Radar is finicky. It is extraordinarily useful for a multitude of tasks, but testing it for some particular tasks is complicated since almost everything interferes with it. That challenge is particularly acute when testing an antenna that is supposed to be used in space, which is why a team from the SENER engineering group in Spain decided to take a novel approach to testing the radar antenna the European Space Agency (ESA) plans to use for EnVision – they suspended it from a balloon.
EnVision is ESA’s mission to Venus that plans to explore everything from the planet’s atmosphere to its core. One of the tools used to do that is the Subsurface Radar Sounder (SRS) – a radar antenna that will bounce radar signals off of Venus’s surface to scan what lies beneath. Specifically, SRS is designed to look for buried craters, structures of tesserae (very deformed sections of terrain), and building three-dimensional images of features beneath the surface of the planet.
SRS isn’t the first instrument to do this either – it is borrowing technology from systems such as RIME, which is planned to launch onboard JUICE, and SHARAD, which first launched on MRO. These predecessor antennas had their own trials and tribulations with testing; however, SRS’s frequency is much lower due to the difficulty in peering through Venus’ clouds. It will operate around 9 MHz, with a 6 MHz bandwidth, which would allow it to penetrate Venus’ surface to a depth of around 600 meters, with a 16-meter resolution. But with lower frequencies comes larger antennas, so the SRS itself is designed to be 16 meters (52 ft) across.
That’s the size of a bus and way too large to fit into the typical test facilities usually used to test this kind of antenna. ESA even has a dedicated Hertz chamber, which would typically be used, but also can’t test at the low frequencies SRS plans to operate at.
Without being able to test inside a dedicated chamber, testing inside at all would be too noisy to collect adequate data. Even outside, the ground itself could interfere with the readings. So, instead of launching a prototype antenna all the way into space, engineers working on the antenna came up with a novel solution.
They attached the prototype antenna to a balloon and had it stay stationary 200 m in the air. That was far enough away from the ground that its interference was minimized, and the balloon itself didn’t cause any interference. However, the test engineers, including Paul Mosley from ESA, also angeled the antenna specifically to point it at the horizon so it wouldn’t collect any stray data from bouncing off the ground.
It also resulted in a photo opportunity that looks like something out of the latest Mission Impossible movie. The contrast of the balloon with a mountain range as a backdrop is striking. It is also holding aloft what appears to be a large box but is a full-size mockup of EnVision itself, with the antennas spread above and below it in a ladder pattern.
SRS isn’t the only sensitive instrument that will be launched with EnVision. And EnVision itself isn’t even the sole new mission heading to Venus in the coming years. NASA will be launching two missions that will join with it – DAVINCI+ and VERITAS. VERITAS’ mission will partially overlap with the data collected by the SRS, as it uses a synthetic aperture radar to survey the surface of our sister planet.
ESA – Aerial antenna for Venus mission test
EnVision – ENVISION/SRS
UT – ESA is Joining NASA With Their own Mission to Venus
Balloon mounted EnVision mock up to test its 16 m antenna.
Credit – SENER