1. GPS Receiver Calibration Demostration - Principle and Data Analysis
Jia-Lun Wang, Shinn-Yan Lin, Yi-Jiun Huang,
Huang-Tien Lin and Chia-Shu Liao
National Time and Frequency Standard Lab.,
Chunghwa Telecommunication Laboratories (TL) , Taiwan
APMP 2012
November 27, 2012
Wellington, New Zealand
Telecommunication Laboratories

2. Purpose of Calibration
Objective: Realize the measurement of UTC(TL) - UTC(MSL), using the local and remote equipment
For this purpose, we must calibrate the local and remote equipment
The colored notation:
Blue: Term to be Measured
Red: Term to be Calibrated
Purple: Calibrated Term
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3. Realize the Measurement of UTC(TL) - UTC(MSL)
Calibrate XS + XR to realize the measurement of UTC(TL) - UTC(MSL)
GPS
GPS XS XS
Local
Ashtech
Z-XII3T
(Z12T)
Remote RX XC XC XR XR XO XO
UTC(TL)
UTC(MSL) XP XP
RINEX to CGGTTS
Cbl Dly = XC
Ref Dly = XP + XO
Int Dly = XS + XR
RINEX to CGGTTS
Cbl Dly = XC
Ref Dly = XP + XO
Int Dly = XS + XR
UTC(TL) - UTC(MSL)
= -
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4. Which term should we calibrate?
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5. Hardware Delay GPS Antenna XS Antenna Delay XC Cable Delay
GNSS Receiver
XR Internal Delay
XO Internal Reference Offset
Source
REF
XP 1-PPS Offset
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6. Delay Measurement with Conventional Scheme
GPS
Antenna
XS Antenna Delay
XC Cable Delay
GNSS Receiver
Measure by Time Interval Counter
XR Internal Delay
XO Internal Reference Offset
Source
Measure by Oscilloscope
RINEX to CGGTTS
Cbl Dly = XC
Ref Dly = XP + XO
Int Dly = XS + XR
REF
XP 1-PPS Offset
Measure by Time Interval Counter
REF - GPS = REFGPS(GNSS Receiver)
Term to be calibrated: internal delay XS + XR
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7. GPS Calibration Exercise Demostration
Motivation:
Demonstrate the GPS receiver calibration procedure
Reduce uB (5ns) of GPS time transfer link
Time Interval Counter (TIC) will be used to specify the voltage level of PPS signal
Goal:
Perform the APMP TC initiative project
Realize the measurement of UTC(TL) - UTC(MSL)
Helpful to the future GPS receiver calibration campaign
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8. Step 1: Equipment Calibration
Objective:
Calibrate the internal delay of the traveling equipment
Data Analysis:
Use P3 (ionosphere free observation), and All-in-view
Cut-off elevation angle 30°
Convert RINEX to CGGTTS by R2CGGTTS V5.0 software, distributed by BIPM and output at CGGTTS format
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9. Cut-off Elevation Angle
Result of the REF - GPS by using PolaRx4 PR0 GNSS receiver
The result will be stable if the elevation angle is large
Elevation angle > 30°
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10. Step 1: Traveling Equipment Calibration
Goal: Calibrate the internal delay, XS +XR , of the traveling equipment
The hardware delay of Z12T are known a-priori, and all the delay are need to measure
GPS
GPS XS XS XC XC
Ashtech
Z-XII3T
(Z12T)
Septentrio
PolaRx4 PRO
(P965) XR XR XO XO
UTC(TL) XP XP
RINEX to CGGTTS
Cbl Dly = XC
Ref Dly = XP + XO
Int Dly = XS + XR
RINEX to CGGTTS
Cbl Dly = XC
Ref Dly = XP + XO
Int Dly = 0
XS +XR
= -
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11. Step 2: Remote Equipment Calibration
Objective:
Calibrate the internal delay of the remote equipment
This is similar with step 1, but the terms to be calibrated are different. One can compare these two cases.
