The ACES mission of the European Space Agency combines optical and microwave-based geodetic observation techniques with highly accurate atomic clocks to achieve a new level of accuracy for geodesy and fundamental physics applications. In addition, the combination of two high-precision measurement techniques provides an even more exciting insight into their errors. Fundamental physics is particularly interested in experiments that require high precision between the results of the successive passes of a satellite. An example of such an experiment is the determination of gravitational redshift. Geodesy applications, in contrast, require both high accuracy and precision. Especially for applications like precise ranging or time synchronization, all possible error influences must be characterized and determined with high precision. Therefore, electronic delays of microwave link terminals pose a challenge to achieving high accuracy. They must, therefore, be calibrated, and the stability of the electronic delays must be monitored. While optical observation techniques can be calibrated sufficiently on the ground, the calibration of microwave measurements before a launch is not precise enough, and continuous monitoring is also not possible. In this study, four calibration methods were tested, all based on colocating optical and microwave measurements onboard a satellite and on the ground. The results of two methods achieved the required accuracy of 100 ps for time synchronization, with a mean error and standard deviation of better than 4 ps and 55 picoseconds, respectively. Correlations between the measured parameters were identified, and the impact of the different approaches on accuracy was investigated. It will be shown that the satellite-based colocation of two different geodetic observation techniques has clear advantages, and the calibration results achieved the required accuracy for geodetic applications in this simulation study.

Today, concepts within the Global Geodetic Observing System (GGOS) aim a Terrestrial Reference System (TRS) with 1 mm accuracy and long-term stability of 0.1 mm/year. GETRIS (Geodesy and Time Reference In Space) is a concept that aims to realize a relativistic reference system based on satellites as an extension to the TRS. This helps with another goal of the GGOS, namely, the connection of different satellite layers with the TRS. For a valuable contribution to the GGOS’s goals, we would expect to achieve precise determined satellite orbits at the level of 1–3 mm and satellite clocks with a picosecond accuracy. The use of increasingly precise links helps to improve the satellite precise orbit determination (POD) and is necessary to synchronize the clocks in the satellite/station network. We analyze a complementary use of high-precision optical dual one-way links at the level of 1 mm precision together with the L-band. In previous studies, we analyzed the benefit for satellite POD, achieving Medium Earth Orbit (MEO) as well as geosynchronous orbit (GSO) accuracies at the low-millimeter level. In this work, we analyze the capabilities for clock synchronization. We compare two different clock types for estimation and prediction. We analyze different satellite constellations and different combinations of measurement links.

Electrostatic space accelerometers are high-precision instruments used in gravity and magnetic fields and in fundamental physics missions. From their first use in space, these instruments show disturbances around the read-out frequency. In recent missions like Swarm and GRACE-FO, these instruments are seriously compromised. Their capability cannot be fully exploited or, in the worst case, not used for science data processing at all. We currently neither understand the mechanism nor the cause of the disturbance. For that reason, every correlation, direct or indirect, between signatures in the accelerometer data and other phenomena in the satellite environment is very important to study. In the GRACE mission, some of the disturbances relate to onboard current switching processes, such as switching of heater and torquer currents. In this paper, we examine the phenomena called twangs. The shape of these disturbances is responsible for their name, because they look like a decaying tone. Some of these twangs seem to relate to changing currents, too. They occur when sunlight hits surfaces of the satellite. This can cause the discharging of satellite parts. Here, we focus on special twangs, which we call cloud-related twangs, as recent investigations related them to cloud coverage. How can clouds in the troposphere influence a satellite, orbiting in the ionosphere? We investigated the troposphere–ionosphere coupling and found that the only coupling which can be responsible for disturbances on satellites and which may affect the accelerometers, are electromagnetic waves traveling in the so-called whistler mode at frequencies in the very-low-frequency (VLF) domain. This special propagation mode is a coupled electromagnetic and electrostatic mode. There, the electric field vector has a component in the direction of propagation due to the interaction with ions and the magnetic field in the ionosphere and magnetosphere. This led us to investigate the similarities between whistlers and twangs. The hypothesis that they are related is very important in our further understanding of disturbances on satellite instruments sensible to electromagnetic pulses in the range of very low frequencies and below.

Atomic Clock Ensemble in Space (ACES) is an ESA fundamental physics mission that establishes a very precise time scale in space. The main clock is a laser-cooled Cesium clock called PHARAO. Two days after launch, the ACES module was mounted on the ISS and now enables time transfer between ground clocks and the clock signal in space using two very precise and accurate synchronization methods: a microwave technique called Microwave Link (MWL) and an optical counterpart called European Laser Timing (ELT). Satellite Laser Ranging (SLR) stations within the International Laser Ranging Service (ILRS), which are equipped with a very precise clock or can establish fiber links to clocks operated at metrology institutes, are motivated to contribute to ELT. In the first evaluation phase, the Wettzell Laser Ranging System (WLRS) will be the main ground station, performing regular clock synchronization experiments between ground and ACES in space. We will provide an introduction to the ACES mission and the timer, which is used for time-tagging the ELT detection events on board the International Space Station (ISS). Furthermore, we will discuss the performance of ELT by showing the results of the calibration campaign completed before the ACES launch.

Unknown effects heavily disturb space accelerometers of gravity field missions. Disturbances in GRACE called twangs could be related to excitations in the ionosphere. This motivated the study of possible interaction between the ionosphere and space instruments. We report on a ground experiment conducted in a plasma chamber, where we reproduced the switching of heaters and the reaction of a shielded and un-shielded receiving antenna. We identified the signature of plasma excitations as typical for a bounded plasma in a cavity, where a neutral layer establishes between the metal surface and the plasma. In addition, we see excitations of smaller frequencies, which would fit to mechanical resonances of the metal shield. The plasma excitations could be shielded, whereas the lower frequencies stay in the response of the receiver. Beneath the presentation of the experiment, we will propose a possible mechanism for the excitation of mechanical resonances of metals by plasma waves disturbing the layer between a metal and a plasma.