(ORDO NEWS) — A test version of the unique satellite navigation receiver was delivered for testing on the Lunar Pathfinder spacecraft.
“This engineering model of the NaviMoon receiver is the very first instrument to be produced as part of ESA’s Luna initiative to develop dedicated telecommunications and navigation services for the Moon,” explains Javier Ventura-Traveset, Head of ESA’s Science Office for Navigation and leader of all activities. ESA on lunar navigation.
“It will be carried aboard the Lunar Pathfinder spacecraft into orbit around the Moon, from where it will perform the farthest navigation fix ever made, over 400,000 km away, with an accuracy of less than 100 m.
This represents an extraordinary engineering challenge, because at such a distance the weak Galileo and GPS signals it uses will be barely visible against the cosmic noise. This demonstration will mark a real paradigm shift for lunar orbital navigation.”
The washing machine-sized Lunar Pathfinder is being built as a commercial mission by Surrey Satellite Technology Ltd, SSTL, in the UK. ESA is funding this system, including a 1.4 kg NaviMoon receiver that will be placed next to the spacecraft’s main X-band transmitter linking it to Earth.
“Getting physical equipment for a mission is always fantastic,” says Lily Forward, systems engineer at SSTL. This engineering sample of the receiver will be integrated into our version of the FlatSat Test Bed mission to test the communications and correct operation of all of our systems before receiving a flight sample of the receiver and antenna later this year.”
This will be SSTL’s first full off-Earth mission, she adds: “Laying the foundation for the many science missions that will come after, Lunar Pathfinder is a communications relay satellite designed to serve assets on both the near and far side.
It will move in an “elliptical lunar orbit” for extended coverage of the South Pole – a special focus for future exploration. Then, at regular intervals, we will orient the spacecraft towards Earth to test the NaviMoon receiver.”
Satnav position fixing by the receiver will be compared to conventional radar by the Lunar Pathfinder X-band transmitter, as well as laser ranging by a retroreflector provided by NASA and developed by KBR.
“This will be the first time these three measurement methods have been used together in deep space,” explains ESA navigation engineer Pietro Giordano.
There is a long legacy of lunar laser ranging going back to the Apollo missions, and the retroreflector we are using is an evolution of NASA’s Lunar Orbiter.” The combination of all measurement techniques will further improve orbit estimation, potentially surpassing the capabilities of radio sounding.
“In principle, this could mean that future missions could autonomously navigate to the Moon using only satellite navigation signals without assistance from the ground.”
Searching for Ultra-
Weak Satellite Navigation Signals The satellite navigation signals used on Earth are already vanishingly weak, equivalent to one pair of car headlights illuminating the whole of Europe. By the time these signals reach the Moon, they have traveled more than 20 times as far, fading out in space like ripples from a stone thrown into water.
“In addition, satellite navigation constellations are not designed to transmit a signal into space, they keep their antennas pointed at the Earth,” adds Pietro. Therefore, we rely on much weaker “side-lobe” signals, like light pouring from the edges of a flashlight. To be able to use these signals, we turned to a satellite navigation specialist whose signal processing techniques turned out to be a truly magical ingredient.”
SpacePNT, based in Switzerland, oversaw the development of the NaviMoon receiver. in 2013 as a scientific challenge,” explains Cyril Botteron, head of the company.
“Combining Galileo’s dual-frequency signals with those of existing GPS satellites is what made this idea feasible. Although, along with the extreme sensitivity required, another big problem is that from the Moon, all satellite navigation satellites are in the same narrow geometry of the sky around the Earth , periodically rotating out of sight”.
SpacePNT’s solution draws on more than half a century of lunar exploration experience. The company installed in the receiver a dynamic software model of all the forces acting on the satellite, including the gravitational influence of the Moon, Earth, Sun and planets, as well as the very slight pressure of the sunlight itself – the pressure of solar radiation – along with factors such as the direction of the radio signal.
Cyril explains: “By the amount of acceleration we experience, the receiver can tell that it is most likely at a certain point in its orbit. Normally, a satellite receiver needs signals from four satellites to fix its position, but with this approach, even less than four signals is still enough to provide useful information, limiting the model to minimize any errors.”
The British company European Engineering & Consultancy, EECL, was tasked with turning the SpacePNT project into fully tested hardware, as well as developing a critical low-noise amplifier that sifts noise to amplify useful signals.
“The amplifier is a high performance dual-frequency satellite navigation diplexer, hand-tuned with the finest components and heat sink technology to further reduce unwanted noise,” says Ben Keniewicz, founder of ECCL.
“Along with being involved in other aspects of the design, we also built, tested and delivered the receiver to SSTL using our space-friendly clean room for assembly and testing.”
Lunar Pathfinder will be ready for launch in late 2024, offering near, far, orbital and polar side services for missions launched in the coming years, laying the foundation for a constellation of interconnected telecommunications and navigation satellites around the Moon.
“Our lunar initiative involves the initial deployment of three to four satellites in lunar orbit, which will provide service for at least five consecutive hours during the day, focusing on the south pole of the moon, where most missions are originally planned,” adds Javier.
“Our system is designed to be expandable, and the idea is to gradually increase the constellation, and most likely to also include surface beacons on the Moon. This will provide full coverage of the lunar surface, higher availability and excellent accuracy.”
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