LATEST CALIBRATION OF GLONASS P-CODE TIME RECEIVERS

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1 LATEST CALIBRATION OF GLONASS P-CODE TIME RECEIVERS A. Fos 1, J. Nwroci 2, nd W. Lewndowsi 3 1 Spce Reserch Centre of Polish Acdemy of Sciences, ul. Brtyc 18A, Wrsw, Polnd; E-mil: fos@c.ww.pl; Tel.: Astrogeodynmicl Oservtory Borowiec, Spce Reserch Centre, Polish Acdemy of Sciences, Kórni, Polnd; E-mil: nwroci@c.poznn.pl, Tel.: Bureu Interntionl des Poids et Mesures, Pvillon de Breteuil, Sèvres, Frnce; E-mil: wlewndowsi@ipm.org; Tel.: Astrct The Russin GLONASS nvigtion system, lthough not yet finished, provides interesting opportunities for interntionl time metrology. Its P-code signl shows n outstnding performnce. In prticulr, it llows clirtion of GLONASS time receivers with n uncertinty elow 1 ns. Our pper reports the ltest differentil clirtion of GLONASS P-code time receivers locted t the AOS nd the BIPM, y mens of portle receiver. Receiver time ises were determined for different GLONASS frequencies. Removing these ises, reching sometimes 10 ns, llows time trnsfer with n ccurcy of round 1 ns. 1 INTRODUCTION Over the lst 50 yers, the ccurcy of tomic clocs hs improved y n order of mgnitude every 7 yers. Tody, they rech stility in frequency of 1 prt in The stility of the interntionl reference time scles, Interntionl Atomic Time (TAI) nd Coordinted Universl Time (UTC), is presently out over period of few wees. Menwhile, the stndrd uncertinty of GPS C/A-code common view, currently the most widely used method of time trnsfer (nd the min method providing the dt for the clcultion of TAI nd UTC), is still few nnoseconds. To enle comprisons of incresingly stle clocs, it is importnt to improve time trnsfer methods. One of the new pproches to time nd frequency comprisons consists of using GLONASS (Glol Nvigtion Stellite System) P-code. Use of GLONASS in stndrd CGGTTS common-view mode ws introduced into time metrology in 1996 y W. Lewndowsi et l. [1,2]. Studies of this pproch hve een continued y J. Azoui, W. Lewndowsi, nd J. Nwroci in 2000 nd 2001 [3,4]. 99

2 2 ADVANTAGES OF GLONASS The GPS nd GLONASS systems shre siclly the sme concept. A sustntil difference etween GPS nd GLONASS is in the signl structure. GPS uses Code Division Multiple Access (CDMA): every stellite trnsmits the sme two crriers modulted y PRN-codes prticulr to ech stellite. GLONASS uses Frequency Division Multiple Access (FDMA): two individul crrier frequencies re ssigned to ech stellite, ut the PRN-codes re the sme for ll stellites. Hence, the GPS crrier frequencies f L1 nd f L2 re invrile: f L1(GPS) = f L2(GPS) = nd the individul GLONASS crrier frequencies f 1 nd f 2 re defined y: f f L1(GLONASS) L2(GLONASS) L L = = where is the crrier numer ssigned to the specific stellite. Both GPS nd GLONASS hve freely ccessile C/A-code tht modultes L1 only. Lie the GPS precision code, the GLONASS P-code modultes oth crriers, ut unlie GPS it is lso freely ccessile. opertor GLONASS Militry Spce Forces (Russi) The GLONASS P-code hs two min *The present numer of GLONASS stellites is elow its nominl vlue. dvntges for high-precision time trnsfer. Firstly, the GLONASS P-code modultion Tle 1. GPS vs. GLONASS. onto oth L1 nd L2 crrier frequencies llows high-precision mesurements of ionospheric delys. Secondly, the GLONASS P-code chip rte is one-tenth tht of the GLONASS C/Acode nd one-fifth tht of the GPS C/A-code. This mens tht GLONASS P-code pseudo-rnge mesurements re much more precise thn GPS or GLONASS C/A-code mesurements. GPS Deprtment of Defense (USA) nominl numer of stellites 24* 24 orit ltitude m m oritl plnes 3 6 oritl inclintion period of revolution ~ 11 h 16 min. ~ 11 h 58 min. ground trc repetition 8 siderel dys 1 siderel dy reference frme PZ-90 WGS-84 time reference UTC (SU) UTC (USNO) ephemeris position, velocity nd representtion ccelertion oritl prmeters signl seprtion Frequency Division Code Division Multiple Multiple Access Access (CDMA) (FDMA) L crrier frequencies L L1/L2 9 / 7 77 / 60 code chip rte C/A P code length C/A (its) P nvigtion messge 2 min. 30 s 12 min. 30 s length 3 DETERMINATION OF GLONASS FREQUENCY BIASES The differentil clirtion of GLONASS time receiver is relized y one-site comprison with portle receiver through common views [5]. Both receivers re connected to the sme cloc nd the ntenns re seprted y t most few meters. Such comprison t short distnce elimintes the common-cloc error nd errors resulting from stellite rodcst ephemerides, ionospheric nd tropospheric delys, nd ntenn coordintes; only the noise of the receivers is oserved. 100

