Future explorers of Mars will always need a way to know where they are, regardless of whether they're rover automatons or flesh and blood humans.

To do that, NASA researchers and scientists alike have been studying the requirements for a potential global positioning satellite (GPS) system around Mars that could also function as a communications network. Their vision is a small flotilla of Mars spacecraft conducting their own science while watching over future robotic or human expeditions, then relaying data back to Earth.

"The science requirements for communications and accurate position fixing are important needs," said Michael Mendillo, an astronomy professor with the Center for Space Physics at Boston University, in an e-mail interview.

Mendillo and a team of researchers studied the effect of Mars' ionosphere would have on a potential satellite navigation system around the planet. The research, which appeared in a recent edition of the journal Radio Science, is part of a broader effort by scientists to develop the satellite infrastructure for an anticipated fleet of rover missions to Mars in upcoming years.

Researchers at NASA's Jet Propulsion Laboratory (JPL), for example, have been laying the groundwork for a Mars Network for navigation and communication.

Previous network designs called for a constellation of small microsatellites called Marsnet to serve Mars explorers while a larger spacecraft, Marsat, would relay data between the planet and Earth.

But current studies expect the system to grow gradually from piggybacked services aboard future science spacecraft to a dedicated platform, such as the anticipated Mars Telecommunications Orbiter set for 2009 - the first satellite specifically designed to facilitate communications with another world.

"Across all those satellites, we'll be able to do positioning [with accuracy] from 10 to 100 meters," Charles Whetsel, chief engineer for the Mars Exploration Program at JPL, told SPACE.com. "So that's comparable to degraded GPS system."

The MTO mission in particular would be the network's centerpiece, working as a Mars-Earth communications hub between satellites and rovercraft, as well as an orbital watchdog for new incoming probes and any outgoing sample return spacecraft.

Red planet roving today

A dedicated Mars GPS system would be a boon for future robots bound for the red planet, allowing them to drive farther without having to stop and check their distance with photographic records.

The Mars Exploration Rovers (MER) Spirit and Opportunity currently on the planet do just that, relying on a "move first and ask questions later" approach.

"We call it a visual odometer," explained Mark Maimone, MER mobility engineer at JPL, adding that each rover uses images from its navigation cameras to track. "It updates the rover's wheel odometer by finding identifiable landscape features in images and tracking them after a drive."

But the process takes up a lot of the rover computer's processing power and can take a long time to complete - about a few minutes per step - during multiple communications periods with Earth.

"Having a Mars GPS would get us to targets faster, instead to taking several communications cycles," Maimone said.

Researchers have also been able to pinpoint changes in rover positioning by studying the Doppler shift in its radio frequency during communications with orbiting spacecraft. Experiments between the orbiting Mars Odyssey, Spirit and Opportunity have proven to be accurate down to about 32 feet (10 meters), researchers said.

That sort of positioning is most likely to be the system of choice for future missions, especially since the key to GPS is multiple assets in orbit, with four satellites required to determine an object's three-dimensional location and time on Earth.

"That's a luxury we won't soon see at Mars with that " said Chad Edwards, chief telecommunications engineers for NASA's Mars exploration program.

In addition to Mars Odyssey, there are currently only two other orbiters around the red planet - Mars Global Surveyor and Mars Express -- with robust communications and Doppler abilities and its unlikely in the near future that four such craft would always be available at any given time, he added.

Navigation and science missions

Earth's GPS satellite constellation not only provides navigation for humans below, but also allows researchers to use its transmissions to probe the ionosphere, Mendillo said, adding that a Mars system could to the same at the red planet.

At the uppermost level of the atmosphere, the thin ionosphere is home to charged particles, auroras and the edge of space. That layer imposes a delay on radio transmissions that researchers can then use to determine the global structure of a planet's ionosphere.

Future missions capable of investigating the Martian ionosphere on a continuous basis would be a "major advance for Solar System atmospheric science," Mendillo said.

In the meantime, NASA officials are ramping up for their next foray to the red planet, the Mars Reconnaissance Orbiter mission expected to launch in 2005. That spacecraft, aimed at taking the most detailed orbital look at Mars to date, is scheduled to be followed by the Phoenix lander in 2007 and the Mars Science Laboratory rover mission in 2009.

"So this infrastructure is key to the future of Mars exploration," Edwards said, adding that future Mars navigation and communications satellites will boost the amount of data sent Earthward for researchers and scientists alike. "It's critical as a support for more ambitious science missions, and for the notion sharing exploration with the public."