GPS Precise Point Positioning Technique: A Case Study in Iranian Permanent GPS Network for Geodynamics

IPGN : Iranian Permanent GPS Network
A dense and wide permanent GPS station network has been established in Iran (Tabriz-Tehran-Mashhad) and other active part of the country by National Cartographic Center of Iran (NCC). Since first of the 2005 this network and is designed both for crustal deformation monitoring and to serve as a highly precise geodetic network in Iran and consist of 107 permanent stations in first phase. Average distance between dense parts is about 25 to 30 km.

Since we have collected about 1 year data, we estimated horizontal crustal displacement and velocity field with respect to the stable Eurasian plate. This new network will bring us more precise information on crustal information and geophysical phenomena such as ionosphere disturbances and water vapour too. Finally this network serve as active controlling. The Software for processing the data is Gamit/Globk V,10.20.

GAMIT
GAMIT is a comprehensive GPS analysis packs developed at MIT and Scripps for the estimation of three- dimensional relative positions of ground stations and satellite orbits. The software is is designed to run under any UNIX operating system supporting X-windows. The maximum number of stations and sessions allowed is determined by dimensions set at compile time and can be tailored to fit the requirements and capabilities of the analyst's computational environment. The primary output of GAMIT is a loosely constrained solution file of parameter estimates and covariance's which can be passed to another software module called GLOBK for combination of data to estimate station positions and velocities and orbital and Earth-rotation parameters.

GAMIT uses double differences by differencing the between station differences also between satellites to cancel completely the effects of variations in the station clocks. GAMIT incorporate difference-operator algorithms that map the carrier beat phases into singly and doubly differenced phases. The GAMIT software uses triple differences in editing the data but not in parameter estimation. A major source of error in single-frequency GPS measurement is the this effect. To correct for the ionosphere delay and provide a check the algorithm used, GAMIT employs both the dual-frequency pseudo-ranges and phase observations.

GAMIT incorporates a weighted least squares algorithm to estimate the relative positions of a set of stations, orbital and Earth-rotation parameters, zenith delays, and phase ambiguities by fitting to doubly differenced phase observations. Since the functional(mathematical) model relating the observations and parameters is non-linear, the least- squares fit for each session may need to be iterated until convergence, i.e., until the corrections to the estimated station coordinates and other parameters estimates and covariance's which can be passed to another software module called GLOBK for combination of data to estimate station positions and velocities and orbital and Earth-rotation parameters.

IPGN Processing Strategy
The processing strategy applied by NCC uses GAMIT software version 10.2 for IPGN (Iranian Permanent GPS Network as follow:

  • IGS final orbits and IERS bulleting B values for EOPs are used.
  • At least (up to 22) IGS stations are included in the processing to link between regional and global solution.
  • Tight constraints on orbital parameters and EOPs.
  • Solid earth tide is consistent with IERS2000.
  • For radiation pressure effects, Berne mode(9-parameter)is used.
  • For ocean title loading , Scherneck model (IERS standards, 1992 ) is implemented. - Sasstamoinen model for the zenith delay and Niell mapping for both the hydrostatic and wet delay are applied.
  • A piece- wise linear zenith model are employed.
  • LC relax combination is used.
  • Elevation-Dependent Model for antenna phase-centers.
  • Ambiguity-fixed and free solutions will be produced for each day.
  • Reference frame Global:
  • Daily solutions are combined with global solutions from SOPAC.
  • Combined daily solutions are transformed into ITRF2000 by minimizing the correction of 8 IGS core stations by estimating 7-parameter transformation with GLOBK.
Observation (Data Set) for PPP
This research used data from 22stations of IPGON which one of them is belong to IGS (tehn). These stations are located on the active part of the country. Data was collected over a period one GPS week (1358) . The initial observation were made on 15jan2006 and the final observation on 21Jan2006 . The reference epoch for processing was 1.1.2000 which the final solution will be referenced to ITRF2000 epoch (2006/0397) . All stations equipped with Ashtech UZ-12(ICGRS)receivers and Ash 701945 B_M precise antenna that collect data in 30" interval.

The strategy adopted for the processing of GPS data of IPGON in PPP mode with Bernese were mentioned in the previous section:

So used IGS final orbit (ITRF 2000) and the epoch refers to each day of observations, and also final clocks and EOP. The basic observable was ionosphere free with 30 second recording interval and elevation musk of 10 deg. The ambiguity weren't solved as integers. Geophysical model specific correction in accordance with IGS and IERS(ocean tide –body tide-pole tide).

Analysis of the Results:
The precision and accuracy of the coordinate of the stations in terms of local geodetic coordinate system (N,E,U) are analyzed.

