Equipment
The field equipment includes one GPS receiver unit and auxiliary devices such as tripod, tribach, and other ancillary equipment.
Experimental Set up, Data collection and Processing
In view of establishing the control point on the top of building, GPS observation was carried out on absolute position mode, also known as single point positioning. In the absolute positioning mode, single GPS receiver is used to find the coordinates of the antenna position. Steps those were carried out for experimental set up, data collection and processing are as follows:
- Antenna setup: The tripod was set above the control point and then attached the tribach. The leveling and centering was then carried out iteratively before fixing the antenna to the tribach. Other components i.e., sensor, controller, battery etc were then connected.
- Initialisation: A mission was then started to take GPS observation in static survey mode. Before starting to take observation a few input parameters such as the sampling rate, sampling type, cut off elevation angle, project and job name, approximate location (29° 52Ë 002 North, 77° 53Ë 522 East and height 268 meter), height of antenna and antenna offset etc for the point were fed through controller.
- Data collection: The data was collected taken during different sessions spread over three consecutive days/nights as given in Table 1.
- Data processing: The collected data was then dumped from controller to computer. After creating a project, the data was brought and processed within SKI 3.2 post-processing software.
Observation and Result
The observation was taken on April 19, 2001 starting from 17hr 31min 55sec to 26hr 46min 55sec i.e., a duration of 7 hr 15 min starting from early evening to late night. On next day i.e, on April 20, the observation session started at noon (14 hr 31 min 30 sec) and continued till mid-night (24 hr 32min 25 sec) for a duration of 10 hr 12 min 55 sec. On April 21, 2001, there were two sessions for observation- first one from early morning (04hr 38 min 20 sec) to noon (14 hr 31 min 30 sec) for a duration of 09 hr 53 min 10 sec and second one from evening (18hr 47 min 30 sec) to night (21 hr 13 min 05 sec) for a duration of 02 hr 25 min 35 sec. On April 22, the observation session was restricted in early morning only from 03 hr 36 min 35 sec to 05 hr 55 min 15 sec for a duration of 2 hr 18 min 40 sec.
The observed data was processed in SKI 2.3 session wise. Only code data of both bands were used in processing. Broadcast ephemerides were used to compute the position of satellite. Hopfield model was used to take into consideration the delay caused by troposphere but no model was used for ionospheric interference. After processing the data, following results have been obtained:
- The geodetic (WGS 84) Latitude of the station point has been found to be very precise with mean (29° 51Ë 45.5274642 North) and standard deviation (0.479622).
- The geodetic (WGS 84) Longitude of the station point has been found to be very precise with mean (77° 54Ë 0.7782962 East) and standard deviation (0.1705042).
- Mean of the geodetic (WGS 84) heights has been found to be 241.80225 meter with standard deviation 2.79805 meter (In this calculation height found in session 2 is considered as Outlier).
The details of the observation and geodetic coordinates are given in Table 1.
Table 1 Observation Session, Period & Duration and Geodetic coordinates (a-priori and calculated)
| SESSION | Detail of Observation | A-priori Geodetic (WGS84)Co-ordinates of Station point | Calculated Geodetic (WGS84)Co-ordinates of Station point |
| Date (April, 2001) | Time | Duration | Latitude(29° 51'+) | Longitude(77° 54'+) | Height (m) | Latitude(29° 51'+) | Longitude(77° 54'+) | Height (m) |
| From | To | Hr | ' | " | Hr | ' | " | Hr | ' | " | ' | " | ' | " |
| 1 | 19th | 17 | 31 | 55 | 26 | 46 | 55 | 07 | 15 | 00 | 45.7988 | 0.8375 | 239.3912 | 45.79879 | 0.83757 | 239.3848 |
| 2 | 20th | 14 | 19 | 30 | 24 | 32 | 25 | 10 | 12 | 55 | 45.4908 | 0.7363 | 214.2942 | 44.77643 | 0.47672 | 119.0427 |
| 3 | 21st | 04 | 38 | 20 | 14 | 31 | 30 | 09 | 53 | 10 | 45.7391 | 0.7054 | 235.2110 | 45.50903 | 0.89779 | 243.4658 |
| 4 | | 18 | 47 | 30 | 21 | 13 | 05 | 02 | 25 | 35 | 46.0577 | 0.8414 | 245.1550 | 46.05767 | 0.84139 | 245.1550 |
| 5 | 22nd | 03 | 36 | 35 | 05 | 55 | 15 | 02 | 18 | 40 | 45.4954 | 0.8380 | 239.1843 | 45.4954 | 0.83801 | 239.2034 |
Discussion
The planimetric position of the station point has been observed quite precisely. Assuming the astronomic coordinates (29° 52
Ë 00
2 North, 77° 53
Ë 52
2 East) of the station point as the geodetic coordinates (since, the meridional and prime vertical components of the deflection to vertical are very small, they can be neglected for further consideration of this study), the observed planimetric position of the station point may be considered quite accurate.
