Selective Availability(SA) satellites at various levels of accuracy denial. Anti-spoofing(A-s) Accuracy degraded 100 m for horizontal 1. GPS fundamental frequency: 8 Technology 0.3m/s for velocity Ons for time 3. P code encrypting Technology The predictable accuracy decreases to 300 m for horizontal position and to 500 m for height. How to Realize Anti-Spoofing? Technology for Denial of Accuracy and Access On March 29. 1996 residential Decision Directive(PDD)on vas released expressing the 2. The rationale for doing this intention to discontinue the use of GPS Selective nding out false signals with the gPs signature to create Availability(A)within a decade in a manner that confusion and caus s to misposition themselves. allows adequate time and resources for our military 3. A-s is accomplished by the modulo 2 sum of the p-code and forces to prepare fully for operations without SA Thus. when A-S is active. the P-code on the li and the L2 carrier is replaced by the unknown Y-code Note that A-S is either on or off. a variable influence of A-S(as is the case with SA)cannot occur. 4. Control Segment 4.1 Monitor Stations Main tasks Operational Control System(Ocs) Each station is equipped with a precise cesium time andard and receivers which continuously measure monitor stations - master control ound control Pseudoranges are measured every 1.5 seconds and Main tasks thed to produce 15-minute interval data which are Tracking of the satellites ansmitted to the master control station. Clock determination and Time synchronization of Upload of the data message to the satellites Colorado Springs Ascension Island in the South Atlantic Ocean The OCS is also responsible for imposing SA on the Diego garcia in the Indian Ocean Kwajalein in the North Pacific Ocean broadcast signals4 1. GPS fundamental frequency: δ Technology 2. Navigation message: ε Technology 3. P code: encrypting Technology Selective Availability (SA) Anti-spoofing (A-S) – The SA has only been implemented in Block II satellites at various levels of accuracy denial. – Accuracy degraded • 100 m for horizontal • 156 m for height. • 0.3m/s for velocity • 340ns for time • The predictable accuracy decreases to 300 m for horizontal position and to 500 m for height. All numbers are given at the 95% probability level. At the 99.99% probability level. How to Realize Anti-Spoofing? 1.The design of GPS includes the ability to essentially "turn off" the P-code or invoke an encrypted code as a means of denying access to the P-code to all but authorized users. 2.The rationale for doing this is to keep adversaries from sending out false signals with the GPS signature to create confusion and cause users to misposition themselves. 3.A-S is accomplished by the modulo 2 sum of the P-code and an encrypting W-code. The resulting code Y-code. – Thus, when A-S is active, the P-code on the L1 and the L2 carrier is replaced by the unknown Y-code. – Note that A-S is either on or off. A variable influence of A-S (as is the case with SA) cannot occur. On March 29, 1996, the Presidential Decision Directive (PDD) on GPS was released expressing the "intention to discontinue the use of GPS Selective Availability (SA) within a decade in a manner that allows adequate time and resources for our military forces to prepare fully for operations without SA". Technology for Denial of Accuracy and Access 4. Control Segment Operational Control System (OCS) monitor stations master control ground control Main Tasks: • Tracking of the satellites for the orbit and • Clock determination and prediction, • Time synchronization of the satellites, and • Upload of the data message to the satellites. The OCS is also responsible for imposing SA on the broadcast signals. 4.1 Monitor Stations Main Tasks: – Each station is equipped with a precise cesium time standard and receivers which continuously measure pseudoranges to all satellites in view. – Pseudoranges are measured every 1.5 seconds and, using the ionospheric and meteorological data, they are smoothed to produce 15-minute interval data which are transmitted to the master control station. Locations: – Hawaii – Colorado Springs – Ascension Island in the South Atlantic Ocean – Diego Garcia in the Indian Ocean – Kwajalein in the North Pacific Ocean