Since the 1980s, GPS time links have been essential to the TAI computation and, until 2006, the Common View (CV) technique has been used for this purpose. Recent advances in obtaining precise satellite orbits and clock parameters now permit us to obtain better results using another technique, which we name All in View (AV). By comparing the GPS CV and AV with the independent and more accurate TW and PPP time transfer techniques, we quantify the gain that can be obtained on a given time link. The AV technique also allows us to choose a more efficient network of GPS links between the tens of laboratories participating in TAI, which further improves the uncertainty in the access to UTC. The BIPM TAI software has been updated and the AV technique has been effectively used since the computation for the month of September 2006.

https://iopscience.iop.org/article/10.1088/0026-1394/45/1/006/pdf

GPS All in View time transfer for TAI computation

Geostationary satellites of the Wide Area Augmentation System (WAAS) offer a novel, robust, and cost-effective means of synchronizing time at widely-separated ground facilities, to levels of ~50ns, without the need for dedicated long-distance wired communication networks. However, reliance on satellite-based signals for time synchronization in high-reliability applications is problematic without explicit hardening against radio frequency interference (RFI). The primary innovations to be discussed in this paper are: (1) adaptive electronicallysteered multi-element antenna arrays and signal processing strategies for RFI mitigation, (2) live signal, synthetic interference, and hardware-in-the-loop testing of jammer cancellation algorithms, and (3) preparations for over-the-air interference tests which will probe the effects of front-end saturation on digital beamsteering performance. We describe a hardware system assembled from readily-available commercial building blocks (data acquisition system, antennas, etc.), and a critical goal of this research is to realize significant GPS anti-jam performance in an open-architecture (or non-defenserelated) platform. Therefore, the central innovations in this research enable adaptive electronic beamsteering with high-dynamic-range signals (14-bit I/Q digitization), employing commercial off-the-shelf (COTS) hardware and computer systems, and targeted for a civilian highreliability, high-volume (many hundreds of deployed systems) GPS timing application.

The WAAS/L5 Signal for Robust Time Transfer: Adaptive Beamsteering Antennas for Satellite Time Synchronization

https://iopscience.iop.org/article/10.1088/1361-6501/ab0f7f/pdf

Evaluation of carrier-phase precise time and frequency transfer using different analysis centre products for GNSSs

The European SBAS (Satellite Based Augmentation
System) EGNOS (European Geostationary Navigation
Overlay Service) is now operated and provides in its
navigation message the time difference between EGNOS
Network Time (ENT) and UTC to users. For this purpose
one EGNOS Earth station has been installed in the
Observatoire de Paris (OP) providing a physical link to
UTC(OP).
Applying EGNOS corrections on GPS measurements
provides a precise time and navigation solution referenced
to ENT. Assessment of the time difference between ENT
and UTC is therefore a key issue for time users.
A new EGNOS system release has been tested since the
beginning of 2008 including some improvements for the
EGNOS time to UTC offset. This paper deals with the
evaluation of the very good performances obtained with
EGNOS and its new release in this time domain.
First the paper details some time aspects of EGNOS and
describes the connection of ENT to UTC(OP). EGNOS
system time requirements are also reminded.
Then two methods of performance assessment are
described.
The first method is based on the time difference between
ENT and UTC obtained by combination of broadcasted
information in Message Type 12 and BIPM Circular T:
?? ENT-UTC(OP) is broadcasted in EGNOS Signal In
Space (SBAS Message #12) using internal delays of
EGNOS Earth station located in the Observatoire de
Paris. By analysing the content of the different
MT#12, we can derive the ENT-UTC(OP) evolution.
On an other side, UTC-UTC(OP) is computed by the
BIPM and available in their Circular T. By
comparing these two pieces of information, an
estimation of ENT-UTC can be established.
The second method consists in an independent estimation
of ENT at user level, in locations where EGNOS
corrections are valid:
?? For this user, an estimate of the difference between
its receiver time and ENT can be produced using
GPS measurements and EGNOS corrections. If the
receiver clock is connected to a UTC(k), an
estimation of ENT-UTC(k) is obtained. UTC(OP)
and UTC(IT) have been chosen in this assessment by
using the data from an IGS station connected to these
UTC(k). This independent estimation of ENTUTC(
OP) is obtained and then compared to the one
broadcasted in EGNOS SIS.
The results of the two methods are given and checked in
regards to EGNOS system requirements.
In BIPM Circular T, there are also information on the
offset between GPST and UTC enabling a differed
estimation of ENT-GPST from which results will be
presented.
To conclude, as EGNOS Network Time is by construction
referenced to UTC(OP) and not directly to UTC, the
expected gain in this chain due to the on-going
improvements in the modernisation of UTC-UTC(OP) in
order to have UTC(OP) closer to UTC is briefly
discussed.

