From jimr@Maia.usno.navy.mil Fri Mar 20 11:41 EST 1998
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From: Jim Ray (USNO 202-762-1444)
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Subject: Summary of responses from IGS/BIPM Call for Participation
To: gpst@Maia.usno.navy.mil
Date: Fri, 20 Mar 1998 11:39:47 EST
Full-Name: Jim Ray (USNO 202-762-1444)
X-Mailer: Elm [revision: 109.19]
Status: RO
From: C. Thomas and J. Ray, Co-Chairs
To: Participants of the IGS/BIPM PILOT PROJECT
TO STUDY ACCURATE TIME AND FREQUENCY COMPARISONS
USING GPS PHASE AND CODE MEASUREMENTS
Circular 2 (20 March 1998)
Subject: Summary of responses from Call for Participation
Dear Colleagues,
For your convenience and consideration, the responses received from you
to the Call for Participation are summarized below. The institutions
responding positively to each of the four main areas of participation are
listed followed by specific comments and suggestions that were sent
organized by topic. (For a full list of participants and institution
acronyms please refer to http://maia.usno.navy.mil/gpst/partic.html .)
If you would like to contribute any additional information, please send
it to us at jimr@maia.usno.navy.mil and cthomas@bipm.fr.
Several additional areas of study were also suggested. There seems to be
considerable interest, for example, in the use of GLONASS data, which fits
nicely with the planned IGEX'98 Campaign (see
http://igscb.jpl.nasa.gov/igscb/mail/igsmail/igsmess.1826 .)
We are planning to organize a meeting of the Working Group members in early
summer. This may be held at the BIPM, Sevres (near Paris), France. We will
send you additional information very soon.
Sincerely,
Claudine Thomas
Jim Ray
1) Deployment of GPS receivers
==============================
[AIUB/OFMET, AOS/BOROWIEC, AUSLIG, BIPM, BKG, BNM-LPTF, CHUNGHWA, DLR, GFZ,
IMVP, JPL, MAO, NIST/BOULDER, NPL, NRL, ORB, ROA, SP/OSO, SSIA, USNO, USP]
Th. Schildknecht et al. (AIUB, OFMET): A common project of OFMET and AIUB to
develop time transfer terminals based on geodetic GPS receivers with
the goal of comparing time offsets with sub-nanosecond accuracy and
frequencies with 10**(-15) over one day accuracy of two or more (GPS
external) clocks was started in 1991. It was clear from the outset
that optimum use should be made of GPS code AND phase measurements
and that only geodetic-type GPS equipment should be used. The
emphasis in this project was put on the comparison of external (as
opposed to receiver internal) clocks.
Today two prototype geodetic time-transfer terminals (GeTT terminals)
are available, an additional terminal will become available in the
near future. The terminals contain modified ASHTECH Z-12 receivers.
A description of the state of the art of the time-transfer terminals
is given in "Short technical description of the GeTT terminals", EFTF
Paper 97, EFTF Paper 98.
After two experiments on European baselines in 1997 (OFMET-NPL,
PTB-NPL) the GeTT terminals will be deployed on a transatlantic
baseline during the second half of 1998. (This will in fact be the
first comparison of the GeTT method with the independent two-way
satellite technique (TWSTFT) on an intercontinental baseline.)
Clearly we wish that these terminals be deployed at National Time
Laboratories. It is assumed that they are connected to the best
available time standards at these sites. Site-log information will be
made available to the IGS Central Bureau (according to the procedure
defined in the Call for Participation). Data will be made available
to the IGS (no restrictions) to allow inclusion into the final IGS
product computation (depending on the selected sites a delivery to meet
the IGS rapid products deadline might be possible).
Further deployments may of course be discussed and fine-tuned with the
management of the IGS/BIPM pilot project.
J. Nawrocki (AOS): Borowiec is also the IGS permanent station with good
results. We are carrying permanent geodetic GPS observations using
Turbo Rogue SR-8000 receiver from September 1993 (point BOR1).
