XIX IMEKO World Congress Fundamental and Applied Metrology September 6-11, 2009, Lisbon, Portugal

1. TESTING THE STABILITY OF
GPS OSCILLATORS WITHIN
SERBIAN PERMANENT GPS
STATIONS NETWORK
Vukan Ogrizović, Violeta Vasilić,
Siniša Delčev, Jelena Gučević
Belgrade University
Faculty of Civil Engineering
Department of Geodesy and Geoinformatics

2. AGROS
 34 stations in Serbian
permanent GPS network
 Trimble receivers (4000, 5700 & NetRS)
 Monitoring center
 Mostly used by surveyors
 m to mm accuracy
Belgrade University
Faculty of Civil Engineering

3. Periodical tests of AGROS
accuracy
Receiver
Phase features
center
Oscillator
stability
Belgrade University
Faculty of Civil Engineering

4. Testing of oscillator stability
RINEX data from 30 stations downloaded from monitoring centre
Observables - Phase L1 measurements for the reference SV
24-hour long session - all referred to the same date
Allan’s variance and PSD calculated
Analysis of obtained graphs
Belgrade University
Faculty of Civil Engineering

5. Measure of frequency stability
 Alan’s variance
N − 2 n +1
1
∑ ( yk + n − y k )
2
σ (τ ) =
ˆ2
2 ( N − 2n + 1)
y
k =1
2
1 N − 2 n +1
 xk + 2 n − 2 xk + n + xk 
=
2 ( N − 2n + 1)
∑ 
nτ 0

k =1  
 Modified Alan’s variance
2
1 N − 3 n +1  j + n −1 
mod σ 2 (τ) = 2 2
y
2n τ ( N − 3n + 1)
∑  ∑ ( xi + 2 n − 2 xi + n + xi ) 
j =1  i= j 
τ2
σ (τ) = mod σ 2 (τ)
2
x y
3
Belgrade University
Faculty of Civil Engineering

6. Type of random process
 Averaging interval
σ 2 ( τ) ~ τ µ ⇒ σ y ( τ ) ~ τ η
y
 Single-sided spectral density
2
Sy ( f ) = ∑h
α = −2
α f α = h−2 f −2 + h−1 f −1 + h0 f 0 + h1 f 1 + h2 f 2
α = −µ − 1
Belgrade University
Faculty of Civil Engineering

7. Block diagram of the VI
Belgrade University
Faculty of Civil Engineering

8. Front panel layout
Belgrade University
Faculty of Civil Engineering

9. Allan variance
0.2
0.15
 Typical graph for 5700
type receiver
σ(τ)
0.1
0.05
0
7
1
4
6
64
16
24
97
38
25
10
40
16
τ (s )
9.01
 However, several stations showed
σ(τ) [ns/s]
6.01
different characteristics, due to
3.01
environmental conditions
0.01
9
1
4
4
16
49
65
37
25
59
41
10
16
τ (s)
Belgrade University
Faculty of Civil Engineering

10. Analysis for station LOZN
1.5
 Correct Allan variance
[ns/s]
1
graph
0.5
0
1 4 16 64 256
(s)
1024 4096 16384
but
 PSD graph indicated
periodical disturbances of
an unknown source with
the period of 15-30 min
Belgrade University
Faculty of Civil Engineering

11. Results
 Functional characteristics of noise processes
 # of FFM (µ = 0): 17
 # of RWFM (µ = +1): 13
Belgrade University
Faculty of Civil Engineering

12. Conclusions and remarks
Older types of receivers showed larger Allan’s variances
Environment affects the results of modern receivers, also
Only FFM and RWFM occur in results
Calibration of GPS receivers is a must
Receivers with 1PPS output are recommended
Belgrade University
Faculty of Civil Engineering

13. The work on this paper was supported by
Serbian Ministry of Science and
Technological Development
Belgrade University
Faculty of Civil Engineering
Department of Geodesy and Geoinformatics