Space communication with Mars
Response time and malfunctions (III)
At perihelion opposition, 56 million km from Earth, at a distance of only 0.37 AU (1 AU = 149.56 million km), it takes 3 minutes and 7 seconds for a signal sent out by the DSN to find Mars reached. But when Earth and Mars are furthest apart at 2.52 AU, it takes 20 minutes and 57 seconds to transmit the same radio signal. Communication takes seven times longer! Because of these time delays, it is impossible to communicate with and control the rover in real time.
When Earth and Mars are in conjunction (opposite sides of the Sun) at a distance of 2.49 A.U. another problem arises. This distance is not as problematic as having the sun in the way as it creates a lot of radio interference that makes communication almost impossible. In fact, for distances less than 10 solar radii from the Sun, the contribution of thermal noise is quite large, and the use of an amplifier in reception increases this difficulty. Therefore, it is very important that the spacecraft going to Mars reach the red planet well before the conjunction, so that engineers and scientists can collect data for a few months before being hampered by communication problems.
Aside from the sun and the distance problem, two other sources of noise interfere with telecommunications: cosmic rays and thermal noise generated by the receiver.
The estimate of signal strength or noise level, also known as "dB under W" or dBW, is a measure of absolute power expressed in watts, rather than a power ratio like the decibel.
If we know the signal power and noise level at the source at the orbiter's distance, we can estimate the signal-to-noise ratio (S/N) according to the bandwidth used.
As in radio astronomy, engineers in space communication estimate that anoise levelof -215 dBW/Hz at 10 GHz is acceptable to the big ears of the DSN network.
For a bandwidth of 100 kHz and a signal close to 2x10-16W or -157 dB (-157 dBW) when receiving, the S/N is only 8 dB. It can be twice as high with ten times less bandwidth, but this configuration is almost useless in practice, except for some digital transmission modes.
But 8dB means that the DSN can theoretically receive such a signal without using error correction protocols, DSP systems, or any BPSK or similar mode (although it does). Under such conditions, the transfer rate is relatively fast, up to 21 KB/s (166 kilobit/s). It's this kind of "small budget" configuration that was used by spacecraft like MGS and other Cassini up until 2005.
Local communications on Mars
Messages sent by rovers are first routed to Earth, but their performance is so weak that even with the DSN's largest antenna, locating and recording them is a complex and full-time task.
Drawing of Mars Phoenix Scout after deploying its solar panels and antennas.
They communicate directly with Earth daily via the HGA, but most of the time rovers primarily connect their information to the nearest spacecraft orbiting Mars, using such as the Mars Odyssey or Mars Global Surveyor orbiter as messengers to relay messages to Earth , as soon as they are within sight of their antennae.
Conversely, orbiters can also send messages received from the DSN stations to rovers. The benefits of using the orbiting spacecraft are first related to the fact that the orbiters are much closer to the rovers than the DSN antennas are on Earth, and then the orbiters have the Earth in their field of view for much longer periods of time than the rovers are on of the earth Earths that are subject to the rotation of Mars.
This type of communication is valuable because orbiters are only 400 km (250 miles) above the surface of Mars and rovers don't have to "yell" as loudly (or use as much energy) to send a message to orbiters as they do is communicating with DSN stations.
The rovers will stay in contact with the earth for a few months using three different systems: a low-gain antenna (LGA), a UHF antenna and a high-gain antenna (HGA). DSN antennas communicate with distant spacecraft via S-, X- and K-band microwaves at frequencies of 2.2, 8.4 and 32 GHz.
X-band antennas used by the rovers
The rovers exploring Mars communicate with each other, orbiters, and the DSN via X-band UHF antennas, which are short-range antennas used at low power. These are like walkie talkies compared to the long range of the low gain and high gain antennas. One UHF antenna is located on the rover and one on the lander petal to aid in intelligence gathering during the critical landing event. Orbiter is tracking the landing process.
