What is a Radar exactly?
How will the Radar work?
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:
- A highly capable S-Band Interferometric Radar: it is able to operate in three distinct modes namely StereoSAR, InSAR and HiRes. Alternative modes are successively used during consecutive orbits to allow different measurements to be made as Venus rotates underneath the spacecraft. Synthetic aperture technique uses the spacecraft’s forward motion to effectively duplicate the resolution of a much bigger antenna.
- SRS (Subsurface Radar System): it operates at a few MHz through a deployable dipole antenna. The wave energy is not completely reflected by the surface interface but can actually penetrate a few hundred meters. Changes in underground dielectric or mechanical properties generate successive echoes that can then be interpreted as buried lava flows, bedrock, faults etc. Global coverage of Venus during nominal mission to resolution of 1km (along track) 5km (cross track) and 20m vertical will be provided.
Focus on SRS:
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.
- StereoSAR (A) Swaths overlap after 12 passes (M)
- InSAR (B) & (C) pass-to-pass InSAR
- HiRes/Multipolar (D) programmable imaging
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.
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).
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.