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Extending Life Via Inclined OrbitWhen it comes to sustaining legacy systems, satellite operators for decades have used life-extending techniques to maximize the value of their on-orbit assets. Placing a geostationary satellite in inclined orbit — what was once called the “Comsat maneuver” — involves the elimination of north-south station keeping, and yields a predictably expanding figure eight. “This extends the on-board fuel and thus the life of the satellite,” says Maleter, who estimates that there are probably 60 to 100 commercial communications satellites now operating with some degree of inclined orbit. “The initial challenge is in deciding how early in the satellite’s life to start this operation, since it requires that Earth stations accessing the satellite be able to track the figure eight motion. Smaller or non-attended antennas typically do not have tracking. How long an operator keeps a satellite in this mode of operation depends on the spacecraft’s role, overall health, service requirements and other factors.”
Inclined orbit satellites often are used for trunking services — which can include TV relay as well as telephony — between large, tracking antennas. This can require upgrades to ground equipment as a result of changing user applications in particular and the desire to squeeze more out of available bandwidth, says Maleter, and it really has little or nothing to do with the satellite in question. “Upgrading to MPEG-2 or MPEG-4, for example, has much more to do with wanting more capacity than with problems related to satellite life,” she says. “In general, cost is king, but the key is whether costs can be recouped through extensions or expansions that add new customers or add value to old customers.”
Another technique for extending the life of satellites on orbit involves using fuel equalization accomplished by fuel rebalancing in satellites with multiple fuel tanks. According to a research paper published in the Journal of Spacecraft and Rockets in 2007, a procedure known as thermal gauging can be used to determine how much propellant is contained in the tanks and rebalance the load using on-board heaters to move the remaining fuel. In essence, the thermal pumping solution entails heating up fuel tanks containing more fuel and keeping the emptier tank cooler so that the helium gas in warmer tanks eventually pushes the liquid fuel into the emptier tank.
The research was conducted using a pair of satellites in geostationary orbit that were decommissioned in 2003. “Publication took sufficiently long, so that the benefits of that specific piece of work have already been exhausted — the satellites in the paper were eventually decommissioned, but fortunately, after maximizing the revenue from them thanks to the propellant rebalancing and gauging work,” says Steven Collicott, a professor at Purdue University’s School of Aeronautics and Astronautics, and coauthor of the paper along with two engineers from Lockheed Martin Space Systems Co. “We are pursuing additional opportunities to deliver propellant gauging and rebalancing to satellite operators and to advise in the design efforts of new satellites. Most new satellites launch have one large fuel tank, so rebalancing needs will likely fade away, but propellant gauging needs will remain.”
Collicott estimates that the overall engineering costs amount to less than a day’s revenue from a modern commercial geostationary satellite, and commercial satellite operators who want to explore this propellant gauging and rebalancing solution can do so via Comsat Technical Services. “Owners and operations should begin to address the gauging question with us as soon as the slightest uneasiness with gauging data emerges,” he says. “The sooner we start on the effort the better we can deliver results and options for the operators and the sooner they can be confident in revised end-of-life schedule predictions. Waiting to start does not improve results and the cost of our analysis (can be) overwhelmed by the expense of a surprise propellant depletion in a revenue-generating satellite.”
According to Boris Yendler, senior thermal system analyst at Comsat Technical Operations/LMMS and coauthor of the report, 10 per cent of existing geostationary satellites could benefit from this solution. “Our technique could be applicable not only to [geostationary] satellites, but also to [low-Earth orbit] satellites as well,” he says, who adds that a plug-and-play software solution is being developed based on this research. “We are in negotiation right now with a company to make a software package which can be sold to customers to estimate propellant remaining.”
The decisionmaking process surrounding the acquisition of a new system or solution is complex. Cost may drive the decision, but when one is attached to a proven piece of satellite-based hardware or software with an extremely reliable track record, taking the next step can be very hard to do indeed.
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