IMPROVEMENTS IN THE SECOND GENERATION

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Fourth International Geodetic Symposium on Satellite Positioning Austin, Texas, April 28-May 2, 1986

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IMPROVEMENTS IN THE SECOND GENERATION GPS SOFTWARE OF THE CANADIAN GEODETIC SURVEY D. DELIKARAOGLOU, D.J. McARTHUR, P. HEROUX AND N. BECK CANADIAN GEODETIC SURVEY SURVEYS AND MAPPING BRANCH

615 BOOTH STREET

OTTAWA, ONTARIO

CANADA KIA OE9

.SUMMARY At the Fi.rSt International Symposium on Positioning with the Global Positioning System (GPS) several areas of investigation and GPS software improvements were identified and recommended. These include: orbit modelling, carrier phase ambiguity parameter resolution, _ atmospheric modelling, differenced vs. non-differenced phase data processing, etc. The Canadian Geodetic Survey GPS software developments established to that date dealt mainly with the determination of single or multiple baselines using the phase observables in a double or triple-difference mode. Recent improvements have led to a second generation software being developed which allows the integrated determination of GPS orbits and multi-station baseline components in a non-differenced as well as differenced modes. In this paper the theory, background and structure of our GPS processing software with its main features and recent enhancements are summa ri zed. 1.0 Introduction The Canadian Geodetic Survey (CGS) Division of the Federal Department of Energy, Mines and Resources has taken an active interest in the geodetic applications of the Global Positioning System (GPS) since 1980. In addition to numerous tests and operational campaigns that have been performed during the past few years, considerable effort has been devoted to studying mutually independent processing techniques and designing software for the reduction and evaluation of GPS code and carrier phase observations. The first experimental VECtor Adjustment (VECA) software package [1J resulted from a simulation package developed partly under contract with the University of New Brunswick. This software has been restructured and is currently been generalized. The result to date is what we consider a

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second generation software (VEGA-2). In this paper, the main features, background and structure of the entire system are described. The status of its implementation and software tests are reported. 2.0 General Overview The VEGA suite of programs is implemented on a HP-1000 and a GYBER mainframe computer. However, in anticipation of our increasing involvement in GPS operations we have also made every effort to achieve as much portability of the software on a microcomputer environment 'for the purpose of maintaining data reduction capability in the field. As a result we have to date streamlined and transported all parts of the software onto HP-9816 microcomputers with minimal effort. This was made poss"ible by using standard FORTRAN language throughout, with occasional exceptions for some preprocessing modules on the HP-9816 (e.g. for dumping collected data from the recorded cassettes to the HP-9816) where the HP-BASIG language was used in order to reduce the coding complexity. The software has been designed with a modular structure so that modifications and enhancements are VECA-2 consists of seven modules as possible with relative ease. shown in Fig. 1. The functions of the major and supplementary modules are briefly described in the following subsections. Most of the discussion about preprocessing aspects pertains to the TI-4100 data since this is the type of receivers currently operated by GGS. We recently received four addi tional Wi 1d-Magnavox 101 recei vers for which a VEGA module to handle data to be provided by the WM-101 receivers has yet to be developed. RFORM/GFORM Module The first step in the processing of the GPS data is the data transfer from the field recording medium to the post-processing computer. The RFORM/GFORM modules perform this function by transferring TI-4100 data from the field cassettes, through a MEMTEGH-5450 XL cassette reader, into the HP-9816. RFORM deals with data recorded with the regular TI-4100 software. GFORM performs the same functJon for data recorded with the Defence Mapping Agency (DMA) GESAR software. A major attractive feature of the RFORM/GFORM module is that while the data is transferred from the cassette onto the HP-9816,

