33rdA nnual Precise Time and Time Znterval (P?TI) Meeting COMMON-VIEW TIME TRANSFER USING THE TWO-CHANNEL FAST-SEQUENCING GPS RECEIVER K+K GPS6-2K P. Hetzel, T. Polewka Physikalisch-Technische Bundesanstalt, Bundesallee 100,38116 Braunschweig, Germany W. Klische K+K Messtechnik GmbH, Braunschweig, Germany Abstract Two 2-channel fast-sequencing GPS receivers (Type K+K GPS6-2K made by K+K Messtechnik GmbH, Braunschweig, Germany) have been studied in co-location and in comparison with the NBS-type receivers used at the PTB for the regular GPS CV- time transfer. The new receivers consist of two channels (the "Tracking Channel" and the "Datu Channel"), a built-in time interval meter, and PC sofware. The "Tracking Channel" either continuously tracks a single satellite or sequentially tracks all satellites in view. Both the code phase and the carrier (delta)p hase are measured and combined to yield a smoothed reading of the received satellite time. The "Daa Channel" reads all the data necessary from the satellites, like almanac, ephemeris, ionospheric model parameters, etc, and acquires new satellites as they arise above the horizon. The PC sofware currently used generates the "CGGTTS GPWGLONASS Datu Format" in either multichannel mode when observing all satellites in view, or in the traditwnal single channel mode when tracking a single satellite according to the limited BIPM schedule. The receiver architecture is described, and measurement results obtained from the K+K receivers are compared with the corresponding results determined by NBS-type receivers. 1 INTRODUCTION In [l] SATMIX time scale comparisons between the FTB Bundesanstalt) (Physikalisch-Technische and the TUG (Technical University Graz) using two single-channel fast-sequencing GPS receivers are described. The time scale differences determined with the SATMIX or Melting Pot Time Transfer method were compared with the corresponding results obtained by the more accurate two- way satellite time and frequency transfer (TWSTFT). Over a period of 1 year the scatter between the two transfer methods amounted to 4 ns, but no significant systematic receiver delay changes of the K+K fast-sequencing receivers were observed. Due to the long-term stability of the receiver demonstrated in this FTB/TUG experiment, an advanced version of this receiver type was developed by K+K Messtechnik the receiver K+K GPS6-2K in the advanced version now contains two channels, a tracking channel, and a data channel which continuously provides all data transmitted in the GPS data stream. The tracking channel can therefore remain in the fast-sequencing mode (FSM) and does not have to be interrupted for the data readout. In addition to the data channel a time interval counter was incorporated in the receiver which allows measurement of the respective time differences between an external lPPS reference and the GPS time as received from the satellites. Along with the advanced version, K+K Messtechnik supplies software which allows an external PC to download the data from the two receiver channels via an RS-232 serial interface and to convert them to the CGGTTS format recommended by the BIPM [2]. 149 Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 3. DATES COVERED NOV 2001 2. REPORT TYPE 00-00-2001 to 00-00-2001 4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER Common-View Time Transfer Using the Two-Channel Fast-Sequencing 5b. GRANT NUMBER GPS Receiver K+K GPS6-2K 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION Physikalisch-Technische Bundesanstalt,Bundesallee REPORT NUMBER 100,38116,Braunschweig, Germany, 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited 13. SUPPLEMENTARY NOTES See also ADM001482. 33rd Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting, 27-29 Nov 2001, Long Beach, CA 14. ABSTRACT Two 2-channel fast-sequencing GPS receivers (Type K+K GPS6-2K made by K+K Messtechnik GmbH, Braunschweig, Germany) have been studied in co-location and in comparison with the NBS-type receivers used at the PTB for the regular GPS CV- time transfer. The new receivers consist of two channels (the "Tracking Channel" and the "Datu Channel"), a built-in time interval meter, and PC sofware. The "Tracking Channel" either continuously tracks a single satellite or sequentially tracks all satellites in view. Both the code phase and the carrier (delta) phase are measured and combined to yield a smoothed reading of the received satellite time. The "Daa Channel" reads all the data necessary from the satellites, like almanac, ephemeris, ionospheric model parameters, etc, and acquires new satellites as they arise above the horizon. The PC sofware currently used generates the "CGGTTS GPWGLONASS Datu Format" in either multichannel mode when observing all satellites in view, or in the traditwnal single channel mode when tracking a single satellite according to the limited BIPM schedule. The receiver architecture is described, and measurement results obtained from the K+K receivers are compared with the corresponding results determined by NBS-type receivers. 