GNSS & Mapping. GNSS (Global Navigation Satellite System) GIS (Geographic Information System)
GNSS for UAV Navigation - ITSNT 2016
Transcript of GNSS for UAV Navigation - ITSNT 2016
GNSS for UAV Navigation Sandy Kennedy
Nov.15, 2016
ITSNT
Sounds Easy Enough …
Probably clear open sky conditions?
» Maybe not on take off and landing
Straight and level flight?
» Not a valid assumption for all applications
How does GNSS fit into the UAV navigation system?
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UAV Navigation: Control System
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GNSS Receiver Host Processor(Flight Control)
Telemetry Rx/Tx
IMU
Actuation of Vehicle Controls
GNSS Telemetry
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Requirements Depend on the ApplicationWhat is the required GNSS positioning accuracy?» Just metres?» Centimetres?» For the whole flight? For landing and take off?
What is the allowed envelope for GNSS antenna and receiver?» Antenna mounting location» Antenna size and weight» Same for receiver
What is the operating environment?» Actual open sky?» Rotor blades?» Buildings/structures?» Launch dynamics, and expectation of accuracy availability
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Antennas – the Neglected Front DoorPositioning accuracy drives antenna specification for phase centre stability and uniform gain pattern
» Across frequencies and across elevation/azimuth angles
» Becomes more challenging with smaller size, but size constraint is typical
» Antenna position is usually not what is required. Needs to be translated to a location of interest (underbelly/wing hook, landing gear)
Frequency support
» Which GNSS signals to support
» L-band corrections?
» Multipath rejection performance?
Mounting constraints
» under rotor blades
» changing orientation from take off to flight 5
Point of Interest vs Antenna Mounting Location
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Changing Orientation
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Antenna Size/Space Constraint
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Telemetry
Some other form of RF onboard the UAV
» For command/control of the UAV
» For delivery of GNSS differential correction data
» UAV reporting back to base
Needs an antenna too
Potential interference source
» Separating antennas may not be possible on small platforms
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IMUProvides the attitude information needed to translate antenna position
Full GNSS/INS solution or just AHRS?
» AHRS typically assumes flat and level and constant velocity operation
» Position bridging needed or just attitude?
May be processed by GNSS receiver or the host processor.
Needs GNSS data to estimate/calibrate the IMU aka converge the GNSS/INS filter.
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GNSS Receiver
Computes position and velocity
» May also compute GNSS/INS solution
» Could provide the input to another sensor fusion filter
» Application specifics dictate what type of solution is needed• Epoch to epoch as uncorrelated as possible, or• As smooth as possible to feed the control system
» SWaP constraints – one or all
May be receiving correction data
» L-band for PPP
» Differential corrections for RTK
» Differential corrections for moving base (relative RTK)
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Landing Scenarios
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Host Processor(s)
Interfaces with the GNSS receiver(s), and possibly other sensors
» A backup GNSS receiver is sometimes used
Performs flight control computations
Actuates the vehicle controls
Manages communications with the user/base
Potential source of interference
» CPU clock frequency creating harmonics in-band
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Payload
Could be inert
» Packages
» Medical products
Could be more electronics
» LiDAR
» SAR
» Cameras
Potential source of interference
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Blood Product Delivery
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Synthetic Aperture Rader
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Common Theme?
Lots of potential for self-interference
» Electronics closely packed together = self-interference
How much degradation is acceptable in your design?
Depends on performance requirements and operating conditions
How much margin do you need to leave for dealing with external interference?
» Autonomous flight means your navigation must be reliable, with maximum availability
» Risk management standards higher
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How to Deal with Self-InterferenceFirst step is to detect and then diagnose the source» Low C/Nos» Frequent lock breaks» Increased phase noise» Slow time to first fix» Difficulty fixing ambiguitiesRF spectrum analyzers are complicated to deploy properly, and interpreting their measurements can be difficult
Could use mechanical solutions» Shield the GNSS receiver card (metal can)
• Won’t help if the antenna is the interference entry point
» Separate GNSS and telemetry antennas• Not always possible due to size constraints
…There’s got to be a better way18
Turn your GNSS Receiver into a Spectrum AnalyzerBuild the detection capability into the GNSS receiver, as we have in OEM7» Gao, F., Kennedy, S., (2016). Demonstrated Interference Detection and
Mitigation with a Multi-frequency High Precision Receiver, Proceeding of ION GNSS+ 2016, Portland, Oregon
Spectrum Analysis via FFT» Ideal for in-band Continuous Wave (CW) interferers which are common in
self interference cases – host processor CPU clock harmonics» More sophisticated shape analysis can determine out-of-band effects
Other detection methods within the receiver signal processing chain» Borio, D. et al (2016) Impact and Detection of GNSS Jammers on
Consumer GradeSatellite Navigation Receivers, Proceedings of the IEEE, Vol. 104, No.6.
