The crossword solution for the term 'trackers, e.g.' is GEOS, which stands for Global Positioning System (GPS) Observation Stations. These stations are used to track and monitor the movements of GPS satellites in order to provide accurate location data. They are also used to measure the Earth's gravity field and to study the Earth's atmosphere. The data collected from these stations is used for a variety of applications, such as navigation, surveying, and mapping.