Positioning Techniques

João F. Galera Monico - UNESP Tuesday 12 Sep

Positioning methods

• Absolute Positioning – Static and kinematic

• SPP and PPP

• Relative Positioning – Static

– Static rapid

– Semi kinematic

– Kinematic • RTK and NRTK

Single Point Positioning (SPP)

• Code based - pseudorange

• Knowing the satellite coordinates , it is possible to compute the coordinates of the user antenna in the same frame

• From a geometrical point of view, only three satellites would be enough

• However, the receiver clock is not synchronized with the satellites system clock – An extra satellite is required

SPP

• Considering the code pseudorange measurements (SF)

This estimable receiver clock is a combination of the true

receiver clock error plus the (frequency-dependent) receiver

hardware delay

TGD Correction (GPS – Galileo - QZSS) SF C/A users has to apply additional P1-C1

(SPP)

The linearized system will be given by: s=1,2, …n>4

n-u = s-4

Comments on SPP

• Satellite positioning from broadcast ephemeris

• Accuracy in the level of 10 to 15 m

• Often referred as navigation solution

– solution for most of mobile

• It can also be computed with dual frequency data

Precise Point Positioning (PPP) • Undifferenced, precise point positioning (PPP)

techniques aim to provide absolute positions in a precision global reference frame.

• It uses pseudoranges and carrier phase observations in combination with precise (IGS) products. – Precise orbits and clocks

• Single-frequency and multifrequency PPP

• One or more constellation.

Precise Point Positioning • The precise (IGS) products (precise satellite clocks)

are based on the ionosphere-free combination, similar to the broadcast satellite clock for most constellations.

• For single-frequency PPP, corrections for the satellite DCBs are required.

– but these are also provided by the IGS or its analysis centers. (Tgd)

• For Single-frequency PPP users ionospheric corrections are essential and these can be obtained from the IGS as well, in the GIM format.

– Can also be estimated •

Precise Point Positioning

Precise Point Positioning

• GLONASS PPP (and also RTK) requires a-priori correction of the receiver- and frequency-dependent interchannel or interfrequency biases, at least for the phase data (L. Wanninger:

Carrier-phase inter-frequency biases of GLONASS receivers, J. Geod. 86(2), 139–148 (2012) ).

– Importante para solução das ambiguidades…

• All other cited errors have to be appropriated treated.

PPP Concept

(a) CORS (global or regional) network determines GNSS

parameters;

(b) satellite-dependent parameters are uploaded by the network and

downloaded by a user;

(c) the user applies the corrections to his data, enabling single-receiver precise positioning

Dennis Odijk

Single Frequency (SF) PPP • It is assumed that identical offsets for the

satellite clock and hardware bias apply to both code and phase observations, as well as a-priori corrections for tropospheric and ionospheric delays.

• And as the satellite clock offsets are based on the Ion-Free combination (as is the case with precise IGS products), the correction terms for code and phase then can be given as:

SF PPP

• The ambiguities is not integer – has bias – Rank defect

• The Ionosphere was taken from a model (GIM for instance)

• We can also estimate the Ionosphere and include constraints if available.

• Accuracy in the level of dm after about 30 minutes of data collection.

Dual Frequency (DF) PPP

DF PPP • With these modifications, the steps to get the

ION-FREE solution is equal to the SF PPP

• Including the ionosphere as unknown, without

combine the observations, is another option (the

state of the art of PPP).

• At such case, the ionosphere will be estimated,

and can be constrained if information is available.

•

International GNSS Service (IGS) RT

• IGS Real Time was developed to provide Real Time PPP

– Several products available.

– Corrections to the broadcast ephemeris and clocks…

– http://www.igs.org/rts/products

PPP with Ambiguity Resolution (AR)

• Several models were developed

• Consist in the determination of fractional part of the satellite phase biases using a network of receivers

• It has to be provided to the user

• Such fractional part (bias) is for some extend constant (at least for few hours/days).

Ambiguity Fractional Part (FCB)

Ion-free versus uncombined PPP

• Iono-free combination, could make the

inclusion of triple frequency observations to be

cumbersome, since many possible dual-

combinations exist.

