Military Use of GPS

Overview Navigation Mission GPS Segments FBCB2 Summary

- Worldwide Navigation Mission - Any weather - 24 Hours/day Provide precise position, velocity, and time to users: - Worldwide - Any weather - 24 Hours/day

System Segments Space Segment User Segment Control Segment

Space Segment Block IIA - 7 1/2 year design life Block IIR - 10 year design life Launched by Delta II from Cape Canaveral

Space Segment 24 satellites in constellation 6 orbital planes, 4 satellites in each plane 55 degree orbit inclination Semisynchronous orbit 12 hour period 12,500 statute miles (10,900 nm) altitude Constellation maximizes 3-D coverage Ensures at least four satellites are visible to user anywhere in the world (usually 6-7 visible)

Space Segment (Constellation Phasing) The GPS constellation is designed to provide 24 hour coverage with graceful degradation.

Monitor Stations & Ground Antennas Cape Canaveral Colorado Springs Kwajalein Hawaii Diego Garcia Ascension Island 5 Ground Antennas Receive/send TT&C Update NAV message and atomic clocks 5 Monitor Stations Collect measurements from GPS satellites in view

The User Segment Authorized Users: Military personnel, military systems, designated allies, and other authorized users Non-Authorized Users: Civilians and commercial firms around the world. Civilian and commercial users of GPS greatly outnumber those of the military, and the system is transitioning to allow Non-Authorized users greater accuracy.

GPS Services Standard Positioning Service (SPS) PLGR Standard Positioning Service (SPS) Intended for Non-Authorized Users Provides decreased location and timing accuracy Horizontal position within 100m or better (95%) Timing accuracy within 340ns of UTC (95%) Actual error based on current DoD Policy Corrupts navigation signal by two methods Epsilon: changes the satellites position Dither: changes the satellites time Precise Positioning Service (PPS) Intended for Authorized (military) users Provides very precise location and time accuracy 3-D position within 16m Spherical Error Probability (50%) Timing accuracy within 100ns of UTC PPS is controlled by special crytographic keys TRIMPACK 46

Military GPS Receivers Small Lightweight GPS Receiver (“Slugger”) Most Models SPS Used during Desert Storm Precise Lightweight GPS Receiver (“Plugger”) Standard Handheld Unit PPS Capable Miniature Airborne GPS Receiver Standard Aircraft Unit SLGR MAGR PLGR

Force XXI Battle Command Brigade and Below (FBCB2) Overview Description: Forms the principal Digital Command and Control System for the Army at Brigade and Below Consists of hardware and software integrated into the various platforms at brigade and below, as well as appropriate Division and Corps slices necessary to support brigade operations Interconnects platforms through a communications infrastructure called the Tactical Internet consisting of existing EPLRS and SINCGARS Radio nets to pass Situation Awareness data and conduct Command and Control 2179 FBCB2 Systems Fielded to 4th for FDD

ONE PLATFORM’S WORTH OF FBCB2 DIGITAL COMPONENTS SUPPORTING SITUATIONAL AWARENESS AND COMMAND AND CONTROL PLGR GPS GPS feeds directly into the FBCB2 computer thus giving precise locations to the user at all times. Because the GPS is completely integrated, the user can actively navigate off of the computer screen map. The GPS also provides a precise location for the user’s icon - generated by the FBCB2 hardware/software. Hardware/Software The FBCB2 hardware and software combine to provide the user with an integrated GIS system. The system allows users to manipulate map data, satellite photography, and digital elevation models. It also incorporates message capability. A key element of FBCB2 is Situational Awareness (SA). The software generates icons depicting actual units, vehicles, or soldiers deployed in the field. EPLRS VHSIC SINCGARS SIP/INC Radios (Single Channel Ground Airborne Radio System and the Enhanced Position Location Radio System) interlink multiple FBCB2s to form a larger network. This associated radio network enables the SA to be transmitted and shared between platforms and command posts.

Common Message Functions (Create/Edit/Send/Manage) Pull down menus allow the user to send various reports or build graphical overlays. When a report is sent, such as a Contact Report, the software automatically updates all other computers in the network with an enemy icon . Similarly, a commander can build a set of graphics depicting unit boundaries, friendly unit locations, known enemy locations, etc and quickly distribute this overlay around the battlefield. Sending a message automatically updates all other FBCB2s in the network.

Combat Messages Spot Report This sample report provides details pertaining to an enemy unit. Once again, information is automatically shared with all others. Enemy location can be quickly entered by clicking the mouse cursor on the computer map where it is located.

Advantages and Disadvantages of GPS Limitations Advantages All-weather Day and night Worldwide direct downlink Passive Common grid Restricted access Survivable Provides 3-D position and velocity Dependent on ground stations Jammable signal Line of Sight from GPS satellites GPS is there when you need it, in all types of weather, 24 hours a day. It is worldwide; the signal is sent directly from the satellite. Because the system is completely passive, the unit receiving the global broadcast does not radiate and, therefore, is not susceptible to detection. Each receiver set can be set to apply many different datums or grids (a datum or grid is a map type). Sometimes 2 different map types will have slightly different coordinates. By using GPS to specify the correct datum or grid, those minor “coordinate differences” can be resolved. A GPS receiver can be used to translate coordinates from one datum or grid to another. It also will coordinate time for all units involved in an operation, allowing coordinated attacks. Because access is restricted, only those individuals or units with access can use it. The GPS constellation is planned for graceful degradation even if the ground segments are taken out. Graceful degradation means that if for any reason the constellation cannot be commanded, the navigation accuracy will worsen slowly over the course of time - it will not be lost immediately. A Block II vehicle will degrade over 14 days. A Block IIA vehicle will degrade over 67 days. Provides the best position and velocity information available.

Summary GPS offers many advantages for navigation The GPS system consists of a Space Segment, a Ground Segment, and a User Segment Selective Availability ensures Authorized users can receive a highly accurate GPS signal while Non-Authorized users (potential adversaries) can be given a degraded signal The most common military GPS receiver is the PLGR, but many other receivers exist as well. GPS is evolving from a standalone capability into an integrated system called FBCB2. FBCB2 integrates GPS and a militarized GIS program to give commanders and soldiers increased situational awareness The three missions of GPS Navigation -- provides precise position, velocity, and time to users. Time transfer -- allows user to calculate time. NUDET Detection -- provides worldwide NUDET detection surveillance. Navigation Theory. C/A Code Code repeats itself every millisecond (1 MHz bit rate). Easy to acquire, assists in acquiring P-Code. Less complex, but no protection measures. Normally only transmitted on L1. P-Code. Code repeats itself every week (10 MHz bit rate). Difficult to acquire w/o handover from C/A Code. Can be encrypted to create a Y-Code. Normally transmitted on both L1 and L2. Y-Code. Encrypted P-Code used in the event of GPS satellite spoofing. Available to authorized users. Standard Positioning Service (SPS). Horizontal position within 100m or better (95%). Timing accuracy within 340ns of UTC (95%). Actual error based on current DoD policy. Cannot decrypt Y-Code or remove SA error. Usually only on one frequency, C/A Code based.