In my last post I made mention of the Curiosity rover, an SUV-size Mars vehicle that is currently exploring Mars and sending back scientific data back to Earth.
An artist impression of NASA single relay communication. By NASA ( [Public domain], from Wikipedia Commons The Curiosity rover, made at a staggering cost of 2.5 billion vehicle has loads of high tech measuring and telemetry devices onboard. There is also at least 17 cameras on the rover. The cameras are there to send back images and videos of the Martian environment, both in colour and black and white. It also helps the drivers at the Jet Propulsion Laboratory (JPL) to navigate the terrain without damage to the expensive machine.
One of the cameras, called the Mastcam, due to its placement on a mast has the highest resolution of all the cameras on board at 2 MP or 1600 x 1200 pixels. That resolution may seem like nothing when you consider that we have digital cameras with up to 120 megapixels or even cell phone cameras with six times the resolution of the $2.5 billion machine. But, take a moment and realise that the camera tech is more than a decade old. The process for scientific procurement started off in April 2004 and ended in November 2008. There is every likelihood the camera is a 2004 technology; it takes a long time to protect the camera and run tests so it can survive the harsh environment of space. The 2MP was the best that year could offer.
Also, the Mastcam can take high definition video of up to 10 frames per second (fps) at a resolution of 720p.
The other day I was watching a live broadcastof the moon landing of the Appollo 11 streamed on July 20th 1969, when a flat-earther or those set of people who disbelieves everything about science asked for a live stream of Curiosity rover's action on the Mars if it was really on the Martian environment. How can we have a live broadcast of an event that occurred nearly half a century ago and not has one that is happening now?
A little background of how communication work
Spacecraft in space communicates with people on ground or earth station through the radio waves. A radio wave forms the basic building blocks for communication, it is sinusoidal in shape and made up of peaks (highs) and valleys (lows).
A wavelength is a complete circle of high and low. The frequency of a radio wave is a result of the number of cycles made in a given second. For instance, a signal is said to be 1kHz (1000Hz), that means the signal undergoes 1000 cycles per second.
A signal, amplitude and frequency modulation. By Berserkerus [CC BY-SA 2.5], from Wikipedia Commons In other to make regular radio waves carry information, we will have to alter some of its attributes. Sometimes, this involves changing with the peaks of the amplitude or the frequency. When the amplitude is the one that changes to accommodate the message we have amplitude modulation, but when the frequency change is the medium of encoding data, we have frequency modulation.
For example, to transmit a picture, the pixels of the images are mapped onto the varying amplitudes of a radio wave, with each peak of different value carrying a specific part of the image; it may be either the light or dark pixel which denotes the bright and dark side of the image. On getting to the receiver, it can then decode these different peaks and extract each pixel information which when stitched together forms a picture or video.
Due to the vast distance (the nearest distance following the elliptical orbit to earth is 33.9 million miles or 54.6 million kilometres) between earth and mars, the rovers use the assistance of two Mars-orbiting satellites Mars Global Surveyor and 2001 Mars Odyssey to communicate with ground station.
An artist impression of commerical communication on Mars with the help of a Mars relay orbitter [Public domain], from NASA Jet Propulsion Laboratory Commons The orbiters, which is close (about 250 miles or 400 kilometres) to the rovers will pick up the signal from the rover, amplify it and send it on the up to 200 million miles journey (320 million kilometres) to the earth station.
On earth, giant dish antenna measuring almost the length of a football field at 70m receives the signal for onward decoding and analysis. This transmission to earth is going to happen provided the Mars orbiter has an unobstructed line of sight path to the dish.
Why streaming video is not happening at the moment
The short explanation is that there is a limit due to the data rate capacity of radio waves. Presently, communication from Mars to earth is as low as 3500 bits per second to as high as 12,000 bits per second (1.5 kilobytes per second). Little wonder it takes up to 5 hours to transmit 60 megabits ( at a rate of 7500 kilobytes per second or 7.5 megabytes per second) of data.
Can't we transmit faster?
Oh yes, we can! But not with radio waves. You see radio wave has a range between 30Hz to 300 gigahertz. The shortest frequency wave is 300 GHz at a wavelength of 1 mm. Now, there is only some small amount of data that the bandwidth of this frequency can accommodate. In other to make it take more, we need to increase the frequency the more. If we increase the frequency of the signal, we have left the confines of radio waves and is now in the domain of the visible light!
In between infrared light and ultraviolet light lies the visible light.[CC0] ,
from Wikipedia Commons The frequency of visible light can handle more data, but the problem is the beams from a laser is narrow and requires a super precise receiver to make that a reality. Though there is a way of making us have a video from Mars by stitching up multiple images; for example, this video shows a 3D motion pictures of made out of 33,000 stitched colourised photos extracted from real images of Mars.
But NASA has not given up on the idea of making use of the faster laser communication. On October 18, 2013, the NASA's lunar communication equipment was able to transmit data right off lunar orbit to earth at an extreme rapid data rate of 622 megabits per second (Mbps) or 77.75 megabytes per second (MB/s).
We are very near to transmitting live videos from Mars, with latency (delay) in the mix.
Maybe by then, my flat earth friend may finally be convinced that we have a rover named Curiosity on Mars!
References
- What are radiowaves?
- Curiosity Rover Cameras
- Why does the $2.5 billion Curiosity use a 2-megapixel camera?
- Cost of NASA's Next Mars Rover Hits Nearly $2.5 Billion
- Radiowaves
- Mars Communications With Earth
- Historic Demonstration Proves Laser Communication Possible
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