Run built in tests that continuously monitor for the presence of interference, and report to the user
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Spectral Analysis via FFT: 3 options to output
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• Pre-decimation: full path view, good for detecting near band interferers lurking in the transition zone
• Post-decimation: the bandwidth of interest for a particular signal (ieGPS L1)
• Post-filter: the band of interest for a particular signal after the application of a notch or bandpass filter
Normal GPS L1 Power Spectrum
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Mitigated In Band CW Interferer – with Notch Filter
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Out of Band Wideband Interferer
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Interference Confirmed – Now to MitigateInterference Toolkit is onboard every OEM7, and is SW configurable on or off
HDR Mode:» High resolution ADC
• More range to the upper end to handle strong interference signals
» Increased front end linearity• Pass band as flat as possible
» Multi-stage gain control• Distribute gain to keep signals in the middle of device’s operational range
Digital Filtering» Notch and bandpass filters
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Example: Interference Toolkit vs Cavity Filter
Out of Band Narrowband Interferer: GlobalStar Satellite voice/data service
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Viewing the Spectrum
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GPS Satellites Tracked – without Mitigation
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Single Point Pseudorange Position – No Mitigation
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GPS Satellites Tracked with Mitigation
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Single Point Pseudorange Position – with Mitigation
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Positioning Errors – Before and After Mitigation
No Mitigation
Northing Easting Height
Std.Dev. (m)
Range (m) Std.Dev. (m)
Range (m) Std.Dev. (m)
Range (m)
OEM628 3.364 126.494 1.164 34.388 5.783 230.399
OEM719 1.420 36.663 1.155 34.350 1.242 32.655
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With Mitigation
Northing Easting Height
Std.Dev. (m)
Range (m) Std.Dev. (m)
Range (m) Std.Dev. (m)
Range (m)
OEM628 0.122 0.566 0.113 0.460 0.291 1.283
OEM719 0.129 0.679 0.114 0.522 0.260 1.231
Multi-Frequency, Multi-Constellation Approach
GPS only is not sufficient for most applications with restricted sky view, or high banking turns
L1 only – doesn’t provide sufficient accuracy and time to fix over longer baselines
So, expand to track everything in the sky.
» Won’t diversity of measurements help overcome interference?
Yes, it will, to a certain extent
» Provided the interference is restricted to a single frequency or signal type
» Provided the interference is at a low enough power that the front end doesn’t saturate
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Example: In-band CW Interferer
Placed an CW interferer at 1582.5 MHz, -44 dBm strength, via conducted input at the receiver input
» Mitigated with HDR mode and a notch filter, 1 MHz wide centred at 1582.5
Computed an RTK solution over a very short baseline (5m), using a “clean” base receiver, without interference
Processed the collected measurements in three ways:
» L1 only (GPS, GLO, GAL)
» GPS L1/L2
» All in view (GPS L1/L2/L5, GLO L1/L2, GAL E1/E5AltBOC)
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Available Satellites
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In Band Continuous Wave Interferer
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Signals Tracked
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L1 Only
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GPS L1/L2/L5
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All in View: GPS L1/L2/L5, GLO L1/L2, GAL E1/E5
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Summary of RTK 3D Position Errors
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Summary – GNSS for UAV Navigation
Protect your GNSS solution
Backups are good, but exploit GNSS to it’s full extent.
Will only help your sensor fusion since GNSS is typically used to estimate errors in other sensors
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Thanks for your attention!
Receiver Design for Interference Robustness
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Filters
Variable Gain Amplifiers
OEM7 Family
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OEM760055x35mm
OEM719OEM7700OEM772071x46mm
OEM729100x60mm