2 Principle of measurements

Although the Generalized Seeing Monitor was the main instrument used in this campaign, we devote little space to its description, which can be found in our previous works.

2.1 Principle of the GSM

In its initial version GSM is presented in Martin et al. ([1994]). The actual instrument is described in Martinet al. ([1998b]) and Tokovinin et al. ([1998b]). Description and performance analysis are also given in Ziad et al. ([1999]). We refer the interested readers to the reports on the previous GSM missions (Martin et al. [1998a] and Tokovinin et al. [1998a]).

The GSM instrument consists of four 10-cm telescopes with their individual image analysis devices to sense the angle of arrival (AA) fluctuations in one direction, called modules. The telescopes are installed on three equatorial mounts; two modules share the same mount and are working as the ESO DIMM. At a distance of 1 m to the south of these modules a third one is installed on a separate mount. The fourth one, with its mount, is located at 0.8 m to the east. The same L-shaped configuration has been used in the previous GSM missions except at Maydanak site.

GSM modules and their mounts were installed on top of 3 concrete pillars, with the height of telescopes 1.7 m above ground. From 3 sides the experiment was surrounded by a wind-protective enclosure consisting of a 2 m high net.

Atmospheric AA fluctuations are measured by each module in the declination direction, with 5 ms time resolution. A typical measurement consists of a 2 mn long data acquisition. It is processed immediately, providing the estimate of the Fried coherence diameter r₀ (from differential AA fluctuations in modules 1 and 2), outer scale ${\cal {L}}_0$(inferred from the covariances of AA and computed via the Von Karman model for the phase spectrum of the turbulent atmosphere), and isoplanatic angle $\theta _0$(from scintillation index).

The ESO DIMM seeing monitor (Sarazin & Roddier [1990]) which operates at the Paranal site continuously was located at a distance of the order of 10 m from GSM. It is installed on a 6 m high tower, in order to avoid the near-ground turbulence effects as much as possible.

2.2 Temperature micro-fluctuation measurements in the surface layer

Turbulence sensing in the SL has been undertaken in order to gain a better understanding of the turbulence effects near the ground. For balloon measurements of the vertical turbulence profiles,very sensitive temperature sensors have been developed in our laboratory. Each probe consists of two thin-wire sensors mounted at a separation of 1 m on a rod. The structure constant of temperature fluctuations, $C_{\rm T}^2$, is calculated from the dispersion of the temperature difference between the pair of sensors. The time resolution of the micro-thermometers with their electronics and transmitting devices is about 5 ms and an integrated value of the structure constant is transmitted to the ground each 1.5 s. The typical noise level can reach a few 10^-8 (Celsius degree)². The refractive index structure constant C_n² is calculated from $C_{\rm T}^2$ using the appropriate values of mean temperature and pressure. C_n² values are obtained with a precision around 10^-20 m^-2/3. Previous results obtained with this equipment can be found in Vernin & Muñoz-Tuñon ([1992]).

Four pairs of sensors were installed on the meteorological mast at the heights of 3, 7, 21, and 31 m above the base of the mast. The upper sensor, however, was damaged, and only the remaining 3 have provided valid data. It must be noted that the base of the mast was somewhat lower (by 1 m) than the ground level of GSM/DIMM location. Hence the sensors at 3 and 7 m are at the same heights as GSM and DIMM, respectively.