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2010 IEEE
714 IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, . 57, . 3, MARCH 2010
Abstract—The calibration of time transfer links is manda-
tory in the context of international collaboration for the real-
ization of International Atomic Time.
In this paper, we present the results of the calibration of
the GPS time transfer link between the Real Instituto y Obser-
vatorio de la Armada (ROA) and the Physikalisch-Technische
Bundesanstalt (PTB) by means of a traveling geodetic-type
GPS receiver and an evaluation of the achieved type A and B
uncertainty. The time transfer results were achieved by using
CA, P3, and also carrier phase PPP comparison techniques.
We finally use these results to re-calibrate the two-way satel-
lite time and frequency transfer (TWSTFT) link between ROA
and PTB, using one month of data.
We show that a TWSTFT link can be calibrated by means
of GPS time comparisons with an uncertainty below 2 ns,
and that potentially even sub-nanosecond uncertainty can be
achieved. This is a novel and cost-effective approach compared
with the more common calibration using a traveling TWSTFT
station.
I. I
G
PS time and frequency transfer is among the most
useful tools for comparison of remote clocks and rep-
resents the basis for the contributions of timing laborato-
ries to the realization of International Atomic Time (TAI)
[1]. It is one of the most accurate techniques in this field;
in the case of precise point positioning (PPP), it is at the
same level of performance as the state-of-art technique,
two-way satellite time and frequency transfer (TWSTFT)
[2]. However, to provide accurate time transfer by means
of a GPS link, it is necessary to carry out calibrations
periodically to verify the long-term stability of the equip-
ment.
Generally there are 2 types of calibration procedures:
absolute and differential ones. The absolute procedure is
carried out by GPS signal simulators [3], [4], and although
some new developments have been introduced during
the last decade, it still is complex and not widely used.
The differential procedure was used, e.g., in 2004, when a
TTR6-AOA GPS receiver from ROA was circulated in a
calibration campaign between selected European labora-
tories that contribute with their data to the computation
of TAI. After visiting 3 European sites, the receiver came
back to ROA, where an unexpected delay change of more
than 6 ns was detected in the closure measurement, indi-
cating the well-known limitations of the equipment that
was used [5].
For the purpose of link calibrations, the Bureau In-
ternational des Poids et Mesures (BIPM) has performed
several differential calibrations for geodetic GPS receivers
contributing to TAI [6], the complete history of which
is now available on the BIPM’s web pages [7]. ROA has
resumed its previous activities and took an initiative to
improve the calibration accuracy of its time transfer link
to PTB, which is presently the pivot laboratory in Europe
for TAI realization. In this paper, we report on the results
of a calibration trip performed in October 2008 with a
focus on the uncertainty budget evaluation. It is a revised
version of [8], which was presented at the EFTF–IFCS
2009 Joint Conference in Besançon, France.
II. T T GPS R
The traveling receiver (TR) comprises of a receiver of
type DICOM GTR50 intended for time and frequency
transfer, its NovAtel antenna, of type GPS-702-GG, with
pinwheel technology for multipath rejection and stable
phase center, and 48 m low loss, easy to handle H155 an-
tenna cable, nearly as flexible as the RG-58 standard.
The TR is basically a Linux PC in a 19¨ chassis to-
gether with a GPS board (Javad GGD-112T) and a time
interval counter. The Javad GPS board supports both
code and phase measurements on both GPS frequencies
L1 and L2. Its internal quartz oscillator is the reference for
pseudo-range measurements and the source of a one pulse
per second (PPS) output synchronized to GPS time. The
difference between this PPS and the PPS input reference
is measured with the time interval counter, which together
with the receiver circuits, and the GPS board, are located
in a thermostated box (based on thermoelectric Peltier
modules) to minimize the impact of their delay tempera-
ture sensitivity.
All components of this setup were transported in a sin-
gle box, which weighs less than 20 kg.
III. C T R
To accomplish the GPS calibration in the differential
mode, we started the common clock measurement at ROA
Improved GPS-Based Time Link Calibration
Involving ROA and PTB
Héctor Esteban, Juan Palacio, Francisco Javier Galindo, Thorsten Feldmann, Andreas Bauch,
and Dirk Piester
Manuscript received May 20, 2009; accepted November 20, 2009. The
involved institutes do not endorse any commercial products, which are
identified in the text for the purpose of scientific clarity only, nor do they
permit any use of this document for marketing or advertisement.
H. Esteban, J. Palacio, and F. J. Galindo are with the Real Instituto
y Observatorio de la Armada, San Fernando, Spain (e-mail: hesteban@
roa.es).
T. Feldmann, A. Bauch, and D. Piester are with the Physikalisch-
Technische Bundesanstalt, Braunschweig, Germany.
Digital Object Identifier 10.1109/TUFFC.2010.1469