# What is the pop-up circular disk with spiral pattern in this NASA animation of the Dragonfly helicopter for Titan? Antenna? Kind, band, target?

At 00:36 in the June 2019 NASA video Dragonfly: NASA's New Mission to Explore Saturn's Moon Titan the animation shows it landing then deploying a circular disk with a spiral pattern of dots at 00:44. There's a zoomed/sharpened detail of a screenshot shown below which reveals a spiral pattern of "T"-shaped little spots circling about 6 times; dense in the middle, sparse along their spiral path at the edge.

@Hobbes' answer to Does the Dragonfly project (quadcopters on Titan) envision attached RTG's or would they be static and revisited for charging? has several newer details and images of the helicopter and they show the disk.

From a labeled heat shield diameter of 3.7 meters we can estimate that end-to-end Dragonfly is roughly 3 meters long(!!) and the disk roughly 80 to 100 cm in diameter.

I'm thinking "high gain antenna" and superficially comparing to Curiosity's and Perseverance's highly articulable HGA. This one looks a heck of a lot thinner, but part of that may be artistic license and part due to it being larger in diameter than those of the rovers.

click images for larger size left: Curiosity's High Gain Antenna (articulated dirty hexagon). Cropped from here. right: Curiosity's High Gain Antenna gain in downlink modes, from MSL Telecommunications System Design.

We can see that Dragonfly is expected to do a lot of communicating each Titan solar day of 15.92 earth days. "Why solar day? on a moon with an opaque atmosphere an radioisotope thermoelectric power so far from the Sun" you might ask? Because from 10 AU the Earth and the Sun are always within about 6 degrees of each other. It's not as close as the 0.4 degrees for the Voyagers but Earth's radio visibility will be synchronized to solar rather than sidereal days on Titan. How well can Voyager 1 separate Earth signals from Solar noise these days?

Source: R. D. Lorenz et al. Johns Hopkins APL Technical Digest, Volume 34, Number 3 (2018), www.jhuapl.edu/techdigest Dragonfly: A Rotorcraft Lander Concept for Scientific Exploration at Titan

Figure 7. Energy management and communication concept of operations. MMRTG continuously recharges the battery, but downlink and especially flight demand significant energy. Activities can be paced to match MMRTG in situ capability while maintaining healthy margins on the battery state of charge.

### Question:

What is the pop-up circular disk with spiral pattern in this NASA animation of the Dragonfly helicopter for Titan? Is it a high gain antenna? If so, what kind, how does it work, what band will it use and "who" will it talk to?

"Bonus points:* Why do the little "T" shapes rotate by only 180 degrees each time the spiral completes a full circle?

Related to spiral and concentric circle antennas:

Related to phasings, phased array antennas and beam forming; no evidence of a spiral pattern of individual radiator elements anywhere!:

Screen shots from Starlink Teardown: DISHY DESTROYED!. See also How does this SpaceX Starlink ground station antenna's gear mechanism move it in both altitude and azimuth?](https://engineering.stackexchange.com/q/40749/6264) and Teardown of “Dishy McFlatface,” the SpaceX Starlink user terminal

click images for larger size

• Could you try to keep your questions a bit more concise? I don't see how 28 (!) links help in any way. Jun 27, 2021 at 8:09
• @asdfex I've tidied up a bit now, but I don't see any actual harm to occasionally consolidating the site's wisdom on a particularly challenging topic like phased array antennas for deep space communication. Some readers may find the topic interesting or need some additional sources for an answer and many of these answers are excellent and worth revisiting.
– uhoh
Jun 27, 2021 at 8:20
• Wild guess on the bonus question: because 180° is good enough (a slot and its 180° rotation are indistinguishable, it only changes the relative orientation of the slots in each pair, which probably doesn't matter) and the smaller angle change between successive slot pairs means they can be packed a little closer. Jun 27, 2021 at 23:29
• @hobbs I think that's likely to be the case as well; slots are slots.
– uhoh
Jun 28, 2021 at 1:16

Quick answer, bit short of time atm:

What is the pop-up circular disk with spiral pattern in this NASA animation of the Dragonfly helicopter for Titan? Is it a high gain antenna? If so, what kind, how does it work, what band will it use and "who" will it talk to?

