The phrase "an observer that" is correct and usable in written English.
It can be used when referring to a person or entity that observes or watches something, often in a scientific or analytical context.
Example: "An observer that records data during the experiment can provide valuable insights into the results."
Alternatives: "a watcher who" or "a spectator that".
As you mention, the time measured by two observers in two different reference frames depends on their relative speeds; in short, an observer that is observing an event in a reference frame moving at speed v will measure the time in that frame to move slower than in her frame by a factor equal to the square root of (1- v/c)2), where c is the speed of light.
Estimates of the states are obtained from an observer that uses the measurements obtained by the tomographic sensor.
The high gain strategy allows the design of an observer that handles the uncertain part of the system.
The kernel of the integrated system consists of an observer that estimates the motion state of the mechanical structure.
This model is used to design an observer that estimates the mechanical quantities from the measured electrical quantities.
The purpose of the former problem is to design an observer that guarantees the asymptotic stability of the estimation error dynamics.
With information on upper and lower bounds of the uncertain parameters, an interval observer that produces an envelope covering all possible state trajectories is presented.
The proposed solution is based on the backstepping control and an adaptive observer that estimates both the (unknown) catenary parameters and the system state.
Then, to tackle this design task, we begin by constructing an adaptive observer that allows estimating both the system state and the unknown catenary equivalent stiffness.
In other cases, the camera is an impartial observer that can help determine which party was at fault.
Here we developed an ideal observer that estimates the positions of cues and also whether they have a common cause.
