18
$\begingroup$

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?

$\endgroup$
  • 2
    $\begingroup$ Interesting. Sounds like a good question for xkcd what if! $\endgroup$ – IanF1 Sep 6 '18 at 17:04
  • 2
    $\begingroup$ 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.) $\endgroup$ – JamieB Sep 6 '18 at 18:04
  • $\begingroup$ @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. $\endgroup$ – gerrit Sep 6 '18 at 18:07
  • $\begingroup$ Is the observer terrestrial, or orbiting? $\endgroup$ – can-ned_food Sep 7 '18 at 3:56
  • $\begingroup$ @can-ned_food Either. $\endgroup$ – gerrit Sep 7 '18 at 9:21
22
$\begingroup$

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).

$\endgroup$
  • $\begingroup$ So what you are saying - maybe not hear what Mars person is talking about, but probably detect that Mars person is transmitting? $\endgroup$ – Prof. Falken Sep 6 '18 at 14:59
  • 3
    $\begingroup$ Yes, that's correct. $\endgroup$ – Hobbes Sep 6 '18 at 15:15
  • 5
    $\begingroup$ @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... $\endgroup$ – Marcus Müller Sep 6 '18 at 17:09
  • 1
    $\begingroup$ I only am capable of transmitting slow Morse (sadly)... $\endgroup$ – Jon Custer Sep 6 '18 at 23:22
  • 1
    $\begingroup$ 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. $\endgroup$ – Perkins Sep 7 '18 at 6:27
13
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ It may be possible to find the sensitivity of the DSN antennas. $\endgroup$ – Hobbes Sep 6 '18 at 17:41
  • $\begingroup$ Some of this answer or maybe this answer or the links therein may be helpful. $\endgroup$ – uhoh Sep 7 '18 at 15:01
6
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ 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. $\endgroup$ – can-ned_food Sep 7 '18 at 3:53
  • $\begingroup$ My phone never tells me "no net found" and stop transmitting. It keeps trying until it empties its battery. $\endgroup$ – Dmitry Grigoryev Sep 7 '18 at 11:14
  • 2
    $\begingroup$ @DmitryGrigoryev it continues to search, but it's not transmitting either, it's only scanning various frequency bands. $\endgroup$ – jcaron Sep 7 '18 at 15:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.