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