All About GPS

AGENDA• Theory of GPS

• System Description, Math, Errors

• Kalman Filtering, RTK, Inertial Navigation

• Operation of GPS

• Accuracy, Usage, Limitations

GPS SYSTEM OVERVIEW

• GPS vs. GNSS• Absolute positioning • GPS, GLONASS, Galileo,

BeiDou, QZSS, WAAS, IRNSS

TRILATERATION

• Receiver estimates range to SV• Using 4 or more ranges, position on surface of Earth can be computed

TRACKING

• Low SNR

• Correlation

• Coherent Integration

RANGING• Basic Ranging Equation:

• Dt = (Position of Satellite - Position of User) * c

• Pseudorange

• Deltarange

EPHEMERIS

• Satellites tell you their location

• Data transmitted as phase shifts encoded on top of correlation code

POSITIONING (MATH!)• Basic Ranging Equation:

• Dt = (Position of Satellite - Position of User) * c

• What are the unknowns?

• 4 unknowns => 4 equations

• Least Sum of Squares

KALMAN FILTERING

• Predict, then correct

• Model all the unknowns (states)

• Residuals, Convergence

COMMON FILTER STATES• Position (x,y,z)

• Velocity (dx,dy,dz)

• Acceleration?

• Orientation?

• Time

• Clock Bias

• Clock Drift

ERRORS

• Obstructions

• Reflections

• Interference

OTHER CONSTELLATIONS

• GLONASS: Russia (24 SVs)

• Galileo: Europe (10/30 SVs)

• IRNSS: India (5/7 SVs)

• BeiDou: China (20/35)

• QZSS, WAAS, GAGAN, EGNOS, MSAS, StarFire, OmniSTAR

SPECTRUM

• L-Band Signals

• Code Types

IMPROVING ACCURACY

CARRIER PHASE TRACKING

• Survey-Grade Receivers

• Expensive HW Requirements

• Complex Algorithms

CORRECTIONS

• Real-Time Kinematics (RTK)

• SBAS

MULTI-ANTENNA ARRAYS

• Estimate Angle of Arrival

• Multipath Mitigation

INERTIAL NAVIGATION• Accelerometer, Gyroscope

• Dead Reckoning: Integration of IMU data

• Strength: Short Interval Dynamics

• Weakness: Time

LOOSE COUPLING

• Navigation with GPS + IMU

TIGHT COUPLING

• Feedback of Inertial data into GPS signal processing engine

ULTRA-TIGHT COUPLING

• GPS and Inertial combined into one system

• Central Navigation Filter

CHALLENGES

CHALLENGES: URBAN CANYONS

• Lots of Multipath, Obstruction

• Multi-constellation helps!

CHALLENGES: JAMMING

• Broadcast noise to “drown out” GPS signals

• Most common type: CW

CHALLENGES: SPOOFING

• Broadcasting fake satellite signals to induce a false location into a receiver

CHALLENGES: LIGHTSQUARED

• LightSquared: Comm System

• Ongoing battle about interference

OPERATION

WHAT DOES GPS TELL YOU?

• Position

• Velocity

• Heading

• DOPs

ACCURACY

• Vertical: Bad - Horizontal: It Depends

• “Standard” GPS: 3-4 m

• RTK: cm possible

INTERFACES: NMEA

• National Marine Electronics Association’s NMEA 0183

• Problematic yet common

• ASCII

INTERFACES: MAVLINK

• Micro Air Vehicle Communication Protocol

• Binary Format

• Supported by ArduPilot, others

INTERFACES: PROPRIETARY

• uBlox, others have proprietary binary formats

• Faster, more consistent

• Not human-readable

TROUBLESHOOTING• No Fix:

• Clear View to Sky?

• Tracking SVs?

• Ephemeris Decoded?

• Poor Fix:

• What are CN0’s?

• Obstructions?

• Interference?

COMMON INTERFERENCE• Cellular Networks, Radios

• FM/TV Broadcasting

• Electromagnetic Fields

• Any other on-board radios, if not properly designed

ALTERNATIVES TO GNSS?

ALTERNATIVES: ELORAN

• Ground-based GPS-like system, currently being debated in Congress

ALTERNATIVES: IOT

• Navigation using connected devices around you

• Ashton Carter (SecDef) statements in 2015

SIDE NOTE: ADS-B

• System for aircraft to communication location to each other and the NAS

• sUAS typically can’t afford beacons in power & weight budgets

QUESTIONS?Thank You!