Thankfully the real rover, some million kilometers million miles away on the Red Planet, is in safer hands. But a Mars day is 40 minutes longer than an Earth day so you get progressively out of synch, so the time I arrive at work changes by an hour every day.
During the Martian night, dozens of scientists, including Gupta, anaylse the images and data coming back from the Red Planet and discuss what they want to do next. Curiosity is the fourth rover since the Pathfinder mission Nasa has run on Mars in recent years, and the procedures have got better with every mission.
Armed with 3D images from its navigation cameras, the drivers work through the Martian night to map out the best, smoothest, route to the next destination. They factor in any stops along the way, to take pictures or operate an instrument, and run a simulation of the journey to double-check they have got it right.
The aim is that when a new day dawns on Mars, the drivers are ready to go. Once either the Odyssey or MRO satellite is in the right place, they send a command to wake up the rover and upload its commands for the day.
Gupta admits that the whole process is more time consuming than he first imagined. Many spacecraft are cruising in space, observing Saturn, the sun, asteroids and comets.
The DSN antennas are extremely busy trying to track all of these space missions at once. A sophisticated scheduling system with a team of hundreds of negotiators around the world ensures that each mission's priorities are met.
During critical mission events, such as landing on Mars, multiple antennas on Earth and the Mars Reconnaissance Orbiter track the signals from the spacecraft to minimize risk of loss of communication. The rover's downlink sessions when the rover sends information back to Earth are generally roughly 15 minutes each, with usually two downlink sessions per relay orbiter ODY, MRO per martian day sol , with two sessions overnight and two sessions in the late martian afternoon. MSPA allows only one spacecraft at a time to have the uplink, and Curiosity commands early in each sol martian day for roughly 30 minutes to provide the instructions for that sol's activities.
During cruise, the Deep Space Network antennas pick up signals from the spacecraft that tell navigators where the spacecraft are located. Engineers cannot physically see the spacecraft with the naked eye or a telescope, and they rely on radio "tracking" to know where the spacecraft are at any given time.
Like a game of "Marco-Polo," the DSN listens for signals from outer space and can detect where the spacecraft is from where the sound comes from. This navigation service is called "tracking coverage" and it includes Doppler, ranging and delta differential one-way ranging, or "Delta DOR.
In order to calculate the speed that a spacecraft is flying, engineers use Doppler data to plot velocity along the line of sight between Earth and the spacecraft. Most people are familiar with the phenomenon of a car horn or train whistle changing its frequency as it moves towards or away from them. Electromagnetic radiation e. The size of the frequency shift, or "Doppler shift," depends on how fast the light source is moving relative to the observer.
Astronomers often refer to the "redshift" and "blueshift" of visible light, where the light from an object coming towards us is shifted to the blue end of the spectrum higher frequencies , and light from an object moving away is shifted towards the red lower frequencies. The Mars Science Laboratory spacecraft commmunicates with controllers on the ground by radio signals.
Ground controllers know the frequency of the signal that is emitted from the spacecraft. However, since the spacecraft is moving away from or towards us, this frequency is being Doppler shifted to a different frequency.
So, engineers or, more accurately, computers compare the received frequency with the emitted frequency to get the Doppler shift. It's then straightforward to find the velocity that would cause the resulting Doppler shift. Ranging is sending a code to the spacecraft, having the spacecraft receive that code and immediately send it back out the spacecraft's own antenna, and finally receiving that code back on Earth. The time between sending the code and receiving the code, minus the delay in turning the signal around on the spacecraft, is twice the light time to the spacecraft.
So that time, divided by two and multiplied by the speed of light, is the distance from the DSN station to the spacecraft. This distance is accurate to about five to ten meters feet , even though the spacecraft may be billion meters away! Delta DOR is similar to ranging, but it also takes in a third signal from a naturally occurring radio source in space, such as a quasar, and this additional source helps scientists and engineers gain a more accurate location of the spacecraft.
In short, there are two ways that Curiosity can navigate the surface of Mars: NASA can transmit a series of specific commands, which the rover then dutifully carries out — or NASA can give Curiosity a target, and then trust the rover to autonomously find its own way there. RSVP can then be used to plot a move go forward 10 meters, turn 30 degrees right, go forward 3 meters — or to pick an end point, which Curiosity will dutifully, autonomously navigate to.
To safely navigate Mars, Curiosity uses its Hazcams hazard avoidance cameras to build a stereoscopic map of its environment, identifies which objects are too large to drive over, and then plots out a course to the end point. Yellowknife Bay is pictured above, along with the step about 2ft high that Curiosity had to cross to descend into the bay.
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