Navigation - Overview

Navigation satellites were one of the earliest military applications of space technology, and among the most useful to military forces on Earth. Warplanes now use navigation satellites to guide them to aerial tankers for in-flight refueling as they fly non-stop from their home bases to conflicts thousands of miles away. Warplanes can also use navigation satellites to guide them to their targets with pinpoint precision, where they can drop their bombs with an accuracy that will rival that of much more expensive smart weapons.

The relatively new concept of Navigation Warfare (NAVWAR) assures access to precise navigation information in a challenged environment. It also selectively denies this information to adversaries while lessening the effect on neutral or friendly operations. The three principal tenets of NAVWAR are to protect the use of GPS by DoD and allied forces in times of conflict within the theater of operations; prevent the use of GPS by adversary forces; and preserve routine GPS service to all outside the theater of operations. Navigation Warfare is focused on validating the technologies and CONOPS for implementing electronic warfare (EW) countermeasures both to protect and prevent the use of satellite navigation systems. The overall goal for the NAVWAR program is to mitigate hostile use of satellite navigation by unfriendly forces on the battlefield (prevention) while ensuring unimpeded use of the Global Positioning System (GPS) for U.S. and Allied forces (protection). Throughout the execution of this program, a multi-Service evaluation team will incrementally assess the viability of developed technologies and their ability to effectively satisfy mission needs. Those technologies which are proven successful will be incorporated into the overall NAVWAR architecture. As CONOPS develop, they must make sure US and allied systems can work together. The US intends to discontinue the use of GPS Selective Availability (SA) by 2006 in a manner that allows adequate time and resources for military forces to prepare for operations without SA.

Airborne Pseudolites (APL) are rapidly deployed GPS Navigation Warfare assets that provide theater-wide coverage to combat the effect of GPS jamming on DoD users ranging from the individual soldier to combat platforms and precision GPS-guided shoot-to-coordinate weapons (e.g., JDAM, JSOW, TLAM, SLAM-ER, AGM-130, etc.). The considerably increased transmit power of the APL fights off the effects of jamming on the DoD receivers. In this approach, the NavWar resources are concentrated on the APLs instead of having to be deployed to each DoD GPS receiver. APLs must overcome the following two critical challenges: (1) Backward compatibility solves the non-Keplerian motion of the UAV platform in a manner that accommodates APL use with a software-only modification to existing DoD GPS receivers (e.g., PLGR, JDAM, etc.); and (2) APL self-positioning provides sufficiently high GPS jam resistance on the APL to continue precision self-positioning using the GPS satellites directly through jamming. DARPA's GPS Experiments (GPX) consist of a two-pronged effort leading up to flight demonstrations of brassboard equipment.

The American Transit navigation satellite network was initiated in the early 1960s, with twelve operational and spare Transit satellites currently in orbit. The Transit navigation satellite network continued operations in 1993, with twelve operational and spare Transit satellites in orbit. Most of the military users of Transit, such as the Navy's ballistic missile submarines that were the original impetus for Transit, will soon shift to Navstar. The Transit constellation will remain in service to civilian users at through 2000 (rather than the previously plan of 1995).⁽¹⁾

The Navstar Global Positioning System (GPS) satellites provide two-dimensional (latitude and longitude) coverage with three satellites in view, and three dimensional (including altitude) coverage with four satellites in view.⁽²⁾ The total of 21 active plus 3 spare GPS satellites is planned for implementation by 1993, which will extend this service on a continuous global basis.

The launch of five Navstar satellites in 1990 brought the total constellation to fifteen active satellites by year's end. Additional launches over the following three years brought the total constellation operational complement of 21 active and 3 reserve satellites by the end of 1993.⁽³⁾ Several older Navstar navigation satellites, launched between 1980 and 1985, continued in service in 1993.

The Coarse/Acquisition (C/A) code broadcast at 1575.42 MHz normally provides approximately 30 meter accuracy, while the precision Code (P-code) at the 1227.6 Y-Frequency provides 10 to 16 meter accuracy. Encryption of the C/A code under Selective Availability, which reduces C/A accuracy to 100 meters, began in mid-March 1990, and ceased temporarily during operation Desert Shield/Desert Storm.⁽⁴⁾ The P-code can also be encrypted, in a format referred to as Y-Code.⁽⁵⁾

This ten-fold improvement in accuracy will revolutionize warfare on Earth. Traditionally Navstar has been regarded as providing navigation support to vehicles and platforms rather than weapons, but this is changing.⁽⁶⁾ Small and inexpensive GPS receivers will be added to the air-launched version of the Navy's Harpoon cruise missile known as the Standoff Land Attack Missile (SLAM), the Block III version of the non-nuclear Tomahawk long-range cruise missile (which begins tests in 1991 for a 1993 initial operational capability), as well as free-fall conventional Inertially Aided Munition gravity bombs.

Traditionally Navstar has been regarded as providing navigation support to vehicles and platforms rather than weapons, but this is changing.⁽⁷⁾ Small and inexpensive GPS receivers will be added to the air-launched version of the Navy's Harpoon cruise missile known as the Standoff Land Attack Missile (SLAM), the Block III version of the non-nuclear Tomahawk long-range cruise missile (which began tests in 1991 for a 1993 initial operational capability), as well as free-fall conventional Inertially Aided Munition gravity bombs.

