Precise Positioning With NovAtel CORRECT Including .

Precise Positioning with NovAtelCORRECT Including PerformanceAnalysisNovAtel White Paper—April 2015OverviewThis article provides an overview of the challenges and techniques of precise GNSS positioning. It provides a description of Precise Point Positioning(PPP), as implemented in NovAtel CORRECT and compares PPP to the Real Time Kinematic (RTK) method that has been used for precise positioningfor over 20 years. The relative advantages of RTK and PPP methods are summarized in terms of the implementation logistics and performance.Finally, sample performance data is presented to illustrate the typical results obtained using NovAtel CORRECT with the TerraStar service.About NovAtel CORRECT NovAtel CORRECT with RTK: Real Time Kinematic positioningfor the most precise measurements (cm level), relative to a localsurveyed base station network. NovAtel CORRECT with PPP: Precise Point Positioning using datafrom the TerraStar correction service to deliver a globally availableand reliable solution with precision approaching that of RTK (subdm level). NovAtel CORRECT with SBAS: Positioning utilizing publicly availableSBAS augmentation data to provide sub-metre solutions.Choosing the best solution for your precise positioning applicationdepends on many factors including performance, cost andimplementation or setup requirements. NovAtel CORRECT is thestate-of-the-art positioning algorithm on NovAtel’s high precisionGNSS receivers that handles corrections from a variety of sources,including RTK, PPP, Spaced Based Augmentation Systems (SBAS)and Differential Global Positioning Systems (DGPS). With NovAtelCORRECT, you can choose the corrections method that best meetsthe requirements and performance objectives of your application.See more ioningPrecise Positioning with NovAtel CORRECT1novatel.com

Achieving high-precision GNSS measurementshave physical issues with sag in the chain, thermal expansion andother factors that would prevent us from trusting the measurementwithin certain limits. Similarly, GNSS signals suffer effects suchas propagation delays and clock precision that lead to rangemeasurement errors. These are described in more detail later.Standard civil GNSS provides a simple and ubiquitous solution forglobal positioning when metre-level accuracy is sufficient. Eventhough this is sufficient for a large number of applications, theneed for high-precision positioning for certain applications has ledto innovations in the way we use GNSS, and methods that providecentimetre, rather than metre-level accuracy.Solving the precision hurdle – carrier phasemeasurementsThe two most important examples of precise GNSS methods areRTK and PPP. As shown in Figure 1, these two methods providesignificantly better accuracies compared to Differential GNSS(DGNSS) or single-point positioning when employing correctionsprovided by GNSS augmentation systems such asSBAS. Figure 1also illustrates the difference in baseline limitations of each system,which constrain their use to within a certain range of base receiversor reference networks. PPP does not have any such limitations and itcan be used anywhere on the Earth.The GNSS signal information is transmitted by modulating an RFcarrier wave in the Gigahertz frequency range. While pseudorangemeasurements are based on the timing of the modulation and theinformation contained in it, both RTK and PPP methods measure thephase of the carrier wave itself to obtain more precise measurements.In the case of the L1 C/A signal, the carrier frequency is 1575 MHz, soone carrier cycle has a wavelength of approximately 19 cm. Moderngeodetic quality GNSS receivers such as the NovAtel OEM628 canmeasure carrier phase with better than 1 mm accuracy. See GNSSMeasurements – Code and Carrier Phase Precision on page 10 formore information.Two hurdles – precision and uncertaintyFigure 1 Comparison of GNSS Augmentation TechniquesThe carrier phase measurements come witha catch however. The measurement does notindicate the full range to the satellite, only thelength of the last fractional wave arriving at thereceiver antenna. Carrier phase measurementson their own are like fine graduations on a tapemeasure which has no labels. We can trackrelative changes in range, caused by movementsof the satellite and the receiver, with greatprecision, but determining the absolute range is acomplex puzzle. The unknown part of the carrierrange measurement is referred to as the carrierphase ambiguity. By analyzing multiple carrierphase measurements from multiple satellites, itis possible to determine the value for the carrier10,000 km Worldwide phase ambiguity that fits best with the observedmeasurements. The methods of determiningthe phase ambiguity differ in that RTK exploitsthe data available from two receivers to simplify the puzzle, whilePPP requires a much greater number of measurements from thesingle receiver to gradually converge on a solution. Once the phaseambiguity is estimated with sufficient accuracy or the integer natureof it is solved, both RTK and PPP methods can estimate the positionwith high accuracy using carrier-phase measurements.Accuracy10 m1m10 cm1 cm10 km100 km1000 kmBaselineStandalone GNSS is limited both in terms of the precision with whichmeasurements can be made and the errors or uncertainty introducedby various physical effects. To understand these limitations, considerthe challenges of making precise distance measurements using thelinks in a long piece of chain.One problem is the links – they serve the purpose well as long asthe precision needed is not much finer than the length of the link,otherwise the very shape and consistency of the links becomes alimitation. In the case of standalone pseudorange-based GNSSmeasurements, the links in the chain are the discrete steps in thechipping code that modulates the RF carrier signal. Precision islimited to about 0.2% of the chip length in the code. For L1 C/A , forexample, this amounts to 2 nanoseconds, or 0.6 metres at the speedof light.This leaves the limitation of uncertainty in the measurement and theneed to mitigate the sources of measurement errors.The second problem with high-precision measurements is theuncertainty in the measurement. In our chain analogy, we wouldPrecise Positioning with NovAtel CORRECT2novatel.com

