1. 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

2. 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 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

3. 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

4. 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= 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

5. 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

6. 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

7. 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

8. 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

9. 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

11. The LNGS system was built by the italian company ESAT

13. 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

14. 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.

15. 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

16. 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

17. 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

18. Master card architectureRx 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

19. (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