*4.2. Data Acquisition*

Except for the GT-2A gravimeter and JAVAD GNSS, the other airborne instruments are synchronously integrated by the Environment for Linked Streams Acquisition (ELSA) system [18], which was developed by UTIG. All integrated instruments can be controlled and monitored and their data are received and recorded by ELSA. ELSA also allows operators to manage data collection and check data quality in real-time on all flights. The system integration relies on an accurate time stamps (10 μs) calibrated with real-time GPS data. Data streams from di fferent instruments with di fferent sampling rates are received by ELSA along with the time and then written into two data files with one for radar data and the other for data from the other instruments [18]. The data files are recorded to a solid-state disk by ELSA. The gravity data are acquired and recorded separately into a USB flash disk by GT-2A's central data unit.

Flight notes are filled out, noting the times of engine start, taxi, takeo ff and other major events. An instrument checklist is filled out every 30 min by FOP members and known or suspected aircraft or instrument anomalies are documented carefully. A base magnetometer and two GPS base stations are operated during each flight; at the end of each airborne survey, all media and flight notes are secured in weatherproof cases and delivered to BOP for data processing.