When rovers communicate directly with the earth, they broadcast messages using both the low-gain antenna (LGA) and the high-gain antenna (HGA), both of which receive the 7.2 GHz uplink signal and the Can send 8.4GHz downlink signal. The LGA is a choked circular waveguide design with a beamwidth pattern of about 70° with a bore gain of about 6 dBic at 7.2 and 8.4 GHz. Although it has a degree of directivity that would satisfy any amateur radio, the LGA is considered a quasi-omnidirectional antenna, according to NASA engineers. It's true that on X-Band we're used to working with beams that are ten times narrower.
Mars Global Surveyor. Document NASA/Corby Waste.
The LGA sends low rate signals to the DSN antennas when the rover orientation is unknown. The omnidirectional UHF antenna communicates via passing orbiters, not only with the mother spacecraft (e.g. Mars Odyssey for the rover Opportunity), but also with other passing orbiters such as Mars Global Surveyor or the future MRO if necessary.
The HGA is a steerable beam aimed directly at any antenna on Earth. The advantage of a steerable antenna is that the entire rover does not necessarily have to change position to talk to the DSN. Just as you turn your head to talk to someone next to you, the rover can conserve energy just by pointing its antenna in the right direction.
About half of all communications will go through the HGA, a 28 cm diameter dish that sends data directly to DSN over the X-Band (8 to 12 GHz). The downlink data rate to the orbiter is fixed at 1.85 kilobits/s or 264 bytes/s using the HGA, while the uplink data rate using the LGA is 0.875 kilobits/s or 125 bytes/s
The noise level received along with the spacecraft data signals is quite high, but with the carefully designed front end of the DSN Block V receiver and the extremely high gain of the large dish antennas, engineers can achieve carrier-to-noise ratios of around 20 to 40 dB- Hz (baseball stadium). At Mars distances (depending on the spacecraft configuration), signals can be easily tracked by phase-locked loops with a loop bandwidth of 1 Hz.
As with the Mars PathFinder spacecraft, the Mars Odyssey Lander HGA is a printed dipole array design that uses a meandering Right Circularly Polarized (RCP) polarizer. It is approximately 28 cm (11 in) in diameter and weighs 1.2 kg (2.5 lbs.). It has a borehole gain of about 20.4 dBic at 7.2 GHz and a borehole gain of up to 25 dBic at 8.4 GHz.
During the voyage, Mars exploration rovers use both the LGA and a separate medium-gain antenna (MGA) used only during the voyage. Antennas on the cruise stage go off with it.
The MGA on the cruise stage has a waveguide (a tube) to the rover that is not used on the surface. The LGA on the cruise stage connects directly to the LGA on the backshell, which in turn connects to the LGA on the rover in a Russian doll design.
Images in 4K
The Curiosity rover benefits from more efficient technologies than the old exploration spacecraft. It communicates with MRO or Mars Odyssey orbiters at a rate of 32 kilobits/s or 0.004 MB/s. On the other hand, the orbit-to-Earth link ranges from ~500 bps to 2 megabps or 0.25 MB/s between MRO and Earth, and at 128 kbps or 256 kbps between Mars Odyssey and Earth. But like modems of their day, these speeds remain very slow when it comes to transmitting high-definition 4K images.
To see:Mars in 4K
To use :File Size Calculator
Mars rein Stereo!
Above the Twin Peaks area in Ares Valles. Below is a close-up of rocks and sand near the lander and a general view of Mars Odyssey. documentsCorby wastefor JPL.
Some people have asked the engineers at JPL why they haven't installed HD video cameras on spacecraft exploring Mars? A 4K HD image contains 3840 x 2160 pixels at a density of 300 dpi and 32 bits per color. This corresponds to 8,294,499 pixels (~8 megapixels) and 31.6 MB of data. Even the fastest 0.25MB/s send takes over 2 minutes (126.6s) to receive a single 4K image. To receive 1 second of video, the DSN would have to wait almost 53 minutes and almost 9 hours for a sequence of only 10 minutes! Not only is it very long, but apart from a few dust whirls every now and then, nothing happens on Mars; the landscapes are inanimate. Rovers also spend most of their time stationary. It therefore makes no sense to send videos from the planet Mars.