J. Luck & R. Govind (AUSLIG): Ashtech Z-12 at Orroral, 5 MHz input from
HP5071A cesium, 1 pps output monitored at 1-minute intervals
against HP5071A designated UTC(AUS) linked by GPS Common-View to
UTC/TAI. Likely to cease at end of July 1998. Ashtech Z-12 or
Z-18 at Mount Stromlo, Canberra, similarly configured, is a
possibility after July 1998.
G. Petit et al. (BIPM): We provide data from an Ashtech Z12-T receiver.
W. Schlueter (BKG): Our Turbo-Rogue (IGS-station WTZR) is connected to our
H-Maser EFOS 13. The Ashtech- and Trimble- Receiver at Wettzell are
operating with the internal frequency so far. The receivers can be
plugged to a Maser. In order not to fail due to the Maser it should
be EFOS 3. All Masers are compared with our Caesium-Masterclock
(UTC(IfAG)). In general the time receiver TTR6 is in use, more over
we started to implement the GLONASS - receiver 3S-Navigation for time
comparisons. The main problem is currently the calibration of all
the cables used. We have to work on that.
P. Uhrich & F. Taris (BNM-LPTF): For time and frequency applications, the
BNM-LPTF finds that there is a need for more details about the atomic
clocks used as local oscillators in the IGS stations. This request
concerns for example:
- Is it a free running oscillator or a frequency steered standard ?
- Any information about the short and middle (< 2 d) term stability ?
- Is the local frequency standard the only reference for GPS data
collection, or is it the local oscillator of the GPS receiver
steered on the local frequency standard ?
The BNM-LPTF is also seequing informations about potential on-board
space GPS receivers data processing.
J. Hahn (DLR): DLR is well equipped with active and passive H-masers,
TTR-4P receiver, (and also 3S GLONASS receivers)
S. Bedrich (GFZ): Available at Oberpaffenhofen are a geodetic TurboRogue
SNR-8000 GPS receiver (IGS station "OBER"), Rb and passive H-maser
clocks, PRARE equipment to generate alternative time transfer data.
S. Poushkin et al. (IMVP): The stations in Mendeleevo (MDVO) and Irkoutsk
(IRKT) are to participate.
M. Golovnia (MAO): There is permanent GPS-station connected with the
Observatory time service (rubidium frequency standart) and
Metrological Time service (hidrogen frequency standart).
The GPS station name is Golosiiv.
The 4-Char ID is GLSV.
IERS DOMES Number is 12356M001.
The permanent GPS-station has GPS receiver - TRIMBLE 4000SSi.
It is planned to use different methods for time comparison,
for example, meteor tracking and GPS.
J. Clarke & J. Davis (NPL): NPL owns two geodesic quality timing GPS receivers,
an Ashtech Z12-T receiver and an Allen Osborne Associates TTR-4P
receiver. We are at present forwarding data from the Ashtech Z12-T
receiver to the University of Bern for analysis. NPL's time scale is
based on two Sigma Tau model MHM 2010 Active Hydrogen Masers. Our
station is not yet registered with the IGS central bureau, we will
do this as soon as possible.
C. Bruyninx et al. (ORB): The permanent TurboRogue GPS receiver in Brussels
(BRUS) belongs, since November 1993, to the IGS network (as a non-
global station) and is therefore included into the ITRF. Precise
reference frequency is provided to the GPS receiver by a hydrogen
Maser maintained by the ROB time laboratory.
A 8-channel Motorola Oncore receiver is also planned to be put into
operation during 1998.
I. Kardos (SGO): The SGO has a permanent GPS-station installed as part of the
IGS network (PENC). The station is connected to the observatory time
service (rubidium) and to the meteorological station.
M. Santos (UFPR): We would like to bring to your consideration the fact that
we have a GPS permanent station close by our Space Geodesy Lab, on
campus of the UFPR. This GPS station (site "Curitiba", four-digit
"PARA") is one of the geodetic markers of the SIRGAS network. It is
also part of the RBMC network (the Brazilian Active Control System).
We have been studying a possible connection to a Cesium clock operating
in another lab on campus (not made yet). Therefore, in a spirit of
collaboration, we could, in principle, make available for this pilot
project dual-frequency GPS data (Trimble 4000 SSi), possibly attached
with the atomic clock.
R. Langley & S. Bisnath (UNB): Precise time and frequency transfer between
moving platforms, including spacecraft.
