Getting data from the Curiosity lander on Mars is no easy task. And it is by far the critical link in its success. Radios on the lander send data back to earth which includes telemetry data about the condition of the various systems as well as those priceless video images. But this wireless data is not real time because of the distance.
Radio waves travel about 186,400 miles per second, the speed of light. On earth, wireless communications is essentially instantaneous because of the “short” distances. We have come to expect no latency or delay in a wireless command or signal. With Mars, the fourth planet from the sun, an average of about 78 million km from the earth, the delay of a radio signal is significant. Since both Mars and Earth orbit the sun at different rates and with the orbits constantly changing, the distance varies considerably. With the average distance between the two about 78 million km or about 48.75 million miles, a radio signal would take on the average of about 4 minutes. NASA indicated that the signal was taking about 14 minutes one way indicating a current distance of about 154 million miles. It has to be frustrating. And over that distance, the signal attenuation has got to be horrendous on the order of 278 dB path loss. It is a miracle we can receive it at all. How do JPL and NASA do it?
The Curiosity has three basic radios on board. Two of them operate in the X band which is typically in the 7 to 8 GHz range. One of these radios uses a low gain omnidirectional antenna that transmits back to earth at a very low data rate but its main function is receiving commands. The other X band radio uses a moveable high gain antenna about one foot in diameter that can be positioned for an optimum view of earth. Data rate is also low. Data rate is as low as 160 bps (yes, bps) but can be higher up to about 12 kbps. Remember error rates are less at lower speeds. No wonder video takes so long to receive even though it is highly compressed
The third radio is a UHF data modem that operates near 400 MHz. It uses an omnidirectional antenna to talk to one of the satellites orbiting Mars. There are two satellites but only one is working and it is used to relay data back to earth. The data rates are higher (about 128 kbps) because the satellites are closer to the Curiosity. The satellite beams the data back to Earth at a higher rate because it uses more power.
Back on Earth, huge antennas of NASA’s Deep Space Network (DSN) receive the signals. DSN consists of three stations spaced about 120 degrees apart around the world so one of them is in constant view of the Mars lander. There is one at Goldstone in California’s Mojave Desert, one near Madrid, Spain and one in Canberra, Australia. Each has an enormous moveable dish antenna about 200 feet or so in diameter. This gives them amazing gain of 70 dB+ at X band frequencies. That gain plus any Curiosity antenna gain and super receive sensitivity at the earth station compensates for the weak signals from Mars.
We take wireless for granted here on Earth. But in space it is still a challenge. The time delay must drive the NASA and JPL engineers nuts. Anyway, congrats to NASA and JPL for a superb engineering effort.