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF ABSTRACT OF PAGES RESPONSIBLE PERSON a. REPORT b. ABSTRACT c. THIS PAGE Same as 7 unclassified unclassified unclassified Report (SAR) 2 RECEIVER ARCHITECTURE The advanced GPS6 2K receiver was designed to provide a versatile instrument which can be K+K used as a stand-alone receiver providing lPPS/GPS or USNO time signals (selectable) and standard frequency, or as a Common-View receiver generating the CGGTTS format recommended by the BIPM. The evaluation of the received data was intentionally made less dependent of the receiver processor and transferred to a standard PC running WindowsTM. Studies of the data processing with other algorithms than those presently used can therefore easily be done with this receiver structure. Figure 1 shows the functional block diagram of the advanced receiver version with two channels. Subject to user choice, in FSM (Fast Sequencing Mode) the tracking channel continuously sequences through all satellites in view, or in SSM (Single Satellite Mode) tracks a single satellite only. In either case both PRN code and carrier (delta) phase are coherently measured and combined to yield a smoothed reading of the received satellite time. The relative weights in combining code and carrier phase data are user-selectable. Due to the carrier-phase smoothing, a dwell time for the observation of 160 ms is used for the observation of one satellite. The sole purpose of the data channel is to read data (almanac, ephemeris, ionospheric correction parameters, etc.) from the satellites and to report these data to the tracking channel. Thus, the tracking channel never needs to interrupt its process of sequentially observing all satellites in view. All clock and timing signals in the receiver are phase-coherently derived from a single fundamental clock signal atfo40.23 MHz generated by the voltage-controlled crystal oscillator VCXO, which in turn is disciplined by the high stability 10 MHz ovened oscillator OCXO. The PLL/Mixer utilizes a three-stage mixing scheme with subsequent integrator to maintain the VCXO signal phase-coherent with the OCXO signal even if the VCXO drifts due to temperature changes or aging of the crystal. The OCXO is continuously phase-controlled by the microprocessor such that the 10 MHz signal is phase- locked to the received satellite signals. Therefore, no input of an external reference clock signal is required and the system can also be operated as a stand-alone GPSDO (Disciplined Oscillator). The time constant to control the OCXO is user-selectable depending on its stability (300 s for the two prototypes studied here). The receiver unit supplies the antennddown-converter unit with DC and the 9* harmonic of the signal (92.07 MHz). In the down converter the 92.07 MHz signal is multiplied once more by VCXO 16 to generafe a 1473.12 MHz signal, which is then mixed with the pre-amplified antenna signal to yield the first intermediate frequency (IF) of 102.3 MHz. In a second stage this 102.3 MHz IF is mixed again with 92.07 MHz resulting in a second IF signal at 10.23 MHz, which is fed down to the receiver unit for further processing. In both units diplexers are used as separatinglcombining filters enable the use of a single coax cable for supplying the converter unit with DC and the 92.07 MHz to signal, and returning the 10.23 MHz IF signal to the receiver. In the data and tracking channels, the 10.23 MHz IF signal is filtered, amplified, and then correlated with the digitally generated PRN code. The resultant “de-spread‘’ signal is narrow-band filtered and mixed withfo to yield a third IF of nominally DC, which in fact is a low frequency signal (up to a few kHz) exhibiting the sum of the frequency deviationfo - fGps and the Doppler shift due to the satellite motion. This signal is passed through a digitally controlled continuous phase shifter with subsequent D/A converter, allowing the microprocessor to establish a numerically phase-locked loop (PLL), thereby tracking the carrier phase with high