The role of the onboard T2L2 instrument is to detect and time-tag the arrival of laser pulses transmitted from ground laser-ranging stations. To achieve this, it performs the following operations:
- It measures the Earth's albedo in order to determine the level of residual light noise beneath which it would not be possible to detect a laser pulse and adjusts the detection threshold accordingly.
- It detects a pulse, that is to say a light signal that stands out sufficiently from the ambient noise.
- It measures the energy of the light carried by the laser pulse.
- It generates an electrical signal from the light signal and time-tags it.
- It stores both a coarse and a precise time-tag for the electrical signal in the instrument's local time base.
To accomplish this, the T2L2 instrument has three principal functions:
- a linear detection device, based on a photo-detector used in linear gain mode, which simultaneously measures the Earth's albedo, detects the light pulse and measures its energy level,
- a non-linear detection device, based on a photo-detector used in Geiger mode, which converts the laser pulse into an electrical pulse,
- a time-tagging device composed of a binary counter and an electronic vernier providing coarse and precise time-tags for the electrical signal. This device is synchronized by an external clock at 10 MHz.
This is completed by an electronic housekeeping system which controls the instrument's operation, collects the scientific and monitoring data, communicates with the satellite and provides the voltages needed for powering the instruments.
The resulting architecture of the T2L2 instrument is illustrated below. The two principal elements, as mentioned, are:
- The two units composing the optics assembly, for the Nonlinear Channel and the Linear Channel, installed on a deck that also holds the Active Thermal Control sub-system. It is located outside the payload module at the end of a mast, facing the Earth.
- The electronics unit, placed inside the payload module on the same wall as the DORIS instrument and synchronized with it via the 10 MHz clock signal.
T2L2's optical unit is made up of three elements:
- The optics unit for the nonlinear channel which collects and filters a part of the laser pulse and conveys it to the electronics unit via a fibre optic cable. It houses the optics responsible for collecting the laser pulse (with a field of view of ± 55°) and for injecting it into the optical fibre.
- The optics unit for the linear channel which collects and filters a part of the laser pulse towards the linear photo-detector, which measures solar noise and generates the signals necessary for detecting and measuring the amplitude of the pulse. It houses the coupling optics and the proximity electronics of the photo-detector which formats the electrical signals.
- The elements of the Active Thermal Control sub-system, thermistors and heaters, necessary for keeping the optical units within their nominal operating temperature range. These are controlled directly by the PROTEUS satellite bus.
It is necessary to keep the two units aligned to within 0.1°. In addition, the ratio of flux received by the linear detection system relative to the flux received by the nonlinear detection system must remain constant (to within 10%) whatever the angle of incidence of the laser beam and the orientation of its polarization.
Since the working principle depends on the use of the LRA, which is an integral part of Jason-2, the optical units must be positioned as close to it as possible. The satellite's attitude must then be taken into account when calculating the time transfer, to compensate for the difference in the optical paths between the laser station and the optical reference point of the LRA on the one hand and between the laser station and the time-tag reference point of the nonlinear optical unit on the other.
T2L2's electronics unit is made up of four elements:
- The Optical Module receives the light collected by the optical unit of the nonlinear channel and focuses it onto the nonlinear photo-detector. It consists of an optical delay line (an optical fibre wound round a drum) and coupling optics for the photo-detector. In order to achieve a good signal-to-noise ratio, it is also fitted with a narrow-band interference filter (2 nm), centred on the wavelength of the laser.
- The Detection Electronics Module controls the two photo-detectors, by adjusting the detection threshold, triggering the nonlinear photo-detector and processing the resulting electrical signals, in particular by measuring the energy of light pulses.
- The Time-tagging Electronics Module, the core of the T2L2 instrument, consists of a logical counter and a vernier which provides both the coarse and precise times for optical or electronic events detected by the instrument. Both the counter and the vernier are controlled by a synthetic clock synchronized with the 10 MHz signal provided by the external clock belonging to the DORIS USO. This module also controls the instrument's entire internal calibration process.
- The Housekeeping Electronics Module includes instrument management functions, time-tagging and detection control functions, acquisition of time-tagging data, digitization of telemetry data, communication with the satellite and power distribution from the 28 V Jason-2 supply.