Finding your way on the Red Planet
By Danielle Masterson
When the National Aeronautics and Space Administration (NASA) lands people on Mars in perhaps 20 or 30 years, it will need to be able to track the location of landing craft on the planet with much more precision than current technology allows.
For that purpose, NASA plans to install at Mars a program similar to the Global Positioning System that locates ground sites on Earth using satellites. But CAS Astronomy Professor Michael Mendillo says the data from such a system could be distorted by the Martian ionosphere. The upper part of the Red Planet’s atmosphere, like that of Earth’s, is electrically charged, which can cause radio signals passing through it to lose power.
“When a representative explained they were going to build this system around Mars, I thought to myself at the time, I wonder if they are aware of what the ionospheric effect would be,” he says. “I had to figure it out myself before I rang any bells.”
Mendillo (GRS’68,’71), in collaboration with three colleagues at the Center for Space Physics, Steven Smith and Jody Wilson, CAS senior research associates, and graduate student Carlos Martinis (GRS’05), as well as researchers at Stanford University and the California Institute of Technology, investigated the subject and recently published “Ionospheric effects upon a satellite navigation system at Mars” in the journal Radio Science. The research was funded partly by NASA.
“We studied observations made on both Earth and Mars, and how the ionosphere can cause errors in determining the precise location of positions,” recalls Mendillo. “We wrote this paper to say, ‘You guys who are planning a satellite navigation system should take this into account on Mars.’ This research shows how the ionosphere on both planets can vary from day to night and year to year,” mostly because of solar radiation at X-ray and ultraviolet wavelengths.
Marsnet, the GPS-like system NASA currently is developing for Mars, will consist of several satellites that orbit the planet and are able to pinpoint exact locations of robotic or manned landing crafts using radio signals. Mendillo says that Mars’ ionosphere, like Earth’s, could impede the radio waves, thereby hampering the system’s ability to determine locations.
“The size of the effect that causes the ionosphere to slow down radio waves, or the magnitude of the error, depends on the frequency of the radio waves,” he says. “If you use high-frequency radio waves, the error would be much less than a meter. At that level, it would not matter what the ionosphere does.”
Mendillo notes that high-frequency radio waves require a lot of energy, and scientists may use a lower frequency, which is “more economical and not as hungry for power. But the lower the frequency, the more important the ionosphere becomes. You shouldn’t just willy-nilly make it a low frequency without knowing how it will be affected by the ionosphere.”
He says NASA should explore the subject further if it considers using low-frequency radio waves in its positioning system. “You could measure the ionosphere and correct the information to the effect,” he says. “Observing the delay the ionosphere has on radio waves can tell you how robust the radio waves are.”
Mendillo’s report concludes that if the positioning system uses dual-frequency signals, the ionospheric issues will be avoided. “For either solution, it is important to realize that the transmissions themselves would provide a new and extremely valuable diagnostic of the Martian ionosphere on a global and continuous basis,” it reads, “providing critical input to ionospheric models and upper atmospheric science, as the current GPS system does on Earth.”
Mendillo, who has studied extensively the solar system and planetary atmospheres, says that he and his BU colleagues will follow up the Radio Science report with more experimentation. “We’re going to continue to analyze some new data from the ionosphere that comes from Mars with a NASA grant,” he says. “We also have another grant to do some modeling of Mars.”
He says the research should help scientists learn more about Mars’ ionosphere, “which is poorly understood.” Much research has been done on the Earth’s ionosphere, but little on Mars’. “We have to understand our own neighborhood first before we start to understand the other planets,” Mendillo says. “We are in the celestial neighborhood and just trying to understand our family of nine planets.”