Thursday, October 1, 2009
Almost half of the current constellation of GPS satellites are at or approaching ‘single thread’ operation, where a critical system failure could render a satellite inoperative. What are the options for replacing GPS satellites?
Brownout" is probably the last term one would ever think of using in association with the Global Positioning System. So when a Stanford University professor briefed a congressional committee in May with a presentation entitled "Mitigation of Possible GPS Brownouts," some might have dismissed this as yet another academic exercise — but only those who didn’t recognize Professor Bradford Parkinson as the man popularly known and respected within the satellite navigation community as "the Father of GPS."
During his former service with the U.S. Air Force, Col. Parkinson was both the chief architect and original program manager of GPS. When Parkinson speaks, the satellite navigation community sits up and listens.
What brought Parkinson to the congressional hearing was the legislators’ need to hear an expert opinion on a Government Accountability Office (GAO) report in April that was severely critical of Department of Defense (DoD) management of space programs, with particular emphasis on GPS.
Essentially, GAO said that due to inadequate planning, poor contractor supervision, diffuse senior leadership, conflicting requirements and a lack of accountability, almost all DoD space programs are either late, over budget or both.
GPS was especially threatened, in a number of major and minor ways. For example, the GAO forecast serious deterioration in navigation service during the decade beginning in 2010.
And while encrypted GPS signals essential to warfighters are already being transmitted, DoD procurement practices have resulted in a situation in which it will be many years before all units have receivers capable of using these signals.
The GPS constellation nominally employs 24 satellites, although currently there are 30 satellites in orbit, providing the best performance users have ever experienced. But that is unlikely to last, because each satellite’s orbital life is unpredictable.
Initially, the spacecraft have contractually promised design lives of between seven and 12 years, depending on progressive technology advances, and failures rarely occur within that period. In fact, many go well past that, with some remaining in orbit for as long as 15 years.
A GPS satellite’s orbital longevity is determined by how long its systems, particularly its solar power panels, can withstand the rigors of outer space. Its several critical systems incorporate triple redundancy for maximum orbital life. Unfortunately, almost half of the current satellites are now at, or approaching, "single thread" operation, where the third and last line of defense of any one of those critical systems could fail and render the satellite inoperative. Such final failures are virtually unpredictable.
In these circumstances, how does DoD plan to avoid GPS brownouts? It intends to employ a two-step process. Initially, failed satellites will be progressively replaced by 12 Boeing-built Version IIF satellites, with the IIFs followed in 2014 by next generation advanced technology GPS III satellites built by Lockheed Martin.
But the GAO didn’t buy DoD’s schedule. The congressional watchdog agency singled out the GPS IIF program as particularly badly managed and running three years late. The originally contracted cost of the program has ballooned from $729 million to an estimated $1.6 billion at its undefined completion date. To cover this overspending, DoD was forced to defer its new GPS ground control center, with a domino effect on other programs.
(At the time of the report’s release, Boeing said it had "taken aggressive steps to resolve the technical issues on IIF with a strong emphasis on Mission Assurance. Design changes were required to ensure performance over the satellite design life and have caused schedule delays, but these changes are in the final phase of implementation, and a fully integrated satellite (SV1) has already successfully completed the thermal-vacuum test program — the most stressing system level test.")
GAO described the promised first launch of the GPS III satellites in 2014 as overly optimistic. DoD had claimed that not only would its new satellites have more advanced capabilities, they would be developed in three years less time than previous, less sophisticated, GPS satellite development programs.
GAO’s analysts used typical GPS satellite failure rates, combined with their assessments of the GPS IIF and GPS III programs, to develop a worst case scenario should GPS III slip by two years. The result was startling.
"The impacts to both military and civil users of a smaller constellation are difficult to precisely predict," the agency said. "For example, a nominal 24-satellite constellation with 21 of its satellites broadcasting a healthy standard positioning service signal would continue to satisfy the availability standard for good user-to-constellation geometry articulated in the standard positioning service performance standard. However, because the GPS constellation has been operating above the committed performance standard for so long, military and civil users have come to expect a higher level of service, even though this service is not committed to them. Consequently, some users may sense an operational impact even if the constellation were to perform at or near its committed standards."
