GPS Policy - Modernization

Policy

GPS Modernization

One of the main components of this modernization is the addition of two new navigation signals for civil use. These signals will be in addition to the existing civilian service broadcast at 1575.42 MHz (L1). The first of these new signals will be a new civil code, called L2C, which will be added on the existing L2 carrier, located at 1227.60 MHz. It will be available for general use in non-safety critical applications. The Block IIR-M satellite, the first to add his capability was launched September 25, 2005. A third civil signal, located at 1176.45 MHz (L5), will be provided initially on GPS Block IIF satellites beginning in 2010, and continuing with the Block III satellites scheduled for launch beginning in May 2014. This new L5 signal is protected worldwide for aeronautical radionavigation use, and will support aviation safety-of-life applications. The addition of L5 will make GPS a more robust radionavigation service for many aviation applications, as well as all ground-based users (maritime, railways, surface, shipping, agriculture, recreation, etc.). L5 benefits include precision approach navigation worldwide, increased availability of precision navigation operations in certain areas of the world, and improved interference mitigation.

The Boeing IIF Series includes a total of 12 satellites. The first satellite launched in May 2010. As of August, 2012 there are two operational IIF satellites in the GPS constellation. A third satellite is planned for launch on 4 October, 2012.

A fourth civilian signal is planned for GPS block III vehicles designated L1C. This signal will be added to the existing L1 carrier and will provide international interoperatability of satellites. Future design of GPS block III satellites will also include Distress Alerting Satellite System (DASS) for search and rescue (source: http://www.gps.gov/systems/gps/modernization/civilsignals/).

The current OCS is being upgraded, as it cannot presently support some capabilities of the newer satellites on orbit. The next generation control segment upgrade will support modernized capabilities for the IIR-M, GPS IIF and GPS III. OCX will add many new capabilities to the GPS control segment, including the ability to fully control the modernized signals, L2C, L5 and L1C. OCX will be delivered in increments. OCX Block 1 will replace the existing command and control segment and support the mission operations of the initial GPS III satellites. OCX Block 1 is scheduled to enter service in 2016. OCX Block 2 will support, monitor and control the navigation signals. Although the GPS OCX is primarily software development, OCX will install new GPS receivers at globally distributed monitoring stations to monitor all GPS signals, and upgrade the ground antennas at all four legacy ground antenna locations.

Precision Navigation Operations Worldwide

GPS offers the capability to provide a safe and efficient seamless satellite-based global navigation system that will be responsive to the future needs of civil aviation. The addition of L5 will provide aviation and other transportation applications with continuous, highly accurate, three-dimensional position information.
The availability of L5 offers the potential of providing precision approach capability throughout the footprint of a satellite-based augmentation system (SBAS) geostationary (GEO) satellite. Even outside the defined SBAS service areas, users in the footprint of these broadcasts, which is essentially most of the world, will be able to use dual frequency avionics (L1 and L5) to provide a precision approach capability with little or no ground infrastructure investment. These dual frequency avionics will use integrity data from augmentation system corrections that are broadcast from Geostationary Earth Orbit Satellites (GEOs) on L1 and L5.
In the event of interference on either L1 or L5, these regions within the footprints, can revert to non-precision approaches using GPS with integrity data from SBAS corrections broadcast on L1 or L5 or GPS with receiver autonomous integrity monitoring (RAIM).

World Map showing regions with GEO footprints

Figure 1: Regions within the GEO footprints represent the area of the world that will gain precision approach capability with the addition of L5. The GEO footprints shown here represent those of the U.S. Wide Area Augmentation System (WAAS), the European Geostationary Navigation Overlay Service (EGNOS), and the Japanese MTSAT Satellite-Based Augmentation System (MSAS).

Increased Availability of Precision Navigation Operations

GPS modernization is compatible with and complements the WAAS. The L5 signal will be available on additional WAAS GEOs launched in September and October 2005. When both L1 and L5 are available, avionics will use a combination of signals to provide the most accurate service possible, thereby increasing availability of the service. These avionics will use ionospheric corrections broadcast by WAAS, or self-generated onboard dual frequency corrections, depending on which one is more accurate.

World map with SBAS service areas

Figure 2: The shaded areas represent conceptual SBAS service areas where L5 will improve the availability of precision approach operations.

Interference Mitigation

Due to the low power of GPS signals, interference has been identified as a challenge related to the use of GPS. Despite this, the technical feasibility of using augmented GPS as the only means of navigation in the aircraft as well as the only navigation service provided by the Federal Aviation Administration (FAA) was confirmed as technically feasible in an independent study conducted by Johns Hopkins University Applied Physics Laboratory. However, the study did recommend efforts to mitigate the effects of intentional interference, unintentional interference, and atmospheric disturbances on GPS and its augmentation systems. The mitigation of interference is not only important to aviation, but to other GPS applications as well. To mitigate the effects of interference and any impact on civil aviation safety-of-life operations, the FAA is planning to use the new navigation signal at L5 (1176.45 MHz) for both WAAS and the Ground Based Augmentation System (GBAS).
The availability of L5 will encourage the development of avionics that can use GPS signals on both L1 and L5 to eliminate ionospheric errors. For example, WAAS will broadcast ionospheric corrections and integrity data on both L1 and L5. In the event of interference on either L1 or L5, avionics will be capable of conducting precision approaches, as currently designed by the FAA, using the GPS and the WAAS ionospheric corrections and integrity data broadcast on whichever frequency is available (either L1 or L5).
Similarly, GBAS will broadcast corrections and integrity data pertinent to both L1 and L5. In the event of interference on either L1 or L5, the aircraft will be able to conduct precision approaches using the data broadcast for the frequency available. The addition of L5 will have a profound impact on both the new and existing applications of GPS for navigation.
For more information on GPS Modernization Activities, please visit the GPS Directorate website.