Every satellite has a bus and payload. The bus is the body of the satellite and is managed by the satellite control operations to maintain a desired location. It carries the payload and is composed of a number of subsystems, such as the power supply, antennas, and mechanical and thermal control equipment. The bus also provides electrical power, stability, and propulsion for the entire satellite. The payload includes the devices the satellite needs to perform its mission. This configuration differs for every type of satellite. For example, the payload for a weather satellite could include cameras to take pictures of cloud formations, while the payload for a GPS satellite would include equipment to pass navigation information from the satellites to receivers on Earth. Monitoring and operating of the satellite payload is done to collect data or provide a capability to the warfighter or civilian user. GPS is a global PNT network consisting of space, ground control, and user equipment segments that support the broadcasts of military and civilian GPS signals. Each of these signals includes positioning and timing information, which enables users with GPS receivers to determine their position, velocity, and time 24 hours a day, in all weather, worldwide. GPS has changed the way the world operates, and underpins military operations as well as major sections of the economy, including telecommunications; electrical power distribution; banking and finance; transportation; environmental and natural resource management; agriculture; search and rescue; and other emergency services. GPS is used by all branches of the military to guide troop movements, integrate logistics support, and synchronize communications networks. In addition, many U.S. and allied precision-guided munitions are directed to their targets by GPS signals. The space, ground control, and user equipment segments are needed to take full advantage of GPS capabilities. The GPS space segment, which accounts for more than half of the total GPS costs in the Air Force’s current budget is a constellation of satellites that orbit approximately 12,500 miles above the earth. According to Air Force officials, current GPS requirements do not specify a number of satellites that are to be in orbit, but rather an accuracy threshold for the system. Based upon the threshold requirement for positioning and timing accuracy, the Air Force derived a constellation size of 24 satellites available 95 percent of the time. Due to the unanticipated longevity of some of the previously launched satellites, the constellation has at times exceeded the derived number of satellites. See figure 1 below for a depiction of the GPS segments. GPS satellites broadcast encrypted military signals and unencrypted civilian signals that can be processed by GPS receivers to identify their location worldwide. Since the constellation became fully operational in 1995, it has consisted of satellites from various generations of development and production, each introducing improved capabilities and additional signals. The latest generation of satellites in orbit—the GPS IIF—broadcasts two military and three civilian signals. Additionally, the GPS constellation currently hosts a nuclear detonation detection system payload to monitor nuclear events on Earth. The GPS ground control segment comprises the Master Control Station at Schriever Air Force Base, Colorado; the Alternate Master Control Station at Vandenberg Air Force Base, California; and various monitoring stations and ground antennas. Information from the monitoring stations is processed at the Master Control Station to determine the accuracy of the satellites’ clocks (for signal timing) and the precision of their orbits. The Master Control Station operates the satellites and regularly updates their navigation messages, transmitting information to the satellites via the ground antennas. The U.S. Naval Observatory Master Clock monitors the GPS constellation and provides timing data for the individual satellites. The GPS user equipment segment includes military and commercial GPS receivers. A receiver determines a user’s position by calculating the distance from four or more satellites based on the time it takes each of the signals to reach the receiver. Military GPS receivers are designed to utilize the encrypted military GPS signals that are only accessible to authorized users; commercial receivers use the civilian GPS signal, which is publicly available worldwide. The Air Force is in the process of modernizing the space segment under the GPS III program, which will incorporate advances over the GPS IIF satellites it is replacing, including a higher power military navigation signal to improve jamming resistance and a new civilian signal to allow users to receive GPS signals in combination with foreign satellite navigation systems. The acquisition strategy is to purchase up to 34 satellites following an incremental approach to replenish the current satellites in the constellation as they reach the end of their operational life. The original strategy was to purchase up to eight satellites in each of the first two increments and up to 18 satellites in the last increment. However, the GPS program is drafting a modified acquisition strategy to streamline the last two increments into one to allow larger buying quantities to take advantage of economies of scale, making this an appropriate time to examine future GPS options. The Air Force has purchased four GPS III satellites to date, the first of which is expected to launch in 2015. The Air Force plans to