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GNSS Precision – New RTKPositioning TechnologySteve Richter – Frontier Precision, Inc.

Outline

The Major Epochs of RTK Innovation

GNSS Positioning concepts

RTK methods, single base and VRS

HD-GNSS, precision based Surveying

xFill, Positioning without a Correction Stream

Summary & Questions

Introduction RTK has quickly become commonplace in Surveying

radio comms

correction streams

on-the-fly initialization

faster processors

Traditional GNSS Initialization techniques have been

effective, but these concepts remain from the 1990’s

No corrections – no precise positions

New technology has emerged for improving field

operation, field confidence, and overall

performance using Precision Based Algorithms

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Objectives

The following presentation aims to:

– Provide some theoretical background on high-

precision positioning

– Highlight the differences of HD-GNSS and

conventional RTK techniques

– Precision based RTK surveying without float/fixed

– Introduce the xFill technique

– Better understand how Precision Based RTK

improves productivity in the field

1988… Trimble introduced the 4000SD

When High Precision Surveying Began!

Ground-breaking technology (literally)

Dual-frequency, 5 L1, 5 L2 channels!

1 Mbyte Internal data storage!

Only 98 lbs (excluding battery), 66Watts!

Post-processing in the office

2.5 lbs (with battery)

... 2012 Trimble R10

Major Epochs of RTK Innovation4000SLD - First Dual-freq Backpackable (1988)

1990 1995 2000 2005 2010 20151985

Nine GPS Satellites in orbit Total Weight: 44 lbs (without car battery)

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Major Epochs of RTK Innovation4000SSE - First RTK with On-the-fly Initialization (1994)

GPS Declared Fully Operational

Total Weight: 15.4 lbs (with everything!)

1990 1995 2000 2005 2010 20151985

Major Epochs of RTK Innovation4800 - First RTK with No Strings Attached! (1997)

Total Weight: 8.5 lbs (full RTK rover)

1990 1995 2000 2005 2010 20151985

Major Epochs of RTK InnovationR8-GNSS, 2005, First GPS/GLONASS RTK

Selective Availability Switched Off

First Modernized GPS (IIR-M) Satellite Launched

Total Weight: 8.2 lbs (full RTK rover)

1990 1995 2000 2005 2010 20151985

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Major Epochs of RTK InnovationR10 – Technology for Productivity (2012)

Precision Based RTK – HD GNSS

31 GPS &24 GLONASS Satellites in orbit

Total Weight: 7.9 lbs (full RTK rover)

1990 1995 2000 2005 2010 20151985

Technical Background- GNSS Positioning Principles

Autonomous Positioning

GNSS Receiver measures

pseudorange (distance) to

each satellite in view

Location of each satellite is known

from orbital parameters

(‘navigation message’)

Position and time determined by

receiver using pseudoranges and

known satellite locations

(trilateration)

Technical Background- Errors affecting GNSS measurements

Satellite orbit and clock errors

directly affect the user-satellite

range measurements

Ionosphere (50-1000km altitude)

causes a frequency-dependent

error in GNSS measurements

Troposphere (< 50km) causes a

delay in the GNSS signals

Multipath is caused

by signal reflection

off nearby objects

Signal masking reduces

tracked satellites and

degrades range

measurement accuracy

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Autonomous Positioning- For Mapping and GIS applications

Autonomous position is normally

estimated using GNSS code

measurements with a typical

accuracy of around 10m

Satellite and atmospheric errors

all directly impact on the

accuracy of the computed user

position (single-frequency)

Differential Positioning- Example, using CMR / RTCM corrections

Satellite and atmospheric errors

are nearly identical for two

closely spaced GNSS receivers

With the differential technique, the

relative position of a rover is computed

with respect to a single reference station

Differential positioning accuracy

is superior to Autonomous

positioning (typically < 1m)Differential corrections are

generated by the reference

station and applied at the rover

Improving Precision- Carrier phase versus code

GNSS pseudorange

measurements are based

on PRN code data

Code measurements

have a precision of a

few decimetersCarrier phase

measurements have

mm-level precision

however they contain

an integer bias

Once the integer bias is resolved on

each satellite, carrier phase

measurements deliver precise range

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Multi-Frequency- Reducing the effects of ionospheric errors

Satellites broadcast on multiple frequency

bands

(e.g. GPS L1, L2 & L5; GALILEO E1, E5, E6)

