Pressure and Temperature : A knowledge of these 2 environmental parameters are of interest to complete the purely acoustic data set. Pressure can be used with the navigation data to reach a precision of about 1m in the instrument position. Temperature can be used as additionnal data in the inversion.
Time base : Travel times measurements with a precision of about 1ms during one year requires a very high stability time base. When operating subsurface mooring, one cannot access the precision of the GPS clock and other time base are used. The time base used here is based on a temperature compensated crystal oscillator. Its power consumption is very low (60mW) for a long term stability of 3.10-9. The crystal behavior in temperature is in-situ compensated from a calibration curve. The key to reach such a stability is a thorough calibration together with a knowledge of the crystal aging history.
Other time base : The use of a Rubidium time base in conjunction with a crystal oscillator can fulfill the high stability requirements at a cost of an expense in energy consumption. The Rubidium osccillator is swicthed on for a short of time. During that time the crystal oscillator frequency is checked against the one delverd by the rubidium one. Such a module is actually under test and the following characteristics can be anticipated.
･ Power supply : 18 to 32V
･ Consumption : typ. 200mW
･ Stability : typ. 5.10-10 to 1.10-9 per year
･ jitter : 8 μs
･ Dimensions : 215mm x 110mm x 100mm
･ Weight : 1.8kg
Figure 3 shows a design of such a time base while figure 4 illustrates the architecture of the time base.
Power amplifier : The power amplifier is a linear class D with a great efficiency (＞ 80%) when working on variable reactive loads in the 50 to 1500Hz frequency range. In order to be operated on batteries, it keeps constant the output voltage when the power supply varies from 50 to 35V. Its ouput power is 500 VA. It can support a 1.4Ω load and is protected against short circuit and overintensity. A shutdown mode put it in a sleep state with a consumption less to 100μA. A more powerful version (1500VA) which will drive the new JHAS transdcuer at 250Hz is under development.
Impedance matching : The aim of this matching circuit is to tranfer the electrical energy from the power amplifier to the transducer with a maximum eificiency. It must reduce the transducer reactive impedance and deliver the required voltage from 28 Vrms at the power amplifier output to 400Vrms on the transducer. A passive circuit (transformer + self + capacitor) was designed for that purpose.
4 Some preliminary results.
The instrument was deployed during CAMBIOS but unfortunately it transmitted only twice due to a failure on a batterie container connector. The container was flooded and no more energy were usable for the transmitting function. The 2 transmissions were recelved at 260 and 560km.