Reducing measurement uncertainty –mitigation of errorsError mitigation – RTK versus PPP methodsRTK is a relative positioning method that provides the position ofone receiver antenna (the “rover”) relative to another receiverantenna (the “base”). If the location of the base receiver is known, anabsolute position of the rover can be estimated. Most error sourcesare common to both the rover and base receivers, and thereforecan be mitigated by differencing measurements across receivers.This reduces the magnitude of the errors significantly when thedistance (baseline) between receivers is not long. The length of thebaseline must typically be 40 km or less to enable RTK carrier-phaseambiguity resolution when ionospheric conditions are not extreme.Refer to Figure 2.A level of precision is only useful to the extent that the measurementcan be trusted to the same level. The sources of errors in GNSSmeasurements include satellite position, signal propagation delaysand timing accuracy in both the satellite and receiver. Typical levelsof uncertainty are shown in Table 1 in metres. The errors must bemitigated to enable centimetre-level positioning using PPP or RTK.Table 1: Source of GNSS ErrorsError SourceError RangeSatellite clock error 2 mSatellite orbit error 2.5 mIonospheric delays 5 mTropospheric delays 0.5 mReceiver noise 0.3 mMultipath 1 mPPP is a point positioning method which provides an absolute positionfor the rover receiver based only on the GNSS measurements availableat a single receiver and globally applicable correction products.Unlike RTK, using data from reference receivers and networks isnot needed for PPP. Therefore, PPP can provide centimetre levelpositioning anywhere on the Earth. Refer to Figure 3.When employing PPP, all significant error sources must be mitigatedwith the best possible accuracy using error models or error correctionsproducts such as precise satellite orbit and clock corrections. Theerror sources to be mitigated when employing PPP or RTK are shownin Table 2.The mitigation of errors is different between RTK and PPP methods.This is partly due to the difference in the means of error mitigationand partly due to the difference in relative versus point positioningmeasurements. The following section discusses error mitigation forRTK and PPP in more detail.Figure 2 RTK System IllustrationPrecise Positioning with NovAtel CORRECT3novatel.com

Table 2: Error Corrections and Models Required for PPP vs RTKCorrection TypePPPRTKPrecise satellite clockcorrectionsRequiredNot requiredPrecise satellite orbitsRequiredNot required forshort baselinesGroup delay differentialRequired ifusing L1 onlyNot requiredRelativity termRequiredNot requiredSatellite antenna phasewind-up errorRequiredNot requiredRequiredNot requiredThe main error sources for PPP are mitigated in the following ways:1. Dual-frequency operation. The first order ionospheric delay isproportional to the carrier wave frequency. Therefore, the firstorder Ionospheric delay can totally be eliminated by using thecombinations of dual-frequency GNSS measurements.Satellite Orbit and Clock2. External error correction data. This includes satellite orbit andclock corrections. In the case of NovAtel CORRECT with PPP, thecorrections generated by TerraStar are broadcast for end-usersby Inmarsat telecommunication satellites.3. Modeling. The tropospheric delay is corrected using the UNBmodel developed by the University of New Brunswick. However,the wet part of tropospheric delay is highly varying and it cannotbe modeled with sufficient accuracy. Thus, residual troposphericdelay is estimated when estimating position and other unknowns.Modeling is also used in the PPP receiver to correct the solidearth tides effect (see next section “Global Versus Local Datum”).Receiver Specific ErrorsReceiver antenna phasewind-up4. PPP filter algorithms. An Extended Kalman Filter (EKF) is usedfor the PPP estimation. Position, receiver clock error, troposphericdelay and carrier-phase ambiguities are estimated EKF states. EKFminimizes noise in the system and enables estimating positionwith centimetre-level accuracy. The estimates for the EKF statesare improved with successive GNSS measurements until theyconverge to stable and accurate values. The typical convergencetime of PPP to under 10 cm horizontal error is between 20 and40 minutes, but it depends on the number of satellites available,satellite geometry, quality of the correction products, receivermultipath environment and atmospheric conditions.Geophysical ModelsSolid earth tidedisplacementsRequiredNot requiredTropospheric delayRequiredNot requiredIonospheric combinations)Atmospheric ModelingFigure 3 PPP System IllustrationPrecise Positioning with NovAtel CORRECT4novatel.com