Multi-frequency carrier phase and

code observations help correct for

ionospheric errors and enable

rapid ambiguity estimation

RTK Modes- Single-Base

Single GNSS base (reference)

station established near work site

and tracks all satellites in view

Satellite and atmospheric

errors are nearly identical for

closely spaced reference and

rover stations

Reference station corrections

delivered to rover via radio

datalink

Rover GNSS applies satellite

corrections supplied by reference

station and obtains cm-level results

RTK Modes- Virtual Reference Station

GNSS Reference stations

established over wide

geographic regions

(e.g. city, state, country)

Reference station spacing

typically 50-100km

Reference stations track

all GNSS satellites in view

Pivot Platform Server concentrates reference

station data and models satellite and

atmospheric errors over network

Network server generates a virtual reference

station next to rover and provides GNSS

corrections via wireless Internet connection

Rover obtains cm-level

accuracy within coverage

region

Internet

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Float / Fixed Solutions

Carrier phase ambiguities are by definition

integer quantities

Need to make use of the integer-nature of the

carrier phase ambiguities to gain utmost

accuracy from GNSS

0 +1 +2 +3-1-2-3

Traditional Ambiguity Resolution

Traditional approach to ambiguity resolution

involves first estimating the carrier phase

ambiguities as floating point (real-valued)

numbers

Hence the term float solution

Code measurements are used to refine the

float ambiguity estimates

0 +1 +2 +3-1-2-3

Traditional Ambiguity Resolution

Direction of

Satellite 2

Direction of

Satellite 1

Direction of

Satellite 3 Wavefront

for Sat 2

Wavefront

for Sat 3

Wavefront

for Sat 1

Integer candidate

locations

Correct integer

candidate

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Traditional Ambiguity Resolution The integer search picks one set of integer ambiguities and then

ignores information still available in the search space

‘Repeated searching’ is effective, but a band-aid

Large disparity between precision of the float solution and the

fixed solution

Incorrect fixing leads to position outliers with low reported

precisions

Po

sit

ion

Err

or

[m]

Time

Float Solution

Correctly

Fixed Solution

Incorrectly Fixed Solution

HD-GNSS involves state-of-the-art techniques for

processing GNSS carrier phase data, including:

– Generalized method for dealing with biases on

carrier phase data

– Using all the information within the search space

to provide statistically optimum ambiguity

resolution

– Improved apriori measurement noise models

– Rigorous generation of aposteriori position

precisions

Techniques have been made possible with the advent

of high performance microprocessors

HD-GNSS Basics

HD-GNSS BasicsAmbiguity Resolution in One Dimension (1D)

Probability Distribution of

estimated Float-Solution

(assumed Gaussian)

Flo

at-

am

big

uity

estim

ate

Most-Probable Integer

Ambiguity is closest to

float ambiguity estimate

Direction of

Satellite

Wa

ve

fro

nts

(In

teg

er

Am

big

uity

Ca

nd

ida

tes)

0 +1 +2 +3-1-2-3-4 +4-5

All integer candidates that

fall within the bounds of the

float solution are considered

in HD-GNSS solution

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Bia

se

d F

loa

t-A

mb

igu

ity

Estim

ate

HD-GNSS Basics Biased Float Solution 1D

Wa

ve

fro

nts

(In

teg

er

Am

big

uity

Ca

nd

ida

tes)

Direction of

Satellite0 +1 +2 +3-1-2-3-4-5 +4

Correct Integer Ambiguity

Candidate has Low

Probability based on

Biased Float-Solution

Must not discard!

Best integer solution based

on traditional ambiguity

resolution approach

Large bias in

Float Solution

HD-GNSS Basics Consideration of biased ambiguities (1D)

Wa

ve

fro

nts

(In

teg

er

Am

big

uity

Ca

nd

ida

tes)

Float-ambiguity

estimate

Bias & Probability of each integer

candidate is considered in the

formation of the HD-GNSS solution

Direction of

Satellite0 +1 +2 +3-1-2-3-4 +4-5

Require an over-determined

solution to be able to assess

the quality of each integer

ambiguity candidate

Precisions reported by

HD-GNSS solution

encapsulate distribution

of integer candidates

Direction of

Satellite 2

Wavefront

for Sat 2

Direction of

Satellite 3

Wavefront

for Sat 3

Direction of

Satellite 1

Wavefront

for Sat 1

HD-GNSS Basics Assessing integer candidate quality (2D)

Quality of each integer

candidate can be made

with an over-determined

number of satellites

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HD-GNSS -Precision Based Surveying

Reported precisions give a statistical measure of the

quality of a position solution

Need to understand that precisions are normally

given at a particular confidence level (e.g. 68%, 95%)

