Flavien Mercier

1 × 10 −16 frequency transfer by GPS PPP with integer ambiguity resolution

For many years, the time community has been using the precise point positioning (PPP) technique which uses GPS phase and code observations to compute time and frequency links. However, progress in atomic clocks implies that the performance of PPP frequency comparisons is a limiting factor in comparing the best frequency standards. We show that a PPP technique where the integer nature of phase ambiguities is preserved consitutes significant improvement of the classical use of floating ambiguities. We demonstrate that this integer-PPP technique allows frequency comparisons with 1 × 10−16 accuracy in a few days and can be readily operated with existing products.

The time stability of PPP links for TAI

In the last few years the BIPM has started using GNSS phase and code observations and the Precise Point Positioning technique to compute time links for the generation of TAI. The estimated instability of such links for averaging time up to 1 month has been taken as 0.3 ns. In this paper, we investigate methods to estimate this instability, following several approaches.

Orbit determination for next generation space clocks

We study the requirements on orbit determination compatible with operation of next generation space clocks at their expected uncertainty. Using the ACES (Atomic Clock Ensemble in Space) mission as an example, we develop a relativistic model for time and frequency transfer to investigate the effects of orbit determination errors. For the orbit error models considered we show that the required uncertainty goal can be reached with relatively modest constraints on the orbit determination of the space clock, which are significantly less stringent than expected from “naive” estimates. Our results are generic to all space clocks and represent a significant step towards the generalized use of next generation space clocks in fundamental physics, geodesy, and time/frequency metrology.

Fixing integer ambiguities for GPS carrier phase time transfer

GPS is widely used for time and frequency transfers between ground stations. It is well-known that the GPS carrier phase is a much more precise observable than the code and is therefore of great interest. However the carrier phase is ambiguous, only the code can provide the absolute time difference between the two stations. Here we propose a rapid method to easily resolve integer ambiguities for accurate time transfer. Using this method, several time transfer results are analyzed for different baselines (from an ultra-short one in common clock configuration to a transatlantic one) and for durations of several days. Moreover our results will be compared to TWSTFT (Two-Way Satellite Time and Frequency Transfer). Finally this method is deemed to be a reliable way to estimate integer ambiguities.

Straightforward estimations of GNSS on-board clocks

In this paper, we present two techniques to estimate GNSS on-board clocks. These techniques only require a single GNSS receiver and allow to evaluate the stability on a given pass of the time difference between the GNSS on-board clock and the clock that drives the GNSS receiver. These methods have been validated using IGS clock products for GPS on-board clocks. We also show that they can be used to characterize ground clocks provided a better space clock is available.

GPS frequency transfer with IPPP

Since many years, the time community has been using the Precise Point Positioning (PPP) technique using GPS phase and code observations to compute time and frequency links. However progresses in atomic clocks imply that the performance of PPP frequency comparisons is now a limiting factor in comparing the best frequency standards. One main limiting factor comes from the effect on the clock solution of the simultaneous resolution of floating phase ambiguities together with other parameters. In this paper we study how to improve the PPP frequency transfer using the Integer-PPP (IPPP) technique implemented by the CNES.

Progress in accurate frequency transfer by GPS and GEO carrier phase at CNES

This paper presents the recent improvements in accurate frequency transfer by GPS and GEO (SBAS) carrier phase at CNES. In a previous paper we described the software used at CNES for GPS frequency transfer using carrier phase and we reviewed the performances obtained with two different clock resolution algorithms. In the best case, we obtained a stability of /spl sigma//sub y/(/spl tau/)=4.10/sup -15/ for /spl tau/=10/sup 4/ s between ALGO and NRC1, both stations being driven by a hydrogen maser. In this paper, we present the improvements performed on this software to process GEO measurements. The GEO satellites provide some advantages and drawbacks that are discussed. For the time being, in our configuration, the GEO carrier phase frequency transfer is limited by code noise and ionosphere correction.

Computation of GPS P1–P2 Differential Code Biases with JASON-2

Abstract GPS Differential Code Biases (DCBs) computation is usually based on ground networks of permanent stations. The drawback of the classical methods is the need for the ionospheric delay so that any error in this quantity will map into the solution. Nowadays, many low-orbiting satellites are equipped with GPS receivers which are initially used for precise orbitography. Considering spacecrafts at an altitude above the ionosphere, the ionized contribution comes from the plasmasphere, which is less variable in time and space. Based on GPS data collected onboard JASON-2 spacecraft, we present a methodology which computes in the same adjustment the satellite and receiver DCBs in addition to the plasmaspheric vertical total electron content (VTEC) above the satellite, the average satellite bias being set to zero. Results show that GPS satellite DCB solutions are very close to those of the IGS analysis centers using ground measurements. However, the receiver DCB and VTEC are closely correlated, and their value remains sensitive to the choice of the plasmaspheric parametrization.

Recent improvements in GPS carrier phase frequency transfer

GPS carrier phase frequency transfer is a convenient method to compare distant ground clocks. It requires multi-channel dual-frequency GPS receivers in both ground stations. In this paper, CNES specific software for GPS carrier phase frequency transfer is used. It overcomes the usual limitation of some GPS solutions : the day boundaries discontinuities. These discontinuities in the clock solution occur if the data are analyzed in daily independent batches. It is also possible to down sample the measurement files. These two functionalities allow us to perform a continuous GPS carrier phase frequency transfer on long durations. Results on different baselines are presented and discussed. For instance, on medium baselines, stabilities reaching 1.10-15 (Allan deviation) on one day are commonly obtained. On transatlantic baselines, stabilities are degraded due to the low number of satellites in common view

Short-term stability of GNSS on-board clocks using the polynomial method

The stability of on-board clocks is an essential issue in the performance of Global Navigation Satellite Systems (GNSS). We developed a straightforward method (the polynomial method) that allows to determine the short term stability of a GNSS on-board clock for a pass over a ground station driven by a clock, the characteristics of which are better than the space clock we want to characterize. In previous papers [1,2], we validated the method on GPS on-board clocks using IGS clock products as reference. In this paper, we continue this work with GLONASS, Galileo, COMPASS/Beidou and QZSS on-board clocks.