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In the question How does Curiosity know how to point and move it's high gain antenna in real time? I show an image of the hexagonal, articulated high gain antenna on the curiosity rover, as well as its radiation pattern.

I'd like to know if a direct link between Curiosity and Earth has ever been made, and ideally when that was and what the received signal strength and data rate was on Earth.


update: Usually it's not a good idea to modify a question very much after an answer is posted, but I just ran across this article and it seems particularly germane.

Space.com's September 2015 article New Antenna Could Give Mars Rovers a Direct Line to Earth says:

Rovers on the surface of the Red Planet today use indirect or relay communications methods. Information is transmitted to an orbiter and then passed on to Earth. [...]

If the long-lasting orbiters expire before NASA's Opportunity and Curiosity rovers, then the rovers will be unable to connect with Earth, essentially putting them out of commission. Although NASA plans to launch a third rover, Mars 2020, to the Red Planet in the near future, no new orbiter is in the works.

Rovers carrying the new antenna could communicate directly with Earth when the rover and Earth are within line-of-sight. An onboard mechanical system would allow the arm holding the antenna to pivot in the appropriate direction. While present-day rovers can only speak to Earth twice a day for about 15 minutes each, the new design could dramatically increase communication time.

Note that Curiosity landed on Mars in August 2012, three years before the article was written.

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The direct-to-Earth (DTE) link via the X-band HGA is used fairly frequently—not daily, but not once a year, either. The Deep Space Communications And Navigation Systems (DESCANSO) Center of Excellence report on the Curiosity Rover telecom system (pre-launch publication, so they still call it Mars Science Laboratory, MSL) goes into excruciating detail on all telecom functional modes, the hardware involved, and their performance. On page 68 it gives the range of data rates possible via the HGA: 7.8125 to 4000 bps. The 4000 BPS probably works only when Mars is at opposition, so the Mars-Earth distance is about as short as it can be.

For larger distances you can scale that roughly as $\frac{1}{r^2}$, subject to the discrete data rates the system is designed to support. When you calculate a data rate using $\frac{1}{r^2}$, the maximum data rate you can actually use is the next lowest system-supported rate. The chart on page 105 suggests what those discrete rates are: 4000, 2000, 1000, 500, 250, 125, 60, 30, 15, & 7.8 (figures approximate—read from a chart!) If for some reason the HGA can't be pointed directly at Earth but must be off-pointed by a small angle (for off-points >5° they won't even try to downlink data) then you have to degrade the on-axis performance using the HGA Downlink Directivity chart on page 65. On page 104 the maximum data rate possible at the maximum Mars-Earth range, with 5° HGA pointing error, to a 34-m beam waveguide (BWG) DSN antenna is quoted at 160 bps, so maybe there's another supported rate between the 125 and 250, and the chart doesn't show them all.

According to the MSL folks I know, downlink via that comm path represents only a few percent of the total data volume downlinked. Relay through the orbiters at Mars (MRO and Odyssey), with their much larger HGAs, yields much higher data rates, so the great majority of the MSL data comes down via those links.

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    $\begingroup$ Ideally I'd like to see data, not theoretical predictions. "...and ideally when that was and what the received signal strength and data rate was on Earth." It turns out DSN makes communications logs available via http, so if it's possible to identify at least a date, then a real, measured strength and data rate are immediately forthcoming. See answers to Is there any publicly available logging, or “Wayback Machine” for DSN Now activity? $\endgroup$ – uhoh Sep 29 '18 at 17:50
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    $\begingroup$ Sounds like it's a matter of taking the time to look it up. Have at it! :) $\endgroup$ – Tom Spilker Sep 29 '18 at 18:00
  • $\begingroup$ Well the idea was to ask in SE first. Without an approximate time frame, trying to download a few months of data one entry at a time is prohibitively slow. It's not a very fast process. So anything that could narrow it down to a one or few day window, even a week, would be extremely helpful. $\endgroup$ – uhoh Sep 29 '18 at 18:03
  • $\begingroup$ The existence of the antenna itself proves that a reasonably useful data rate was certainly expected by NASA. The purpose behind asking the question was to find confirmation that it worked. So the "pre-launch publication" is quite interesting and will be a great comparison, but I'm really asking for evidence that it has been used. $\endgroup$ – uhoh Sep 29 '18 at 18:07
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    $\begingroup$ The Space.com reference to "...the rovers will be unable to connect with Earth, essentially putting them out of commission." is a significant exaggeration. Though decreasing the time-averaged data rate by nearly two orders of magnitude, greatly decreasing the prospect for serendipitous science, it does not "put them out of commission." It appears nobody else, including NASA, is on fire about this, and I know of no other source but the DSN logs (notably, no categorized records), so you'd probably have to slog through those logs yourself. $\endgroup$ – Tom Spilker Oct 3 '18 at 15:40
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downlink power is in dBm (same units as displayed in the web page) That's after the gain from the aperture, which is different for all the dishes. You can find the actual gain (and more) by searching for the 810-005 document

https://deepspace.jpl.nasa.gov/dsndocs/810-005/

You'll want to look at the "Space Link Interfaces" to get antenna gains. In round numbers, 70m is about 74 dBic gain 34m is about 68 dBic The numbers vary a bit depending on whether they have the dichroic plate in, which aperture you're on, etc.

