In astronomy, larger distances are both a blessing and a curse. They can cause issues like longer communication times, which also requires more powerful equipment, and positioning uncertainty that can affect the outcomes of measurements, especially in the outer reaches of the solar system. However, they can also be useful for a specific type of measurement called interferometry, where two systems a far distance apart can provide accurate location measurements to a third system – the same principle that GPS uses. A new paper looks at potentially using the same technique to track deep space probes rather than cars on a freeway and finds that, while it is around the same accuracy level, it is able to provide that same location data for more than double the amount of time.
The key to this system is the use of geostation satellites. Called the Radiometric Interferometry for Deep Space Navigation using Geostationary Satellites (RINGS) concept, it uses satellites in geostationary orbit to provide location data to deep space probes throughout our solar system, rather than the traditional Very Long Baseline Interferometers (VLBIs) located on Earth itself that typically provide location data to deep space probes.
On paper, RINGS has several advantages over traditional VLBIs. The distance between the contributing satellites is an order of magnitude larger than any VLBI located on Earth – 80,000 km compared to 8,000 km. A larger distance between the base stations of an interferometer means more accurate location calculations, which is why the VLBIs on Earth are so far apart to begin with.
RINGS’ GEO satellites also don’t have to contend with atmospheric interference, which can distort the phase of the radio signals used to convey distance data. But perhaps its greatest advantage is that RINGS’ uptime is 98%, whereas a VLBI on Earth is affected by the planet’s rotation, making its uptime to any given location in the sky closer to 49.7%
However, RINGS does have some disadvantages of its own. One is a Doppler shift between the spacecraft itself and the motion of the satellites that make up the location measurement system. On Earth, the components of the VLBI will be stationary compared to one another, but in space, the two GEO satellites can drift either farther apart or closer together, confounding the practicalities of utilizing them for phase measurements, a critical component in location calculations.
Another issue is the stability of the clock in a GEO satellite. On Earth, there are no weight constraints, so many VLBI base stations use a hydrogen maser – a highly stable form of clock capable of extremely accurate timekeeping. GEO satellites, on the other hand, typically rely on Rubidium clocks that have an average drift of one picosecond every thousand seconds. That may not seem like much, but over very long periods of time, that drift can cause a significant discrepancy between the clocks of the three systems in the RINGS structure, introducing further error.
But perhaps the most difficult part of RINGS is accurately predicting where the GEO satellites themselves are. The positions of VLBI base stations on Earth are well known and understood down to a few centimeters. However, the position of GEO satellites are only known within a few 10s to 100s of meters. This doesn’t affect measurements like GPS too badly, but once the distances start to become a few AU, that uncertainty can start to add up. The paper’s authors, based at the Technion in Haifa, suggest there are improvements to bring that uncertainty down to .5 m. That’s a big assumption, but otherwise the accuracy of the entire system would be blown out of the water, and it does seem feasible from a physics standpoint, though it would require further work.
Ultimately, with those assumptions, the accuracy of RINGS is within the same order of magnitude as terrestrial VLBI systems, though slightly worse overall. However, it does still have the advantage of being available 98% of the time. Whether or not that is an attractive enough proposition to convince an organization to spend the time and effort to launch a system of GEO satellites specifically for deep space tracking remains to be seen, but now at least, we have some technical guidance on what that system would look like.
Learn More:
M. Golani, Y. Rozen, & H Rostein – Radiometric Interferometry for Deep Space Navigation using Geostationary Satellites
UT – One of NASA’s Radio Dishes Can Now Track Space Lasers Too