However, the observed values for geodetic (WGS 84) height of the station point have been found to be very erratic. The mean calculated height (241.80225 meters) of the point is quite away from the geodetic height (207.6677 meters) [Considering, the geodetic undulation of the station point with reference to WGS 84 ellipsoid is –60.3323 meters and height above datum (geoid) as 268 meters]. It has also been observed that there is a great variation in geodetic height between a-priori and calculated values and depends on the duration of observation as well as on the period of the session. The variation is prominent if the observation session either starts or finishes at afternoon i.e., when the ionospheric disturbances are most severe. It has been also been found that geodetic height increases from its a-priori value as the observation session spreads from morning to early afternoon i.e, ionospheric disturbances changes form stable state to most severe state. And it decreases if the condition gets reverse. The
degree of variation is more in case observation taken form most severe disturbance state to stable state of ionosphere. These variations are attributed to the different types of errors involved in GPS observation.
Generally, the position indicated by the GPS at a given time does not coincide with the exact position of the apparatus. In fact, the difference between the coordinate readings and the true values are caused by two types of errors:
- systematic error due to the GPS system itself (receiver noise and resolution offset, receiver hardware offset etc.), which remains the same irrespective of the measurement date and point measured; and
- random error that differs with each measurement, due to atmospheric conditions, muti-path and shadowing effect, presence of water vapor etc.
To obtain accurate geodetic height, single point positioning can not be applied directly. Rather, some other better method may be tried.
Conclusion
GPS system can be used reliably for establishing precise planimetric position of a control point. Thus, planimetric change in position of any object or phenomena can be studied very precisely by single point pointing of GPS receiver. For this no previous information is necessary other than very approximate location of the station point.
Since ionosphere is activated by solar radiation, its disturbances are much more severe on+ GPS observation during the day time than at night. So, GPS data should be preferably be collected at night than during day time, in single point positioning.
A further study can be done on the accuracy of single point positioning by using the precise ephemerides (in calculating the position of satellites).
However, other methods like relative positioning etc may be adopted which minimises the different errors. Moreover, post processing of relative positioning data takes into consideration the
phase data of GPS observation and thus, may improve the accuracy of control point location. Local atmospheric models may be adopted to keep atmospheric errors least.
Acknowledgement
The authors are grateful to Dr. P.K.Garg, Coordinator, RSPE section of Civil Engineering Department, IIT Roorkee for providing available facilities and infrastructure and other laboratory staff and helpers for their help and support in carrying out the experiment.
References
- Clark, D, 1968. “Plane and Geodetic Surveying Vol. II”, Constable & Company Ltd., London pp 684.
- Leica GPS- System 300 “Guidelines to Static and Rapid Static GPS Surveying”, May 1995.
- GPS Positioning Guide - Geomatics Canada, Geodetic Survey Division Ottawa, Canada
- Vanicek, p and E.J. Krakiwsky, 1986 . Geodesy: the Concepts, Elsevier Science Publishing Company Ltd pp 697.
- Defense Mapping Agency, 1987. GPS UE Relevant WGS-84 Data Base package, Geoid height approximates in meters.
- Arnaud, M and A. Flori, ”Bias and Precision of Differential Sampling Methods for GPS Positions”, Photogrammetric Engineering and Remote Sensing, Vol. 64, No. 6, June 1998, pp. 597-600.
- Kaplan, E.D., 1996. “Understanding GPS: Principles and Applications”, Artech House, USA pp 554.