D. Blitzkow & E. da Fonseca Junior (USP): We have an operational continuos GPS
station in Presidente Prudente (lat = -22 07' 11", lon = -51 24' 20")
which is part of the continuos brazilian network. We don't have at the
moment an external frequency standard.
2) GPS data analysis
====================
[AIUB/OFMET, AOS/BOROWIEC, AUSLIG, BIPM, BNM-LPTF, CHUNGHWA, DLR, GFZ, IMVP,
JPL, NIST/BOULDER, NRL, NTU, ROA, SP/OSO, U. COLORADO, UNB, USNO, USP]
Th. Schildknecht et al. (AIUB, OFMET): For the IGS/BIPM project it was
essential to upgrade the [Bernese GPS] software to allow for
zero difference processing. This actually happened in September 1995
(see Annual Report 1996 of the CODE Analysis Center). Since January
1997 the CODE satellite clock estimates are of comparable accuracy as
those of other IGS Analysis Centers. Internally, not only satellite
clocks, but also receiver clocks are estimated using several
combinations of code and phase zero difference observations. In
summary we believe that the present version of the Bernese GPS software
is very well capable of producing interesting results for the IGS/BIPM
Pilot project.
Within the framework of this project we plan to perform two different
types of analysis:
(1) We will deliver (the satellite and) receiver clock estimates as they
emerge from the daily CODE routine processing. In this routine
processing up to 80 globally distributed (IGS) stations are used to
generate satellite and receiver clock estimates with a sampling rate
of 5 minutes. Currently only "smoothed code" observations are used
but it is planned to use the code and phase observation
simultaneously in the near future. In a first phase clock estimates
will be available together with the final CODE products only which
means with a delay of about 5 days.
(2) A special routine time-transfer routine processing will be set up.
In this processing we will use the CODE and/or IGS products like,
e.g. orbits, EOPs, station coordinates, and troposphere delays, to
determine receiver (and satellite) clock corrections. In this
special processing we will focus on a selected set of stations,
like e.g. receivers at timing-laboratories and of course our own
receivers (the GeTT-terminals). In this setup code and phase
measurements will be used! The results will be based on either the
"rapid" or "final" products depending on the requirements of the
project.
Furthermore we also would like to point out that, starting 1 January,
1999 our group will be "ex officio" involved in this analysis part of
the project (Tim Springer will be the new IGS Analysis Center
Coordinator).
J. Luck & R. Govind (AUSLIG): Routine daily orbit solutions, as IGS Associate
Analysis Centre, including receiver clock estimates. Comparison of
estimated clocks against 1 pps results on suitably configured receivers
as above. Extension of strategies to characterize satellite clocks
similarly.
G. Petit et al. (BIPM): We develop a competence in the analysis of data from
a network of GPS receivers for time and frequency comparisons.
J. Hahn (DLR): Our main interest will be in the area of processing algorithms
to use carrier phase smoothing, and in the calibration task where still
more detailed investigations are needed.
C. Bruyninx et al. (ORB): Clock comparisons by single differences on phases
is foreseen.
3) Analysis of instrumental delays
==================================
[AIUB/OFMET, AOS/BOROWIEC, AUSLIG, BIPM, BKG, BNM-LPTF, CAGLIARI, DLR, GFZ,
HAYSTACK/CFA, IMVP, JPL, MAO, NIST/BOULDER, NPL, NPLI, NRL, ORB, ROA, SGO,
SP/OSO, UNB, USNO, USP]
Th. Schildknecht et al. (AIUB, OFMET): Calibration of delays in cables,
temperature dependent delays, etc., were and are of major interest in
the context of the joint OFMET/AIUB project. We emphasize at this
point that the control of these delays is absolutely mandatory for
time transfer. (The corresponding requirements are much less
stringent for frequency transfer.)
Many of the development of the OFMET/AIUB time transfer project are
intimately related to issues listed in this area. We may of course make
available our papers dealing with these topics (see "Short technical
description of the GeTT terminals", EFTF Paper 97, EFTF Paper 98). We
would be very much in favour of the establishment of a special working
group dealing with instrumental delays, etc., and we would be willing to
contribute to such activities. It might be a good idea to organize a
session dealing with that topic at one of the future major "time
conferences".
