Mission
Principle Behind the Experiment
T2L2 uses very short pulses of light, propagated between the clocks to be synchronized.
The ground-based clock and the space-borne clock to be synchronized are linked respectively to a laser station and the onboard T2L2 instrument (see illustration below). The onboard instrument consists of a photo-detection system, a time-tagging device and a retro-reflecting device. The laser station transmits laser pulses towards the satellite whose retro-reflecting device reflects back some of the photons towards the station. The station records the time of departure and return of the pulses, while the T2L2 instrument notes the moment of their arrival on board the satellite. For a given series of pulses transmitted by station A, it is then possible to calculate any offset XA between the clock at A and the clock on the satellite. Any offset XB between a clock at a station B and the satellite-borne clock can also be calculated. It then becomes possible to deduce the time-transfer between stations A and B from the difference between XA and XB. The transfer can take place with both stations in sight of the satellite and firing their pulses in the same time interval, or with the stations in sight one after the other. In the latter case, the "noise" from the onboard clock during the time between the observations of A and B must be accounted for in the performance budget.
The T2L2 system is designed to handle time-transfers with an accuracy of one picosecond.
Principle behind the T2L2 experiment
For the version of T2L2 carried on Jason-2, the onboard clock and the retro-reflection device are not part of the T2L2 instrument but are already available on the satellite. The onboard clock signal is provided by the local oscillator of the DORIS instrument and the retro-reflector by the satellite's Laser Reflector Array (LRA), a pyramid of nine cube-corners used for laser ranging.
Mission
The T2L2 system is expected to improve the efficiency of systems for comparing ground-based and space-borne clocks by one or two orders of magnitude, opening up new prospects not only for Time / Frequency metrology but also for Fundamental Physics and many other fields.
T2L2 will make it possible to synchronize remote clocks located around the globe, to monitor the satellite-borne clock and to perform either Space to ground or ground to ground time transfers, whether or not the stations have the satellite in view simultaneously.
T2L2 uses the propagation of very short pulses of light, between the clocks to synchronize them.
The T2L2 instrument will be carried as a technology passenger on the Jason-2 satellite, which must be placed on the same orbit as Jason-1, whose mission it will pursue, at an altitude of 1336 km with an inclination of 66°. At this altitude, the orbital period is 113 minutes. The satellite passes over the same spot once every ten days and the distance between the ground tracks of successive passes is 250 km.
Jason-2 moves on a Low Earth Orbit, which means it is visible to a laser station for a period of 15 to 20 minutes. Let us take 1000 seconds as a convenient round number for our calculations. The periods of visibility can be grouped into sequences of several passes. Depending on the latitude of the station, these sequences will either contain six passes separated by the orbital period (approximately 120 minutes) every 24 hours or three passes every 12 hours. Depending on the position of the stations, the satellite may be visible simultaneously from more than one station (mainly on the scale of a continent) with the additional possibility of "close" observations (when stations are under the same orbital arc with non-visibility of only a few minutes between them).
T2L2 as a system is not liable to saturation and the number of simultaneous users, which could be as many as 20 in areas with a high concentration of stations such as Europe, is only limited by the onboard storage capacity for telemetry data.
A typical observation session might be described as comprising the following elements:
- Pulses will be transmitted ideally at night (according to the user station's local time); daytime transmissions should not be ruled out, however, as there is no a priori technical difficulty.
- As soon as the satellite becomes visible for the laser station, the telescope picks it up (by estimating its position from the satellite's ephemeris) and the pulses are transmitted for as long as possible (the more data that can be accumulated for statistical analysis, the more time transfer efficiency improves).
The objectives of the T2L2 mission on JASON-2 fall into one of three categories:
- Technological objectives for validating the operation and efficiency of T2L2.
- Scientific objectives concerning the use of the measurements taken by T2L2 for comparing the clocks.
The principal fields concerned are Time / Frequency metrology (establishing timescales, calibrating other comparison devices, etc.) and Fundamental Physics. - Complementary objectives related to the contribution of T2L2 to the Jason-2 mission.
This involves independent characterization of the DORIS USO particularly concerning radiation and the South Atlantic Anomaly (SAA), improving the choice of locations for ground beacons in this region and participating in laser ranging by allowing measurements to be made even without a return signal.
These objectives have also been ranked by priority. The experimental nature of this first T2L2 mission means that the first priority must be the technological objectives including those related to the Jason-2 mission, whose success will depend essentially on the project and science teams of the T2L2 mission. The value of the scientific results will also depend on the involvement of scientists with an interest in the T2L2 products, principally those specializing in Time / Frequency and Fundamental Physics.
| OBJECTIVE | T2L2 | ||
| Objective | Classification | Performances | |
| Technological | T2L2 : Validation of Time Transfer Validation of performances, stability and accuracy Ultra-precise laser ranging Unidirectional telemetry | Principal Principal Principal Principal | 10-15 on one pass 10-16 on one day |
| Scientific | Time / Frequency metrology Calibrating Time / Frequency links Characterizing onboard clocks Comparing ground-based clocks Timescales | Secondary Secondary Secondary Secondary | 10-16 on one day 10-16 on one day 10-16 on one day 10-16 on one day |
| Fundamental Physics Drift in the fine structure constant  Gravitational redshift Isotropy of the speed of light | Secondary Secondary Secondary | 10-16 on one day 10-16 on one day 10-16 on one day | |
| Earth Observation Atmospheric propagation | Secondary | TBD | |
| Geodesy | Secondary | TBD | |
| Very Long Base Interferometry (VLBI) | Secondary | TBD | |
| Complementary | Jason-2 : Characterizing the DORIS oscillator Improving the localization of beacons / SAA Contributing to Laser Ranging | Principal Principal Secondary | 10-15 on one pass 10-15 on one pass TBD |
