The LNGS UTC system Some notions concerning GPS The LNGS UTC clock D.Autiero, Strasbourg 22/1/03 Some notions concerning GPS The LNGS UTC clock Interface to the OPERA timing system

The Global Positioning System (GPS) The GPS system was built and it is controlled by the US militaries (DOD), it is composed by at least 24 satellites orbiting at 20000 Km, the budget for maintenance is 500 M$/year Depending on the place and the time of the day on average between 5 and 8 satellites are visible by an observer on the earth

Each satellite is equipped with a Cs atomic clock and transmits its time and position (computed with its ephemeris): e.g.: x1,y1,z1,t1 The observer on the earth, getting the quadrivectors of 4 satellites has to solve a system of 4 equations in order to find his own time and position: x0,y0,z0,t0 (x0-x1)2 + (y0-y1)2 + (z0-z1)2 = c2(t0-t1)2 (x0-x2)2 + (y0-y2)2 + (z0-z2)2 = c2(t0-t2)2 (x0-x3)2 + (y0-y3)2 + (z0-z3)2 = c2(t0-t3)2 (x0-x4)2 + (y0-y4)2 + (z0-z4)2 = c2(t0-t4)2 Then x0,y0,z0,t0 can be converted in the more familiar Latitude,Longitude,Altitude and time

At least 4 satellites are needed, if only 3 satellites are visible one can still compute the position by imposing the constraint of being on the surface of the earth (geoid model), in this case the altitude cannot be measured In case n (n>4) satellites are visible the system becomes overconstrained and the precision improves as 1/sqrt(n) If the observer knows already his position x0,y0,z0 (fixed observer) then just one satellite is needed in order to measure t0 One assumes the speed of the ligth in vacuum (c) but this is not exactely the case since the ionosphere is a medium: The satellite emits on two frequencies: L1=1.575 GHz (C/A code) L2=1.2276 GHz (P(y) code) in order to model and correct for the real speed of the ligth in the ionosphere (differential GPS) The typical resolutions of a GPS measurement (single time fit) are: 22 m horizontal accuracy 200 ns UTC time

The satellites needs to know their ephemeris and the atomic clocks need continously to be readjusted from the ground stations with a complex procedure. Main control at Colorado Springs. Relativistic corrections are not neglegible and are correctly taken into account

GPS signals can be anytime denied on regional basis The system has been running for many years unchanged, the main change happened in 2000 by disabling the SA (about a factor 10 improvement) GPS signals can be anytime denied on regional basis

Typical commercial high precision GPS system (4Keuro) Has an accuracy of about 1 microsecond (limited by the quality of the local clock) People do not need to distribute the signal underground at 8 km distance It has a square wave output at 1 Hz (1 PPS) to tell when each second start The date (down to the second) is sent out in the IRIG-B format

The LNGS UTC clock For the physicists who like to have an accuracy of 100 ns and like to work underground at many Km from the antenna something more sophisticated is needed ….. The LNGS UTC system exists since the MACRO experiment started data-taking (1987). It is among the duties of the lab to provide UTC signals to the experiments in the caverns GPS antenna GPS receiver Slave clock(s) Master clock 8 Km opticql fiber Underground LABs External LAB

The GPS receiver is coupled to an atomic clock (rubidium oscillator at 10 MHz) called master clock The atomic clock generates the local time scale in between GPS syncronizations Stability: Short term fluctuations 3x10-12/100s Long term drift (aging, temperature) 1 microsecond/day Each second the atomic clock is readjusted on the GPS, the UTC time scale is known with a precision better than 100 ns Two working modes: Position: averages over a sufficient number of measurements to find the position of the antenna within a few m 2) Time only: once the position is known can be used as input in order to get a very precise measurement of the time If the GPS signal is not available one relies only on the atomic clock

The LNGS system was built by the italian company ESAT

Signal distribution to the underground labs The UTC time scal is known at better than 100 ns. Every ms (1000 better than the 1 PPS standard) a synchronization pulse and the time/date string are sent through the 8 Km long monomodal optical fiber (1310 nm) to 6 slave clocks in the caverns. The ligth pulse is converted in electric serial pulses. Slave clocks have to regenerate the local time with TCXO oscillators with a stability of 1 PPM (100 times better than what needed to keep in track within 1 ms) The time is then available in a 80 bits formatand it is known at the level of 100 ns MACRO was reading it with CAMAC modules BOREXINO with VME

How to interface to the existing system ? The original master card for the time distribution was foreseeing a 1 PPS input and IRIGB input (then one needs onboard a very high stability oscillator stable at the level of 100 ns/1s The slave clock has a 1 PPS output, then one has to convert the date string in IRIGB format The other possibility is to bypass the slave clock and build a card directly reading the fiber, this would allow to use a less sophisticated oscillator We asked ESAT which proposed a new slave clock for us for 10 keuros ESAT is also available in helping us to develop the master card directly interfaced to the fiber (we are missing of experience on the high stability oscillator part). We are going to meet at the beginning of february.

Clock distribution SM1 SM2 PCI card Node card i Master card 0 MLVDS O/E converter 1:2 splitter Optical fiber Master clock PCI card

PCI card architecture Hot Link 923 APEX 20KE PLX 9080 Hot Link 933 Inputs from GPS receiver (pps, 10Mhz, analog Irig B, digital Irig B) Optional for Lab tests 10Mhz 5.10E-11 OC-050 Vectron Int. O/E Converter Must be define Optical fiber from the master clock Master clock date EPC2 Hot Link 923 Optical fiber to the O/E converter Data+clock mixed TX HFBR1116T Local bus APEX 20KE PECL PLX 9080 Hot Link 933 RX1 HFBR2116T Optical fiber From the SM1 RX2 HFBR2116T Optical fiber From the SM2 EEPROM To the station

Schéma optique Rx=? nm Rx=1310 nm Tx=1310 nm Tx=1310 nm Rx=1310 nm Fibre optique Master Clock Rx=? nm O/E converter Rx=1310 nm MLVDS Tx=1310 nm Tx=1310 nm Vers SM1 HFBR1116T 1:2 Rx=1310 nm HFBR2116T HFBR5205T Rx=1310 nm HFBR2116T Fibre optique Multimode 62.5/128 O/E converter HFBR5205T MLVDS Rx=1310 nm Vers SM2 Carte PCI Tx=1310 nm

Master card architecture Rx Tx Bus n0 Data + clock mixed clock deserializer clock data HOT Link 933 Clock From node EPLD EPM7256 data data HOT Link 923 Bus n1 SN65MLVD 202D clock RJ45 data serializer Clock From node

(reset, reboot, incr cptr, …) Node Card architecture Differential signals Clk data Clk from node RJ45 On the Node card Input clock x 10 = 100Mhz Clock 10Mhz EPLD EPM7128 Node address Commands sent to the FPGA (reset, reboot, incr cptr, …) Address serialized Address requested