J. Luck & R. Govind (AUSLIG): A limited opportunity exists to study
correlations with environmental Parameters while receiver is at
Orroral. Agreement-in-principle has been reached to measure
inter-channel delays and variations, once a GPS Simulator is
installed in South Australia.
G. Petit et al. (BIPM): We try to calibrate and assess the stability of
receiver hardware. Accurate frequency transfer: by monitoring the
time variation of calibration delays, using temperature controlled
antennas.
Accurate time transfer: by absolute calibration of instrumental
delays. The goal is to obtain a reference station with a calibration
uncertainty of 1 ns. This will allow to obtain a similar reading
accuracy in TAI.
W. Schlueter (BKG): The main problem is currently the calibration of all
the cables used. We have to work on that.
J. Clarke & J. Davis (NPL): NPL hope to actively participate in this part of
the study. We intend to perform "zero baseline" common-clock
measurements between our two receivers to determine their delay
stability.
C. Bruyninx et al. (ORB): The L1-L2 differential instrumental delays of the
GPS receiver in Brussels have been determined and are continuously
monitored since 1993. This work is done within the frame of a study
of the ionospheric disturbance on the GPS signal. This long experience
demonstrates the relevant contribution of the ROB with respect to the
analysis of instrumental delays as proposed in the IGS/BIPM pilot
project.
4) Time transfer comparisons
============================
[AOS/BOROWIEC, ATSC, AUSLIG, BIPM, BKG, BNM-LPTF, CAGLIARI, CHUNGHWA, DLR, GFZ,
IEN, IMVP, JPL, KRISS, MAO, NIST/BOULDER, NPL, NPLI, NRL, NTU, ROA, SP/OSO,
SSIA, U. COLORADO, USNO]
A. Chen & J. Green (ATSC): We are responsible for maintaining NASA Deep Space
Network(DSN) Timing and Frequency system. Currently, time comparisons
between DSN and NIST are done by using TTR-5 and TTR-6 GPS receivers
made by AOA in the common view mode.
J. Luck & R. Govind (AUSLIG): Existing time transfers by GPS Common-View,
Multichannel, and MCCV at Orroral. The potential exists to tie in
with existing TWSTFT between NML-CRL and NML-NIST but there are many
difficulties unlikely to be resolved within six months.
Coordination between areas of participation, and between cooperating
institutions within Australia.
1. VLBI "Time Syncs" between Tidbinbilla (equipped with IGS TurboRogue
driven from hydrogen maser) and Goldstone,CA have been discontinued. It
may be possible to persuade JPL to resurrect them (and correct some
anomalies) for this Pilot Project.
2. There may be sufficient other sites combining IGS station, VLBI
station and timing laboratory to warrant establishment of a Topical
Sub-Group to study comparisons of GPS Carrier Phase against VLBI. For
example, within Europe, and the KeyStone system with CRL in Japan.
3. LASSO/T2L2 could be considered between e.g. Graz, Grasse, Wettzell
and Matera (new SLR system) if any on-board packages can be deployed or
reactivated. It is also conceivable that SLR time transfer will occur
between Canberra and CRL, Tokyo - but not yet. Although it is probably
premature to consider SLR as a serious candidate for time transfer
comparisons, developments should be monitored.
G. Petit et al. (BIPM): The goal is two compare primary frequency standards
to 1.d-15 in one day or less. This will allow a better comparison
and a more accurate use of their data in the elaboration of TAI and
TT(BIPM).
A. Niell et al. (HAYSTACK, CFA): comparison with VLBI
F. Cordara (IEN): We are instead participating in the international two-way
synchronization network from INTELSAT geostationary satellite and in
the CCTF working group devoted to two-way methods from satellites, where
I hope we will soon be able to contribute with regular measurements.
J. Levine (NIST/BOULDER): Developing methods for comparing the next
generation of primary frequency standards, which are expected to
have accuracies of 10-15 or better.
J. Clarke & J. Davis (NPL): NPL owns three NIST type common-view GPS receivers
which are used to follow the BIPM common-view time transfer schedule.