Maximum usable distance
The rover's telecommunications system has been functionally tested at JPL to a distance of 250 meters from its base transmitter. It performed reasonably well in ambient conditions typical of a warm August day (35 °C or 95 °F). Engineers increased the distance to well over 700 meters, with the interval showing an abundance of multipath reflections. The radio modems also worked well under these conditions and did not lose communication.
However, under certain conditions, the quality of the communication link degrades. Especially at the lowest acceptable operating temperature of -30°C (-22°F), the bit error rate (BER) can cause a communication failure problem due to an operating frequency shift. If the rover remains in the lander's line of sight and the radios are kept at a warmer operating temperature, the maximum usable distance over which the rovers can communicate should be at least 700 meters.
The real limitation on how far the rover can be driven is based on the stereo imaging range of the Lander IMP camera. Above about 10 meters, the IMP camera resolution may not be able to provide good enough stereo coverage of a given location to assist the rover navigation team in driving the rover.
Engineers could rely on the rover to get its navigation information from its own stereo cameras, but this method is certainly more difficult to plan for and implement. If this process is performed, the rover traverses would most likely be on the order of 2 meters each, as this is approximately the distance a beam can be projected from the rover cameras.
However, given the bank of interesting geological formations near the lander, scientists were initially content to remain in close proximity to the lander. However, they quickly ventured much further (in 2010, the Spirit and Opportunity rovers each covered over 20 km), but scientists kept in mind that the rovers should remain higher in altitude than the lander and in line of sight. Under different circumstances, communication between probes was established via the orbiter.
Transponders for space
A transponder is a communications system onboard a relay satellite that receives and retransmits the signal, often on different downlink and uplink frequencies. This system is activated when the ground stations are out of sight of the orbiting satellite or rover and direct communications are lost.
General Dynamics has worked with JPL to provide the spacecraft terminal for X- and Ka-band telecommunications using DSN. The SDST's flexible design, which makes extensive use of MMIC technology (for multipliers and amplifiers), multi-chip modules, and a signal processing ASIC, offers the ability to address the telecommunications needs of almost any space mission in a smaller, lighter, and less expensive package to previous designs.
The transponder shows the next performances :
- X-Band-Uplink: 7,145-7,235 GHz
- X-Band-Downlink: 8.400-8.500 GHz
- X-Band TX/RX ratio: 880/749
- Ka-Band-Downlink: 31.800-32.300 GHz
- Ka-band TX/X-band RX ratio: 3360/749
The X-Band receiver will display upcoming performances:
- Noise figure: < 2.5 dB at 25°C
- Carrier-Tracking-Signalbereich: -70 bis -156 dBm
- Carrier loop bandwidth (two-sided): 20 Hz nominal at threshold (extended to 200 Hz, strong signal).
- Carrier loop attenuation factor: 0.5 at 0 dB loop S/N (Type 1, 2nd order loop)
- Tracking range: > 200 kHz.
- Range filter type: 3-pin Chebyshev, other options available
- Ranging filter bandwidth (3 dB): 1700 kHz nominal, other options available
- Temperature stability: approx. 3 ppm (-20°C to +60°C)
Telecom Relay Capabilities
Using the HGA, the downlink data rate to earth varies from about 12 kilobits/s (1.75 KB/s) to 3.5 kilobits/s (0.5 KB/s), a rate that is about five times slower than a standard 56K modem. But the data rate to the orbiters is much faster, at a constant 128 kilobits/s (18.2 KB/s), twice the average data rate of a 56K modem.
