In traditional space TT&C (Tracking, Telemetry and Command) ground station system, TT&C signals transfer via the point to point special signal cable or switch matrix. This method calls for putting forward the backup infrastructure in the preliminary stage of system design, and then designing a great lot of signal transmission cable. Meanwhile, it is likely to be effected by electromagnetic interference in the analog signal transmission proceeding. In addition, not only this method is poor scalability, but also reduce the flexibility of the resources reconfiguration among TT&C ground systems.
In order to overcome the problems exposed in the original frame such as complex link relation, limited scalability and electromagnetic interference in cable network, TT&C signals transfer is proposed through Ethernet. As is known to all, such as range measurement, velocity measurement and coherent demodulate, TT&C signals transfer require high standard for the phase continuity. However, there are dropping of packets, jitteriness of data or transmission delay in network business, so the phase continuity is damaged. They would influence TT&C system performance.
The paper presents a TT&C signals network transmission & switching technology which can effectively settle above problems. It would adopt signal compression and recovery, broadband network, multi-frequency & multi-signal framing, phase-coherent signal network transmission and recovery technology, etc.
With the aid of sparsity of wideband signal in frequency domain, TT&C signals are compressed and accomplished multi-frequency & multi-signal framing, besides transferred on network. Symbol block length depends on signals type. The low-rate transmission use short frame, simultaneously, the high-rate transmission use long frame. It avoids high packet frequency and low network transmission efficiency troubles.
The phase-coherent signal network transmission and recovery technology use data buffer mechanism and time domain frame deletion at the receiver. It is taken frame continuity validation, frame sorting, leakage frame detecting by frame counting. When leakage frame or jitter buffer underflow, it would insert time domain deletion-frame to replace original transmission frame, in order to keep signals phase continuity. Detecting signals amplitude, frequency continuity of transmission frame. When an exception occurred, it can be solved by insert time domain deletion-frame or complete local modification. Size of data cache is chosen according to routing distance and network quality.
This paper proposes a new idea and solution to achieve high standard for the phase continuity on network transmission, and effectively solve the problems. Finally, compared with the traditional methods, the new space TT&C signals transfer technology based on Ethernet proposed by this paper achieves the same performance index values.

We describe the aim, results and outlooks of the TRP project entitled “Novel Deep Space Station Architectures for Frequency Generation, Conversion and Distribution” (NOSAF), a study carried out by Menlo Systems GmbH and Time Tech GmbH. The purpose of this study was to propose a new architecture for the frequency and time (F&T) distribution system inside ESA’s three deep space stations (DSS) facilities in order to replace the current system.
The proposed architecture for F&T system implements an optical distribution system with an improvement in stability of two orders of magnitude. The F&T signals distributed to the end-users are intrinsically coherent and the ECM issues associated with coaxial cables are eliminated. The system provides full online monitoring of the performance by continuous phase tracking, as well as traceability to UTC. Furthermore, the deployment of an optical distribution system makes it feasible to add higher performing sources at a later stage. It also allows to extend the length of the distribution links to up to several kilometres, which will make possible the installation of future antennas outside of the existing DSS buildings, with the antennas being connected to a single ensemble of references.
The chosen F&T distribution scheme relies on the transmission through optical fibres of the pulse train from a mode-locked laser whose repetition rate is locked to the RF reference of the station. The pulse train at the far-end of the fibre is stabilised using a scheme based on an interferometric approach where the optical phase difference between local and back-reflected pulses is measured and used as an error signal in a feedback loop controlling an adjustable delay unit. This way, the disturbances caused by external temperature changes or acoustic vibrations are compensated. The main advantages of the chosen stabilisation scheme are its high performance in terms of short- and long-term stability, its lower complexity compared to other schemes, its robustness and its ability to support link lengths up to 2 km.
Another advantage of a pulsed optical F&T distribution system is the possibility of choosing any harmonic of the laser’s repetition rate (typically 100-250 MHz) as an extracted frequency. The dissemination of a higher frequency (e.g. 10 GHz) would reduce the signal degradation often present in the frequency conversion chains leading to the communication frequency bands, which are in the 2-35 GHz range.
The selected architecture is in parts similar to a Menlo Systems F&T distribution system. Its design is based on many years of experience and it has been installed at several facilities, for instance at the Elettra synchrotron in Trieste (Italy) and at the geodetic observatory in Wettzell (Germany).
This work is performed under ESA contract 4000119932/17/D/AH.

The ground stations of ESA’s deep space tracking network are equipped with frequency and time (F&T) reference systems, which are based on redundant hydrogen masers. These masers are used to generate the 5 MHz, 10 MHz, 100 MHz, 1PPS and IRIG reference signals, which are distributed to the user equipment. The 1 PPS signals of the masers are continuously monitored by comparing them to the 1 PPS output signals of GPS receivers.