incrementally increase the capabilities of the GPS III satellites as technology maturation occurs and funding allows. The ground control segment is being modernized under the Air Force’s Global Positioning System Next Generation Operational Ground Control System (GPS OCX) program. GPS OCX is also being developed in three increments and will eventually allow DOD to take full advantage of the capabilities offered by the various satellites. The first increment is to deliver a capability to launch and initiate on-orbit testing of GPS III satellites; the next increment is to deliver a capability to command and control GPS II and GPS III satellites; and the final increment is to deliver a capability to make military and international signals operational. Software challenges have delayed availability of the command and control capability, which was previously planned for 2015 and is currently expected to be operational in late 2016 (final block is planned for 2017). Each of the military services is managing modernization of its user equipment through the joint Military GPS User Equipment program, which is to develop modernized military GPS receivers that deliver improved capabilities for accurate, reliable, and available PNT service where current receiver performance might be compromised (e.g., jammed) or unavailable. The 2004 U.S. Space-based PNT policy established a coordinating structure to integrate input from, and delineate the respective roles of the military and civilian departments and agencies for program planning (including identification of system requirements), resource allocation, system development, and operations. As part of the coordinating structure, an executive committee advises and coordinates among U.S. government agencies on maintaining and improving U.S. space-based PNT infrastructures, including GPS and related systems. The executive committee is co-chaired by the deputy secretaries of the DOD and the DOT, and includes members at the equivalent level from the Departments of State, Commerce, Homeland Security, the Interior, and Agriculture; the Joint Chiefs of Staff; and the National Aeronautics and Space Administration (NASA). The National Coordination Office for Space-based PNT provides day-to-day support for the executive committee. The Air Force broadly addressed all committee reporting requirements in its assessment of future GPS options for the space segment. These requirements include evaluation of system capabilities, implementation approaches, technical and programmatic risks, and estimated costs. However, each of the options presented and evaluated is based on a GPS constellation of 30 satellites, whereas the Air Force’s requirement is for a 24-satellite constellation. The 30-satellite constellation assumption has a significant effect on the cost of the options studied. It also raises questions about what constellation size the Air Force is committed to fielding and maintaining in the future. The Air Force GPS report identified and assessed nine options for the space segment of a future GPS which were developed as part of a six- step process the Air Force used to conduct the study, to include: Define the purpose, scope, and decision criteria. Assess the user requirement or capability in operational context. Develop a broad trade space of all available options. Review the trade space with GPS Senior Advisory Group to select a small set of promising options and to develop criteria for assessing the down-selected options. Analyze the options and assess them against the criteria using modeling and analysis tools, and conduct a risk assessment for each option. Integrate all the findings and develop recommendations. According to program officials, they focused the report on options for the space segment because it is the most costly part of the GPS. The space segment accounts for more than half of all GPS program costs in the current budget. For the space segment, we found that the Air Force’s report addressed all four Committee requirements for each of these nine options. Specifically, the report identified system capability, implementation approaches, technical and programmatic risks, and estimated costs, as provided for by the Committee. Each of the options for the space segment is assessed to determine how quickly it could be fielded, implementation approaches, technical and programmatic risks, and space segment costs (to include the cost of launch). GPS program officials stated that the cost analyses that support the nine space segment options were not high fidelity estimates but instead estimated at a high level. Although this may be expected given the limited time provided to complete the study and prepare the report, the high-level cost estimates are not at a level that would support programmatic decisions. Table 1 below identifies our evaluation relating to the Committee requirements as well as our observations. Of the nine options the Air Force identified for the space segment, several relied on a core constellation of 18 to 24 GPS III satellites that would be augmented with other satellites to reach a total of 30. The last two options do not rely on a core GPS III constellation but would also consist of 30 satellites. See table 2 for a description of each option as well as information on the results of the Air Force’s risk and cost assessments. Technical risk, for some of the options, includes the incorporation of technologies that are new to GPS. Programmatic risk includes areas such as establishing requirements, budgeting for funds, and obtaining approval of the acquisition strategy.