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

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 Telecommunication Laboratories

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) = REFGPS(Z12T@TL) - REFGPS(RX@MSL) Telecommunication Laboratories

Which term should we calibrate? Telecommunication Laboratories

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 Telecommunication Laboratories

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 Telecommunication Laboratories

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 Telecommunication Laboratories

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 REFGPS(Receiver@Lab) at CGGTTS format Telecommunication Laboratories

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° Telecommunication Laboratories

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 = REFGPS(Z12T@TL) - REFGPS(P965@TL) Telecommunication Laboratories

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. Telecommunication Laboratories

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 P965@MSL XC XC RX@MSL 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 = REFGPS(P965@MSL) - REFGPS(RX@MSL) Telecommunication Laboratories

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 1@TL Step 2@MSL 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 Telecommunication Laboratories

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), dongua@cht.com.tw, if any problems Thank you for your listening and advice Telecommunication Laboratories

Calibration Values (incomplete) TL’s standard: Z12T (last calibration by BIPM @ 2005) X: -2994428.02 m, Y: 4951309.28 m, Z: 2674496.91 m XC = 119.8 ns XP + XO = 52.0 ns XS + XR = Traveling Equipment: PolaRx4 PRO #965 XC = 176.25 ns XP + XO = Traveling Equipment: PolaRx4 PRO #966 XC = 175.38 ns

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 Telecommunication Laboratories

Total Delay Ref: Z. Jiang et al, 2011 IFCS-EFTF Telecommunication Laboratories

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) = REFGPS(Z12T@TL) - REFGPS(RX@MSL) - dX(Z12T,RX) Telecommunication Laboratories

Which term should we calibrate? Telecommunication Laboratories

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) Telecommunication Laboratories

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 Z12T@TL P965@TL 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) = REFGPS(Z12T@TL) - REFGPS(P965@TL) Telecommunication Laboratories

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 P965@MSL RX@MSL 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) = REFGPS(P965@MSL) - REFGPS(RX@MSL) + dX(Z12T, P965) Telecommunication Laboratories

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 1@TL Step 2@MSL 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) Telecommunication Laboratories

Calibration Values (incomplete) Total Delay of the Time Link Link Z12T-P965: dX(Z12T, P965) = 189.2 ns Link Z12T-P965: dX(Z12T, P966) = 192.7 ns Link P965-P966: dX(P965, P966) = 3.5 ns