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The Curiosity rover has a compact X-band high-gain antenna (HGA) that allows for low data rate communication directly with Earth, but is used mostly for higher data rates to spacecraft orbiting Mars, which then relay the data back to Earth with their much larger dish antennas.

From Section 2.2, Surface Operations of Article 14 of the DESCANSO Design and Performance Summary Series Mars Science Laboratory Telecommunications System Design one can see that the HGA needs to point in the correct direction within a few degrees;

The HGA sits on a 2 degree-of-freedom gimbal, with 5-deg system pointing accuracy (including rover attitude knowledge), and is 0.28 m in diameter. Table 2-4 shows that the downlink gain is about 4 dB lower and the uplink gain about 3 dB lower, at 5 deg off boresight.

How does Curiosity get its "rover attitude knowledge"? Also, since the position of Earth is moving with respect to Mars, as is that of the satellites around Mars, How does Curiosity know how to slew the antenna correctly in real time? Does it cary an ephemeris which is updated from time to time via uplink?

Curiosity's High Gain Antenna

above: Curiosity's High Gain Antenna (articulated dirty hexagon). Cropped from here.

below: Curiosity's High Gain Antenna gain in uplink and downlink modes, from MSL Telecommunications System Design.

Curiosity's High Gain Antenna gain Curiosity's High Gain Antenna gain

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  • $\begingroup$ Also, even more important, with no magnetic field, attitude is indeed difficult to get. $\endgroup$ – Prakhar Apr 14 '18 at 12:43
  • $\begingroup$ I'm not sure if this is how it's done but technically you could use an accelerometer to calculate two of the robots rotational positions. Then, combined with a clock and an ephemeris, Earth's location (if visible) would be along a circle in the sky. Then you just need to scan along this circle for the best reception. Once you find earth, can then recalibrate a gyro or other inertial navigation systems. $\endgroup$ – Dragongeek Apr 14 '18 at 13:48
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    $\begingroup$ Earlier Mars rovers used a sun-finding camera + accelerometers to fix their attitude; I couldn’t find an equivalent paper on Curiosity. pdfs.semanticscholar.org/55bf/… $\endgroup$ – Russell Borogove Apr 14 '18 at 18:11
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    $\begingroup$ From what I can tell, the high gain talks to earth at a low data rate, and the omni antenna can hear the DSN at a low rate and talk to Mars orbiting relays at high rates. To track earth I'm guessing a sun tracker and ephemeris is plenty. A start tracker could probably find earth directly, but that's probably overkill. Navcam could serve as the sun tracker, since they're already doing object detection onboard. All guesswork though. $\endgroup$ – Saiboogu Apr 14 '18 at 18:34
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    $\begingroup$ From the link provided by @Russel I would like to highlight one interesting statement in the paper. The rovers did not have gyrocompassing which can looks for mars rotation vector. The rotation vector can be used with position to calculate the coarse attitude if camera failed at anytime. $\endgroup$ – Prakhar Apr 15 '18 at 10:40
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This answer is based on a book called "The Design and Engineering of Curiosity"

https://doi.org/10.1007/978-3-319-68146-7

On chapter 6 it is mention that

The Hazcams and Navcams are flight spares or build-to-print copies of the engineering cameras of the same names on the Mars Exploration Rovers; this not only saved money in hardware, but made it significantly easier to use a modified version of the same rover driving software for Curiosity as for Spirit and Opportunity.

We can assume that the Surface Attitude Position and Pointing (SAPP) is similar to the one used in the Mars Exploration Rover. Because of that, we can rely on a paper called "Mars Exploration Rover Engineering Cameras"

https://doi.org/10.1029/2003JE002077

It seems that the attitude information is collected from two main sources: an inertial measurement unit and the rover cameras. The IMU provides the rover's roll and pitch angles (Nadir vector) by measuring gravity acceleration. The NAV cameras are used to calculate the Sun position.

If the rover position is known it is possible to calculate the sun position in an inertial frame (J2000) using a well-known model. Combining this knowledge with the Nadir vector and sun vector measurement can be used for estimating the robot's attitude and the antenna attitude as well.

enter image description here

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  • $\begingroup$ Thanks for your answer. The Engineering Cameras paper looks great! I'll give it a read. $\endgroup$ – uhoh Apr 22 '18 at 20:56
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    $\begingroup$ Bottom line: accelerometers give down, Sun gives azimuth. On-board ephemerides of Earth, Mars, and the location of the rover on Mars, all of which are kept up-to-date by the operations team, allow the HGA to be slewed to track Earth in the sky. There is one more critical instrument and calibration required: the current time. $\endgroup$ – Mark Adler Apr 23 '18 at 0:46
  • $\begingroup$ @MarkAdler any thoughts, or corrections? space.stackexchange.com/a/30829/12102 $\endgroup$ – uhoh Sep 21 '18 at 23:39

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