The precession analysis is based on two criteria
  1. The formal standard deviations of the coordinates.
  2. The repeat abilities of the estimated coordinates.
The formal standard deviation of parameters is obtained from (variance and covariance matrix) of parameters. The daily repeatability provide a more realistic measure of precession for station coordinates. It is given by the expression:



n: number of occupation days, Ri is the estimated coordinate and ∑i is the formal error of the coordinates for day i and Rm is the weighted mean of the coordinates of the station. Fig 3,4 illustrate the precession of the station coordinate components(N,E,U)in term of and repeatability respectively.


Fig 2- precession of the station coordinates



Fig 3- repeatability


Comparison(Relative Positioning and PPP)
The main benefit of PPP is that its strengths are exactly in the areas of the weaknesses of relative processing and vice versa. Rather that being competitors, they complement each other perfectly.

The accuracies of relative positioning degrade with distance from the base station . Of course, this is not the case with PPP since it does not relay on a base station.

The main draw back with PPP is that there is a delay of approximately two weeks from the time of data collection to the availability of the precise satellite coordinates and clocks. Consequently, the most precise positions are not available until two weeks after the data were collected. A less precise solution is possible however, using the rapid satellite position and clock solutions. Due to double-differencing, the degradation of the coordinate accuracy is less when using the rapid products in relative positioning.

Relative positioning is a more logical choice for this task since it takes advantage of the cancellation effect when double differencing. That is, any errors common to the network partially or totally cancel- troposphere, ionosphere, tidal and non-tidal loading each fall into this category. This many not be the case with PPP since each point is processing independently of the others. Ambiguity resolution is also simpler in relative processing. Great care must be taken, however, to ensure that correct antenna .

Phase centre models are employed when mixing antenna type in relative processing, while PPP relies only on a single antenna. Fig4 shows the coordinate difference between two processing mode for IPGN.


Fig 4


Conclusion
In this research 22 stations of IPGN were processed with Bernese V.5 software and compare the result with those obtain in ITRF2000 with. The result shows the general agreement. The results shows that the standard deviation of the PPP for this GPS week (1358). Solution is better than 4mm for daily solutions, and the repeatability is about 2mm, 3.5mm,4.5mm for N,E,U components. The difference between the coordinates of PPP and those obtain in relative mode in ITRF 2000 are due to the ambiguity resolution for PPP and combination of solution in Gamit/Globk software. The difference between baseline computed by PPP and relative mode is better than 3mm. Farther analysis recommend for longer observation and also velocity estimation and plate rotation vector with the PPP.

References:
  1. Gao, Y . and Shen, X.(2001), "I improving Ambiguity convergence in Carrier Phase-Based Precise Point Positioning", Proceedings of ION GPS, Salt Lake City.
  2. Gao, Y.and Shen, X.(2002),"Ambiguity Pseudo-Fixing in Precise Single Point Positioning", ION 58th annual Meeting/CIGTF 21st Guidance, Albuquerque, NM
  3. Hofmann et al(1997),"GPS:Theory and Practice". 4th revised ed., Springer Wien New York 389p.
    4. .
  4. King, M.: Edwards, S. and Clarke P. "Precise Point Positioning: Breaking the Monopoly of Relative GPS Processing".
  5. Kouba,J.(2003)"A Guide to using international GPS Services (IGS) Products".GSD,Ottawa, Ontario Canada, February.
  6. Kouba,J.and Heroux,P.(1995),"GPS Precise Point Positioning with a Difference. "Geomatics'95,Ottawa,Ontario,Canada, June 13-15
  7. Kouba, J. and Heroux, P.(2000), "GPS Precise Point Positioning) using IGS Orbit Products", GSD,NRCan.
  8. Roulston, A. Talbot N.and Zhang K.(2000), " An Evaluation of Various GPS Satellite Ephemeris, "Proceedings of ION GPS 2000,Salt Lake City, UT.
  9. Salam, M.A.Gao , Y. and Shen , X. (2002), "Analyzing the Performance Characteristics of a Precise Point Positioning System", Proceedings of ION GPS,Portland
  10. Shen ,X.(2002), "Improving ambiguity Convergence in Carrier Phase-based Precise Point Positioning" MSc. Theses, Department of Geometrics Engineering, University of Calgary.
  11. Witchayangkoon, B.(2002), "Elements of GPS Precise Point Positioning", M.Sc.Thesis, the University of Maine December
  12. Zhong-yi,C,Cheng. H; Xiao-Gong,H.(2002), "Solution of Regional GPS Network Using Precise Point Positioning with Undifferenced Data", Chinese Astronomy and Astrophysics 26,p 69-80.
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