In addition NPL routinely operates a Two-Way Satellite Time and
Frequency Transfer (TWSTFT) earth station. We regularly perform TWSTFT
time transfers with seven other laboratories in Europe and North
America, including USNO. We are also setting up a 3S Navigation
R100-40 GLONASS receiver. We hope to forward GLONASS data to BIPM soon.
R. Beard (NRL): We occasionally conduct experiments with other groups or
locations that might be of general interest and could contribute
that data.
I. Kardos (SGO): There are plans for studies of ... new methodes for clock
comparisons.
5) GPS receiver design
======================
J. Luck & R. Govind (AUSLIG): It would be of value for the Pilot Project
Working Group to produce a paper explaining exactly how the receiver
clocks are set in each geodetic receiver - Ashtech, TurboRogue,
3S R-100, Trimble, etc. - when running from internal oscillator,
from external frequency standard; what is the (hardware) relationship
between internal clock (the one estimated in geodetic processing) and
1 pps out; time constants and mechanisms for driving receiver clocks
against GPS time; extent of carrier phase smoothing of code signals;
and similar matters.
J. Levine (NIST/BOULDER): Discussions with manufacturers on how geodetic
receivers might be modified to improve their performance in time-
transfer experiments.
Possible design of a geodetic-type time transfer receiver using an
existing commercial GPS "engine".
6) Timescale generation
=======================
J. Nawrocki (AOS): Time Lab at Astrogeodynamical Observatory in Borowiec
participates in the creation of TAI and UTC time scales (acronym
AOS Borowiec in Circular T published by BIPM).
J. Levine (NIST/BOULDER): Potential Improvements in TAI that could be
realized using geodetic-receivers or improved software.
Designing methods that would further automate the work of the BIPM
so as to reduce the labor that is required to process the clock data
from the timing centers and national laboratories.
C. Bruyninx et al. (ORB): The time laboratory of the ROB participates to the
realization of TAI by sending the measurements of its clocks to the
BIPM. In 1997, data from 3 Cesium clocks and one Maser clock were
submitted to the BIPM. The ROB clocks are located in thermostatised
basements with limited humidity and temperature variations. The time
transfer to TAI is presently done using a NBS type receiver.
McCarthy et al. (USNO): Investigate formation of new near-realtime timescales
using GPS time transfer techniques and a weighted combination of
contributing clocks in the IGS network.
7) GLONASS data
===============
J. Luck & R. Govind (AUSLIG): IGEX'98 would appear to be a great
opportunity to justify inclusion of GLONASS in this Pilot Project
- unless it would complicate it unduly.
W. Schlueter (BKG): In general the time receiver TTR6 is in use, more over we
started to implement the GLONASS - receiver 3S-Navigation for time
comparisons.
J. Hahn (DLR): DLR is well equipped with active and passive H-masers,
TTR-4P receiver, (and also 3S GLONASS receivers).
S. Poushkin et al. (IMVP): We propose to start simultanously a compain for
GLONASS observations in the same areas.
J. Clarke & J. Davis (NPL): We are also setting up a 3S Navigation R100-40
GLONASS receiver. We hope to forward GLONASS data to BIPM soon.
C. Bruyninx et al. (ORB): From mid-1998 a double frequency GLONASS/GPS
(3S-Navigation : R-100/40T) will be used for both time transfer and
participation to the upcoming IGEX-98 GLONASS observation campaign.
8) Site-dependent effects
=========================
I. Kardos (SGO): There are plans for studies of site dependent effects
( multipath, atmosphere, receiver specific instrumental delays )
and of new methodes for clock comparisons.
K. Jaldehag (SP) & J. Johansson (SP/OSO): Studies of site dependent effects
such as multipath, atmosphere, and antennas.
9) Other capabilities and comments
==================================
Y.S. Song (KRISS): For the maintenance of national time and frequency
standard, KRISS is, at present, equipped with 3 cesium atomic
clocks(HP model 5071A), 1 hydrogen maser(Sigma Tau model 2002) and
2 GPS timing receivers(AOA model TTR-5 and TTR-6) for the
international time comparisons by GPS common views. No geodetic
GPS receiver is maintained at our laboratory now.
R. Langley & S. Bisnath (UNB): Assess performance of GPS receivers for
spacecraft time and frequency control.