By Colonel William T. “Bill” Cooley, U.S. Air Force, Director, Global Positioning System
The cliché “time flies when you’re having fun” accurately describes how quickly the past six months have passed for me. In a program as challenging, rewarding, and mission-critical as GPS, it is full-speed ahead all the time. As the GPS director, I am acutely aware of the importance of time — particularly high-accuracy time. Since declaring initial operational capability in December 1993, the extremely precise time service delivered by GPS has enabled numerous technological advances around the globe. While extremely proud of the accomplishments over the past 20 years, I look forward to the next 20 years, as GPS brings on new signals and continues to deliver on its promise of a worldwide free positioning, navigation, and timing (PNT) service. This article elaborates on the GPS Directorate commitment, along with the 2nd Space Operations Squadron (2 SOPS), to provide unparalleled space-based PNT accuracy, availability, and reliability to the civil community.
The first commitment, arguably the most important for users everywhere, is the commitment to high accuracy for space-based PNT. After speaking at the ION GNSS+ conference and meeting many of you in Nashville this September, I realized that some users remain worried that selective availability (SA) — the intentional degradation of public GPS signals — could return and negatively impact GPS signal quality. SA was discontinued in May 2000 to provide an increased level of accuracy to all users around the globe. Since that time, the U.S. government has adopted a policy to no longer use SA and, furthermore, in 2007 removed that function from the upcoming GPS III satellites. The GPS Performance Standard for the Standard Positioning Service reflects our commitment to accuracy by ensuring the signal-in-space (SIS) user range error remains low: better than 4-meter accuracy. Figure 1 shows the record-setting SIS accuracy and how GPS meets and far exceeds this guarantee: consistently better than 1-meter accuracy! The 2 SOPS operators who command and control the GPS satellites do a masterful job ensuring global PNT accuracy is as good as it can be.
The quality of these services, however, does not permit the GPS enterprise to rest in the comfort of past success. We are dedicated to updating and modernizing the program.
The second commitment I’d like to highlight includes the GPS pledge for constellation sustainment and service availability. This is a guarantee to maintain a constellation of at least 24 satellites continuously broadcasting the GPS signals, providing a low dilution-of-precision value around the globe. Current efforts to meet this commitment range from replenishing unhealthy satellites to deploying improved, modernized satellites and corresponding ground support. For example, the GPS IIF satellites are rapidly becoming an integral part of the GPS constellation. With four IIFs on-orbit and a fifth soon to be launched, the constellation continues to exceed the 24-satellite requirement.
The third commitment — to modernize the GPS constellation with new signals — is best illustrated by, but not limited to, the modernized GPS IIF and GPS III satellites that are beginning to populate the constellation. In addition to four GPS IIF satellites now on orbit, the remaining GPS IIFs are either ready for launch or in final testing.
Down the road, GPS III satellites are completing development and progressing through early production. They will add the fourth civil signal, L1C, for civil users worldwide. Earlier this year, the GPS III program shipped the GPS non-flight satellite testbed (an engineering development unit) to Cape Canaveral; it successfully communicated with the next-generation operational control system (OCX), essentially as it would for launch and on-orbit check and control of functional GPS III satellites. The operational portion of the GPS ground segment, OCX Block 1 is on track to begin operations in 2016, modernizing the control segment by providing mission operation control of all legacy signals, as well as L2C and L5; later, OCX Block 2 will support L1C.
GPS civil users will soon be able to take advantage of the L2C and L5 signals that broadcast the modernized civil navigation message (CNAV); this will effectively eliminate the need to access the L1 and L2 P(Y) signals through codeless or semi-codeless techniques. These codes permit civil users to access dual-frequency solutions for precision applications (using dual-frequency enables ionospheric corrections for highly accurate PNT solutions). Compared to the L1 C/A signal, L2C features faster signal acquisition, enhanced reliability, and greater operating range. L5 will provide for increased safety-of-life applications with broadcast power even greater than L1 C/A and L2C combined, increased bandwidth, and advanced signal design. Regardless of the early availability of L2C and L5, semi-codeless users will be able to access P(Y) code — unhindered and unaffected by fully tested and vetted flex-power operations — until the end of 2020. Overall, these modernization efforts emphasize a commitment that availability surpasses simply putting satellites on orbit.
Finally, the GPS Directorate is committed to providing a high-quality service that is highly reliable. We achieve this by fielding first-rate satellites, conducting extensive test campaigns before deploying new capabilities or launching new satellites, and working closely with the operators at 2 SOPS, ensuring our constellation delivers top-quality PNT signals. An example of diligent testing is the recent live-sky test of the CNAV message on all GPS IIR-M and IIF satellites this past June. An example of a modernization feature that will enhance reliability is the capability of GPS III satellites to autonomously monitor the atomic clocks that drive the signal, thereby protecting users from clock instability and resulting signal errors.
Our demonstrated commitment to deliver accurate, available, and reliable space-based PNT allows innovators around the world to invest confidently in the creation of a multitude of GPS and GPS-enabled technologies that shape the way we live. GPS and its related technologies go far beyond letting you find the deli down the street and “checking in” to let your friends know what you’re up to on Facebook; it tracks financial transactions, enables precision farming, and allows accurate real-time updates on natural disasters such as earthquakes and tsunamis with capabilities that organizations like the NASA Jet Propulsion Laboratory and the International GNSS Service provide using GPS. The GPS Directorate is keenly aware that innovators invest their time and talent because they know they can depend on GPS availability. Our commitments are not just “feel-good” words; they are our mission and promise to the world.
I am very proud of the men and women in the GPS Directorate and thrilled to be part of this great team as the new GPS program director. The commitments listed here provide a glimpse into the services provided by the GPS enterprise. I am excited about bringing modernized signals to the world, but more excited to learn how the PNT community will use these signals to develop new innovative and useful applications. The world is easier to navigate because of GPS and GPS-enabled technologies, all of which rely on services the GPS enterprise provides: accuracy, availability, and reliability. We are committed to delivering these services as we modernize and improve the enterprise to continue GPS as the gold standard of space-based PNT.
Colonel William L. cooley is Director, Global Positioning Systems (GPS) Directorate, Space and Missile Systems Center, Air Force Space Command, Los Angeles Air Force Base, California.