Overview

For a technology that measures millimetres and nanoseconds, it might seem counterintuitive that GNSS can support cutting-edge science on the planetary-and even interplanetary-scale.

But with the aid of two types of NovAtel antenna designs, that is happening today. After all, a GNSS receiver is helpless without the antenna element that pulls satellite signals into its digital processing channels.

The Atomic Clock Ensemble in Space (ACES), a payload destined for installation on the International Space Station (ISS), will use two exceptionally accurate clocks along with GNSS receivers coupled with a NovAtel antenna for a variety of experments, including a test of the theory of relativity.

Back on Earth, an innovative dual-antenna design is making it possible to map subterranean formations thousands of metres below the surface without ripping up the surface ecosystems.

Keeping Up with Einstein

ACES will use two atomic clocks-PHARAO, a cesium cold-atom clock developed and provided by CNES, the French space agency, and a space hydrogen maser (SHM), which is being contributed by the Swiss. The time kept by the clocks whirling by in orbit will be compared with that maintained by laboratory clocks on the ground to try and detect any differences.

The idea is to test the relativity theories of noted physicist Albert Einstein, says Dr. Achim Helm, the project integration manager of the project for Germany's Astrium Space Transportation, which is building ACES for the European Space Agency (ESA).

“Time is one element that you can measure with very, very high precision, and now the clocks are in a range where we can really see and check whether Einstein is right,” says Helm. “We (also) want to look to see if some physical constants really are not constants. If we can prove this, then physics has to be rewritten.”

“These clocks are so sensitive that they can determine their position in space-time,” explains Robert Tjoelker, supervisor of the Frequency

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 703-GGG-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 spatial 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-the-shelf 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.