Christian Jayles

The current evolutions of the DORIS system

Abstract DORIS was developed for precise orbit determination and precise positioning on Earth. Three new satellites fitted out with dual-channel second-generation receivers have been recently launched. Jason-1, ENVISAT and SPOT-5 acquired a real autonomy thanks to DIODE real time on-board orbit determination software. Today the DORIS system has built up a global network of 55 stations. In order to reach new accuracy goals for Jason-1 and ENVISAT, it was decided to improve the long-term stability of the antennas when necessary. Third-generation beacons deployed from the end of 2001 offer new features and greater reliability. The satellites relay acquired and stored data at regular intervals to SSALTO, the new DORIS mission control center. DORIS data from the different satellites are currently available in the two Data Centers and used by the International DORIS Service Analysis groups.

DORIS-DIODE: Jason-1 has a Navigator on Board

DIODE (Doris Immediate On-board orbit DEtermination) is a series of real-time orbit determination software, which process one-way up-link Doppler measurements performed by a DORIS receiver on a satellite. The DIODE software are embedded within the DORIS receivers, and they provide orbit and time determination to the user as well as technical parameters to adjust the tracking loop within the instrument. After a first successful flight on-board SPOT4, the second generation of the family operates on-board Jason-1, with more efficient and more accurate algorithms. Similar versions have been embarked onboard SPOT5 and ENVISAT. The accuracy is between 10 and 30 centimeters RMS for the radial component, and about 50 centimeters RMS in 3D. With several Failure Detection and Incident Recovery (FDIR) enhancements implemented in the software, DIODE/Jason-1 has experienced only one anomaly in July 2004; its availability is 99.7%, after two years and a half in-orbit. This article describes the DORIS/DIODE element of the Jason-1 system. It summarizes the main results obtained from the various verification activities that concerned all parts of this navigation and time-tagging Jason-1 subsystem.

DORIS/DIODE: Real-Time Orbit Determination Performance on Board SARAL/AltiKa

One amazing heritage of the current altimetry missions, Jason-2, CryoSat-2 (without mentioning their predecessors TOPEX-Poseidon, ERS, Jason-1, and EnviSat) is that DORIS using DIODE On-Board Orbit Determination software calculate orbits in real-time with accuracy. For example, accuracy has been improved to 2.7 cm RMS on board DORIS/Jason-2 compared with the final Precise Orbit Ephemerides (POE) orbit, generally known to have less than 1 cm accuracy on the radial component. Simultaneously, an efficient integrity team on-ground continually monitors the health of the DORIS system. In February 2013, SARAL/AltiKa was launched hosting a DORIS DGXX receiver with the latest LV11 software as previously used in Jason-2 and CryoSat-2. DORIS on-board SARAL has since been permanently producing results efficiently every ten seconds without exception, including during manoeuvring phases. Spacecraft, ground-system, and users are provided with real-time information on the satellite position: the accuracy is approximately 3.0 cm RMS on the radial component, which is a major break-through for Near Real-Time (NRT) processing. These results are detailed in the paper. Future DORIS/DIODE versions will be used on-board Jason-3 and Sentinel-3.

Characterization of an ultra stable quartz oscillator thanks to Time Transfer by Laser Link (T2L2, Jason-2)

The T2L2 experiment (Time Transfer by Laser Link), on-board Jason-2, with an orbit at 1335 km, since June 2008 allows the clock synchronization between ground clock (generally H-maser) and space clock (quartz Ultra Stable Oscillator (USO) DORIS) with a stability of a few picoseconds over 100 seconds. In common view, when two laser stations see T2L2, the time transfer stability is less than 10 picosecondes over few seconds. In order to perform non-common view time transfer for synchronizing distant ground clocks, it is important to precisely characterize the on-board oscillator at least on 10,000 seconds (maximal flight time between two distant stations). The key is to study the space environment on the Jason-2 orbit, to separate deterministic and stochastic behaviors of the USO (shift and drift). We show that T2L2 is able to provide accurate frequencies, which are deduced from the ground to space time transfer over each laser station (few 10-13). Since 2008, these time transfers helped us to create an on-board frequency data base. The major contributors to these frequency variations on 10,000 seconds are temperature and space radiation especially due to the South Atlantic Anomaly (SAA) (in which Jason-2 pass through). Aging can be considered as a linear drift during 10,000 seconds and the effect of radiation like a very small shift over each SAA overflight. The effect of the temperature is drived by the on-board temperature measurement. A model is realized to represent these effects on USO with a RMS of few 10-13 over 10,000 seconds. Space phenomena are also playing an important role in long term. Actually, if we consider both accumulation dose received by radiation and aging, we can explain 99.9 % of the global frequency variation of the USO since the beginning of the T2L2 mission.

DORIS-class oscillator under radiations: The Jason family of satellites

The Jason family of satellites, which were launched in 2001, 2008 and 2016, embarked an ultra-stable-oscillator (USO) as the on-board frequency reference (short-term stability of a few 10−13 at 10-100 seconds) for the Doppler Orbitography and Radiopositioning Integrated on Satellite (DORIS) tracking system. All three satellites shared the same orbit (circular, 66°, 1335 km) and platform (PROTEUS) which is 3-axes stabilized. And all three oscillators have been perturbed by repetitive radiation exposures above the so-called South Atlantic Area, which implied deleterious frequency variations of various levels roughly from 10−12 to 10−11. The primarily objective of this work is to compare the frequency response of these USOs to radiations. On the assumption that all environmental parameters (proton flux from the SAA, position, orientation of the platform, etc.) are identical, we demonstrate that the oscillators present the same behavior with a pseudo-periodic frequency variation at 59 days whilst enhancing a different sensitivity. From all the perturbation sources, a dedicated exposure to radiations is identified due to a very particular situation — platform and attitude law, orbit, satellite anisotropy.