GPS Beamforming with Low-cost RTL-SDRs file3 Streamed 60 seconds of data from each RTL-SDR (2016)...

GPS Beamforming with Low-cost RTL-SDRs

Wil Myrick, Ph.D.

September 13, 2017

GNU Radio Conference 2017

2

Recap from GRCon 2016

Timelineof Signal Processing Experience

MWF Invented by Dr. Scott Goldstein and Dr. Irving Reed (1996)

GNU Radio Initial Release (2001)

Completed MWF based GPS STAP Ph.D. Thesis with Dr. Michael Zoltowski (2000)

Revisited GPS Work and GNU Radio (2013)

Asynchronous Distributed Sensor Processing with FM

SoOPs (2014)

Embedded GPS Research

(2015)

+

GNSS-SDR GPS Processing

Adjust the frequencies and phases based on tracking

information from the RTL-SDRs

3

Streamed 60 seconds of data from each RTL-SDR

(2016)

Demodulated navigation data for each detected

channel (2016)

Tracked delay, frequency, and phase from each

RTL-SDR (2016)

Time, frequency, and phase alignment based on

RTL-SDRs sharing no common external references

RTL-SDR based GPS beamformer with GNSS-SDR

GPS position estimates from beamformed RTL-

SDRs

GNSS-SDR on Raspberry Pi 3

From GRCon16 to GRCon17

http://gnss-sdr.org/documentation/how-gnss-sdr-works

GNU Radio

Receiver based

on GNSS-SDR

Framework

(CTTC)

• Built on GNU Radio blocks

• Supports real-time positioning based on RTL-SDRs

• Supports custom algorithm integration

4

Robust Positioning, Navigation, and Timing (PNT)

Robust PNT

# GPS Receivers + # Other RF Sensors

Anti-Spoofing

Anti-JamMultipath Ro

bu

st P

NT

Spatial Beamforming enables a variety of solutions that provide Robust PNT on single platforms

IoT Devices

IoT devices with low SWAP-C could share PNT information potentially providing Robust PNT solutions for a group of IoTDevices

Explore Distributed GPS Beamforming with a Variety of SDRs

Standard Beamforming Solutions

Fusion of SDR Measurements

ENSCOTCR-D

Low Swap-CRTL-SDR

5

GPS Beamforming with RTL-SDRs

Asynchronous SDR Receivers

GNSS Signals

Low-Cost RTL-SDRs Lack External Clock Inputs

6

Yacht Spoofing

UAV Spoofing

Typical Approaches to Synchronizing RTL-SDRs

Multi-RTL Approach

Other References: http://superkuh.com/rtlsdr.html#clocks.align.multirtl

Images from https://github.com/ptrkrysik/multi-rtl

“Multi-RTL is GNU Radio block that transforms cheap multiple RTL-SDR receivers into multi-channel receiver” Reference: https://github.com/ptrkrysik/multi-rtl

Typical approaches

to leveraging

multiple RTL-SDRs

involve hardware

modifications so a

single clock source

is shared between

the RTL-SDRs

7

Overall RTL-SDR “Software” Synchronization Approach

Coarse Bit Synchronization based on PRN

Fine Chip Code Synchronization based on PRN

IF I&Q Samples

A/D Asynchronous Snapshot Files

GPS Beamforming based on Carrier Phase Extraction

IF I&Q Samples

A/D

IF I&Q Samples

A/D

RTL-SDR Software Synchronization

GPS Extraction

GNU Radio GNSS-SDR Receiver

W. Myrick, M. Picciolo, J. Scott Goldstein,“A COTS based Asynchronous Distributed Array Processor utilizing Reduced-Rank STAP,” IEEE Radar Conference, May 2013

Asynchronous Snapshots

Bit and Code Synchronization

Carrier Phase and Code Frequency Extraction

8

RTL-SDR “Software” Synchronization Approach

9

Asynchronous Snapshots with RTL-SDRs



Asynchronous Snapshot Files



GPS Extraction


RTL-SDR1_file_asyncRTL-SDR2_file_asyncRTL-SDR3_file_asyncRTL-SDR4_file_async

Script to capture 60 seconds of GPS data from RTL-SDRs

RTL-SDR1_file_syncRTL-SDR2_file_syncRTL-SDR3_file_sync

RTL-SDR4_file_async

RTL-SDR data files are synchronized relative to RTL-SDR4 afterRTL-SDR software synchronization

Synchronization of RTL-SDRs relative

to RTL-SDR4

10

GPS Acquisition across RTL-SDRs






GPS Extraction


RTL-SDRs provide varying GPS acquisition results

RTL-SDR 1 RTL-SDR 2 RTL-SDR 3 RTL-SDR 4

13 13 13 13

15 15 15 15

29* 29* 29* 29*

2 - 2 2

5 5 5 5

20 20 - 20

* PRN 29 was used as reference transmitter for RTL-SDR synchronization

11

Coarse (Data) and Fine (Code) Synchronization






GPS Extraction

Leverage RTL-SDR 4 as the reference


19 ms

24 ms

62 ms

12

Coarse (Data) and Fine (Code) Synchronization






GPS Extraction

Leverage RTL-SDR 4 as the reference


13

Carrier Phase Extraction on RTL-SDRs






GPS Extraction

Differential phase of PRN 29 relative to RTL-SDR 4


Clock Drift

14

Carrier Phase Extraction on RTL-SDRs (PRN 29)






GPS Extraction

Differential carrier phase with clock drift compensation


15






GPS Extraction




16






GPS Extraction




17






GPS Extraction




18






GPS Extraction




2

19






GPS Extraction




20

GPS Beamforming RTL-SDR Performance






GPS Extraction


RTL-SDR Beamforming Performance per PRN

Beamformer Gain

RTL-SDR 4

4.14 dB4.47 dB2.89 dB

5.06 dB

Ideal Gain: ~6dB

5.39 dB 4.56 dB

21

GPS Acquisition across RTL-SDRs






GPS Extraction


PROMPT SCREEN

GNSS-SDR operating on Raspberry Pi 3

22

RTL-SDR Beamformer Comparison (SDR 2)

RTL-SDR 2 Only Best RTL-SDR Beamformer

23



24



25

Summary and Next Steps

# GPS Receivers + # Other RF Sensors

Ro

bu

st P

NT

Explore Distributed GPS Beamforming with a Variety of SDRs

Fusion of SDR Measurements

ENSCOTCR-D

Low Swap-CRTL-SDR

Initial results show feasibility of GPS beamforming utilizing RTL-SDRs without hardware modification

“Software Synchronization” preprocessing approach of RTL-SDRs allow exploration of GPS beamforming leveraging existing GNSS-SDR processing architecture

Plan to explore distributed GPS beamforming with a mixture of SDRs in a variety of environments

Ideas to Reality™

Wilbur L. Myrick, Ph.D.

[email protected]

(703) 321-4504

Questions?

GPS Beamforming with Low-cost RTL-SDRs file3 Streamed 60 seconds of data from each RTL-SDR (2016)...

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