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12. Step 2: Remote Equipment Calibration
Goal: Calibrate the internal delay, XS + XR, of the remote equipment RX
Need to measure the hardware delay of the remote equipment
GPS
GPS XS XS XC XC XR XR XO XO
UTC(MSL) XP XP
RINEX to CGGTTS
Cbl Dly = XC
Ref Dly = XP + XO
Int Dly = XS + XR
RINEX to CGGTTS
Cbl Dly = XC
Ref Dly = XP + XO
Int Dly = 0
XS +XR
= -
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13. Measure XS, XP and XO for traveling equipment (TE), PolaRx4 PRO
Measure XS, XP and XO for remote equipment (RX), Javed EURO-80
Set up TE and start GPS observation to get RINEX
Set up RX and start GPS observation to get RINEX
Calibrate antenna coordinate of TE from RINEX
Calibrate antenna coordinate of RX from RINEX
Convert RINEX to CGGTTS using antenna coord., XS, XP and XO
Convert RINEX to CGGTTS using antenna coord., XS, XP and XO
Calibrate XS + XR for TE according to the local standard Z12T
Step
Step
Set up TE and start GPS observation to get RINEX
Calibrate antenna coordinate of TE from RINEX
Convert RINEX to CGGTTS using antenna coord. XS, XP, XO and XS + XR
Calibrate XS + XR for RX according to TE
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14. Thank you for your listening and advice
Discussion
The P1 and P2 internal delay are required to be calibrated to perform calibration by P3 observation
If the calibration point is different from UTC, then one should only measure the delay of these two
Overall uncertainty evaluation can refer to the report from BIPM and EURAMET, who provide very excellent analysis
Antenna coordinate is ITRF and calculated from IGS product
One can download data and perform data analysis from TL’s FTP site, ftp://ftp.stdtime.gov.tw
Do not hesitate to contact me, Yi-Jiun Huang(TL), if any problems
Thank you for your listening and advice
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15. Calibration Values (incomplete)
TL’s standard: Z12T (last calibration by 2005)
X: m, Y: m, Z: m
XC = ns
XP + XO = 52.0 ns
XS + XR =
Traveling Equipment: PolaRx4 PRO #965
XC = ns
XP + XO =
Traveling Equipment: PolaRx4 PRO #966
XC = ns

16. BIPM New Calibration Scheme
Motivation:
No delay measurement are required
The same architecture with conventional scheme
Calibrate the remote equipment which is no need to power-off
BIPM expects lower uB (< 2ns) by conducting new calibration scheme due to no delay measurement
Goal:
Perform calibration by BIPM’s new calibration scheme
The traveling equipment and remote equipment will be calibrated w. r. t. local equipment
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17. Total Delay Ref: Z. Jiang et al, 2011 IFCS-EFTF
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18. Realize the Measurement of UTC(TL) - UTC(MSL)
Use total delay of the link, dX(Z12T,RX), to measure UTC(TL) - UTC(MSL)
GPS
GPS
Local
Ashtech
Z-XII3T
(Z12T)
Remote RX
X(Z12T)
X(RX)
UTC(TL)
UTC(MSL)
RINEX to CGGTTS
Cbl Dly = 0
Ref Dly = 0
Int Dly = 0
RINEX to CGGTTS
Cbl Dly = 0
Ref Dly = 0
Int Dly = 0*
UTC(TL) - UTC(MSL)
= - - dX(Z12T,RX)
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19. Which term should we calibrate?
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20. New Scheme: Delay Measuremet NOT Required
GPS
Antenna
 X, Total Delay
XS Antenna Delay
XC Cable Delay
GNSS Receiver
XR Internal Delay
XO Internal Reference Offset
Source
RINEX to CGGTTS
Cbl Dly = 0
Ref Dly = 0
Int Dly = 0
REF
XP 1-PPS Offset
REF - GPS = REFGPS(GNSS Receiver) - (XO + XP) + (XS + XC+ XR)
:= REFGPS(GNSS Receiver) X
Term to be calibrated: total delay of receiver, X
(or total delay of the time link, dX)
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21. Step 1: Calibrate Total Delay Locally
Goal: Calibrate the total delay of the time link, Z12T-P965, dX(Z12T, P965)
Delay measurement and previous calibration results are not required
GPS
GPS
X(Z12T)
X(P965)
UTC(TL)
RINEX to CGGTTS
Cbl Dly = 0
Ref Dly = 0
Int Dly = 0
RINEX to CGGTTS
Cbl Dly = 0
Ref Dly = 0
Int Dly = 0*
dX(Z12T, P965)
:= X(Z12T) - X(P965)
= -
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22. Step 2: Calibrate Total Delay Remotely
Goal: Calibrate the total delay of time link Z12T-RX, dX(Z12T, RX)
, using calibrated dX(Z12T,P965)
GPS
GPS
X(P965)
X(RX)
UTC(MSL)
RINEX to CGGTTS
Cbl Dly = 0
Ref Dly = 0
Int Dly = 0
RINEX to CGGTTS
Cbl Dly = 0
Ref Dly = 0
Int Dly = 0*
dX(Z12T, RX)
= - + dX(Z12T, P965)
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23. Measure XS, XP and XO for traveling equipment (TE), PolaRx4 PRO
Measure XS, XP and XO for remote equipment (RX), Javed EURO-80
Set up TE and start GPS observation to get RINEX
Set up RX and start GPS observation to get RINEX
Calibrate antenna coordinate of TE from RINEX
Calibrate antenna coordinate of RX from RINEX
Convert RINEX to CGGTTS using antenna coord., XS, XP and XO
Convert RINEX to CGGTTS using antenna coord., XS, XP and XO
Calibrate dX(Z12T, TE) according to the local standard Z12T
Step
Step
Set up TE and start GPS observation to get RINEX
Calibrate antenna coordinate of TE from RINEX
Convert RINEX to CGGTTS using antenna coord. XS, XP, XO and XS + XR
Calibrate dX(Z12T, RX) for RX according to dX(Z12T, TE)
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24. Calibration Values (incomplete)
Total Delay of the Time Link
Link Z12T-P965: dX(Z12T, P965) = ns
Link Z12T-P965: dX(Z12T, P966) = ns
Link P965-P966: dX(P965, P966) = 3.5 ns