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How Reentering Spacecraft Are Tracked: NASA vs. JSpOC

On a previous post I mentioned that contrary to what a lot of the recent news reports imply, NASA itself is not in the business of tracking uncooperative or (in the case of UARS) dead spacecraft. NASA normally will determine a satellite’s orbit (called “Orbit Determination”) via communications with the spacecraft. Both ranging (measuring the time for signals to get up and back from the spacecraft) and range-rate (measuring the doppler shift of signals) are used to track a satellite. Sometimes spacecraft also have on-board systems (such as GPS receivers) that help. Once a spacecraft is dead NASA quickly runs out of options for tracking. NASA doesn’t have a lot of radar or other assets to track dead objects, that’s done by the Joint Satellite Operations Center or JSpOC.

Vandenberg AFB’s site has a description of the JSpOC here, Vandenberg on the JSpOC
and there is a wikipedia entry here: Wikepedia on the JSpOC

NASA’s Final UARS press release clearly says JSpOC was tracking it: NASA UARS Press Release

“The Operations Center for JFCC-Space, the Joint Functional Component Command at Vandenberg Air Force Base, Calif., which works around the clock detecting, identifying and tracking all man-made objects in Earth orbit, tracked the movements of UARS through the satellite’s final orbits and provided confirmation of re-entry.

“We extend our appreciation to the Joint Space Operations Center for monitoring UARS not only this past week but also throughout its entire 20 years on orbit,” said Nick Johnson, NASA’s chief scientist for orbital debris, at NASA’s Johnson Space Center in Houston. “This was not an easy re-entry to predict because of the natural forces acting on the satellite as its orbit decayed. Space-faring nations around the world also were monitoring the satellite’s descent in the last two hours and all the predictions were well within the range estimated by JSpOC.” ”

The JSpOC provides information on their re-entry work, including how they do extra tasking and pay more attention to reentering vehicles: JSpOC Reentry Tracking

“Objects are tracked throughout their orbit life, with the results posted in the Space Catalog. When an object appears to be re-entering within seven days, orbital analysts in the JSpOC will increase sensor tasking (monitoring) and begin to project a refined re-entry time and location. At the four-day point, a monitor run is accomplished three times a day. Messages indicating the calculated re-entry time and location are transmitted to forward users and customers at the four-, three-, two- and one-day points. Starting at the 24-hour point, the object is monitored at the highest level of scrutiny, with processing at the 12, six and two-hour points. Again, ground traces and messages are transmitted. The object is monitored throughout re-entry.”

JSpOC can only monitor an object through the actual atmospheric re-entry if it happens within view of a sensor. From earlier on the same link we can see that the Space Surveillance Network (SSN) has a limited capability:

“The SSN uses a “predictive” technique to monitor space objects, i.e., it spot checks them rather than tracking them continually. This technique is used because of the limits of the SSN (number of sensors, geographic distribution, capability, and availability). ”

They do have one space based sensor, SBSS… (looks like its cameras can be controlled by ground operators – now that would be a cool job!)

The Space Based Space Surveillance (SBSS) satellite is the follow-on to the Midcourse Space Experiment (MSX) satellite. SBSS satellite provides the only space-based sensor in the U.S. Air Force SSN with the ability to detect debris, spacecraft or other distant space objects without interference from weather, atmosphere or time of day. SBSS satellite uses a visible sensor mounted on an agile, two-axis gimbal, which allows ground operators to quickly move the camera between targets without having to expend time and fuel to reposition the entire spacecraft.”

But according to Boeing the SBSS is in a low-Earth 630 Km sun-synch orbit, so it can’t always see everything – check out: BOEING SBSS

So we know JSpOC did as much tracking as they could, and very likely have a set of SP (special perturbations, their high-precision orbital elements) for the last segment of UARS’ flight. Sure would be fun to have those.

The JSpOC is tasked with tracking spacecraft, but is not the early warning system of the Missile Defense Agency (MDA). MDA may have been able to track the spacecraft as well, which our friend Marco looks into here at SatTrackCam