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We certainly cannot see a person walking on Mars from Earth.

But what if the person is trying to use a cell phone, walkie talkie, or satellite phone? Although he won't have much luck using any of those devices, can we detect the signal from Earth (including Earth orbit), even when the antenna on Mars is omni-directional / not actively directed at Earth?

gerrit
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    Interesting. Sounds like a good question for xkcd what if! – IanF1 Sep 06 '18 at 17:04
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    For cellphones, there's a tech question here too -- I don't know that they will transmit at all if there's "no signal". Cellphones are constantly receiving information from the towers around them so they know who to talk to and how. If there are no towers around to chat with, I'm not sure the cellphone will transmit at all. If you were writing a story about trying to detect someone who found themselves stranded on mars, a walkie talkie might work better. (Walkie talkies mostly use more power than cellphones too, which would make them easier to detect.) – JamieB Sep 06 '18 at 18:04
  • @JamieB Good point about cell phones. I suppose satellite phones are more like cell phones here, so it'd have to be a walkie talkie. – gerrit Sep 06 '18 at 18:07
  • Is the observer terrestrial, or orbiting? – can-ned_food Sep 07 '18 at 03:56
  • @can-ned_food Either. – gerrit Sep 07 '18 at 09:21
  • Does detecting from Earth by the way of controlling relay orbiters around Mars count? – Alice Sep 07 '18 at 10:51
  • @Alice No, for then the question is obviously yes. I mean a direct detection from Earth. – gerrit Sep 07 '18 at 11:02
  • The signal from a walkie talkie is not well suited for the noise reduction methods necessary for detection on Earth. The data rate of modulated speech signal is too high. A digital signal is needed with a very low and very constant data rate to allow effective noise reduction by signal integration over a longer time interval. A starting sequence with known shape to synchronize the receiver on Earth to the timing of the signal from Mars. – Uwe Sep 07 '18 at 12:16
  • @Uwe Great info. Could you edit that into your answer? – gerrit Sep 07 '18 at 13:04

3 Answers3

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Maybe.

Let's start with a known system that can communicate with Earth directly from Mars' surface: Curiosity's low gain antenna. This is driven by a 17 W transmitter and has 6 dB antenna gain (so 48 dBm), which is enough to communicate at low speeds (10-50 bps) with a 34 m DSN antenna on Earth.

Compare this to a cell phone: this has a 3 dB antenna gain and up to 1W transmitter power, giving 33 dBm.

Then we need to know the link margin Curiosity's LGA has. I have no number for this, but it seems to be pretty low (page 120 of the PDF), with communication only possible when Earth is near zenith as seen from Curiosity.

My conclusion: you might be able to detect that a transmission is going on using a 70 m DSN antenna, but the achievable data rate is much too low to support communication (much lower than the phone can function at, it needs a few kbit/s to support a phone call).

I've ignored a few effects (satphones and old analog cellphones may use a stronger transmitter, transmission frequencies other than the 8 GHz used by Curiosity).

