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Friday, January 24, 2014

US Lab Developing Technology For Space Traffic Control

As reported by Network WorldScientists at the Lawrence Livermore National Laboratory say they have tested technology that could eventually help them monitor and control space traffic.
The driving idea behind the project is to help keep satellites and other spacecraft from colliding with each other or other debris in Low Earth Orbit. 
Recently the Lawrence Livermore team  said they used a series of six images over a 60-hour period taken from a ground-based satellite to prove that it is possible to refine the orbit of another satellite in low earth orbit.
Specifically, the Livermore team refined the orbit of the satellite NORAD 27006, based on the first four observations made within the initial 24 hours, and predicted NORAD's trajectory to within less than 50 meters over the following 36 hours. By refining the trajectory of NORAD 27006 with their ground-based payload, the team believes they will be able to do the same thing for other satellites and debris once their payload is orbiting earth, the team stated.
The technology used to redirect NORAD 27006 and refine its orbit are being developed for Livermore's still developing Space-Based Telescopes for Actionable Refinement of Ephemeris (STARE) mission.  Ultimately STARE will consist of a constellation of nano-satellites in low earth orbit, that will provide data and work  to refine orbits of satellites and space debris to less than 100 meters, the team stated.
According to the Livermore web site, "Each nano-satellite in the constellation is capable of recording an optical image of space objects (debris or assets) at various range and relative velocities as scheduled by the ground infrastructure based on their closest approach distance (typically less than 1000m). The ground infrastructure processes the data received from multiple observations of the objects and reduces the positional uncertainty on the probability of collision to a level typically less than 100m, warranting taking actions such as moving assets. For an 18 nano-satellite constellation, STARE has the capability to reduce the collision false alarm rate by 99% up to 24 hours ahead of closest approach."
In a 2011 paper about STARE, the Livermore scientists wrote: " STARE is a proof-of-concept mission whose goal is to improve upon the orbital ephemerides [the position of astronomical objects] obtained by ground based instruments for a small population of satellites and debris to the level where a predicted collision is actionable. To do this, two Cubesat satellites will be launched into a 700 km polar orbit where they will image other satellites at optical wavelengths during closest approach. The images will then be processed along with Global Positioning Service (GPS) data to refine the position and trajectory of the targets. If successful, the mission will pave the way for a small constellation of similar satellites capable of refining ephemerides for all of the satellites and debris pieces involved in close approaches."
According to Livermore scientists, accurately predicting the location of a satellite in low earth orbit at any given time is hard because of the uncertainty in the quantities needed for the equations of motion. Atmospheric drag, for instance, is a function of the shape and mass of the satellite as well as the density and composition of the unstable atmosphere. These uncertainties and the incompleteness of the equations of motion lead to a quickly growing error in the position and velocity of any satellite being tracked in low earth orbit.
To account for these errors, the US Space Surveillance Network (SSN) must repeatedly observe the set of nearly 20,000 objects it tracks; however, positional uncertainty of an object is about 1 kilometer. This lack of precision leads to approximately 10,000 false alarms per expected collision. With these large uncertainties and high false alarm rates, satellite operators are rarely motivated to move their assets after a collision warning is issued, the team stated,
The STARE mission aims to reduce the 1 kilometer uncertainty down to 100 meters or smaller, which will in turn reduce the number of false alarms by roughly two orders of magnitude, the Livermore team stated.