Module 2 : Global Positioning System
  Lecture 13 : GPS Measurements Techniques II
Accuracy and observation times
  • The specific time period for an observation is called a session. The following factors determine the length of a particular observation:
    • Length of baseline
    • Number of visible satellites (affects geometry)
    • Relative geometry of satellites and change in geometry
    • Signal to noise ratio (SNR) of received satellite signal.
  • General guidelines for length of observations for conventional static surveying and single frequency receivers are given below:

Table 13. 1 Session length for static survey using single receiver

Baseline (km) Session (min)
0-1 10-30
1-5 30-60
5-10 60-90
10-20 90-120
Why long session lengths are required for carrier phase surveys?
At each observation epoch, the carrier phase is measured to a mm accuracy or even better. Although one can measure the dm, cm, and mm precisely, the observation must last long enough to determine the meters by ambiguity resolution. For short baselines (< 1 km), it can be resolved in 5-10 minutes with L1 only phase. With dual frequency, long base lines (> 15 km) are accurately measured with just 2 minute data. Table 13.2 provides guidelines for determining baseline occupation time in static surveys (NAVSATR Global Positioning System Surveying, 2003).
 
Table 13. 2 Static baseline occupation time (NAVSTAR Global Positioning System Surveying, 2003)

Baseline Length
(km)
Recommended Minimum Observation Time (minutes)
Satelllites in View/Single- or Dual-Frequency Receiver
4
5
 6 or more satellites in view

Single
Dual
Single
Dual
Single
Dual
1-10 km
60 min
20 min
36 min
12 min
24 min
8 min
10-20 km
75 min
25 min
45 min
15 min
30 min
10 min
20-50 km 1
105 min
35 min
75 min
25 min
60 min
20 min
> 50 1
180 min
60 min
135 min
45 min
90 min
30 min
1 Dual-frequency receivers are recommended for baselines greater than 20 km
Source: USACE GPS Field Review Group (September 2002)

 

Thumb rule: Assuming expected horizontal accuracy of 10 mm, clear visibility, and clean data the basline observation time can be taken from the following thumb rule (NAVSTAR Global Positioning System Surveying, 2003):

  • For single frequency receiver: Baseline observation time = 10 minutes + 1 minute/km
  • For dual frequency receiver: Baseline observation time = 5 minutes + 0.5 minute/km
 
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