Hello,

I would like to know exactly how communications between NASA and the
Mars Rovers are accomplished. By communications, I mean:

1.What type of signal modulation is used?
2.How long does it take to send and receive signals?
3.How much power does it take at both sides?
4.Are any satellites involved? If so, what role do they play?
5.Any other cool and interesting facts about the process you find!

Any source would be fine. Journals, links, books, whatever! No such
thing as too much information! I have an engineering background, and
dont mind reading the complicated stuff. It's fun!

Thanks in advance!

---

Clarification of Question by stargazer71-ga on 02 May 2004 02:24 PDT

Update: Thanks to all of you for the great help! I really thing that
it will take some time to digest the information on the
page->"mars_relay_comm.html". And I suspect, I will learn all I ever
wanted to know about it (maybe more!).
An especially thorough anwser, wedgeotronic!! I really appreciate it!

Since I have my answer in comment form, I will be closing the question
soon. Unless there is someone wishing to knock me off my feet with an
even more amazing answer, I will close the qustion with a big THANK
YOU to my Commentors. You rock!

Here are a few pages from the Mars Rovers Web site:

http://marsrovers.jpl.nasa.gov/mission/spacecraft_rover_antennas.html

Communications With Earth
http://marsrovers.jpl.nasa.gov/mission/communications.html

X-band radio waves used by the rovers to communicate
http://marsrovers.jpl.nasa.gov/mission/comm_xband.html

How Fast and How Much Data the Rovers Can Send Back
http://marsrovers.jpl.nasa.gov/mission/comm_data.html

hlabadie-ga

I didn't read the links hlabadie provided so the links are much more
likely corect than my guesses. 1 the data is digital, the carrier
likely FM, but SSB = single sideband would work and several other
unlikely technologies. Often the up link is quite different from the
down link to reduce the chance of interference.  2 If Mars is 93
million miles from Earth now, it takes 500 seconds = 8.3333 minutes
each way. Likely Mars is about half that far awy today, but the
distace increases almost a million miles per day, I think. 3 I'll
guess 100 watts for the down link, so we need a large radio telescope
to receive the signal noise free. With most home type radios and a
yagi antenna with 10 elements, you would likely not even detect the
signal. Watts are cheap for the uplink, so perhaps 45,500 watts (The
Aricibo transmiter unless they have up graded) which the very high
gain transmitting antenna makes Mars think we are sending perhaps 1000
gigawatts. Likely millions of times more than needed, but why risk
loosing a command to save $1000 per month on the electric bill? The
same home antenna and radio might pick up a usable signal from the
uplink transmitter propagated by back scatter or forward scatter
depending a lot on the time of day and your location. We can use the
Arecibo transmitter only about one hour per day, so we may only use it
for back up. Most radio telescopes are fully steerable, but not
Arcibo. 4 No satellites, except NASA rebrodcasts by satellite for TV
news feeds all over the world. Your cable company may give you one of
the NASA satellites 24/7   Neil

On second thought, several radio telescopes in various parts of the
world receive the signal from Mars.  Earth rotates, so Mars is below
the horizon about 12 hours per day at each location. The data is sent
by satlelite back to USA. Costly modifications and a good hacker would
be needed to get this data even with the satellite equipment TV
stations have, I think.   Neil

> 1.What type of signal modulation is used?

There are currently 5 functional spacecraft operating around or on 
Mars.  These are the Mars Global Surveyor (Orbiter, NASA), Mars 
Odyssey (Orbiter, NASA), Mars Express (Orbiter, ESA), MER-A aka
"Spirit" aka "Marvin" (Rover, NASA), and MER-B aka "Opportunity" aka 
"Daffy" (Rover, NASA).  These use a variety of different modulation 
schemes and communication methods to communicate to and from Earth or 
with each other.  Communications from Mars to Earth or vice versa use 
X-band communications.  A variety of advanced modulation and coding 
schemes are available for use at any given time.  A typical scheme is
to use phase-shift keying with Viterbi coding.  Communications for 
the rovers can either be direct to Earth when it is above the local 
Martian horizon or via the three satellites through a UHF relay.  Mars 
Odyssey is the primary relay as it supports both uplink and downlink 
of data, telemetry, and commands (MGS does not support downlink to the 
rovers).

There are tons of details on the Mars rover relay communications
here:
http://www-mgcm.arc.nasa.gov/mgcm/micromet/mars_relay_comm.html


> 2.How long does it take to send and receive signals?

Typically the signal delay is dominated by the light-travel time 
between Earth and Mars.  At present Mars is 324 million km away from 
Earth, which corresponds to a delay of about 18 minutes at the speed 
of light.  However, for the rovers, when using the relay method to 
communicate the delay can be anywhere from a few hours up to 24 hours.


> 3.How much power does it take at both sides?

From the spacecraft side, transmission power ranges from a low of only 
a few watts for a low powered lander (e.g. 5 Watts for Beagle-2) up 
to around 25-Watts for one of the orbiters.  This buys up to about 
256 kbits/s of data throughput from Mars.  How much power is 
transmitted from Earth depends on the circumstances.  Some stations in
the Deep Space Network can transmit at up to 400 kiloWatts.  Actual
uplink power to Mars is usually less than that though, typically in the 
low kiloWatts to tens of kiloWatts (e.g. up to 20 kW for Mars 
Pathfinder).


> 4.Are any satellites involved? If so, what role do they play?

No satellites in Earth orbit are involved.  However, the Mars orbiters 
do function as relays for the rovers.


> 5.Any other cool and interesting facts about the process you find!

The ground stations used for communications to and from Mars are either
part of the Deep Space Network for NASA missions or the ESTRACK network 
for ESA missions.  The DSN is comprised of numerous radio dishes 
ranging in diameters from 26 meters up to 70 meters at complexes in 
Goldstone, California, USA; Canberra, Australia; and Madrid, Spain.
ESTRACK has a similarly far flung, worldwide network of dishes.  The
most advanced receivers use cryogenic, travelling wave ruby maser 
ultra low-noise amplifiers, cooled by liquid Helium to 3 Kelvin.
Slightly less advanced units use high electron mobility transistor
based low-noise amplifiers cooled to 15 K.

Here are some links which you may find useful or interesting:

DSN:
http://deepspace.jpl.nasa.gov/dsn/index.html
http://deepspace.jpl.nasa.gov/dsn/faq-data.html

ESTRACK:
http://www.esoc.esa.de/pr/facilities/estrack.php3


Mars Global Surveyor:
http://marsprogram.jpl.nasa.gov/mgs/

Mars Odyssey:
http://marsprogram.jpl.nasa.gov/odyssey/

Mars Express:
http://www.esa.int/SPECIALS/Mars_Express/

Mars MER Rovers:
http://marsrovers.nasa.gov/home/index.html