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IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring”, Hyderabad, India, 2002 
  
EFFECT OF TIME OF OBSERVATION ON THE POSITIONAL ACCURACY OF DUAL 
FREQUENCY GPS RECEIVERS IN STAND ALONE MODE 
B. Narender', P. J ayaprasad, Anjum Mahtab, S.K. Pathan, Ajai 
Forestry, Land use and Photogrammetry Group, Space Applications Centre, Ahmedabad — 380 015 (India) 
KEY WORDS: GPS, Dual frequency receivers, IGS stations, Geodetic surveying and positioning 
ABSTRACT: 
Global Positioning System (GPS) has revolutionalised the area of navigation and surveying by giving precise position on the surface 
of the earth. The GPS system consists of 24 satellites, placed in six orbital planes with orbital inclination of 55°. In the present paper 
an attempt has been made to study the accuracy of coordinates obtained by dual frequency receivers in stand-alone mode. The 
observations with dual frequency receivers were taken at two stations, viz. point A and point B. The observation at point A was 
carried out for 55hr 22min 50sec. The coordinate of point A was established by network adjustment with the six IGS (International 
GPS service for Geodynamics). The coordinate of point B was calculated with respect to point A using common observation of 24 
hours. Precise ephemeris was used for the base line network adjustment of point A with IGS stations and also for baseline 
computation of point B with respect to point A. The effect of observation time on the accuracy of the position of coordinates as 
established by dual frequency receivers in stand-alone mode was studied by taking four sets of observation, spanned over 24 hours 
i.e. from 0000 to 0600, 0600 to 1200,1200 to 1800 and 1800 to 2400 hours. Broadcast ephemeredes were used for the processing in 
stand-alone mode. From the study it was concluded that error in latitude is always more compared to longitude. It is observed that 
  
error in height is more compared to planimetry. 
1. INTRODUCTION 
Global Positioning System (GPS) has revolutionalised the area 
of navigation system and surveying by giving precise position 
(Latitude, Longitude and height) on the surface of earth. The 
GPS System consists of constellation of 24 satellites, which 
were launched by U.S. Department of Defense (DOD). The 
satellites are placed in six orbital planes with orbital inclination 
of 55°. The GPS satellites transmit two fundamental carrier 
waves called L1, L2. The frequency of L1 is 1575.42 MHz and 
that of L2 is 1227.60 MHz. There are two important types of 
GPS observable, one is pseudo range and other is carrier phase. 
The pseudo range is a measure of distance between satellite and 
receiver at the epoch of transmission and reception of the 
signals.This is generally used for navigation. The carrier phase 
is the difference between phase of carrier signal transmitted by 
satellite and the phase of receiver oscillator at the epoch of 
measurement. This technique is generally used for. surveying. 
(Leick, 1990). 
Surveying with GPS has become popular due to the advantage 
of speed, versatility and economy. The numerous limitations of 
the terrestrial surveying like requirement of inter- visibity of 
survey stations, 3D position parameters etc. could be overcome 
by using GPS techniques. Enormous surveying with GPS has 
been carried out by scientific and professional teams. Corbenu 
et. al, 2000 used two dual frequency GPS receivers for 
landslide monitoring. Abidin et.al, 1998 used dual frequency as 
well as single frequency receivers to establish second and third 
roder networks. The relative accuracy of network was 2-3 cm. 
GPS gives coordinates in WGS-84 coordinate system. To 
establish GPS control network, at least one point with precise 
GPS coordinate is required. To establish a precise point in 
WGS-84 coordinate system, single point observations with dual 
frequency receivers in static mode can be made for more than 
  
* Corresponding author: (naren_br@yahoo.com) 
645 
24 hours. The observations are to be corrected with IGS 
(International GPS service for Geo dynamics) station 
coordinate observations, which can be downloadable from 
Internet. The IGS stations coordinate are available in RINEX 
(Receiver Independent Exchange Format). 
In the present paper, an attempt has been made to study the 
effect of observation time on accuracy of point coordinates as 
obtained by dual frequency receivers (which can track both L1, 
L2 signals) in stand alone mode. 
2. METHODOLOGY 
A 12-channel dual frequency GPS receiver has been used in the 
present study. The observations with dual frequency receivers 
were taken at two stations, henceforth, referred as point A and 
point B. The base line distance between two stations was 
approximately 650 km. The two control points were chosen 
taking care of good characteristic for GPS observation such as 
clear view of sky, no obstruction above the cut-off angle, no 
reflecting surfaces that could cause multi-path, possibility to 
leave the receiver unattended. The observation at point A was 
carried out for 55 hr 22 min 50 sec. Common observation of 24 
hours was taken between point A and B. Data from the six IGS 
(International GPS service for Geo dynamics) stations 
synchronous to the observation made at the two experimental 
stations (point A and B), were downloaded from internet. The 
coordinate of point A was established by network adjustment 
with six IGS stations. The coordinates of point B were 
calculated with respect to point A using common observation of 
24 hours. Precise ephemeris, downloaded from Internet, was 
used for the base line network adjustment of point A with IGS 
stations and also for base line computation of point B with 
respect to point A. 
The accuracy of computed coordinates of point ‘A’ was 
calculated with the help of back computation of IGS stations,