A 24-satellite constellation meets civil aviation’s needs, but at an international satellite navigation conference last year, a United Airlines spokesman said even a reduction to 23 satellites would negate several GPS-based procedures.
On the other hand, military needs could still be met with a 21-satellite constellation when supplemented by inertial systems. However, an 18-satellite constellation, with outage periods of several hours duration, would be unacceptable for virtually all aviation applications.
Not unexpectedly, DoD disputed the analysis and assured the GAO that there are a number of remedial measures available, should they be needed, although it did not reveal them.
In light of the above, Parkinson’s careful choice of the word "mitigate," i.e., to make less severe with regard to brownouts, instead of the more optimistic "prevent," had disturbing implications. Unlike DoD, however, Parkinson openly discussed four potential recovery scenarios, in order of feasibility and implementation readiness.
The first is the simplest, yet most interesting. It turns out that there are five GPS satellites that have been shut down due to the inability of their solar panels to provide sufficient power to support the satellite’s full requirements, but which remain in their former orbital positions in the constellation. The key words here are "full requirements," because besides providing civilian navigation and timing services, each GPS satellite also carries a number of less well known military devices, one of which is a nuclear detection system (NUDETS) that continuously monitors the earth below for nuclear explosions. Upon detection of such an event, NUDETS transmits down the time, location and, it is understood, the intensity of the explosion. If these satellites were reactivated with their power-hungry military systems turned off, Parkinson proposed, they could serve as stopgap navigation signal sources as long as their power lasted. He said the reactivation process was routinely practiced for short periods by U.S. Air Force Space Command technicians, but added that while this rejuvenation would be virtually free, the satellites might only have "a few more years" of operational life remaining. "This may alleviate some of the pain, but it will not solve the problem by itself," he cautioned.
Parkinson’s second scenario would see the acceleration of the GPS III program. This would bring new signals to the constellation, including the internationally compatible L-1C satnav frequency, plus the civil GPS L-5 frequency that appropriate user receivers would compare with the current GPS L-1 civil frequency to cancel out ionospheric effects, the last remaining source of major position errors.
Earlier introduction of GPS III satellites would also bring transmissions of almost 10 times the power of those presently in orbit, and lessen vulnerability to GPS jamming by adversaries. However, accelerating the program would have serious funding and budgetary impacts in the present DoD management structure where, Parkinson observed, "many can say no, and no one can say yes."
His third scenario was a modified version of the second, where several of the GPS III satellites nearing completion are closed up earlier by leaving off the military elements, in a configuration he called IIIS, for Simplified or Spartan. This would not seriously affect the "fully configured" satellites further down the production line, and could have several benefits.
First, the IIIS satellites would allow earlier launches, bringing new signals and higher-powered transmissions to the constellation. Leaving off the military equipment could save about $75 million per satellite and also decrease overall weight to the point where dual satellite launches could be feasible.
"If the first IIIS satellite could be launched by 2013," Parkinson said, "it would constitute a significant insurance policy against brownouts."
Unfortunately, it appears that neither this nor Parkinson’s first reactivation scenario would be attractive to DoD planners, since it would result in fewer NUDETS-equipped satellites in orbit, particularly with actual nuclear weapons testing under way in North Korea and the potential of the same in Iran.
The fourth scenario — adding additional IIF satellites to the current Boeing contract — was included by Parkinson only to acknowledge its existence, and to recognize that it might be proposed by others. But that, he asserted, is a non-starter.
"The design is a dead end, many of its components are obsolete and it would be both expensive and risky while not meeting operational requirements," he said. Parkinson also pointed out that the IIFs are still untried and that, ominously, they "may have further congenital defects."
In concluding, Parkinson said "GPS is so much a part of today’s embedded infrastructure that brownouts must be avoided.... To avoid this risk will require the best efforts of DoD, Congress, the GPS Program Office, the Air Force Space Command and the contractors."
