What a difference eight years can make! My September 2006 GPS World article “Managing the GPS Constellation for Today’s Needs,” dealt with GPS performance issues many high-precision users then faced. Demanding applications of real-time precision positioning, such as precision agriculture and machine control, did not find enough satellites in view to support their needs.
I posed the question: Is the problem with the number of usable GPS satellites, or with growth in the demands of the user community? The 2006 answer was: a little bit of both.
Now the issue has pretty much gone away. Users have adapted to incorporating other GNSS signals, initially GLONASS and now BeiDou. Russia’s commitment to operate GLONASS at full capacity developed into today’s operation of its 24-satellite constellation. China’s similar declaration led to deployment of 16 satellites to date toward an eventual constellation of 35. Europe is likewise poised to offer global services with Galileo. GPS’s days as the sole provider of ubiquitous, accessible services appear to be over. One of my 2006 recommendations was for GPS decision-making authorities to support an aggressive program to replace aging satellites.
This has been done. GPS went from one IIR-M satellite in 2006 to the present seven IIR-M satellites and seven IIFs. They provide a new civil signal on L2C, and the IIF provides a new civil safety of life signal on L5.
Let’s look at the differences in service between 2006 and 2014 as shown in the table. GPS RMS user range error (URE) has been cut in half, and the number of usable GNSS satellites has gone up by 39 percent (44 to 61).
Today GPS and GLONASS operate at full capacity. GPS exceeds its marks by providing 31 satellites broadcasting signal-in-space range errors in the half-meter range, even as Block IIF satellites add L2C and L5 signals. GPS high-precision users also employ space-based augmentation systems services such as WAAS, EGNOS, and QZSS. Internet-connected GPS receivers, including those in cellular phones, use Assisted-GPS to provide near instantaneous times-to-first-fix.
One drawback to GNSS is its undependability when subjected to blockage, interference, or spoofing.
GNSS services should be made more resilient, and PNT users must diversify their positioning sources. We are now moving into a hybrid world, in which PNT services go far beyond “just GPS” to multi-GNSS services augmented by other PNT technologies, including assisted GNSS, inertial sensors, and terrestrial positioning services. Although diversifying PNT sources increases cost, it may not be as much as some might think. At a recent PNT symposium at Stanford University, Greg Turetzky of Intel predicted even consumer-grade receivers used in automobiles, tablets, and smartphones will embrace all GNSS, despite the added cost in chip size and power.
Setting aside the larger PNT discussion and considering only GPS, what challenges must GPS address to remain the cornerstone of PNT services? Here is my list of the top issues GPS faces today.
Signal Vulnerability. Since the issue of GPS vulnerability was raised in the 2001 Volpe Report, this issue has not changed, but the stakes have risen much higher. There is greater dependence on the GPS service than ever before, with over a billion users. It is generally conceded that for many applications, reliance on GPS in its current form is insufficient and even risky. Brad Parkinson espouses the mantra of protect, toughen, and augment GPS, focusing on steps necessary to strengthen its service.
Numerous methods are being explored and implemented to protect and toughen GPS: increased signal power on modernized satellites, improved antennas, and authentication of the signal against spoofing. The U.S. Department of Transportation is actively seeking ways to protect GPS spectrum through public workshops on GPS adjacent-band compatibility.
The GPS civil signal remains open to malignant spoofing by nefarious forces. Various methods are being proposed to counter this threat. It may seem that adding signal authentication is a bit too late, since civil GPS signals have already been defined in interface specifications, but it turns out this may not be the case. At the Stanford Symposium, Col Matt Smitham of the GPS Directorate stated that now is a good time to play with the civil navigation message implementations to explore features like authentication. “This is the time to do this, change the message types” he said. Thus, there is an opportunity to counter this threat.
Gaps in Service. A low-power service that has limited operation in many settings, GPS does not provide full functionality at the Poles, nor does it work indoors or underwater. This issue is exacerbated by society’s demands for PNT services anywhere. The 2008 National PNT Architecture identified these gaps as a primary concern, encouraging numerous actions to resolve them.
Split Leadership. Although the Space-Based PNT Executive Committee and its National Coordination Office provide a mechanism for establishing high- level policy and providing outreach, they fall short of meeting other essential needs for acquiring, operating, and sustaining GPS. Funding for GPS is split between a number of departments and agencies including the DoD, DOT, the FAA, and NASA. The net effect is prioritization decided by individual departments and agencies, but not by the GPS leadership itself. Thus, some programs get funded by Congress, such as satellite and control system acquisition and the FAA’s NextGen program, but others do not. Civil signal monitoring and complementary PNT services to support increased PNT resilience have not been adequately funded. GPS operations experience the tragedy of the commons: GPS civil signal formats are defined but service standards and management protocols are not.
How to Manage GPS for Today
◾ Resolve GPS vulnerabilities by strengthening the system and augmenting the service. Take the lead in addressing system vulnerabilities, including mitigating jamming and interference, and installing protections against spoofing. Hold forums on authentication means and methods, and fund research demonstrations using pre-operational civil signals.
◾ Work to close the gaps in service. Implement reduced-cost-impact, easily accessible complementary technologies to fill GNSS gaps. Implement civil signal monitoring using alternative networks until the Next-Generation Operational Control System incorporates civil signal monitoring requirements.
◾ Establish even closer cooperation between military and civilian leadership to provide unified funding, acquisition, and operations. Ensure a unified message to Congress for multi- agency funding needs. Work together to implement new civil signals, including operational protocols. Set dates cooperatively and meet them.
The GPS program produced a revolution in ubiquitous positioning, navigation, and timing that cannot be stopped. Care must be taken to ensure its services continue to benefit mankind while its vulnerabilities do not cause undesired harm to its users. With thoughtful planning and execution, GPS leaders will succeed.