RFORM/GFORM is decoding the incoming data stream which is in the Once standard TI-4100 integer and double precision binary form [2J. the file names and header data have been entered interactively, RFORM allows the operator to select a processing window by entering the start and end times of the period he is interested in extracting data from the input cassette(s). An option for compressing the data by selecting an output data rate different than the original recording sampling rate is also available. The output of this process is a set of ASCII files of edited observables, of decoded and scaled ephemeris data, and of an almanac. Most recorded measurements are included in the RFORM measurement file after editing for: - non-integer seconds time tags; - valid tracking mode; - poor measurement quality; - signal strength; - large L1 - L2 code difference. ARLUT Modul e ARLUT is a utility program that reads TI-4100 data in the GPS standardized Exchange Format of the Applied Research Laboratory of the The output of ARLUT is al so a set of University of Texas [3J. measurement, ephemeris and almanac files in the same format as those Like RFORM/GFORM, ARLUT allows the option of output by RFORM. compressing the data by selecting an output window selectable by the ope rator. GPSPLOT Module The GPSPLOT module is a graphics routine that is used to plot code and carrier beat phase measurements for both L-band frequencies from all satellites in selectable combinations from a RFORM/GFORM output file. This utility gives the operator a visual representation of large cycle slip occurrences in the data either on the HP-9816 screen, or on a printed hardcopy. SOAP Modul e One of the major problems with the otherwise very precise GPS carrier beat phase data is the frequent occurrence of cycle slips, which need to be detected and rectified before the data is used for positioning computations. Even in the absence of cycle slips, the

full utilization of the carrier phase measurements is limited by the ability to resolve the cycle ambiguity problem. The automatic detection of existing cycle slips in carrier beat phase data and the Solution of the Ambiguity Problem is dealt with in the SOAP module. An overvi ew of the a1 gorithms used for thi s purpose wi 11 be di scussed in some detail in section 3.1. OF I LE Modu1 e Although code-tracking GPS receivers like the TI-4100 are capable ~of generatin~' positions in real time, the accuracies of these (point) positions are limited to about 10-15 m. Such abso1ute accuracies while useful for navigation are inadequate for most surveying tasks. Post-processing of the collected data offers the ability to model biases in the observations, or to remove or reduce them by differencing the observations and processing the data acquired simultaneously on a network of sites in order to obtain accurate relative positions. The DFILE module accepts two output SOAP files of data at two different sites and creates a file of differenced obse rvab 1es of: - carrier beat phase di fferenced across receivers (single difference) carrier beat phase differences across receivers, then across satellites (double difference) carrier beat phase differenced across receivers, then across satellites, then across time (triple difference) code differenced across receivers (single difference code). SORTO Modu1 e The SORTO module is a fast merging and sorting routine which can take up to six output files from DFILE and sorts the available observations sequentially according to their respective time tags. Thi s ut i1 ity is used ma in 1y when all observati ons from one mu1tistation session are to be processed in the network mode in the ma in program VECA. VECA Module VECA is the main module of our GPS processing software. It currently accepts P- or CIA-code and carrier beat phase data from individual baselines or several baselines observing in the same session. The main processor is structured in such a way that in the ll

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future the observations could be biased ranges, single, _ double or triple differences or any combination. At this time only the single and triple difference observation equation models have been implemented in VECA. Our alternative processor DIPOP [4] can be used, if we wish to process double difference observations. Satellite orbital data for VECA is usually in the form of broadcast ephemerides. However, other alternatives are poss"ible. For arcs of 3 to 4 hours duration, sho rt-a rc orbi ts can be evaluated qui te effici ently by means of least squares approximations of the second derivatives X, y, of the GPS satellites evaluated from a low degree and order geopotential model [5,6]. The satellite positions and velocities are then represented analytically by Tchebychev pol yn om i a1s • VECA carries out a sequential least-squares parametric adjustment of the data provided by DFILE. It essentially acts as a filter with the input to the filter being the observations collected over one or more baselines and the sequential output being the receiver coordinates of the participating sites and various nuisance or bias parameters such as relative receiver clock offset, drift and ageing; - phase ambiguities; and satellite orbit bias parameters (up to six parameters per arc). Currently the HP-9816 version can handle data for which estimation of up to 100 parameters (shared between coordi nates, clock and ot her The mainframe version can of nuisance parameters) may be required. course handle several stations, although we have never processed data from more than 6 stations simultaneously. Currently we are at the process of expanding the capability of VECA in that respect, so that in the future several sessions would be adjusted in one step. The main thrust in this direction is to have all observations of a campaign divided into sessions, with the transition from session to session being characterized by a complete change of all nuisance parameters. Thus a session may conceivably be thought of as lasting a few hours or a few days, if common parameters are assumed for this s pan and so 1ved fo r in one VECA run. •