An orbiter that flies over the rover is close to the sky to communicate with the rovers about 8 minutes per sol (Martian day). In this time about 8.5 MB of data (about 1% of a CD-ROM capacity) can be transferred to the orbiter. The same amount of data sent directly from the rover would take between 1.5 and 5 hours to transmit directly to Earth! Rovers can only transmit directly to Earth for a maximum of three hours per sol due to power and thermal limitations, although Earth may be in sight for much longer.
The best rate is held by the Curiosity rover, which transmits between 100 and 250 megabits/s or between 12.5 and 31.25 MB/s of data per sol to the orbiter. If needed, Curiosity can transfer its data directly to Earth, but transferring the same 31.25 MB would take almost 20 hours!
The communications network between spacecraft exploring Mars and the DSN stations.
But there is a downside to this system. As Mars rotates on its own axis, it carries the rover with it, and it will sooner or later disappear from Earth's view. Hopefully orbiters can see Earth for about 2/3 of each orbit, or about 16 hours per sol. They can therefore send much more data directly to Earth than the rovers, not only because they can see Earth longer, but because they can operate their radio much longer since their solar panels are bright most of the time and they have larger antennas than rovers .
So far, for the smallest landers like the stillborn Beagle 2 or Netlander, the UHF relay telecom function is passive and has no way of communicating directly with the earth.
Unlike big explorers like Mars Exploration Rover (MER) or Smart Lander, the UHF-Relay-Telekom can handle 10 times more data for the same amount of energy in return.
In all cases, as we have said, this is the orbiter providing navigation to assist with descent maneuvers upon arrival as well as land operations. Circumferential relays also connect to the night side of Mars, which is hidden from Earth's view.
This short relay connection from surface to orbit is far more efficient than a direct connection to Earth. The communication difficulties in space with the earth increase with the square of the distance. In the worst case, the maximum range is 2.7 A.U. or 400 million km ! In comparison, the in-situ connection only lasts between 1000 and 6000 km. Under such circumstances, the power loss to Earth exceeds 266 dB (at 56 million km) compared to the in situ link on Mars. Here your wattmeter should read 10-26W at the reception !
Note that reception was no better with the New Horizons spacecraft that visited Pluto in 2015. The power of their transmitter was 12 W, but upon reaching the DNS Newtork, the signal was reduced to 3.45x10-22W, or about thirty thousandths of a billionth of a billionth of a watt!
This loss must be compensated for by the antenna gain at both the transmitting and receiving locations.
(in free space)
LdB= 92,4 + 20 Log (FGHzx Dkm)
Example. 56 million km from Earth, a signal broadcast from Mars in X-band at 8.4 GHz shows a loss of 266 dB, a power ratio >1024.
Antenna gain ccalculation
(Antenna dish for transmission)
GdB= 18 + 20 Protocol (FGHzx Dm)
Example. At 8.4 GHz, a 2.5 m diameter HGA dish antenna gives a transmission gain of 44 dB, a power ratio >2x104.
This huge difference in performance is difficult to manage even with the largest DSN antenna DSS-14 with a diameter of 70 m. The orbiter thus offers a first major benefit by providing a surface-to-orbiter-to-earth relay link with a reduced amount of power. The second advantage is that in the same time slot, the rate and volume of data transmitted has increased drastically in just a few years, as the next table shows.
New Horizon (Pluto)
In the future, orbiters will fly at higher altitudes to allow longer communications with the surface rover, extending coverage from 2 hours to 6-12 hours. The only downside to such high orbits is the longer range of inclination for users. This is compensated for by using a medium gain and steerable antenna that offers 13–15 dBi of gain.
Finally, since 2005, Mars Reconnaissance Orbiter (MRO) and new explorers have been using even lower-loss receivers to increase signal-to-noise ratio. For example, they operate with 2dB less filtering and radio loss compared to the older UHF relay radio design that was onboard Mars 2001. A concatenated Reed-Solomon code also enhances the channel for short-range communications with a further 2dB of gain, complemented with Turbo Decoders that add another 0.5dB of performance improvement to this set.
