Home Articles Issues regarding integration of GPS data for NRIS database generation

Issues regarding integration of GPS data for NRIS database generation

P. M. Udani1, R. K. Goel2
Informatics Applications Division Space Applications Center,
Ahmedabad – 380015.
1[email protected], 2[email protected]

Introduction
For the management of National Resources, computerized digital databases are generated for different administrative units like village, district , state and for different themes like landuse, roads, canals, elevation points etc. At present, thematic maps generated by different agencies is the main source of inputs for the creation of National Resources Information System (NRIS) district databases. Dependency on GPS data and satellites data including stereo pair is much less. It is required to update, validate and enhance databases periodically for reliability of GIS outputs and integrity of data analysis. To meet such requirements GPS technology provides facility for data collection pertaining to new developments and also for checking the accuracy of existing databases.

After removal of Selective Availability (S/A), the applicability of GPS data in facility mapping, infrastructure planning and GIS database generation will increase by many folds. In context of NRIS, it is necessary to prepare fresh base maps for many districts using IRS data and control points provided by GPS observations due to restriction policy of digitization of SOI maps and non availability of current maps. Thus, GPS is an important source for NRIS database generation.

This paper discusses interface issues and data integration problems in context of authors experience regarding usage of GPS receivers for NRIS database validation and enhancement

GPS System
The NAVSTAR GPS (Navigation Satellite Timing and Ranging Global Positioning System ) is a satellite based radio navigation system providing precise three dimensional position , navigation and timing information to suitably equipped users on a continuous basis. GPS receivers measures code or carrier phase or both to provide meter level accuracy in point positioning mode and up to few centimeters in differential mode. The brief description of GPS system components viz. space segment, control segment and user segment is given below.

Space Segment
The space segment consists of 24 satellites arranged in six different orbital planes of inclination 55 Deg. w.r.t. equatorial plane. These satellites are orbiting the earth at a height of 20200 km from the surface of the earth and have periodic time of 12 hours. Minimum 4 satellites are visible for positioning on ground/sea/air at any time throughout the year. Each satellite is transmitting coded signals known as pseudo random noise (PRN) signals modulated on L1 (154 * 10.23= 1575.42 MHz (= 19.05 cm)) and L2 (120 * 10.23 = 1227.60 MHz (=24.45 cm)) carrier frequencies. Transmitted signals on both frequencies are modulated with navigation and system data including satellite ephemeris, atmospheric propagation correction data and satellite clock bias information. The L1 signal contains both, P- code and C/A code. The L2 signal contains P code only. The P code frequency is 10.23 MHz which corresponds to wavelength of 29.31 meter with period of 267 days and 7 days for a satellite. The C/A code frequency is 1.023 MHz which corresponds to wavelength of 293.1 meter with period of 1 millisecond. Data signal frequency is 50 bps and its cycle length is 30 second.

Control Segment
The Operational Control Segment for GPS consists of the Master Control Station near Colorado Spring (USA), three Monitor Stations and ground antennas in Kwajalein, Ascension and Diego Garcia, as well as two more monitor stations in Colorado Spring and Hawaii. The tasks of the control segment are to:

  • monitor and control the satellite system continuously for uploading data into the satellites.
  • predict the satellite ephemerides and the behavior of the satellite clocks.
  • periodically update the navigation message for each particular satellite.

User Segment
This relates to various types of GPS receivers like navigation, survey, single frequency, dual frequency etc.

Objectives
To study the role of GPS data in NRIS database content validation and enhancement with respect to following objectives.

  • Determine location accuracy and alignment of NRIS database elements like roads, railway lines, canals and selected area features like water bodies ,forest boundaries etc.
  • Determine height accuracy of elevation points and contours in databases
  • Provide accurate control points for registration and rectification of satellite images
  • Provide accurate control points for registration of cadastral map
  • Provide control points for DEM generation and evaluation
  • Error modeling of GPS observations.
  • Provide control points and Spatial Framework for base map generation

Experiment Conducted
GPS satellites are available for all 24 hours and observation can be taken at any place at any time. For better accuracy planning is required with respect to identifying area of interest, distribution of points, data collection strategy, availability of required no of satellites with good GDOP value and above 15 degree of elevation angle. Detailed procedure followed during experiment is as below,

  • Preparation of map for different coverages for which data are to be collected
  • Selection and marking of features/control points on map with identification code
  • Total time of data collection and assigning time slot for different area
  • Preparation of satellite visibility chart
  • Logistics planning
  • Selection of reference points position and rover points positions for data collection
  • Determining initial approximate coordinates of observation points in WGS 84 system

GPS observations were taken at well distributed points within map sheet no 46 F/9 using two LEICA SR 9400 GPS receivers one working as reference receiver and other working as rover receiver. During first schedule simultaneous observations were taken at reference point (continuously for 6 hours) and at seven rover points(for 30-40 minutes). Similar observation pattern was followed for second schedule.

Data Processing
Specific mission parameters like ambiguity resolution limit, models for atmospheric and ionospheric correction, baseline length limit etc. were defined before data processing. The coordinate solutions were determined using phase and code measurement.

The computed WGS 84 coordinates were converted to Everest Datum using computed local seven parameters. Heights above ellipsoid were converted to MSL value using external geoid model.

Results

  • For all observed points WGS 84 coordinates are given in table-1.
  • Differences in NRIS database coordinates and coordinates computed using local 7 parameters are given in table-2 for selected 8 points with larger base-length. Differences are within 10 -15 meter limits.
  • Differences in GPS coordinates and coordinates estimated using 7 parameters (for whole Indian region) and Geoid model are given in table-3. Comparatively larger errors are found while using global parameters for Indian region.

