Curiosity Rover: RF And Microwave Technology Communicates With The Martian Planet
By Paul Kruczkowski, editor
Last week NASA’s Jet Propulsion Laboratory gave us engineers who moonlight as “space geeks” something to get excited about when they successfully landed the Curiosity rover on the surface of Mars. The x-band and UHF transceivers aboard Curiosity provide the communications for all phases of the mission, including the spacecraft’s navigation to Mars, its landing, and now its exploration of the planet. Curiosity is receiving commands and providing photos, videos, and eventually the test data from samples of the Martian surface directly to the NASA Deep Space Network (DSN) on earth or relaying them through the Mars Odyssey and Mars Global Surveyor (MGS) satellites that orbit Mars.
The DSN — the earth end of the communications link — consists of three deep-space communications facilities strategically placed around the world: at Goldstone, in California's Mojave Desert; near Madrid, Spain; and near Canberra, Australia to enable constant observation of spacecraft as the Earth rotates on its axis. These sites have enormous 70 meters (about 76 yards) and 34 meters (about 37 yards) antennas allowing engineers and scientists to communicate with the many spacecraft exploring our solar system. The Curiosity shares time on the DSN, which uses a sophisticated scheduling system that ensures that each mission´s priorities are met.
X-Band Guidance To Mars
Curiosity’s x-band radio and the DSN work together during flight to achieve periodic spin axis pointing updates to make sure the antenna stays pointed toward Earth and that the solar panels stay pointed toward the sun. Both also provided engineers a navigation platform for radio tracking of the spacecraft to determine its position at any given time by using Doppler, ranging, and delta differential one-way ranging, or "Delta DOR."
Delta DOR yields the most accurate location of the spacecraft because a well-known position reference like a quasar is used. The difference in the distance between the spacecraft and two DSN locations is measured, which provides a skyward angle for the spacecraft relative to the stations. The angular separation between the spacecraft and quasar is determined by subtracting the quasar angle to the station. The resulting angle is accurate to about ten nanoradians which translate to a position accuracy of 1 km (0.6 mile) even as it approaches the Martian atmosphere.
The speed of the spacecraft is determined by plotting its velocity along the line of sight between Earth and the spacecraft. A known x-band signal is sent by the DSN antenna and one is returned from the spacecraft. Computers compare the received frequency with the known emitted X-band frequency to get the Doppler shift of the signal, and then the velocity that would cause the resulting Doppler shift is determined.
Ranging is used to determine the distance from the ground station to the spacecraft. A code is sent to the spacecraft, and it is immediately sent back to earth. Knowing the speed of light and the delay in sending the signal back from the spacecraft, the distance can be calculated to within 10 meters (33 feet).
Multiple Links To The Surface
Now that the rover has landed, it will go through a period of system and payload tests but will eventually become a laboratory on wheels designed to search for the organic building blocks of life. The rover can receive commands and send data collected by the various payloads directly to earth DSN stations on its X-band transceiver using either its low-gain omnidirectional or its high-gain antenna. The rovers can only use its X-band link to earth for, at most, three hours a day due to power and thermal limitations, at data rates that vary from about 3,500 to 12,000 bits per second.
The rover also has a UHF transceiver that enables two way communications with the Mars Odyssey and Mars Global Surveyor (MGS) orbiters, which are 250 miles above the surface of Mars and can relay commands and data to and from earth. An orbiter is above the rover for about 8 minutes each day and can receive approximately 60 megabits of data from the rover in that time compared to the 1.5 to 5 hours it would take to send that 60 megabits directly to the DSN. In addition, the orbiters can send much more data to the DSN because their antennas are larger, they can see Earth longer, and their x-band radio can operate longer since their large solar panels receive light most of the time.
The mission is highly dependent on the microwave and UHF communication systems. Without these systems the rover would not be on Mars today, and it is the link to control Curiosity as it navigates the planet. In addition, all of the test results from the collected samples are meaningless if they are not communicated back to engineers and scientist on earth. The rover’s payload contains all kinds of new scientific equipment including its x-band microwave radio that operates at higher frequencies than many older spacecraft radios and requires less power and smaller antennas. The success of the mission will be measured by the amount and types of organic compounds that Curiosity finds on the Martian surface, however, the amazing photos and videos that will be received from the rover will fuel debate on what the next mission should be and what technology is required. Those photos can be seen on the Mars Science Laboratory website and they will surely transport NASA scientist and engineers — and yes, us “space geeks” — to Mars.