11

The GPS Software-Defined Receiver(a.k.a., the software radio)

Michael S. Braasch, Ph.D., P.E.Associate Professor of EE

Avionics Engineering CenterOhio University

22

Overview

• GPS Ranging (time-of-transmission, etc)• Spread-Spectrum Review• Traditional GPS Receiver Operation• Limitations of the Traditional Design• Software Radio Fundamentals• GPS Software Radio Advantages• The Road Ahead…

33

Signal is TransmittedFrom the Satellite

t

t

Signal is Received by the User

(A)

(B)

MSB:98010306Not to Scale

A: Signal is Transmitted at Time = TOT

B: Signal is Received at Time = TOR

TOT TOR

44

Spread-Spectrum Signal

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Spread-Spectrum Ranging

• The pseudorange is given by the speed-of-light times the difference of time-of-reception and time-of-transmission: PR = (TOR-TOT)*c

• When the signal is received, the receiver looks at its own clock to mark the TOR

• TOT is encoded by the satellite onto the signal through the navigation (NAV) data and the pseudorandom noise (prn) code

66

Spread-Spectrum Ranging (cont’d)

• Time-of-transmission (TOT) is decoded from the signal by synchronizing a copy of the prn code (generated within the receiver) with the code which is in the received signal

• This synchronization is achieved by exploiting the properties of the correlation function of the prn code

• R(☺☺☺☺) = c(t)c(t+☺☺☺☺)dt

77 MSB:98010301Not to Scale

1☺

☺

(a)

(b)

1

N- 1 N

1

1

-1

-1

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Traditional Spread-Spectrum Tracking

• The correlation function is an even function of code offset

• As a result, one does not know if the locally-generated code is too early by a certain amount or too late by the same amount

• In a traditional receiver, tracking is achieved through the use of a so-called delay-lock loop (DLL)

99 MSB:98010302Not to Scale

☺

(b)

+ d 2

- d 2

RE (☺) RL(☺)

(a)

D (☺) = RE(☺) - RL (☺)

☺

- d 2

+ d 2

-d d

d-d

1010

MSB:98010307

FromA/D Converter

90

Commands FromMicroprocessor

CarrierNCO

Delta-rangeAccumulator

Q Sample Processing(Same as for I)

3-bit Shift Register

L P E

Accumulate& Dump

Accumulate& Dump

Accumulate& Dump

1E

P

L

ToMicroprocessor

Commands FromMicroprocessor

CodeNCO

PRN CodeGenerator

Time-of-TransmissionRegister

1

1

1111

Pre-amp1 2

RF FrontEnd

31 4

ClocksLO and Clocks

LO

Interrupts!

Analog-to-Digital

Converter

Hardwareand

Software Signal

Processing

Pseudoranges,delta-ranges,carrier-phase,

navigationdata

NavigationProcessing Position,

Velocity,Time,

Fault Detectionand Isolation

External Sensors(INS), Loran-C

Barometric Altimeter)

FrequencySynthesizer

ReferenceOscillator

1: Single-frequency designs process L1 (1575.42 MHz) signal only;Dual-frequency designs process L1 and L2 (1227.6 MHz) singals

2: Several types available: Single-bit (hard-limiting), Multi-bit, Adaptive thresholds

3: Single-channel designs sequentially process each satellite tracked;Multi-channel designs multiplex the A/D output into parallel channels with each tracking separate satellites

4: Integration of GPS and external sensor data may occur outside of the GPS receiver

MSB:98010304Not to Scale

1Antenna

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Limitations of Traditional Receiver Design

• Signal processing is performed exclusively in the time-domain

• Frequency and other transform-domain processing techniques are not possible

• Phase and delay-lock loop tracking techniques are cursed with the noise vs. dynamic tracking error trade-off

• In general, any kind of block-data processing techniques are not possible

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Software Radio Fundamentals

• NO DEDICATED DIGITAL HARDWARE PROCESSING ! ! ! ! !

• Flexible baseband processing through the exclusive use of software

• Computational Speed versus Flexibility trade-off:» ASIC: great speed; zero flexibility» Desktop PC: low speed; great flexibility» FPGA’s, DSP’s: in-between

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GPS Software Radio Advantages

• FFT-based signal acquisition• Signal Quality Monitoring (correlation function

analysis, etc)• Interference detection and mitigation via

spectrum estimation/time-frequency estimation• Frequency domain tracking (looking towards

total elimination of the feedback loops)• Deep GPS/INS integration• Yet-to-be-discovered block-processing

applications

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Ohio University Version 0.1

1616

Raw Data from the A/D Converter

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Spectrum of Raw Input Signal

1818

Energy Detector vs. Code Offset

1919

Energy Detector vs. Code/Freq

2020

Frequency Offset vs. Time

2121

Pseudorange (partial) vs. Time

2222

Carrier-phase vs. Time

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The Road Ahead…

• To date, most of the advantages of the GPS software radio concept have been applied to signal acquisition

• Novel tracking algorithms (such as frequency-domain approaches) are just now being explored

• Interference detection and mitigation• Joint time-frequency estimation• Deep GPS/INS integration• Other block processing techniques

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Contact Information

Michael S. Braasch, Ph.D., P.E.Ph: 740-593-0105Fax: 740-593-1604Email: braaschm@ohio.edu

Curt CutrightPh: 740-597-1316Email: cutright@ieee.org