Remote sensing instrumentation: devices that emit electromagnetic radiation of a specified wavelength or band to actively illuminate the Earth's surface are called what?

Introduction / Context:
Remote sensing systems are categorized as passive or active based on whether they rely on external illumination (e.g., the Sun) or generate their own radiation (e.g., radar, lidar). Recognizing this distinction is foundational for understanding data characteristics and use cases.

Given Data / Assumptions:

• Devices provide their own electromagnetic illumination at specific wavelengths or bands.
• Earth's surface is observed by measuring backscatter or return signal.
• Examples include synthetic aperture radar (microwave) and lidar (laser).

Concept / Approach:
Active sensors transmit energy and record the returned signal, enabling night-time and all-weather observations (especially in microwave). Passive sensors record naturally emitted or reflected radiation and cannot be described as 'providing illumination'.

Step-by-Step Solution:
Match the definition: devices that illuminate the target must be active.Eliminate 'passive sensors': they do not emit, they only detect.Eliminate 'sensonrs': misspelling and not a category.Therefore, select 'active sensors'.

Verification / Alternative check:
Radar altimeters, scatterometers, and lidars are canonical active systems in Earth observation.

Why Other Options Are Wrong:

• Passive sensors: rely on solar or terrestrial radiation, not on-board illumination.
• 'sensonrs': not a valid technical term.
• None of these: incorrect because 'active sensors' is the correct category.

Common Pitfalls:

• Assuming all optical imagers are passive; many lidars operate in optical wavelengths but are active.

Final Answer:
active sensors