The future of space communications will be loud and clear!
For more informations
satellite reception(on this page)
Properties of space probes+ Link to NASA Press Kit, UHF Satcom
Amateur DSN group(Yahoo!)
DSN network, Nasa
Parkes Observatory's support for the Apollo 11 mission(500-KB-PDF).
Back to radio astronomy
book page1-2- 3 -
X-band radio waves used by the rover to communicate
The rover communicates with the orbiters and the DSN through radio waves. They communicate with each other through X-band, which are radio waves at a much higher frequency than radio waves used for FM stations.
Most often, Curiosity sends radio waves through its ultra-high frequency (UHF) antenna (about 400 Megahertz) to communicate with Earth through NASA's Mars Odyssey and Mars Reconnaissance Orbiters.Can you communicate with someone on Mars? ›
Talking to Mars
To send communications over such a great distance, you need a powerful radio. And missions like rovers need to be small and light, so there isn't room to strap a huge antenna to them. To circumvent this issue, Mars has a system for relaying communications, called the Mars Relay Network, or MRN.
How will the first humans on Mars communicate with Earth? After all, the minimum distance between Earth and Mars is 50 million kms away at its closest point. Right now, it takes between 5 and 20 minutes for communication from Earth to reach Mars and vice versa.Can Mars support life as we know it? ›
The low atmospheric pressure combined with cold temperatures also mean liquid water is not stable at the surface. Life as we know it cannot exist in these conditions.How will humans be able to breathe on Mars? ›
The atmosphere on Mars is made up of mainly carbon dioxide. An astronaut on Mars would not be able to breathe the Martian air and would need a spacesuit with oxygen to work outdoors.Do we have the technology to send a man to Mars? ›
There are some in development, but the very word development tells us that they are developing the technology. Nobody has a vehicle that can land humans on Mars. Mars has a very thin atmosphere, which means it doesn't serve as an efficient braking mechanism as Earth's atmosphere does.How long would Mars communication take? ›
It generally takes about 5 to 20 minutes for a radio signal to travel the distance between Mars and Earth, depending on planet positions. Using orbiters to relay messages is beneficial because they are much closer to Perseverance than the Deep Space Network (DSN) antennas on Earth.Can Elon Musk send people to Mars? ›
Elon Musk's company has set itself the ambitious task to make space travel more accessible for humanity. SpaceX is developing a reusable rocket called Starship that aims to take people to the Moon, Mars and elsewhere.Can you use your phone on Mars? ›
The cell phone wouldn't survive the Martian conditions. The radiation level far too high. It may receive signals close up, but not from Earth, and not from long distance without repeaters (cell phone towers).
Someone from Mars is called a Martian.Can you FaceTime someone on Mars? ›
Depending on the distance between Earth and Mars, the communication delay can reach up to 22 minutes one way, making it highly impractical for Mars astronauts to FaceTime with anyone millions of miles away on Earth.How does NASA communicate in space? ›
The Short Answer: Spacecraft send information and pictures back to Earth using the Deep Space Network (DSN), a collection of big radio antennas. The antennas also receive details about where the spacecraft are and how they are doing. NASA also uses the DSN to send lists of instructions to the spacecraft.How does NASA communicate with Mars perseverance? ›
After landing on Mars, the Perseverance rover will rely on the Mars Relay Network orbiters overhead to keep in touch with engineers on Earth, just like the two current NASA missions already on the surface of the Red Planet--the Curiosity rover and InSight lander.Why Has NASA stopped communication with Mars? ›
Mission controllers at the agency's Jet Propulsion Laboratory (JPL) in Southern California were unable to contact the lander after two consecutive attempts, leading them to conclude the spacecraft's solar-powered batteries have run out of energy – a state engineers refer to as “dead bus.”