The shifting sands and lack of visible landmarks in Saudi Arabia accentuated the utility of Navstar for ground forces.⁽⁸⁾ Given the limited availability of military GPS receivers, the military ceased encoding GPS signals⁽⁹⁾ (to reduce the accuracy of civilian receivers to 100 meters, versus the 16 meter encoded military standard), and several thousand commercial receivers were quickly purchased for shipment to Desert Shield forces.⁽¹⁰⁾

During operations Enduring Freedom, Noble Eagle and Iraqi Freedom, GPS contributions increased significantly. During OIF, the GPS satellite constellation allowed the delivery of 5,500 GPS-guided Joint Direct Attack Munitions with pinpoint precision (to about 10 feet) and with minimal collateral damage. This was almost one-fourth of the total 29,199 bombs and missiles coalition forces released against Iraqi targets. GPS continues to fill a crucial role in air, ground and sea operations guiding countless service members and equipment to ensure they are on time and on target.

The GPS Program Office is working to enhance many of the capabilities provided by the current satellite based position and navigation equipment. In April, 2014 the pre-operational broadcast of navigation messages began for additional civil signals (L2C and L5). Applications such as mapping, aerial refueling and rendezvous, geodetic surveys, and search and rescue operations will benefit from these enhancements.

Robust Surface Navigation via Signals of Opportunity (RSN)

The Defense Advanced Research Projects Agency"s (DARPA) Special Projects Office (SPO) released two Broad Agency Announcements (BAAs) during late April 2005 for the Robust Surface Navigation via Signals of Opportunity (RSN) and Sub-Surface Navigation via Signals of Opportunity (SSN) programs. These two programs, combined, represent the concepts of interest stemming from the Request for Information (RFI) released 11 Feb 2004 entitled Geo-location and Navigation via Signals of Opportunity (NAVSOPP). The overall goal of these programs is to extend our geo-location and navigation capability to areas where GPS does not provide adequate capability, and to provide these services under conditions of GPS denial.

Phase 1 of each program will consist of identification and assessment of potential exploitable signals, followed by analysis and performance modeling, measurement-based concept validation, and preliminary design. In Phase 2 the program performers whose designs are selected from Phase 1 will conduct final design, build, and test of prototype receivers and other supporting infrastructure for navigation using the signals of opportunity and special beacons, where required. This will include demonstration of a non-form/fit functional prototype system in multiple environments. The specific goal of the RSN program is to develop technologies that will enable our warfighters to have GPS-like navigation under conditions of GPS denial and in difficult environments where GPS does not provide adequate performance.

These environments include inside buildings, urban canyons, and under dense foliage. Since GPS signals do not penetrate or are obscured by multipath propagation in these environments, RSN expects to use existing terrestrial and space based signals (such as broadcast TV, AM/FM radio, and cellular phone networks) as a basis for navigation. A key desired feature is the ability of the system concept and architecture to exploit these signals as globally and as non-intrusively as possible. DARPA envisions the capability to span the range of user platforms from airborne fast-movers to sensors and ground forces in urban interiors. It should use a layered approach that uses, for example, signal-of-opportunity "calibration reference" ground stations at maximum practicable standoff for wide area coverage, but that can easily and smoothly transition to more intrusive active beacons as necessary to augment the signals of opportunity when they are inadequate for the task.

The specific goal of the SSN program is to develop technologies that will enable warfighters to have GPS-like navigation and stationary asset geo-location in sub-surface environments such as underground facilities, tunnels, mines, and unimproved caves. Ideally, SSN would use existing "signals of opportunity," as RSN does. However, DARPA realizes that very few signals penetrate the ground enough to be usable. Thus the SSN program recognizes that purpose-deployed signal beacons and inertial navigation sensors or other augmentation will take on a greater role, and that the characteristics of these solutions are likely inconsistent with a more general receiver and architecture that make it a broadly applicable and efficient global solution for all above-ground navigation, as in RSN.

References

1. Danchik, Robert, et al, "The Navy Navigation Satellite System (TRANSIT)," Johns Hopkins APL Technical Digest, volume 11, numbers 1 & 2, 1990, pages 97-101.

2. "Sluggers Pinch Hit for Army GPS," Military Space, 24 September 1990, page 1, 8.

3. "Magnavox Prepares for GPS Buildup," Military Space, 25 September 1989, page 3-5.

4. Kiernan, Vincent, "Air Force Alters GPS Signals to Aid Troops," Space News, 24 September 1990, page 1, 35.

5. Klass, Philip, "Inmarsat Decision Pushes GPS to Forefront of Civ Nav-Sat Field," Aviation Week & Space Technology, 14 January 1991, page 34-35.

6. NATO AGARD (Advisory Group for Aerospace Research and Development), Tactical Applications of Space Systems, Avionics Panel Symposium 16-19 October 1989, (AGARD-CP-460, NTIS N90-27438), contains several papers that provide an excellent review of the status of Navstar users.

7. NATO AGARD (Advisory Group for Aerospace Research and Development), Tactical Applications of Space Systems, Avionics Panel Symposium 16-19 October 1989, (AGARD-CP-460, NTIS N90-27438), contains several papers that provide an excellent review of the status of Navstar users.

8. Moore, Molly, "US Training, Tactics Shift With Desert Sand," The Washington Post, 25 November 1990, page A1, A25.

9. Kiernan, Vincent, "Air Force Alters GPS Signals to Aid Troops," Space News, 24 September 1990, page 1, 35.

10. "Sluggers Pinch Hit for Army GPS," Military Space, 24 September 1990, page 1, 8.