Lec 7 - GPS

NAVSTAR GPS

NAVigation Satellite Time And Ranging Global Positioning System; developed by US DoD for precise navigation; proposed by US Air Force in 1973; Initial Operational Capability: 08 Dec 1993; Full Operational Capability: 17 Jul 1995

GPS Segments

Space Segment (satellite constellation), Control Segment (controls satellites), User Segment (users with receivers)

Space Segment

32 operational satellites (31 active, 1 reserve), 8 spares, 10 more planned; cost ~

Control Segment

tracks satellite orbits, performs time synchronization, uploads navigation message, manages Denial of Accuracy (DOA); Master Control Station at Falcon AFB, Colorado Springs; monitor stations in Hawaii, Ascension Island, Diego Garcia, Kwajalein

User Segment

comprises all users with GPS receivers

GPS Signals

L1 frequency (1575.42 MHz) carries navigation message and SPS code; L2 frequency (1227.60 MHz) used for PPS; codes: C/A Code (Coarse/Acquisition on L1, for civilian SPS), P-Code (Precise on L1 & L2, for PPS), Navigation Message (50 Hz data on L1-C/A with orbit, clock corrections, etc.)

Minimum Satellites

至少需要4颗卫星进行观测: 3颗用于3D坐标, 1颗用于校正接收机钟差

Denial of Accuracy (DOA)

US military methods to limit full system accuracy: Selective Availability (S/A - adds noise to clock & navigation message), Anti-Spoofing (AS - encrypts P-code to Y-code); sometimes turned off

Sources of Error

Satellite Clock error (~30 cm per nanosecond drift), Receiver Clock error (corrected via 4 satellites), Satellite Orbit error (monitored, meters-level), Atmospheric Errors (ionosphere >10m, troposphere; use dual-frequency receivers), Multipath (reflected signals), Geometric Dilution of Precision (GDOP - better with larger tetrahedron volume/more satellites)

Satellite Mask Angle

trade-off: higher angle reduces atmospheric refraction but decreases satellite visibility; low angles have more refraction

Selective Availability (S/A)

DoD-introduced error to degrade civilian accuracy to ~100m; removed in May 2000

Differential GPS (DGPS)

corrects GPS inaccuracies; uses a base station at known point to compute errors and a rover to apply corrections; accuracy: meters (moving) to sub-meter (stationary)

DGPS Types

Post-Processing (no real-time link, data processed later), Real-Time Processing (on-the-spot correction, requires special equipment/services)

DGPS Positioning Modes

Static, Kinematic

GPS Receivers Types

Recreation-Grade (casual use, least accurate), Navigating (portable, long battery), Mapping-Grade (more features, higher accuracy, more data storage), Survey-Grade (extreme accuracy, larger, expensive)

Uses of GPS

Airplane/Boat Navigation, Continental Drift monitoring, Surveying, Precise Timing, Iceberg Tracking, Archaeological Expeditions, Mobile Multimedia

Developments

Total stations with GPS, Multi-GNSS receivers (using GPS, GLONASS, BeiDou, Galileo)

Multi-GNSS

Uses multiple systems (GPS, GLONASS, BeiDou, Galileo) to increase visible satellites from ~12 to ~30; improves availability and accuracy

GPS Community Base Station in UP

Located on Melchor Hall rooftop; above a Coast and Geodetic Survey control point