (Galaxy-3C (2002), Spaceway-1 (2005), Spaceway- 3 (2007), DirecTV-10 (2007), and DirecTV-12(2009)). From these measurements, light curves and time-resolved colour indices were generated. The results suggested that older satellites had an increased spectral refectance at longer wavelengths. This was consistently observed in the R-I colour index. In the process of conducting this experiment, it was found that that the light curves of identical GEO satellites shared some similarities, but diff ered in structure and magnitude. In addition, a modeled specular re ection analysis technique was developed to support the aim of the thesis. These results indicated that it is possible to constrain the physical origin of many of the observed light curve features. Overall, this work contributes to the fi eld of space situational awareness by expanding the current capabilities of multi-colour broadband photometry to measure the e ffects of the space environment on the spectral reflectance of satellite materials.
La photom etrie couleur a large bande a et e grandement utilis ee pour caract eriser satellites arti ciels en orbite terrestre, mais relativement peu d' etudes ont et e consacr ees a la mesure des eff ets de l'environnement spatial sur la r efectance spectrale des mat eriaux retrouv ees a la surface de satellites. Les etudes qui ont et e entreprises avaient principalement pour objectifs d' etudier des satellites inactifs non-stabilis es ayant pour cons equence que la g eom etrie d'illumination et d'observation etait variable et inconnue. Donc, il n' etait pas possible de conclure quelles surfaces des satellites r ef echissaient la lumi ere solaire au t elescope. A n de contrer ce probl eme, une exp erience observationnelle fut mise sur pied dans laquelle les sujets etaient tous des satellites g eostationnaires (G EO) ayant une orientation stable et donc une g eom etrie d'illumination et d'observation connue. Le but de cette exp erience etait de d eterminer si la photom etrie couleur a large bande pouvait ^etre utilis ee pour etudier les eff ets de l'environnement spatial sur la r efectance spectrale des G EO qui ont une orientation xe avec une g eom etrie d'illumination et d'observation. Lors de 16 nuits entre les mois de juin et octobre 2015, des mesures de photom etrie couleur ont et e prises de cinq satellites G EO actifs ayant des plateformes semblables mais qui ont et e plac es en orbite terrestre a diff erentes dates (Galaxy-3C (2002), Spaceway-1 (2005), Spaceway-3 (2007), DirecTV-10 (2007), and DirecTV-12(2009)). A partir de ces mesures, des courbes photom etriques et des indices de couleurs furent cr ees. L'analyse de l'indice de couleur R-I indique que les satellites ayant pass e le plus de temps en orbite terrestre d emontrent une augmentation de leur r efectance spectrale dans les longueurs d'onde plus longue. L'analyse des courbes photom etriques a par ailleurs montr ee que les satellites G EO ayant une plateforme semblable ont des signatures photom etriques similaires hors avec des di erences dans leur structure et intensit e. De plus, pour mieux comprendre les r esultats, unmod ele de la r efexion sp eculaire fut elabor e. Ce mod ele fut utilis e a n de contraindre l'origine de certains traits observ es sur les courbes photom etriques. En g en eral, le travail pr esent e dans ce document contribue au domaine de la connaissance de la situation spatiale en elargissant le spectre d'information pouvant etre recueillie de la photom etrie couleur a n de mesurer les eff ets de l'environnement spatial sur la r efectance spectrale de satellites.
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