NovAtel's Annual Journal of GNSS Technology Solutions and Innovation

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Aces/geophones and Timing Advanced Instrument Development Group at NASA's Jet Propulsion Laboratory. JPL is developing one of the ground reference clocks. "The distortion in space-time due to the mass of the Earth has to be corrected in any atomic clock that is flown in space," Tjoelker continues. "So the orbit of the ISS entity eccentricity—or the variations of [the space station's] distance from the center of the Earth— cause shifts in clocks due to the theory of relativity." These shifts can be observed with atomic clocks and measured at very high levels of precision. To find the answers, however, researchers need to have a precise position for the clocks in orbit. The clocks are orbiting the planet at a certain known velocity, explained Helm. This velocity causes relativistic effects, and researchers can model these relativistic effects. "As an input parameter for that, we need to know the position and velocity of the ISS and this is done with a GNSS receiver," adds Helm. But that is not the only reason to include a GNSS antenna element—the NovAtel 703GGG—in the payload. The antenna is critical to ACES' secondary mission—proving that GPS signals can be used to do remote sensing. If successful, the new capability could be deployed on any low Earth orbit (LEO) satellite and used as an independent data source for information on rising ocean levels. "You measure the direct signal coming from the GNSS or the GPS satellite and then you compare it to the same satellite signal that has been reflected from the oceans' surface," explains Helm. "In the simplest way, you just take the run time. It's like a ping from a submarine," he says "I know where my receiver is and I know where the GPS satellite is—then I can calculate geometrically where the reflection point is. … (From that) I can deduce the height of the ocean." Though active radar altimeters like those on the NASA's Jason satellite provide higher ocean height resolution, says Helm, they are limited by the satellite orbit. "They cannot cover a large area in the short time (period) or the larger spa- For more Solutions visit http:/ / tial resolution." ACES might make it possible to improve the resolution of GNSS-based remote sensing. Many satellites will be generating multiple GNSS signals, and eventually many satellites could be carrying an ACES-inspired payload. Because a receiver for the GNSS signals would be passive, the antenna and the instrument package could be small and require little power. Such receivers could ride as a secondary payload on a variety of satellites. One of the missions of ACES is to prove the concept. That's where NovAtel's patented Pinwheel® antenna technology comes in. "The idea is that you get so many [GNSS range] measurements [that] you'll generate a lot of data," says Helm. "You get a large and timely spatial coverage of the ocean surface, which is very important to get in order to fit into numerical models to understand this interface between atmosphere and ocean." "The Pinwheel design is kind of unique because it is a wideband antenna with a very high phase center accuracy—a very tight phase center offset and phase center variation," said Gordon Ryley, NovAtel's product manager for antennas. "Because it is a wideband element, it pretty well tracks every available GNSS signal." The NovAtel 703-GGG antenna bound for the ISS also has high near-multipath rejection—on par with that used by surveyors, says Ryley. That is, the antenna can ignore the confusing signals that occur when the satellite signal bounces off a nearby surface, in this case the hull of the space station or its solar panels. The ACES project is using a commercial off-theshelf receiver, says Helm, "because only (commercial receivers) have the possibility, really, to use Galileo, GPS, and GLONASS—the newest signals. So we are working with the L1/E1, L2, and L5/E5a GNSS signals—with all three frequencies." With its ability to receive such a wide range of signals, the NovAtel antenna could eventually help researchers to follow changes in ocean currents and other features of the marine environment such as tsunami signatures. Atomic clock frequency instability & inaccuracy level which is the equivalent of 1 second in 349 million years The ACES fight model (FM) #02 GPS-703-GGG antenna element from NovAtel, taken when Astrium performed its incoming inspection of the part. Image credit: Astrium photo, 2012. 2013 velocity 11

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