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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 ….
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