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What is a Radar exactly?

How will the Radar work?

Artistic concept of EnVision in orbit around Venus
Figure 1 : Artistic representation of EnVision in orbit around Venus, showing the relative orientation of the radar antennas

RADAR (RAdio Detection And Ranging) used commercially since the beginning of the 20th century and in space since the 1970s to detect, track and map.
Radar systems are peculiar as they actively send electromagnetic pulses and analyse the echo. This places additional power constraints but boasts the incredible advantage of being able to obtain scientific data regardless of ambient illumination, night/day side, weather phenomena and other perturbations. 
Radar wavelength has the fundamental benefit of being able to penetrate the thick Venusian Atmosphere with low attenuation or scattering. Compared to UV, Optical or IR measurements made by Venus Express that were particularly adapted to sounding the different layers of the atmosphere of Venus, EnVision will complement these findings by concentrating on the surface and subsurface.
As proposed, EnVision will actually deliver two complementary Radar systems:

Focus on SRS:

SHARAD subsurface radargram from Mars

Figure 2 : SHARAD radargram over a portion of western Medusae Fossae Formation, a low density pyroclastic deposit spanning across the crustal dichotomy of Mars. The deposit labelled "North Hill“ (nh) is about 500 m thick [Carter, L. M., and 12 colleagues, (2009). Shallow radar (SHARAD) sounding observations of the Medusae Fossae Formation, Mars. Icarus, 199(2), pp.295--302]

SRS will focus on mapping the vertical structure of geologic units by exploring the subsurface properties of tesserae, plains, lava flows and impact debris. SRS also provides information on the surface in terms of roughness, composition and dielectric properties at wavelength completely different from those of VenSAR, thus allowing a better understanding of the surface properties. Moreover, a fusion of the InSAR data (both intensity, topography and displacement variables) with the sounder data would result in an exceptional capability to understand the link between the surface and subsurface processes on Venus. Depending on the final design of the system, the sounder could also acquire measurements for characterizing the ionosphere.

Focus on VenSAR:

The antenna measures 5.5m by 0.6m, weighs 130kg and boasts 24 phase centres for sending the signals and receiving the echoes. Wave front forming allows the beam to be steered without requiring moving mechanisms making for a more durable but also responsive and precise system. It is EnVision’s prime payload in terms of mass, power consumption and system level constraints as well as data output.

Magellan Stereo SAR of Venuschange interferograms pass to pass from Earth Observation satellites

Figure 3 : (left) Processed Magellan data to create stereo views of Venus landscape. Because VenSAR is designed from the ground up to operate in this mode, the resolution and subsequent scientific value will be game changing. (right) Example of change interferograms overlay routinely produced by Earth Observation satellites [Meyer, Sandwell].

Details of the three operating modes are summarized below, see Figure 4 for the resulting mapping strategy:

  1. StereoSAR (A) Swaths overlap after 12 passes (M)
  2. Data from the far swath of pass A and the near swath of pass M provides image of the same area under two different look angles providing a sense of depth very valuable for interpretation. This mode will provide topography and altimetry of the entire planet, computing digital elevation models at 20 m resolution.

  3. InSAR (B) & (C) pass-to-pass InSAR
  4. Pass to pass technique combines de data acquired from two consecutive orbits. From the swath overlap change interferograms are produced.  Decoherence data at 20 m resolution from up to one third of the surface is planned. Vertical surface displacements down to a few cm will be observable.  Pass to pass technique allows for the first results to be obtained within weeks, not month of beginning of science operations. To uncover longer term changes, all raw data is archived on the ground and further post processing combines acquisitions of different mapping cycles (a cycle is one Venus day or 243 days Earth days).

  5. HiRes/Multipolar (D) programmable imaging
  6. Extreme resolution (<5 m) for high priority targets (up to 10% surface). It is also possible to alternate between horizontal and vertical polarization, helping differentiate the chemical properties of surface minerals.
    Acquisitions can be targeted by ground control commands but also through autonomous on-board response based on VEM data, and passive emissivity data. This is especially useful if VEM detects the signature of a volcanic eruption. High resolution imaging will help confirm the discovery and repeat imagery will provide a complementary wealth of information on the on-going process. 

Mapping strategy of EnVision at Venus
Figure 4 : Mapping Strategy to alternate InSAR modes in each consecutive orbit as Venus slowly rotates underneath. Nadir pointing SRS is shown in brown. Orange spotlight represents field of view of Venus Emissivity Mapper.


Related questions:
What world-wide firsts will EnVision accomplish during its mission?
What is the legacy of Venus Express?
Is EnVision technically feasible?