resolution. The PRN code generators of both channels are clocked with the fo signal, while numerically controlled phase shifters allow adjustment of the PRN code phase to the’ satellite range and to 150 compensate for the Doppler shift as determined by the carrier tracking loop while tracking a satellite. Finally, thefo signal is divided down to yield the lPPS output signa,] which represents either GPS system time or UTC(USN0). Thefdl Hz divider is set by the software to secure proper timing. In addition, the lPPS can be delayed or advanced with 1 ns resolution to compensate for cable delays. Due to the phase coherence of the VCXO and the OCXO signals, the lPPS output has a fixed phase relationship to the 10 MHz output signal, with <1 ns jitter. The receiver unit includes a built-in time interval counter (TIC) which is basically an instrument of its own, except that it reports the measured TI data to the data channel via a standard RS-232 serial interface. The TIC is started by lPPS from a local lPPS reference clock and stopped by lPPS/GPS as generated from the receiver. The 10 MHz OCXO signal serves as time base signal. Internal arming circuitry ensures proper operation even with asynchronous, possibly coincident pulses. Resolution is 50 ps, accuracy typically 500 ps. Once per second all data obtained in the two channels and the TIC necessary for the common-view processing are transmitted to the PC: date, time, position, the TIC measurement result, the current OCXO time and frequency control-loop errors, azimuth, elevation, model parameters of the ionospheric and tropospheric delay, satellite-clock correction, almanac, ephemeris, the differences between the GPS time as received from a single satellite, and the averaged time as received from all satellites in view. Although the system is designed to allow for any kind of data processing in the the current PC software version supports the Common View Time Transfer using the BIPM PC, format as described in “Technical Directives for Standardization of GPS Time Receiver Software” [2] and “CGGTTS GPS/GLONASS Data Format Version 02” [3]. In the PC software, the user can choose between the fast sequencing mode (tracking all satellites in view according to the regular International GPS Tracking Schedule, like a multichannel receiver), or the single-satellite mode (tracking satellites according to the limited BIPM Tracking Schedule, like a single-channel receiver). 3 MEASURING ARRANGEMENT FOR THE STUDY OFSEVERAL GPS RECEIVERS Figure 2 shows the block diagram of the measuring arrangement for studies of two K+K type receivers KKl and KK2. In co-location with three NBS-type receivers, the GPS time as received with KK1 and KK2 has been compared with the corresponding results obtained by three NBS-type receivers NBS, TTR5, and TTR6. The caption of Figure 2 specifies which receiver types and models are assigned to the abbreviations KKl, KK2, NBS, TTR5, and TTR6. Each receiver includes a time- interval meter to determine the time difference between a local reference pulse and the GPS time received. In this study all measurements are related to the common reference UTC(PTB)/lPPS distributed to the receivers by the distribution amplifier DA. The NBS-type receivers performed selected satellite observations per day according to the limited 48 tracking schedule recommended by the BIPM. The K+K type receivers did the same when operated in the SSM. In the FSM they observed all satellites in view covering all 89 daily track intervals according to the regular International GPS Tracking Scheduleo f the BIPM. As the artificially limited tracking schedule is a sub-schedule of the regular International GPS Tracking Schedule and the full track length of 13 minutes is the same in both cases, the results obtained from a receiver in SSM should be the same as those obtained in FSM if only data are selected which correspond in track time. 15 1 4 MEASUREMENT RESULTS Figures 3 and show the daily mean values of the differences AT(GPS) between the GPS time 4 T(GPS) received with various receivers, e.g. NBS - KK2 stands for: NBS - KK2 [UTC(PTB) - T(GPS(KK2))I - [UTC(PTB) - T(GPS(NBS))] = AT(GPS). E To avoid overlapping curves, arbitrary constants C have been added to At(GPS). Figure 3 shows the receiver delay changes of KK2, TTR5, and TTR6 related to NBS. The noise of the NBS-type receivers for succeeding days is lower than that of KK2; however, systematic receiver delay changes of about 3 ns can be seen for NBS-TTR5 and up to 11 ns for TTR6. Omitting the data points when KK2 was operated