Hobbes
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  • So what you are saying - maybe not hear what Mars person is talking about, but probably detect that Mars person is transmitting? – Prof. Falken Sep 06 '18 at 14:59
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    Yes, that's correct. – Hobbes Sep 06 '18 at 15:15
  • I must be missing something.... One transmits at 48 dBm, the other at 33. So you need to add 15 dB to your link budget, which means multiplying the radius by 5, not 2. That's a pretty big antenna! And the gain of a DSN antenna varies with he frequency (not favourably at all in this case), which makes things worse. Also the Curiosity antenna is probably somewhat pointing up, while a cellular antenna will send its signal more sideways. Satellite phones are probably a different story though. – jcaron Sep 06 '18 at 16:40
  • True, but instead of transmitting information we're just detecting the carrier, I expect that gets you a few dB worth of link budget. – Hobbes Sep 06 '18 at 16:50
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    @Hobbes but phones don't simply use a carrier – you'd be trying to detect a frequency hopping (GSM), spread-spectrum (UMTS) or OFDM (LTE) (OFDM looks like an elevated white noise floor within bandwidths) signal under heavy doppler shift. I don't think this would work out... –  Sep 06 '18 at 17:09
  • So my 5W handheld ham radio with a fairly simple Yagi antenna would do just fine. Interesting. – Jon Custer Sep 06 '18 at 19:54
  • @Jon Custer: Your 5W handheld ham radio with a fairly simple Yagi antenna may do fine if used to transmit slow Morse code signals. Of course with very stable speed to allow a low error rate detection. – Uwe Sep 06 '18 at 21:09
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    I only am capable of transmitting slow Morse (sadly)... – Jon Custer Sep 06 '18 at 23:22
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    Analog cell phones definitely did use a stronger transmitter. My dad's old Motorola brick used to work on Boy Scout hiking trips as long as they were on top of a tall enough mountain to have something vaguely approaching line-of-sight. (with the whip antenna at least. The one-inch stubby he used in town wasn't worth much.) I remember the manual saying something like 8 watts, but I'm not sure. Earlier portable phones were more like 20, but they were car-mounted. – Perkins Sep 07 '18 at 06:27
  • I rather doubt it. Didn't the Curiousity used an aimed antenna, whereas mobile phones just emit everywhere in the hope there's a receiver somewhere ? – Gloweye Sep 07 '18 at 07:28
  • Curiosity has several antennas. They normally use the high gain antenna (which has to be aimed), but it also has a low gain antenna for emergencies. That's the one I used in my calculation. – Hobbes Sep 07 '18 at 07:42
  • If you use Aricebo instead of one of the big DSN antennas, you should be able to pick up the entire conversation. – Mark Dec 28 '18 at 23:46
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To know if a signal can be received, you need to check your link budget.

Here we have:

  • send power: at most 1W (30 dBm)
  • $+$ transmit antenna gain: 2 to 3 dB.
  • $-$ free space loss: $$20 log_{10}\left(\frac{4\pi df}{c}\right)$$ The distance between Earth and Mars varies between 54 and 401 million km. Mobile phone frequencies vary from 800 MHz to 2.1 GHz, so loss is somewhere between 245 and 270 dB.
  • $+$ receive antenna gain: $$10log_{10}\left(\frac{4\pi^2r^2\eta f^2}{c^2}\right)$$ Let's consider a 70m antenna (that's already a biiiiig antenna), the gain is between 54 and 63 dB depending on the frequency

The end result is received power at -149 dBm in the best case, -183 dBm in the worst case. If you controlled the sending device and could send a signal with a bandwidth of a few Hz, and your receiver bathes in liquid nitrogen, then it could be possible. But with cell phones using bandwidth on the order of MHz, that seems quite unlikely.

jcaron
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Cell phones using timeslots for different users should not transmit on their own. They have to search for towers around and establish a connection to the nearest one. When a call is established, the tower assigns atime slot to the phone. The phone may transmit only within the assigned timeslot. If the phone would transmit at any time outside the the assigned time slot, it would disturb the data exchange with other cell phones within the same tower's area.

But on Mars there are no towers for cell phones. A cell phone would search for towers around but will not find any. It would display "no net found" and not transmit anything. Therefore no transmitter signal could be detected from Earth, even with a very sensisitive receiver.

But even if there would be a cell phone tower there on Mars, the signals transmitted by the cell phones are to weak to be received on Earth. The modulated data rate of the signal is much too fast for so huge distances. With a very slow data rate of some bits per second or even several seconds per each bit, receiption of such a weak signal might be possible. Detection of a signal weaker than the noise from space is not that easy.

Uwe
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  • The ionosphere would interfere too, wouldn't it? I mean, it wasn't the primary reason that the certain range was assigned to cellular telephone signals, but off the top of my head, I seem to remember that they'd be reflected rather than pass through. – can-ned_food Sep 07 '18 at 03:53
  • My phone never tells me "no net found" and stop transmitting. It keeps trying until it empties its battery. – Dmitry Grigoryev Sep 07 '18 at 11:14
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    @DmitryGrigoryev it continues to search, but it's not transmitting either, it's only scanning various frequency bands. – jcaron Sep 07 '18 at 15:35