 

Conclusion
Accuracy obtained using LIECA SR 9400 single frequency GPS receiver measuring code or carrier phase is of the order of 10-15 meters in point positioning mode with 5 hours of observations. Accuracy of similar order is obtained at rover points also within distance of 2-3 kilometers for observations of 30 minutes.

  • Due to limitation of single frequency LIECA receiver observations for more then 45 minutes are required for base-length of 5-10 kilometers for resolving ambiguity.
  • The main source of error for single frequency receiver is ionospheric delay. Therefore, observations for base-length more then 10 kilometers could not be taken due to system limitation for compensating for ionospheric error
  • It was possible to identify and correct abnormal height observation due terrain familiarity

The desired accuracy of GPS observations can be obtained by appropriately taking care of following issues

  • Selection of day and time of observation
  • Duration of observation
  • Selection of reference point and corresponding rover points
  • Datum conversion
  • Height conversion

Future Issues

  • Develop GUI based interface for on line tracking of GPS data and converting it into ARC/INFO coverage
  • Create base maps using GPS observations

Acknoledgement
We are thankful to Shri A.R Dasgupta, DD SITAA for providing us opportunity to explore feasibility of using GPS data for NRIS project.

Table-1: Coordinates Derived Using GPS (WGS-84 Datum)
Point 17 is Considered as Reference Point for Baseline Processing

SR. NO. LATITUDE LONGITUDE HEIGHT ABOVE
ELLIPSOID
17 22 50 28.09682 73 40 24.38946 87.1877
08 22 48 08.85339 73 37 02.93546 69.3931
11 22 51 44.32849 73 37 37.03182 68.2053
13 22 54 36.81210 73 38 08.98172 65.8170
15 22 53 47.64277 73 36 01.11677 74.3100
50 22 51 44.11065 73 31 58.76126 51.1162
14 22 54 21.33026 73 37 24.27028 89.8122
16 22 52 56.72526 73 33 54.93078 71.5811
Point 3 considered as Reference Point for Baseline Processing
SR. NO. LATITUDE LONGITUDE HEIGHT ABOVE
ELLIPSOID
03 22 47 01.64677 73 33 49.27532 54.8687
01 22 47 44.28205 73 30 09.98229 31.9430
05 22 47 16.51705 73 41 02.23118 98.4480
06 22 48 54.08300 73 33 18.23283 54.5433
07 22 47 08.08902 73 36 40.59074 63.9192
08 22 48 08.58790 73 37 02.80601 66.4169
09 22 45 11.27817 73 40 31.15743 84.8819

Consistency of GPS measurement is apparent for point no. 8

Table – 2 Comparison of Coordinates in Everest Datum

  • The coordinates in first raw are from NRIS database
  • The coordinates in second raw are from GPS measurement
  • SR NO LATITUDE LONGITUDE HEIGHT DIFF. IN LAT.(meters) DIFF. IN LANG. (maters) DIFF. IN HEIGHT(meters)
    03 22 47 00.292
    22 47 00.885
    73 33 52.524
    73 33 52.481
    116.00
    110.04
    -17.77 01.27 5.96
    05 22 47 14.915
    22 47 17.722
    73 41 04.103
    73 41 04.701
    155.00
    153.42
    -24.20 -17.94 1.58
    06 22 48 52.992
    22 48.53.125
    73 33 20.955
    73 33 21.492
    100.00
    110.13
    -3.99 -16.08 -10.13
    08 22 48 06.375
    22 48 07.706
    73 37 05.374
    73 37 05.685
    121.00
    121.74
    -39.94 -9.29 -0.74
    09 22 45 09.741
    22 45 10.702
    73 40 34.750
    73 40 33.678
    135.00
    139.44
    -28.96 32.15 -4.44
    13 22 54 35.190
    22 54 35.249
    73 38 10.065
    73 38 11.753
    130.00
    122.50
    -1.77 -52.39 7.5
    17 22 50 27.251
    22 50 26.968
    73 40 27.362
    73 40 26.922
    140.00
    142.88
    8.50 13.17 -2.88
    50 22 51 43.538
    22 51 42.856
    73 32 00.678
    73 32 02.159
    95.00
    107.34
    20.44 -44.41 -12.34

    Differences are within Range of 10-15 meters because of Local Parameters being used for conversion

    Table -3 comparison of coordinates in WGS-84 datum

  • The coordinates in first raw are estimated using Geoid model and seven parameters of transformation for Indian region
  • The coordinates in second raw are from GPS measurement
  • SR NO LATITUDE LONGITUDE HEIGHT DIFF. IN LAT.(meters) DIFF. IN LANG. (maters) DIFF. IN HEIGHT(meters)
    03 22.78394
    22.78389
    73.56469
    73.56368
    70.750
    54.868
    5.5 108.54 15.88
    05 22.78800
    22.78792
    73.68456
    73.68395
    109.84
    98.448
    8.64 65.55 11.39
    06 22.81524
    22.81498
    73.55592
    73.55499
    54.772
    54.543
    27.54 100.01 0.23
    08 22.80229
    22.80245
    73.61825
    73.61748
    75.806
    66.417
    -17.71 83.37 9.39
    09 22.75323
    22.75313
    73.67640
    73.67532
    89.803
    84.882
    10.91 111.29 4.92
    13 22.91029
    22.91002
    73.63622
    73.63571
    84.915
    65.817
    29.27 54.65 19.09
    17 22.75323
    22.75313
    73.67641
    73.67372
    94.881
    87.187
    30.78 68.69 7.69
    50 22.86261
    22.86225
    73.53363
    73.53285
    49.798
    51.116
    39.20 83.59 -1.32