3 GLONASS stellite ntenn 1 ntenn 1 lortory receiver portle receiver The is for the frequency with reference frequency No. 6 is given y B dt dt ( i ) ( i ) = 6 where ( dt i ) is the men vlue of the GLONASS P-code common views using the frequency over the whole intervl of computtion. CLOCK Figure 1. Scheme of differentil clirtion. The most recent differentil clirtions of GLONASS equipment were mde in the lte 1990s. We detil mesurements tht too plce t the Bureu Interntionl des Poids et Mesures (BIPM), the Astrogeodynmicl Oservtory of Spce Reserch Centre, Polish Acdemy of Sciences (AOS, Borowiec), Royl Oservtory of Belgium (ORB, Brussel), nd the Nederlnd Meetinstituut, Vn Swinden Lortory (NMi-VSL, Delft). Receiver No. ws chosen s reference; it hd een compred with portle BIPM receiver No in 1997 [5]. L. BIPM BIPM AOS ORB VSL Locl Receiver Ser. No (BIPM D) 0030 (AOS) 0022 (ORB) 0018 (VSL 18) Portle Receiver Ser. No (ROVER) Period 11/09 23/10/97 31/08 02/10/98 11/01 24/01/99 02/02 12/02/99 15/02 27/02/99 Numer of Common Views 899 (L1) 739 (L1) 698 (L2) 922 (L1) 412 (L1) 398 (L2) 807 (L1) 783 (L2) Tle 2. Some clirtions of GLONASS P-code receivers mde in the 1990s y the BIPM. The GLONASS P-code frequency ises re shown in Figure 2. It cn e seen tht they differ not only for every GLONASS frequency, ut lso for ech receiver. B / ns B / ns Figure 2. GLONASS P-code frequency ises with respect to frequency No. 6: ) receiver No 0030 (AOS) L1 ) receiver No 0018 (VSL) L1 receiver No 0025 (BIPM D) L1 receiver No 0018 (VSL) L2 receiver No 0025 (BIPM D) L2 101

4 receiver No 0022 (ORB) L1 receiver No 0022 (ORB) L2 Figure 3 shows n exmple of one-site comprisons of two receivers. The GLONASS C/A code multichnnel dt re noisier then the GPS dt. However, the GLONASS P-code single-chnnel comprison with frequency ises removed shows outstnding performnce. Corresponding time devitions re shown in Figure 4. dt / ns c d MJD ises Figure 3. One-site comprisons t the VSL: GPS C/A-code multi-chnnel; GLONASS C/Acode multichnnel; c GLONASS P-code single-chnnel; d GLONASS P-code single-chnnel with frequency removed. c d Figure 4. Time devitions for one-site comprisons: GPS C/A-code, GLONASS C/A-code, c GLONASS P-code (L1) without corrections for GLONASS frequency ises, d GLONASS P-code (L1) with corrections for GLONASS frequency ises. cii

5 4 APPLICATION OF CALIBRATED GLONASS FREQUENCY BIASES We pplied the frequency ises otined from the dt from Jnury nd Ferury 1999 to the time trnsfer etween AOS nd NMi-VSL mde in Ferury Using the mesurements with rodcst ephemerides nd modeled ionospheric delys, the 1-dy root-men-squre (RMS) of GLONASS P-code common view fter corrections for frequency ises is out 3.7 ns, which is the sme s for GPS. t [ns] c 0 d , , , MJD Figure 5. [UTC (AOS) UTC (VSL)] compred y: GPS C/A-code, GLONASS C/A-code, c GLONASS P-code (L1) without corrections for GLONASS frequency ises, d GLONASS P-code (L1) with corrections for GLONASS frequency ises (with rodcst ephemerides nd modeled ionospheric dely). 5 CONCLUSIONS The use of GLONASS P-code promises further improvement in the stility of time trnsfer. Therefore, it is very importnt to repet clirtions of GLONASS receivers in order to determine the frequency ises of those currently in opertion. After corrections for ionospheric dely, derived from IGS mps or P-code doule-frequency mesurements of the ionosphere, nd precise ephemerides, cloc-comprison uncertinties of less thn 1 ns cn e otined [3,4]. New GLONASS clirtion exercises, using the new genertion of receivers, re progrmmed for the ner future. 103

6 REFERENCES [1] W. Lewndowsi, J. Azoui, A. G. Gevoryn, P. P. Bogdnov, J. Dnher, G. de Jong, nd J. Hhn, 1997, First Results from GLONASS Common-View Time Comprisons Relized According to the BIPM Interntionl Schedule, in Proceedings of the 28 th Annul Precise Time nd Time Intervl (PTTI) Applictions nd Plnning Meeting, 3-5 Decemer 1996, Reston, Virgini, USA (U.S. Nvl Oservtory, Wshington, D.C.), pp [2] W. Lewndowsi, J. Azoui, G. de Jong, J. Nwroci, nd J. Dnher, 1998, A New Approch to Interntionl Stellite Time nd Frequency Comprisons: All-in-View Multi- Chnnel GPS + GLONASS Oservtions, in Proceedings of the ION-GPS Meeting, Septemer 1997, Knss City, Missouri, USA (Institute of Nvigtion, Alexndri, Virgini), pp [3] J. Azoui nd W. Lewndowsi, 2000, Test of GLONASS precise-code time trnsfer, Metrologi, 37, [4] W. Lewndowsi, J. Nwroci, nd J. Azoui, 2001, First use of IGEX precise ephemerides for intercontinentl GLONASS P-code time trnsfer, Journl of Geodesy, 75, [5] J. Azoui, G. de Jong, nd W. Lewndowsi, 1997, Differentil time corrections for multi-chnnel GPS nd GLOANSS time equipment locted t 3S Nvigtion, BIPM nd VSL, Prt 1, Rpport BIPM-97/6. 104