As you rightfully guessed, it is also mentioned in the answer you linked to:

It is a HGA - high gain antenna - and it is used for direct communication to Earth.

The two blocks on top are for the navigation camera suite, as such the dish is articulated.

The articulation of the dish will help to build up a visual panorama.

https://www.lpi.usra.edu/opag/meetings/feb2020/presentations/Turtle.pdf

As for the antenna:

Single layer circular apertures also known as radial line slot arrays (RLSA).

Linearly polarized arrays are also provided by changing slot array arrangement.

For the circular pattern, there's concentric and there is spiral, which is used here:

Radial line slot array (RLSA) antennas are a type of cavity or waveguide antennas.

These antennas were firstly developed for satellite receivers as an option besides parabolic antennas.

Unlike parabolic antennas, RLSA antennas have an advantage of having feeders at the back of the antenna, so that the feeders do not block out incoming signals.

The other advantage is their flat shape so that they are more compact.

The (slot arrangement) technique aims to produce a uniform-aperture distribution. This antenna has a double-layer cavity and exhibits a good linear polarization. Ando also proposed a beamsquint technique to improve the poor reflection coefficient in linearly polarized RLSA antennas

The structure of RLSA antenna consists of a radiating element, a cavity, a background and a feeder.

The radiating element usually is a circular plate made of metals, such as aluminium, copper or brass.

The radiating element consists of many slot pairs.

One slot pair acts as one antenna element so that all the slot pairs form an array antenna.

The background is a metal plate just like the radiating element, but the background does not have slots.

The cavity is a dielectric material that has the form of a tube.

Together with the radiating element and the background, the cavity operates as a circular waveguide that guides the signal from the feeder to propagate in radial direction.

The feeder is a part of RLSA antennas used to feed signals from a transmission line into the antenna.

For the resultant radiation from each slot to combine at boresight to produce linear polarization, the slot excitation phases are required to differ by 0 or 180 degrees.

Therefore, the slot spacing is chosen to be half of the guide wavelength.

At a guess, it operates at the X-band Deep Space Network (DSN) frequencies.

Referencing Cassini:

this is a link that might yield more:

https://deepspace.jpl.nasa.gov/dsndocs/810-005/201/201C.pdf

And this says:

X-band capability is required for communication through the atmosphere of Titan

Although Ka-band downlink has a clear capacity advantage, there is a need to maintain multiple band downlink capability. For example, three-band telemetry during outer planet atmospheric occultations allows sounding of different pressure depths within the atmosphere. In addition, S-band is required for communications from Venus during probe, balloon, lander, and orbit insertion operations where other bands cannot penetrate the atmosphere. X-band capability is required for communication through the atmosphere of Titan, and also for emergency spacecraft communications. The committee recommends that all three DSN complexes should maintain high power uplink capability in X and Ka-band, and downlink capability in S, Ka, and X-bands.

• Overwhelmingly excellent answer, thank you! I see you are not even finished, yet, but don't forget the "'Bonus points:' Why do the little 'T' shapes rotate by only 180 degrees each time the spiral completes a full circle?" I don't think it has any impact on polarization, a slot is a slot, but there may be some other subtleties...
– uhoh
Jun 27, 2021 at 13:26

I am the designer of this antenna at JHUAPL. It is actually still under development, and the slot pattern shown is an artist's rendition and would not result in a working antenna. The current baseline antenna is similar to the antenna JHUAPL built for DART:

Technical details can be found in:

https://ieeexplore.ieee.org/document/9330400

The previous posters answers are pretty good. The RLSA antenna has a single feed in the center which produces a radial wave which travels from the center of the antenna to the edge of the antenna. Along the way energy leaks out of the slot forming a beam. Each T-slot pattern forms a single antenna pair which produces circular polarization. Each slot pair needs to be phased correctly to form a beam. To phase the elements they are arranged as a spiral. Consider a single diameter across the antenna. Elements on opposite sides of the center are oriented 180 degrees to one another. To correct for the geometry, the phase of the elements on opposite sides of center needs to be 180 degrees out of phase. To do this they are moved 180 degrees in electrical length farther from the center. Walking around the antenna like this forms the spiral.