Indian Antarctic Programme: Identify what was installed during the Eleventh Indian Antarctic Expedition (Nov 1991 – Mar 1992). Focus on the scientific instrument commissioned in that season. Choose the correct item installed.

Given data

• Expedition: Eleventh Indian Antarctic Expedition, Nov 1991 – Mar 1992.
• Task: Identify what was installed.

Concept/Approach
Contemporary records of India's Antarctic activities attribute the installation of a SODAR system during the Eleventh Expedition to study lower-atmosphere wind and turbulence profiles.

Reasoning
• SODAR uses acoustic pulses and echo profiles to infer boundary-layer structure. • The permanent stations 'Dakshin Gangotri' (earlier) and 'Maitri' (commissioned mid-late 1980s) do not match the 1991–92 installation event.

Common pitfalls
Assuming a station was newly established in 1991–92; station work in that season focused on instrumentation like SODAR rather than inaugurating a new base.

Final Answer
SODAR (Sonic Detection and Ranging) instrument