ESA - European Space Agency patch.
October 2, 2020
Just as we navigate our way around Earth's surface using the connection between our phones and navigation satellites high above us, our missions use the very same satellites to navigate their way in space.
SMART-1 in orbit
To pinpoint a location accurately, a receiver – in our phones or on a spacecraft – needs to collect and combine signals from at least four navigation satellites. The receiver determines its distance from each of the satellites by measuring the time that it takes for the signal to travel from the satellite to the receiver.
Navigation satellites orbit 22 000 kilometres above Earth's surface. As they point in the direction of Earth, any spacecraft between them and Earth are served well by their signal. But around ten years ago, engineers started demonstrating that spacecraft outside the orbit of navigation satellites could also navigate in space using 'spill over' signal from the satellites.
Side lobe satnav signals
Navigation satellites aim their antennas directly at Earth. Satellites orbiting above the navigation (GPS in this image, but Europe's own navigation system is Galileo) constellation could only hope to detect signals from Earth’s far side. Now spacecraft can make use of signals emitted sideways from navigation antennas, within what is known as ‘side lobes’. Just like a torch, they shine energy to the side as well as directly forward.
Then in 2012 two Discovery & Preparation studies explored a seemingly radical question: could this spill over signal even be used to navigate our way around the Moon, and if so, what kind of receiver would we need to build to be able to use these signals?
The studies were very successful, finding that indeed, the signal from navigation satellites orbiting Earth could be used to navigate the Moon's surface. But with the signal being so weak, they found that a new type of receiver would need to be built, and at the time there was no clear application for this.
Fast-forwarding eight years, and ESA has invested in the development of such a receiver, and is exploring whether it could be demonstrated on the Lunar Pathfinder mission. ESA is collaborating with Surrey Satellite Technology Ltd and Goonhilly Earth Station on this mission, which will provide exciting new opportunities for science and technology demonstration. In particular, it will help lay the groundwork for providing navigation services around the Moon, currently studied through two ESA NAVISP activities and culminating in the Moonlight initiative.
Lunar Ride and Phone Home Service
"We have now accurate simulation results that show that navigation signals may be used at Moon orbit and provide good performances," adds Dr Javier Ventura-Traveset, Head of the Galileo Science Office and in charge of coordinating all GNSS Moon activities for ESA's Navigation Directorate. “And with an innovative receiver in Lunar Pathfinder, we could have the first ever experimental evidence of this. This is exciting!
"Furthermore, we are also studying how existing navigation constellations may be complemented by additional Moon-orbiting satellites, providing additional ranging signals for an optimal navigation service including Moon landing and Moon surface operations. This is being done as part of the ESA NAVISP program and through the ESA Moonlight initiative."
"The Discovery & Preparation studies have been eye-openers and they are currently being followed up by a NAVISP activity aiming to develop the highly sensitive spaceborne navigation receiver planned to fly on board Lunar Pathfinder," notes ESA Radio Navigation Engineer Pietro Giordano. "This technology will enable improved performances and much more cost-effective ways to navigate and operate missions to and around the Moon."
It is thanks to the pioneering Discovery & Preparation studies that ESA was confident enough to invest in the new receiver. This success story demonstrates the importance of investigating in blue sky research where real-world applications are not immediately apparent. Discovery & Preparation specialises in such research and is therefore pivotal in laying the path for ESA’s future activities.
Related links:
ESA NAVISP: https://navisp.esa.int/
Moonlight initiative: https://www.esa.int/ESA_Multimedia/Images/2020/07/Moonlight_logo
Lunar Pathfinder: https://www.esa.int/Science_Exploration/Human_and_Robotic_Exploration/ESA_signs_collaboration_agreement_for_commercial_Lunar_missions
Surrey Satellite Technology Ltd: https://www.sstl.co.uk/
Goonhilly Earth Station: https://www.goonhilly.org/
Discovery and Preparation: https://www.esa.int/Enabling_Support/Preparing_for_the_Future/Discovery_and_Preparation
SMART-1: https://www.esa.int/Enabling_Support/Operations/SMART-1
Images, Animation, Text, Credits: ESA/SSTL.
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