GUS Type 1 Maintainer Training Purpose of the WAAS
GPS • GPS Satellites constantly orbit the earth; at least four should be visible anywhere on Earth. • 12-hour orbit (11,000 nm altitude, in view for up to 6 hours) • 6 orbital planes, located 60 ° apart in right ascension and inclined at 55° to the equator. • Broadcast signals include: Ranging, Ephemeris (own orbit), Time, Almanac (other orbits), Health and Status
Distance Point A Point B Range Navigation • Distance measured as mi or km • Or distance measured using time Distance = Time X Velocity
Ionospheric Delay Changes the Speed of the signal • GPS signal isn’t traveling in a straight line through a vacuum • Ionosphere’s electron content causes a curved path and a propagation delay • Causes a range error because signal takes longer to arrive • Can be determined by measuring two different freq L1 and L5
GPS Error Sources Type of Error Definition Amount Drift, short-term and long-term, that prevents satellite broadcasts from being perfectly synchronized Clock 3 meters 4 meters Ephemeris Error in measurement and calculation of satellite’s orbit Troposphere Effects of lower (ground to 8-13 km) atmosphere’s temperature, pressure, and humidity on signal propagation 2.4-25 meters Ionosphere Effect of upper (50 km to 500 km) atmosphere’s free electrons on signal propagation 10-30 meters Multipath Error caused by signals reflected from surfaces near the receiver’s antenna (such as buildings); these reflections interfere with or are mistaken for the primary signal 2-100 meters
The Theoretical View • Satellite orbits are perfect, so the satellite position is absolutely correct. • Ranging signals from all satellites are perfectly synchronized, so receiver offset is a constant • Broadcast signals are not distorted by passing through the atmosphere
The Realistic View • Satellite orbits are out of date, so the satellite position is uncertain. • Ranging signals from all satellites are out of step, so receiver offsets are all different • Broadcast signals are delayed by the ionosphere and troposphere; noise is present everywhere
Using Differential GPS to reduce GPS errors • Receiver is placed at very accurately surveyed location • Position is measured using GPS satellites • Range correction is calculated by base, since it knows exactly where it is • Correction is transmitted to neighboring receivers More info at: http://www.trimble.com/gps/dgps.html
WAAS Functions • Measures pseudoranges to satellites and verifies the GPS satellites are giving valid data. • Measures and models atmospheric effects (signal delays) • Calculates corrections (pseudorange, satellite position, satellite clock, Ionospheric delay) • Determines residual errors (uncertainty) in the corrections
WAAS Functions - Continued • Broadcasts the corrections and associated residual errors using geosynchronous satellites • Monitors, configures, and directs repair - both automatically and manually
Safety Is Paramount • Hazardously Misleading Information (HMI) must be avoided • Indicated error more than true error, pilot won’t land with GPS/WAAS , even though it’s safe—misleading but not hazardous • Indicated error less than true error, pilot will land with GPS/WAAS, but it’s not safe—misleading and hazardous.
Two System Modes • Continuous Service • All algorithms performed • All messages broadcast • Military Emergency • All algorithms performed (as in Continuous Service) • Fast Correction message altered; range correction = 0 and UDRE = maximum, to deny full accuracy to enemy
GEO Satellite GPS Satellites GCCS GEO Satellites GUS (3) GUS Primary GUS Backup GUST Primary GUST Backup WRS (38) (3) Correction and Verification Subsystem WRS = 3 WREs Operations and Maintenance Subsystem WAAS/GCCS System Diagram TCN WMS (5) (3 C&V Capable) O&M (2)
WAAS/GCCS Architecture • Three GEO satellites (CRE/AMR/CRW) and six GUSTs (BRE, WBN, HDH and SZP, APC, and LTN). • A primary and a redundant GUSTs are implemented with a dedicated GEO satellite. • The two GUSTs are operationally independent and geographically separated. • Mitigate simultaneous loss due to natural disasters. • Redundant GUST radiates into a dummy load while receiving GEO and GPS satellite signals. • A GUST consists of the SGST, RFU, antennas, and supporting equipment.
Terrestrial Communication Network (TCN) • Private Network • Two Independent Rings with diverse paths • 10.20x.1.xxx and 10.20x.2.xxx • Four Communication Nodes - ZLA (Palmdale, CA), ZDC (Leesburg, VA), ZAU (Chicago, ILL), ZTL (Hampton, GA) • One Distribution Node – ZSE (Seattle, WA) • Each Site connects to two different Communication Nodes.
Wide-area Reference Station (WRS) • Contains three identical Wide-area Reference Equipment (WRE) subsystems • With three independent measurement threads • Each WRE: • Collects and validates data • Independently transmits measurements to the three WMSs with C&V cabability (ZDC, ZTL, and ZLA)
Wide-area Master Station (WMS with C&V capability) • Selects independent wide-area reference equipment (WRE) data streams • Calculates corrections for each space vehicle (SV) • Determines residual errors • Ensures proper safety bounding of user position error; adjusts user differential range error (UDRE), grid ionospheric vertical error (GIVE) • Schedules and formats messages to users • Sends messages to all ground uplink stations type 1 (GUSTs) for broadcast to GEO • Automatically controls GUST switchovers
GEO Uplink Subsystem Type 1 (GUS Type 1) • Relay WAAS transmission from WMS to the WAAS satellites • Transmit combined WAAS message and ranging signal • Generate proper C/A code ranging signal • Provide closed loop control to maintain a “GPS like” WAAS Signal-in-Space (SIS) • Validate GEO message transmission • Monitor and report GUST status to WMS
O&M Subsystem • Monitors all WAAS/GCCS subsystems • Displays alarms and alerts • Allows operator to reconfigure subsystems • Change operating mode • Download software • Allows operator initiated corrective maintenance procedures.
Summary Purpose of WAAS • Satellite Navigation • WAAS Functions • WAAS Architecture