Performance Assessment of the Time Difference Between EGNOS-Network-Time and UTC

The United States LORAN-C network has been significantly upgraded in recent years so that it might better serve as a backup to the Global Positioning System (GPS) both for navigation and timing applications. This paper examines the potential role of the enhanced LORAN-C network by comparing it not only to GPS, but to the other network and wireless distribution systems that anchor the time and frequency infrastructure for the United States. It then ranks enhanced LORAN-C amongst these systems as a reference source for time-of-day, precise time synchronization, and frequency. The rankings are primarily based on the estimated accuracy and stability that can be obtained with each system; but other factors are discussed including availability, coverage area, acquisition time, reliability, redundancy, and traceability to national and international standards.

/pdf/the-potential-role-of-enhanced-loran-c-in-the-national-time-20qkthe9g8.pdf

The Potential Role of Enhanced LORAN-C in the National Time and Frequency Infrastructure

This paper extends the program (Ref. 1) to evaluate the WAAS potential as a time transfer and a time distribution system. This paper reviews the performance of using dual frequency GPS satellite measurements
corrected by the WAAS broadcast data. In our previous work (Ref. 1) we found that the code and carrier measurements made on the signal coming from the WAAS Geostationary satellite were at least a magnitude noisier
than GPS. We attributed most of this error to multipath errors and to the fact that the WAAS signal is narrow band (2MHz) and cannot be improved by modern multipath-resistant correlation techniques. This paper examines the use of a directional antenna focused on the
WAAS satellite. The directional antenna should eliminate the effects of multipath and could potentially provide a very clean continuous signal for time and frequency transfer. A feed Hornand directional parabolic dish antennas were tested over a short baseline at USNO. The directional dish antenna was found to produce 25% cleaner data than the feed horn. Because of the very high signal to noise ratio level attained by using the high gain dish antenna, we found that the receiver maintained phase lock for days. Data from the directional dish antenna was collected at USNO and NRC. Results from this 700Km
base line revealed ns level performance for time transfer and 1E-15 level frequency transfer after only 8 hours of observation.

The Use of the Wide Area Augmentation System (WAAS) as a Time Transfer System

: Because of the role of time as a secondary mission of the Wide Area Augmentation System (WAAS), the FAA has been interested in developing the use of the WAAS for time distribution. An economical, evolutionary approach to the development of the WAAS for time distribution has been pursued. Several groups have been participating in a program to develop a Time Distribution System (TDS). Data presented to date indicate that the WAAS is probably not at the same level as GPS for time distribution. Recent WAAS data indicate that the daily average values of the differences between UTC (USNO) and WAAS Network Time (WNT) range between +20 ns and -25 ns. Recent GPS data from USNO obtained with a single frequency GPS Time Transfer Unit indicate that 2-day average values of the differences between UTC (USNO) and GPS Time range between +6 ns and -15 ns. Because of foreseeable improvements in the not-too-distant future, it seems reasonable to expect that the WAAS will soon exceed the performance of GPS for time distribution. Among these improvements are operational improvements within the WAAS as it matures, and improved orbits for the geostationary satellites as more stations inaugurate satellite-based augmentation system (SBAS) observations for time.

Time Distribution Capabilities of the Wide Area Augmentation System (WAAS)

Capabilities of the WAAS and EGNOS using High Gain Antennas for Time Distribution and Frequency Transfer

Using precise point positioning (PPP) we show the potential for comparing frequency standards with time stabilities of under 300 ps in flicker phase noise for time periods of 1 to 10 d. This leads to comparisons at 1 part in 1015 or better at 10 d. We compare short baseline PPP results with 2 hour data from the NIST Measurements System. We compare results from the NIST - USNO baseline with hourly two-way satellite time and frequency transfer results.

/pdf/frequency-transfer-using-precise-point-positioning-26m4700gi2.pdf

Pat Fenton

Papers

The Use of the Wide Area Augmentation System (WAAS) as a Time Transfer System

Time Distribution Capabilities of the Wide Area Augmentation System (WAAS)

Capabilities of the WAAS and EGNOS using High Gain Antennas for Time Distribution and Frequency Transfer

Frequency transfer using Precise Point Positioning