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

: GPS timing plays a critical role in modern practice of time errors estimate and synchronization. Big noise of the GPS-based measured data and inherent non-stationary of a time error cause major difficulties here. In spite of theoretical separation of the application fields for the filters (stationary and non-stationary signals), GPS-based time error processes require more explicit practical answer. Indeed, what process may be practically treated as a stationary one and, to opposite, how to recognize a non-stationary case? In this report we answer these questions by numerically and show that for the same transient time the following filter should be used to get the best accuracy for the known initial fractional frequency offset y(sub 0) (time error rate) of oscillator, namely an average smoother for |y(sub o)| < r(sub 1), the Wiener filter for r(sub 1) less than or equal to |y(sub 0)| less than or equal to r(sub 2), and the Kalman filter for r(sub 2) < |y(sub 0)|, where r(sub 1) and r(sub 2) are coordinates dependent on the required accuracy. We prove this conclusion by the example of a time error estimate of the rubidium standard based on the reference tinting signals of the Motorola GPS UT+ Oncore Timing receiver.

GPS-Based Time Error Estimates Provided by Smoothing, Wiener, and Kalman Filters: A Comparative Study

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 European SBAS (Satellite Based Augmentation System) EGNOS (European Geostationary Navigation Overlay Service) provides in its navigation message the time difference between EGNOS Network Time (ENT) and UTC. For that purpose, an EGNOS ground station was installed in the Observatoire de Paris (OP) and is connected to UTC(OP). Applying EGNOS corrections on GPS measurements provides a precise time and navigation solution referenced to ENT. Therefore the assessment of the time difference between ENT and UTC is a key issue for time users. A new EGNOS system release has been tested since the beginning of 2008, it includes some improvements in the timing functions. This paper deals with the evaluation of the performances obtained by these functions.

Performance assessment of the time difference between EGNOS-network-time and UTC

Global Navigation Satellite Systems (GNSS) keep a central role in the international timekeeping. American Global Positioning System (GPS) is a navigation system that has proven itself to be a reliable source of positioning for both the military community and the civilian community. But, little known by many, is the fact that GPS has proven itself to be an important and valuable utility to the timekeeping community [1]. GPS is a versatile and global tool which can be used to both distribute time to an arbitrary number of users and synchronise clocks over large distances with a high degree of precision and accuracy. Similar performance can be obtained with Russian Global Navigation Satellite System (GLONASS) [2]. It is expected in the near future satellites of a new European navigation system GALILEO might bring some important opportunities for international timekeeping. This paper after an introduction to international timekeeping focuses on the description of recent progress in time transfer techniques using GNSS satellites.

International Atomic Time Scales and Related Time Transfer

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

: The FAA is currently developing the Wide Area Augmentation System (WAAS), a GPS Satellite-based Augmentation System (SBAS) that will be used for navigation within the National Airspace System (NAS). WAAS consists of a multitude of ground reference stations and several geo-stationary communication satellites. The geo-stationary satellites will be broadcasting a "GPS"-like signal that can be used as a supplementary navigational signal and that will also contain GPS health status and correction data. The WAAS also offers a significant tool to the timing community. There are several unique advantages of using the WAAS or any SBAS for frequency and time transfer: (1) The geo-stationary satellites are always in view. They never set like the GPS and GLONASS satellites. This offers the ability for multiple sights to be permanently "phase-locked" together by locking themselves to the common WAAS signal. (2) They do not have any intentional distortions like the GPS SA dither. (3) WAAS system computes and broadcasts orbit and clock corrections for all GPS satellites in view thereby substantially removing the effects of SA on GPS. (4) The geo-broadcast signals are cesium-based, generated and controlled by the WAAS network of cesium-based GPS receivers. (5) The WASS network computes a real-time observational ionospheric delay model for the coverage area. This model is broadcast over the WASS signal so that an L1 single frequency user can make local corrections. This paper describes an initial attempt at time transfer using the WAAS test transmissions currently broadcast from the INMARSAT Atlantic Ocean Region - West (AOR- W) satellite. The clocks being compared are the master clock at USNO Washington and the master clock at NRC Ottawa. A 30-day data set is examined. It was obtained from dual frequency GPS/WAAS NovAtel receivers that were set up at both institutions. Comparisons are made between UTC(USNO), UTC(NRC), GPS time, and WAAS Network Time (WNT).

Edward Powers

Papers

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

Time Transfer Using WAAS: An Initial Attempt