East

North

Up

3D Error Ellipsoid

East

North

2D Error Ellipse

Reported horizontal and vertical precisions are

a function of:

– Satellite geometry

(more satellites = improved precision);

– Measurement errors (primary multi-path)

(smaller measurement errors = improved precision)

– The term “initialized” is redefinedRTK started – precisions dependent on the environment

HD-GNSS -Precision Based Surveying

Excerpt from RTK Market Requirements Document Jan 2003:

“User-selectable accuracies:

The RTK customer should be able to specify a particular application, which

automatically drives different elements of the RTK engine and produces appropriately

accurate positions”

HD-GNSS -Precision Based Surveying HD-GNSS delivers seamless convergence to the same traditional

‘fixed’ precisions levels – fast!

An important aspect of the scheme is that it delivers

corresponding converging precisions

The polarized switch from ‘float to fixed’ is gone (and these terms!)

An environment that caused a ‘bad init’ reports higher precisions

Po

sit

ion

Err

or

[m]

Time

Float Solution

Incorrectly Fixed Solution

HD-GNSS

Solution

Correctly

Fixed Solution

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HD-GNSS -Positioning advantage example, 11km

HD-GNSS SolutionFloat Solution

HD-GNSS Solution produced

where conventional fixed

solution would have failed44 sec

HD-GNSS -Positioning advantage example, 11km

HD-GNSS Solution

Float Solution

HD-GNSS Solution produced

where conventional fixed

solution would have failed

44 sec

HD-GNSS - Summary

Precision-based measurements

Start measuring sooner

Shorter occupation times

Measure with confidence in

more challenging

environments

(Trimble R8/R7/R6/R5/R4)

(Trimble R10)

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xFill Extension Service- Basics of field operation

There are times when reference

GNSS corrections are blocked

Rover positioning normally

ceases soon after reference

GNSS correction interruption

The Trimble RTX technology

delivers precise GNSS orbit and

clock data to users via satellite

xFill helps to maintain precise

rover positioning while normal

GNSS corrections are blocked

xFill- RTX Network Concepts

Trimble has established a

global GNSS receiver tracking

network ~ 100 stations

GNSS data from the tracking station network are

processed at a central RTX control center (server)

to produce cm-level satellite orbits and clocks

RTX – Satellite

Orbits and

Clocks

xFill Extension Service

RTX Control

Center

RTX L-band

Satellite

Virtual Reference

Station (VRS) correction

R10 Rover

Physical Reference

Station correction

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RTX L-Band Delivery Services

RTX Service L-Band Frequency [MHz]

Bandwidth [bits / second]

Coverage

RTXWN 1557.8615

600

Western North America

RTXEN 1557.8590

600

Eastern North America

RTXEA 1539.9525

600

Europe, portions of Russia, Africa and

the Commonwealth Independent

States

RTXAP 1539.8325

600

Russia, Asia and Australasia

RTX services provided via Inmarsat satellites in

equatorial, geosynchronous orbits

GNSS errors

for the rover

Differencing Errors

Usual method when primary stream available

Errors common to single base / rover cancel

VRS stream errors similar to rover using

interpolation – appears as a short baseline

GNSS errors for the

reference station / VRS

Residual errors in

difference GNSS

Modelling Errors

Rover must operate autonomously

RTX service provides satellite clock & orbit

Modeled errors removed at rover

Local atmospheric effects estimated

GNSS errors

for the rover

GNSS errors

modeled in RTK

stream

GNSS atmosphere

algorithms used in

RTX

Residual errors

in Trimble RTX

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SBL/VRS Position SBL/VRS PositionPosition Outage

Primary Stream Blocked

Radio or Cellular Modem signal unavailable

Physical Reference

Station Correction

SBL/VRS Position SBL/VRS PositionxFill

xFill using RTX L-band stream

Rover generate positions autonomously

Physical Reference

Station Correction

RTX L-band

Satellite

Field Considerations

xFill starts seamlessly without convergence

Effect of loss of L-band RTX stream

– xFill continues for 20-secs (current time-out)

– Resumes without delay for up to 5-minutes

Effect of loss of GNSS satellites

– Loss and Gain OK if 4 satellite maintained

– Drop to 3 satellites requires radio-link to re-start

– Precisions affected by satellite geometry (PDOP)

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xFill Positioning Example

xFill Positioning Example

Summary

Shift from traditional Initialization Algorithms

HD-GNSS sets a new industry benchmark for

RTK technology and performance

Precision Based GNSS allows you to measure

more confidently and be more productive

xFill seamlessly bridges correction-stream

drop outs (radio or GPRS modem)

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Questions?