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3.0 System Capabilities and Functions At the First International Symposium on Positioning with the Global Positioning System, several areas of investigation and software improvement pertinent to pre-processing and parameter estimation were identified and recommended. Amongst them were carrier beat phase cycle slip detection and ambiguity resolution, orbit modelling and atmospheric modelling. In the sequel, a brief overview of the CGS efforts and the software improvements achieved since that date along these lines are' presented. 3.1 Carrier Beat Phase Ambiguity Resolution Carrier beat phase measurements lead potentially to the most precise information about the receiver-to-satellite ranges one can _ obtain from GPS. The problem with uti1jzing this potential however is one of ambiguity: it is very difficult to locate accurately the cycle of the carrier whose phase is being measured. The present GPS receiver systems are capable of providing phase measurements with an internal precision of about 1-5 mm, with the overall accuracy of these measurements (including the influence of the atmosphere) being at the 1-4 cm level [7]. However the success of achieving this level of positioning accuracy from carrier phase measurements hinges on the capability to resolve the aforementioned cycle ambiguity. The problem is further compounded by the occurrence of frequent cycle slips which need to be detected and rectified before the final parameter estimation. In an effort to make the entire process as automatic as possible, our practical approach to this problem ~th the TI-4100 data has been somewhat different from the more common approaches of using piecewise continous polynomials and manual (and hence tedious) editing procedures (e.g. [8J). Since the TI-4100 receiver can continuously track the carrier phase between epochs during a satellite pass, the ambiguity problem reduces to recovering two unknown cycle ambiguities -one for L and one for L • The fact that TI-4100 receivers provide 1 2 carrier beat phase measurements on both L1 and L2 offers a natural choice in using both signals to resolve these ambiguities. Furthermore, what comes handy in this connection is that P-code delays are also provided on L and L • These are much less precise than the 2 1 J

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phase measurements but are unambiguously measured. Hence a combination of the two types of observables makes a truly synergistic mix. The approach implemented in the SOAP module is essentially a two step process. First, we use an algorithm similar to the one originally proposed in [9J and used with SERIES-X data [10J. For each observation epoch where P-code and carrier phase measurements are Javailable on the L and L , the formulation of two observation 1 2 equations fof'> carrier beat phase measurements kN

4>L + NL = ( T + -

e

f2

L = Ll ' L2

) + TL

(1 )

L

one for L1 and one for L2 , and similarly observation equations for the P-code delays kN

for two

corresponding

e

(2)

f2

L

yield through a recursive solution [cf. IIJ ambiguities on L and L • They are given by 1 2 NL 1

= 4.091

NL 2

= 3.967

estimates of the cycle

f L TL - 3.967 f L2 TL2 1 1

4>L +

L1

(3a)

f L TL - 4.091 f L2 TL2 1 1

4>L + 5N L 2 2

(3b)

with similar expressions for oN and L1

1

QN

oN L

2

oN L = 4.091 f L T - 3.967 f L T*L - 4>*L 1 1 L1 2 2 1 oN L = 3.967 f L TL - 4.091 f L T*L - 4>*

(4a)

(4b) L2 2 1 1 2 2 where T is the true (but unknown) delay, kNe is a factor dependent on the electron content of the ionosphere and T*, and 4>*, represent respectively residual instrumental and atmospheric errors in the P-code delays and carrier beat phases on L and L2 • Equations (1) to (4) 1 a1so apply to the case where TL and 4>L can be thought of as across-receivers single difference observables.

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practical terms, the success of achieving from this c~nbination an ambiguity resolution at the desired sub-cycle level hinges on the capability of reducing the effect of the instrumental errors ­ ~

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