in FSM, the standard deviation of a daily value from the mean of NBS-KK2 over the period shown amounts to 1.5 ns. The changeover from SSM to FSM, however, results in a shift of about 10 ns. As can be seen in Figure 4, the data for NBS-KK1 show a similar behavior when changing over from SSM to FSM. From MJD 52165 to MJD 52177, both receivers behave as expected when operated in SSM. Attempts to understand the reason for the delay shifts have been made (minor software modifications and corrected antenna coordinates). Unfortunately, data for KK2 are missing between MJD 52217 and MJD 52225 due to a hardware failure of the down- converter. But as can be seen from the last values, the changeover behavior from SSM to FSM of both receivers seems to have been improved, though the scatter of the data points is still too high. Figure 5 gives the timing instabilities of the receivers TTR5, TTR6, and KK2 related to NBS and calculated from the data of Figure 3. 5 CONCLUSION AND FUTURE WORK In SSM operation the K+K type receiver can be used for common-view time transfer achieving a delay instability of about 1.5 ns. An apparent shift of the receiver delays and an increased scatter of the GPS time received caused by changing from SSM to FSM could not yet be removed completely. Future work will, therefore, focus on finding and removing the cause of the apparent phase shifts of the received GPS signals when changing from SSM to FSM and vice versa. Another important part of the work will be to further reduce the receiver noise. Special attention will be devoted to a careful study of the impact of various parameters within the GPS receiver software, in particular the relative weight of carrier Doppler frequency and PRN code phase in the carrier-smoothing algorithm and the time constant of the control routines. OCXO 6 REFERENCES [l] P. Hetzel, D. Kirchner, T. Polewka, and H. Ressler, 2000, “SATMZX Time-Scale Comparison Using a Single-Channel Fast-Sequencing GPS Receiver with Carrier-Phase Smoothing,” in Proceedings of the 3lSt Annual Precise Time and Time Interval (FTTI) Systems and Applications Meeting, 7-9 December 1999, Dana Point, California, USA (U.S. Naval Observatory, Washington, DC), pp. 383-392. [2] BIPM Report 9316, 1993, “Technical Directives for Standardization of GPS Time Receiver Sohare. ” [3] BIPM Document, 1998, “CGGTTS GPSIGLONASS Data Format Version 02, submitted to the ” 7” CGG’ITS Meeting, November 1998, Reston, Virginia, USA. 152 Figure 1: Functional block diagram of the K+K GPS6-2K receiver. The diplexers serving as combininglseparatingf ilters allow the use of a single cable for the up- and down-signal. Phase- coherent down conversion of the received GPS signal use harmonics of the 10.23 MHz-VCXO-signal which drives code generators and correlators in the Tracking and Data channel. 153 1 ?PSI UTY v l v l I Y l Y l Y Figure 2: Block diagram of the setup for studies of K+K-type and NBS-type receivers. KKl, KK2: K+K-type receivers, model GPS6-2K, made by K+K Messtechnik GmbH; NBS: Original NBS-type receiver, made by Rockwell Collins; TTRS, TTR6: NBS-type receivers, models TTRS/SN156 and TTR6/SN420, made by Allen Osborne Assoc.; DA: lPPS distribution amplifier; PC 1, PC2, PC3 connected with GPS receivers via RS-232 interface. 110 120 130 140 150 160 iro 180 im 200 210 220 230 240 - MJD 52000 Figure 3: Daily mean values of the differences AT(GPS) + C between the GPS time T (GPS) obtained with the receivers KK2, NBS, TTR5, TTR6. NBS - KK2 [UTC(PTB) - T(GPS(KK2))l - [UTC(PTB) - T(GPS(NBS))] I NBS - TTR5 [UTC(PTB) - T(GPS(TTR5))I - [UTC(PTB) - T(GPS(NBS))] E NBS - TTR6 [UTC(PTB) - T(GPS(TTR6))l- [UTC(PTB) - T(GPS(NBS))] E Full symbols: Single satellite mode (SSM); open symbols: Fast sequencing mode (FSM). For clarity different constants C added. 154 n \5 30 + E E 20 9 10 0 -1 0 160 170 180 190 200 210 220 230 240 - YJD 52000 Figure 4: Daily mean values of the differences AT(GPS) + C (C=Constant) between the GPS time T(GPS) obtained with the receivers KK2, NBS, TTRS, TTR6. NBS - KK1 [UTC(€TB) - T(GPS(KK1))l- [UTC(PTB) - T(GPS(NBS))] NBS - KK2 [UTC(PTB) - T(GPS(KK2))l- [UTC(PTB) - T(GPS(NBS))] E KKl - KK2 [UTC(PTB) - T(GPS(KK2))I - [UTC(PTB) - T(GPS(KK1))I E Full symbols: Single satellite mode (SSM), open symbols: Fast sequencing mode (FSM). For clarity different constants C added. RECEIVER INSTABILITY 1. OEa n E P 2 3 =- 1.OEQO 0 E 1.OE-10 1.OEtO4 l.OE46 I.OE100 1.OE47 Averaging Time, T, Seconds Figure 5: Time deviation of NBS-TTR5 (circles), NBS-KK2 (diamonds), and (NBS-TTR6 (squares). 1551156