This approach gives a synchronisation to UTC that is accurate enough for all the missions supported so far. However, triggered by the demanding time accuracy requirement for the GAIA mission, a critical analysis of the time inaccuracies in the F&T system was done including a calibration campaign in the Cebreros Deep Space Station. The result showed that the synchronisation accuracy was worse than expected.

Therefore, and in order to improve the connection of the ground station timescales to UTC, TimeTech has assembled a transportable GNSS calibrator including a state-of-the-art time and frequency transfer receiver. The components are integrated in a small portable rack. The core element is a GTR55 GNSS Time and Frequency transfer reviver. It includes two time interval counters (TIC) for the determination of the stations’ UTC reference points and to reference these points to the user interfaces. An PicoScope 6403C is used for monitoring the quality of the 1 PPS signals, ensuring that the trigger levels of the TICs are set properly, and to avoid false triggering. The GNSS receiver, the TICs, and the PicoScope are controlled by a built-in computer, which also stores all measurement data. The transportable GNSS calibrator shall be used to determine the delays of the existing installation in the deep space ground stations. For this tasks its internal delays (incl. antenna and antenna cable) have been determined by comparing the transportable GNSS receiver with respect to the reference receiver at Physikalisch Technische Bundesanstadt (PTB) by using the two-way time and Frequency transfer (TWSTFT) link between TimeTech and PTB.
The results of the determination of the delays of the transportable GNSS receiver by using the calibrated TWSTFT link between TimeTech and PTB and the outcome of the calibration campaign in Malargue Deep Space Station will be presented and the uncertainty estimation will be discussed.

All European Maser (AEM) project status

Xavier Vernez1, Serge Grop2, Bryan Leu1, Pierre Mosset2, Sylvère Froidevaux1 and Pascal Rochat1,2

1T4Science and 2Orolia Switzerland, rue Vauseyon 29, 2000, Neuchâtel, Switzerland
E-mail: vernez@t4science.com

T4Science and Orolia united their over 15 years of experience in space and ground hydrogen masers to manufacture a robust, high performance and low cost all European active hydrogen maser (AEM) for ground application. This development was funded by the European Space Agency over two projects and led to the commercialization of a new instrument: the IM4000.
The AEM is an ultra-stable atomic frequency standard used as a frequency generator in precise positioning, precise time keeping and other applications requiring a frequency source with outstanding frequency stability performance for averaging times in the range between 1 and 100’000 seconds.

In a hydrogen maser, a beam of hydrogen atoms is magnetically state-selected such that the higher energy, low-field-seeking hyperfine states flow into a storage bulb situated inside a microwave cavity resonant with the 1420 MHz hyperfine transition. The atoms reside inside the storage bulb for about 1 s, during which they interact coherently with the microwave field. The microwave field stimulates a macroscopic magnetization within the atomic ensemble, and this magnetization in turn stimulates the microwave cavity field. This continuous coherent interaction is referred to as the active maser oscillation [1].
A simple approach of a classical full-size aluminum cavity was adopted. The advantages are the followings:
• Aluminum cavity offers an excellent loaded quality factor; besides sapphire loaded cavity or ceramic full-size cavity, no real other alternate solution exists to keep the loaded quality factor higher than 25000, which is essential to sustain the self-oscillation for an active hydrogen maser.
• Aluminum cavity provides superior mechanical reliability.

However, because of the interaction between the atoms ensemble and the microwave cavity, any fluctuations of the cavity frequency will directly affect the hydrogen transition frequency and degrade the frequency stability of the instrument. To minimize this effect, the cavity is thermally regulated to achieve residual temperature fluctuations in the order of m°C. Nevertheless, this temperature control is not enough to achieve the long-term performances required for applications, for example, time keeping. A second correction is implemented to adjust in real-time the cavity frequency towards the hydrogen hyperfine transition frequency. This system is called Auto-Cavity Tuning (ACT) [2].
The AEM studies were conducted in 3 different phases. The first feasibility study of a prototype device demonstrated the functionality of the full Maser operation as well as the ACT principle. The main limitations were identified and were corrected in a second phase which end-up with a full operational Maser achieving most of the required specifications, although there were some remained limitations in order to meet the long term performances.

The third ongoing phase will improve the overall performances in order to be aligning with the AEM requirements.
The main improvement will be done on the thermal sensitivity on specific ACT electronic compensation and the signal to noise ratio of the critical signal compensation in order to reduce the compensation noise. The integration of the electronic will also be reworked in order to guarantee a better thermal regulation of temperature-sensitive parts.
According the recent tests to detect the most sensitive parts of this ACT, Orolia & T4Science are confident that the final stability & drift goals can be achieved using the same principle.
In this development stage, we obtained the thermal sensitivity & drift close to the final goal though we need further improvements by adjusting the design of electronics.

[1] M. A. Humphrey: Precision measurements with atomic hydrogen masers, PhD thesis, 2003
[2] D. Goujon et al.: Development of the space active hydrogen maser for the ACES mission, European Frequency and Time Forum, 2010