• In order to provide a suitable representation for

triple frequency receivers uncombined phase

biases model compatible with the RTCM SSR

framework may be better

– RTCM (Real-Time Maritime Services)

– SSR (State Space Representation)

•

The PPP-Wizard software package

uses uncombined phase biases model

(http://ppp-wizard.net/)

‘PPP-RTK’ integer PPP improved by atmospheric corrections from local/regional RTK networks

PPP-AR

SSR Computation

• In RTK or NRTK systems, the main errors affecting the GNSS measurements are monitored by a GNSS reference station or a CORS dense network, and provided to the users as range corrections

– OSR (Observation-Space-Representation)

– OSR is not able to dissociate the different errors impacting GNSS measurements

SSR Computation

• State-Space-Modeling (SSM) is the

representation of all relevant physical

effects by a mathematical model

– The parameters that are estimated in

real-time using CORS network

–Knowing the temporal and spatial

behavior of these effects, it is possible to

optimize the use of all observations

coming from the CORS network

SSR Computation

• The state vector of such State-Space-

Representation (SSR) is provided by the

integrated and optimized SSM.

– This representation is applicable to PPP-RTK

….

….

Real Time PPP activity

• Install the BNC (BKG Ntrip Client) – https://igs.bkg.bund.de/ntrip/bnc

• Get access to data and one kind of IGS corrections • Choose a station available in real time and

perform real time PPP.

• Provide Analysis and comment of the results

"IGS-IP" <igs-ip@bkg.bund.de>

Real Time Kinematic (RTK)

• Concept: a base station provide corrections

(observations) to a rover receiver in real time.

• It is mainly carrier based (DD observables)

• The aim is to reduce and remove errors

common to a base station and rover pair

• All data processing is carried out at the rover.

– Ambiguity fixed

– Accuracy of up to 1 cm + 1 ppm.

Real Time Kinematic (RTK)

• It is a relative positioning method

(2017) Springer Handbook of Global Navigation

Satellite Systems Editors: Teunissen, Peter J.G., Montenbruck, Oliver (Eds.)

Drawback of RTK

• Deteriorate with distance from base station

(2017) Springer Handbook of Global Navigation

Satellite Systems Editors: Teunissen, Peter J.G., Montenbruck, Oliver (Eds.)

troposphere

From RTK to Network RTK (NRTK) • It is based on the use of several widely spaced

permanent stations.

• Depending on the implementation, positioning

data from the permanent stations is

regularly communicated to a central processing

station.

• On demand from RTK user terminals, which

transmit their approximate location to the central

station, the central station calculates and

transmits correction information or corrected

position to the RTK user terminal.

• The benefit of this approach is an overall

reduction in the number of RTK base

stations required.

• Depending on the implementation, data may be

transmitted over cellular radio links or other

wireless medium.

• The distance dependent errors can be accurately

modeled using the measurements of an array of

GNSS reference stations surrounding the rover site.

• Therefore, the solution to the distance limitation of

RTK lies in multibase techniques which became

popular under the name network RTK, sometime

abbreviated to NRTK.

•

• Network RTK technique enabled the establishment of positioning services which serve larger regions or whole countries.

– One possibility is to create a VRS (Virtual Reference Station) close to the user

– The user than perform the relative positioning.

•

Stations RTCM-VRS RTCM-VRS

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NMEA NMEA

Bidirectional Link

VRS

Relative positioning

RTK experiment - Usina Guarani

Mendonça; Monico; Motoki (2012)

São Paulo State NRTK.

Statistical results from an experiment of station Tupã (SP-Brazil).

Estação de

Coleta Base/RTK

Tempo de

inicialização Início Fim

Pontos

coletados

TUPÃ

VRS 1min 24 seg 13:07:01 as 13:18:17 205

ARAC (GNSS) 84,13km 8 min 4 seg 13:24:52 as 13:43:41 205

VRS 2 min 23 seg 13:47:55 as 14:08:33 205

ARAC_S (GPS) 84,13 km 12 min 19 seg 14:18:35 as 14:44:30 205

Comments on NTRK

• It works quite well for very stable ionosphere

–We can say the same for PPP and PPP-RTK

• In the equatorial region during some period we have problems due to the behavior of the ionosphere, mainly due to the Ionosphere Scintillation – after the sunset….

Comments on NTRK

• Several research can be carried out on these topics ….