Improved Space Object Awareness with Advanced Sensing Technologies

A range of space -based sensing technologies, encompassing wavelengths from the ultraviolet (particularly appropriate when observing sunlit targets against the relatively dark background of the atmosphere in the “solar blind” wavelength region) to Very Long Wavelength InfraRed, or VLWIR (which, with LWIR, collects most of the thermal radiation from a satellit e), can be adapted to provide improved space object awareness capability. Here, a challenging problem of geosynchronous belt surveillance is described, and a combination of technologies is proposed to respond to this challenge. I. Introduction ertain prope rties of the geosynchronous belt (apart from its long range from Earth and LEO platforms) challenge the “space situational awareness” (SSA) of satellites in the belt, particularly if continuous coverage of a given target or targets is required over a compl ete 24 hour period of revolution. Visible light observations from the ground can exploit large telescope collecting optics for high sensitivity using shorter wavelengths of light that result in relatively high angular resolution for the measurement of sat ellite position. The solar illumination of the Earth -facing side of objects observed from the night side of the Earth coincides well with high nighttime (dark sky) sensitivities for this type of observation. However, the sun -illuminated side of geosynchr onous satellites faces away from Earth for satellites near noontime longitudes, becomes a crescent viewing condition for mid -day viewing in general, and daytime observations from the ground are dramatically limited by the high level of daytime sky brightne ss. Ground based telescopes on the night side of Earth need to work through high zenith angles (and high atmospheric airmass) to “reach” the satellites that are even within 35 degrees or so of the noontime meridian, corresponding to outages of 4 to 5 hour s. These conditions are not favorable to the continuous type of detection and monitoring that is envisioned for the total “situational awareness” of satellites in the geosynchronous belt. As a proposed way of filling this gap and enabling continuous (24 hour) surveillance of a geosynchronous satellite or satellites, an economical space -based infrared sensor is proposed. This sensor would (1), take advantage of the vacuum of space to enable cooling of the telescope mirror surfaces for orders of magnitude reduction in telescope thermal emission and the associated noise floor that affects target signal to noise ratio, (2), potentially open wavebands blocked by the Earth’s atmosphere where satellite thermal emission remains strong (e.g., wavelengths of 13.5 m icrometers and longer; see Fig. 1), and (3), observe geosynchronous satellites with their “backs towards the sun”; that is, detect the thermal emission from the non -solar illuminated sides. (The only difficulty for this type of observation occurs at the e quinoxes with the sun directly behind the target; this will be discussed in additional detail, below). An alternative approach that achieves similar capabilities might comprise visible light sensors in polar orbits having large apogee distances, such that observations of geosynchronous targets made near apogee allow for the partial viewing of the illuminated satellite surfaces. II. Considerations of utility Below we mention some specific considerations for space -based infrared surveillance of that part of the geosynchronous belt that is invisible to visible light ground based sensors during the day. As is the case for observations using large, ground based telescopes, these observations are all based on unresolved detections, since targets would be “point sour ces” as viewed by such a system.