The Electra-fication Of Deep Space

A team at NASA’s Jet Propulsion Laboratory (JPL) is using the agency’s first software-defined radio (SDR) system in deep space to communicate with spacecraft studying Mars.

The system, dubbed Electra, is a fully redundant UHF relay solution aboard the Mars Reconnaissance Orbiter, which was launched in 2005 and is expected to operate until 2010. In addition to its SDR activity, the spacecraft will perform the first NASA tests of Ka-band transmissions from deep space as well as provide two-way X-band traffic.

In a nutshell, Electra marks the start of a new era in deep space, and the satellite industry will want to monitor Electra’s progress for a variety of reasons.

Electra’s use of SDR and an adaptive data rate algorithm which allows radios to autonomously configure the link data rates in real time as channel conditions change is a true 21st century advancement in deep space. Gone for good is the possibility that an adverse signal-to-noise ratio might disrupt or impede communications over links that must operate flawlessly on a daily basis millions of miles away. This give overall mission performance a big boost in the process, and the option to incorporate newer encoding techniques as well as to transmit and receive arbitrary complex modulations — all reprogrammable in flight — makes Electra a standout in all respects.

While tests of the relay capabilities have been ongoing, Electra will begin operating in permanent relay mode once the Phoenix lander prepares to touch down on the surface of Mars in May, says Tom Jedrey, NASA’s manager for the Electra payload. Phoenix’s ultimate destination is the Red Planet’s northern polar region, where Phoenix will become NASA’s first mission to actually probe the layer of ice that lies just beneath the surface of Mars. Phoenix will perform water analysis, including a search for any microbial presence along with weather monitoring.

Electra will play an important role in Phoenix’s entry, descent and landing operations. Using UHF-based open-loop recording, the entry, descent and landing transmissions from Phoenix will be sent to Earth for demodulation. Post landing, Electra will be used to command Phoenix and will collect telemetry data at a rate of 128 kilobits per second each time the Orbiter passes over the Phoenix landing site, says Jedrey. Phoenix will have no direct communications link to Earth.

While JPL awaits the completion of Phoenix’s journey, Jedrey points out that the Electra SDR toolset already has solved one big problem on the Orbiter: excessive instrument-generated interference in Electra’s receive bandwidth that could disrupt the Orbiter’s instrument operations. However, new signal processing techniques involving filtering and scaling have been uploaded to Electra that enable relay communications without interfering with spacecraft operations.

“Having this reprogrammable Electra radio at the heart of the relay infrastructure around Mars gives us both the ability to infuse new protocols and new coding strategies and to deal effectively and quickly with unanticipated mission scenarios,” says Chad Edwards, chief telecommunications engineer for the Mars program at JPL.

Not only can Electra relay data in support of Mars surface operations, but by using its open-loop recording capability, Electra can listen in on UHF transmissions from the Mars Odyssey orbiter near the time that the raypath between Odyssey and the Orbiter spacecraft is occulted by Mars and then relay that data to Earth where it is analyzed to evaluate surface properties and atmospheric properties. This form of remote monitoring can occur anytime one orbiter rises above or sets below the Martian horizon as viewed from the other orbiter.

The Electra system weighs 5 kilograms, and the team has reduced this to 3 kilograms for another system that will be used aboard the Mars Science Lander, which is scheduled to arrive at Mars in 2009. In preparation for this mission, a new software and firmware load will be sent to Electra to allow the two radios to communicate as efficiently as possible.

“Not only will this maximize the data return from [the Mars Science Lander], it will also reduce the operations load significantly,” says Edwards. “While currently the telecom team must analyze each pass prior to determining which fixed data rate will provide the best overall data return, with the [adaptive data rate] algorithm, the two radios will work this out on the fly in real time.”

Future uploads to the Orbiter and the Mars Science Lander are planned in order to evaluate new forward error correction techniques. An upload to allow the Mars Science Lander to transit and decode with a low density parity check encoder also is in the planning stages.