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  • $\begingroup$ Thanks for the information, and welcome to Space! Generally this should be a comment rather than a new answer to the question, but the Stack Exchange system will not let you do that until you reach 50 reputation "points". That won't take very long though. update: in fact, it didn't take very long at all ;-) $\endgroup$ – uhoh Nov 27 '18 at 1:03
  • $\begingroup$ I've done a bit of link budget calculation in this answer but the units used in the raw XML files I'm downloading are not clear. Since the units of frequency are not even consistent between downlink and uplink (MHz vs Hz) how can you be sure of what units are actually used in the XML data? Clearly there are conversions being used when reporting on the website (MHz→GHz & Hz→GHz for example) $\endgroup$ – uhoh Nov 27 '18 at 1:13
  • $\begingroup$ And so I've asked Understanding the units in the Deep Space Network XML data? as a new question. $\endgroup$ – uhoh Nov 27 '18 at 1:27
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Got'cha! 2018-11-23 01:06 UTCCanberra's DSS36 was in the process of "teardown" after talking directly with Mars Science Laboratory (Curiosity).

Numbers first, then data. From this answer:

$$ P_{RX} = P_{TX} + G_{TX} - L_{FS} + G_{RX} $$

  • $P_{RX}$: received power on Earth
  • $P_{TX}$: transmitted power by Voyager
  • $G_{TX}$: Gain of Voyagers transmitting antenna (compared to isotropic)
  • $L_{FS}$: Free space Loss, what we usually call $1/r^2$
  • $G_{RX}$: Gain of Earth's receiving antenna (compared to isotropic)

The gain of a dish or other full-aperture antenna $G_{RX}$ can be estimated by

$$G_{Dish} \sim \left( \frac{\pi d}{\lambda} \right)^2 e_A$$

where $d$ is the diameter of the dish, $\lambda$ is the wavelength, which is the speed of light of 3E+08 m/s divided by the frequency of 8.4E+09 Hz or about 0.036 meter (3.6 centimeters), and $e_A$ is some aperture efficiency term between 0 and 1 for a realistic dish, which we'll set to 0.8 at both ends arbitrarily. For the [Deep Space Network][5]'s largest diameter dish antenna of 70 meters, this becomes about 1.9E+07 which after applying $10 \times \log_{10}$ becomes about 73 dB.

For Curiosity's flat phased array, let's use 20 cm for the active area diameter, which gives 24 dB. That compares nicely to the 24 dB shown in How does Curiosity know how to point and move it's high gain antenna in real time?.

The Free Space path loss is calculated by calculating the fraction of an expanding spherical wave (from an isotropic radiator) that would be received by an area similar to one square wavelength. The exact equation in dB is:

$$L_{FS} = 20 \times \log_{10}\left( 4 \pi \frac{R}{\lambda} \right).$$

The reason the fraction flipped, but a minus sign did not appear outside is because by convention, loss is expressed in positive dB, and then subtracted by the minus sign in the "master equation". Currently Mars is close to 1 AU from Earth, so $R$ is about 1.5E+11 meters. That makes $L_{FS}$ about 274 dB.

Let's be generous and give Curiosity's High Gain Antenna 20 Watts, or 13 dBW. The final math becomes

$$ P_{RX} \ dBW = 13 \ dBW + 24 \ dB - 274 \ dB + 73 \ dB = -164\ dBW$$

The small amount of data I have so far is pretty noisy itself, but the values cluser around -150. The problem is that so far we don't know for sure if the units are dBW or dBm in the raw XML files. See Understanding the units in the Deep Space Network XML data? for more on that problem. If those were dBm, then the ~-140 to -160 dBm would be -170 to -190 dBW and agree nicely with this calculation as an upper limit.


I grabbed a screen capture just after the little wiggly radio wave icon stopped moving. I'll check for more data now via Missions at or soon-to-be at Mars and their DSN "codenames"?

enter image description here

below: I've used Deep Space Network data accessed as explained in this answer to look at communications directly between Earth and the Mars Science Laboratory (Curiosity Rover). Here is a plot of some uplink and downlink data points. For uplink power I assume the scale is straight kilowatts, and for downlink, that's probably dBWatts, but I'm still looking into it. These are X-band frequencies, uplink in Hz and downlink in MHz.

Horizontal axis is relative (sidereal) days, the data ends about 12 hours ago roughly.

Deep Space Network contact DIRECT with Mars Science Laboratory

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