IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring", Hyderabad, India, 2002 
  
  
by considering IGS stations as rover stations and point A as 
reference station. The accuracy of point B was also calculated 
in a similar way. Observations were taken in static mode for 
more than 24 hours and network adjustment was carried with 
IGS stations to establish precise WGS-84 coordinates of the 
two stations. 
The effect of observation time on the accuracy of the position 
coordinates as established by GPS observation in stand-alone 
mode was studied by taking four sets of observation, each 
having duration of six hours. These four sets of measurements 
spanned over 24 hours i.e. from 0000 to 0600, 0600 to 1200, 
1200 to 1800 and 1800 to 2400 hours. The processing for these 
sets of observations was carried out with out correcting with 
IGS stations. Broadcast ephemeredes were used for the 
processing in stand-alone mode. 
Leica SKI-Pro software has been used in the present study for 
post processing of GPS observations. 
3. DATA ANALYSIS 
The exercise was carried out for two different purposes: i) to 
establish accurate WGS-84 coordinates at two different stations 
by single point processing and ii) to study the effect of 
observation time on the accuracy of point coordinates as 
obtained by dual frequency receivers in stand alone mode. 
3.1 Single Point Processing 
Dual frequency receiver in single point processing mode were 
used to establish two different points in accurate WGS-84 
datum. For this purpose, the observation at point 1 was carried 
out for 55hr 22min 50sec starting from September 26, 2001 at 
10 hr 35 min to September 28,2001, 17 hr 57 min 50 sec. 
Common observation period of 24 hr is taken between point A 
and point B, starting at 09 hr 17 min 30 sec, October 5,2002. 
The coordinates of point A in WGS-84 coordinate system were 
computed after base line network adjustment with 6 IGS 
stations viz, Indian Institute of Sciences (IISC) Banglore 
(India), Bahrain (bahr), Chumsyh (chum), Talas (tala), Poligan 
IVTAN 2 (pol2) and Kitab (kit3). The coordinates of point A 
were computed by taking IGS stations as fixed control points. 
The data for each IGS station (observation file, navigation file) 
in RINEX format and also precise ephemeris was downloaded 
from Internet. The coordinates of point B were calculated with 
respect to point A, of common base line observation for 24 
hours. 
The following parameters were used for baseline processing of 
point A with respect to IGS stations as well as the baseline 
processing of point B with respect to point A. 
Cut-off angle (deg.) - 15? 
Ephemeris - Precise 
Tropospheric model - Hopefield 
Ionospheric model - Automatic 
The accuracy of the computed coordinates of point A was 
calculated with the help of back computation of coordinates of 
IGS he accuracy of the computed coordinates of point A was 
calculated with the help of back computation of coordinates of 
IGS stations viz. IISC, bahr, chum, tala, pol2 and kit3 by 
considering IGS stations as rover stations and point A as 
reference station. The difference of latitude, longitude and 
height between computed IGS coordinate and actual IGS 
646 
station’s coordinate were calculated. The difference of latitude, 
longitude and height are given in table 1. 
The accuracy of point B, was calculated with the help of back 
computation of coordinates of IGS stations viz. IISC, bahr, 
chum, tala and pol2 by considering IGS stations as rover 
stations and point B as reference station. The computed 
coordinates of IGS stations were compared with the actual 
coordinates. The results are given in table 2. 
Table 1. Difference between Computed IGS station coordinates 
and actual IGS Station coordinate with respect to point A 
  
  
S. Referen- Rover difference diffrence difference 
No ce IGS latitude longitude height 
station station (cm) (cm) (cm) 
1 Point A IISC 3 0.57 -57.16 
2 Point A Tala -1.35 0.78 -0.52 
3 Point A Bahr 3.93 11.16 -4.52 
4 Point A Kit3 0.3 6.54 S.24 
S Point A Pol2 -1.71 -1.59 -6.88 
6 Point A chum 0.51 -6.81 5.51 
  
Table 2. Difference between Computed IGS station coordinates 
and actual IGS Station coordinate with respect to point B 
  
  
S. Referen- Rover difference diffrence difference 
No ce IGS latitude longitude height 
station station (cm) (cm) (cm) 
1 Point B IISC 6.3 -12.93 -36.66 
2 Point B Tala -13.44 -8.19 15.55 
3 Point B Bahr 0.75 13.86 33.96 
4 Point B Pol2 -12.78 -6.30 15.02 
5 Point B chum -11.31 -16.44 13.23 
  
3.2 Effect of Observation Time 
To study the effect of observation time on positional accuracy 
as obtained from dual frequency receiver in stand-alone mode, 
analysis was carried out by taking four sets of observations, 
each having duration of 6 hours, spanned over 24 hours i.e. 
from 0000 to 0600, 0600 to 1200, 1200 to 1800 and 1800 to 
2400 hours. The processing for six hours observation was done 
with out correcting with any IGS stations. Broadcast 
ephemerides were used for the processing in stand-alone mode. 
The analysis was carried out for point A and point B, where the 
the accurate WGS-84 coordinates were calculated as described 
in 3.1. The results of analysis for point A and point B was given 
in Table- 3 and Table-4. 
The following parameters were used for single point processing 
for point A and point B. 
Cut-off angle (deg.) - 15° 
Ephemeris - Broadcast 
Tropospheric model - Hopefield 
Code frequency = L2