How likely is the threat of future GPS brownouts? As one senior industry official told Avionics, "If Brad Parkinson says there’s a threat, you’d better believe him, no matter what DoD says."
Interestingly, the federal government earlier this year released a report that it had kept under wraps since 2007, apparently relenting because of continuing Freedom of Information requests.
The report contained the unanimous view of a panel of independent experts, chaired by Parkinson, that the very necessary and only practical backup to GPS failures is eLoran. The report’s release was not connected to the GAO’s report and shortly afterward the White House Office of Management and Budget stated that it was canceling all further Loran expenditures to save money. Observers wondered how much impact the reduction of $35 million per year on Loran would have on the economic stimulus package and other big-ticket programs.
Unquestionably, any reduction in active satellite numbers without some backup provision would be noticeable in poorer GPS performance, although the loss of one or two could possibly be compensated with the Wide Area Augmentation System (WAAS) over most of the United States, since its two satellites each broadcast a pseudo range signal identical to those from the normal GPS space vehicles.
There’s also the chance that by 2015 or so, the first few of Europe’s Galileo satellites will be in orbit. The international L-1C frequency signals mentioned earlier should allow L-1C capable receivers to combine GPS and Galileo to work seamlessly together. However, that would also assume that sufficient GPS III or IIIC units (but not IIFs, which don’t carry L-1C) plus Galileo satellites could provide usable fix geometry, which seems unlikely.
If without such help, the GPS constellation deteriorated much further — down to say, 21 satellites — there would be a very serious impact on Automatic Dependent Surveillance-Broadcast (ADS-B), Required Navigation Performance (RNP) and other key civil GPS applications. And fewer than 21 satellites would almost completely reduce the system’s usefulness.
The GAO report, "Global Positioning System, Significant Challenges in Sustaining and Upgrading Widely Used Capabilities," is available at www.gao.gov.
Task Force Concerned By GPS Schedule Risk
Noting the Next Generation Air Transportation System (NextGen) is "highly, if not totally" GPS-centric, the RTCA NextGen Mid-Term Implementation Task Force in a draft report called upon the U.S. government to maintain the launch schedule of current-generation satellites and consider accelerating the launch of GPS Block III satellites.
"Given an unreliable GPS constellation, our vision for NextGen will be at grave risk, and thus our aviation transportation system will deteriorate," the Task Force states in its draft final report, released Aug. 20. The final report was not available at this writing.
The RTCA Task Force was established earlier this year at FAA’s request to develop industry consensus on steps to achieve NextGen operating efficiencies between now and 2018. (See article, Page 30). It relies on enabling technologies such as Required Navigation Performance (RNP) that, in turn, rely on GPS.
In its draft final report, the Task Force notes the level of GPS performance over the last decade has far exceeded the Department of Defense "guaranteed" level of 24 satellites, available 95 percent of the time — and has been on the order of 30 satellites. Civil and commercial aviation users, it says, have proposed a level of service more reflective of that actual historical performance.
"Aviation community users can accrue credit for a more robust level of accuracy and availability if a higher level of service is assured by the government," the draft report states. "With higher GPS constellation assurances, the performance requirements of aircraft equipment would not be as conservative, and users would be able to attain real benefit from GPS-centric equipment. This benefit would precipitate additional NextGen equipage and thereby (justify) the governmental spend on the GPS constellation reliability. This additional level of service would only need to be guaranteed until the next generation of satellites became airborne, in approximately 2015 (Block IIIs) and the compatible global systems also fly. In theory, the level of service would only improve over time."
Referencing recent concerns over GPS launch schedule risk, the Task Force said it "vigorously" suggests that:
The administration maintain the launch schedule for current generation satellites.
The administration appropriately ensures that next generation satellites are launched on schedule, with full functionality.
The administration consider accelerating the launch of Block III satellites in coordination with industry and DoD to ensure global harmonization with other constellations.
FAA clearly communicate, and all appropriate departments understand, that NextGen is highly if not totally GPS-centric. —Bill Carey