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An article on RT: Russia to stop ferrying US astronauts to ISS from April 2019, is accompanied by a photo of a ground-side radio array consisting of four separate parabolas merged into one pointable element, seemingly tracking the ascending Soyuz.

Four parabola radio array

Why would one subdivide the array like that? There does not seem to be an immediate advantage over a single, larger parabola.

Here are some overlapping possibilities I could think of:

  • Economics: It may be cheaper/faster/less finickiy to build four separate smaller parabolas instead a single large one with the same surface.
  • Technological/Physical limitation: Maybe emitters and receivers need to be put on different parabolas, so you need at least two parabolas in any case.
  • Redundancy: You have four, so no problem if one receivers or emitters go down, scheduled or otherwise.
  • Sturdiness: The supporting grid might be sturdier on this construction for the same budget.
  • Environmental: Wind or temperature swings give too many problems with a large parabola.
David Tonhofer
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3 Answers3

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It's a very simple phased array antenna. Other examples include cell phone antennas,

cell phone antenna array

this French satellite tracking antenna,

French satellite tracking array

and the famous Very Large Array (VLA) in New Mexico.

Very Large Array

You create a phased array by spacing more than one antenna at regular intervals. The individual antennas can be almost any type: dipole rods (cell tower), helical (French example above), or dishes (VLA and the Russian tracker). There is more than one way to arrange the antennas: a simple line, a rectangular grid, a wye (VLA), a triangle (cell tower), or a diamond (French tracker, Russian tracker). The antennas can be mounted in a fixed orientation (cell tower), individually steerable (VLA), or steerable as a group (French tracker, Russian tracker).

The advantage for received signals is that it can locate the direction of the signal. This is because one of the antennas will be ever-so-slightly closer to the radio source than another antenna. This causes the sinusoidal signal of one antenna to be slightly out-of-phase with that of another antenna. (That's why it's called a "phased" array.) By comparing the phases of the received radio signals, one can calculate the direction of the radio source. This is how cell phone towers know where your phone is, even when you turn GPS off. So, the primary use of these arrays in tracking spacecraft is determining the spacecraft's location.

For transmitted signals from the array, you can do essentially the reverse process, and electronically steer the radio beam. It's not quite as vital for the Russian array in your question -- as the entire array can be mechanically steered -- but you can change the orientation much more quickly electronically than mechanically.

You mention increasing the size of a dish antenna. That is indeed helpful for boosting weak signals. However, it isn't much of an issue with an antenna so close to the launch site.

DrSheldon
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  • Lovely answer - except I have a minor issue with the cell tower piece. Typically cell towers do not know what direction you are in as they don't direction find this way. The only do triangulation using signal strength between towers. – Rory Alsop Sep 04 '18 at 06:30
  • @RoryAlsop: I'm pretty sure they triangulate basing on ping times, not signal strength. They can still detect your position quite accurately in varied terrain like forest, where the signal strength of the towers in range is very randomized. – SF. Sep 04 '18 at 09:05
  • Cell Site Location Information (CSLI) is not through ping times, no. It is entirely signal strength. Stingrays use this data to accurately triangulate a phone. They do not use ping times. – Rory Alsop Sep 04 '18 at 12:26
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I did some research in Russian media and found this (in Russian)

Looks like the answer by @Dr Sheldon is right about the antenna's form.

It's tracking anthenna for receiving of rocket's telemetry.

quote from the link:

А сейчас телеметрию стартующих "Союзов" принимают на комплекс МКА-9 с антенной "Ромашка"

translation:

In present time the telemetry of Soyuz rockets is received by MKA-9 complex with "Romashka" antenna ("Romashka" means "Chamomile")

enter image description here

The antenna is situated to south from Gagarin's start on Baikonur, I suppose it's here:

enter image description here

coordinates: 45.909721, 63.334086

I've also found frontal view of similar antenna:

enter image description here

source

P.S. Found one more nice photo

enter image description here source

Heopps
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  • I love these links, and these images, thank you! If there is any way to confirm that there are four of them in order to sense the direction in order to move the antenna more accurately, that would be better, thought it may not be possible. By the way, the image to which "Обратите внимание на антенны под телескопом" refers may be an answer to the question What were those motorized human-piloted platforms with helical antennas called (tracking launches)? – uhoh Sep 05 '18 at 15:42
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    @uhoh nothing is said about it in the link, but I think it's most probable. It could be especially useful for off-nominal flight, where rocket's trajectory can become unpredictable (remember Proton launcher disaster in 2013). In such case direction determination is valueable, I suppose. – Heopps Sep 05 '18 at 19:45
  • Okay thanks! On the last part of my previous comment, are you interested in posting an answer there? If not, I'll probably do it because it seems to be exactly what I'm asking for, but since I'm just relying on google translate it would be better if someone who had a better understanding of the link would do it. – uhoh Sep 05 '18 at 23:24
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    Wow that tracker-and-rocket-and-dust image might be right out of a "Martian Chronicles" movie. – David Tonhofer Sep 06 '18 at 20:45
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    @uhoh there is no any specific info about that telescope-atenna, only photo. – Heopps Sep 07 '18 at 12:01
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Aha! update

I found another quad parabolic dish array that uses electronic conical scanning!

From NASA Technical Note TN D-6723 Apollo experience report: S-band system signal design and analysis which I found here

The high-gain antenna consists of an 11-inch-diagonal wide-beam horn flanked by an array of four 31-inch-diameter parabolic reflectors, as shown in figure 5. Transmitting beam widths of 40.0°, 11.3°", and 4.4° are selectable by manual switch. Reception and transmission gains corresponding to these beam widths are listed in table I. The antenna tracks by using electronic conical scan where the angle-tracking information is encoded as amplitude modulation (AM) on the phase-modulated signal received from earth. This error information is extracted within the USB equipment by a narrowband coherent amplitude detector and routed back to the antenna system, thereby providing angular displacement control.

enter image description here

I'm going to build on @DrSheldon's hypothesis and @Hobbes's comment that by subdividing a single dish into four, it allows for a small amount of fast electronic steering by dynamic phasing of each of the four signals before combining them in order to track the direction of motion of the rocket.

That makes a lot of sense since the target is moving, and in a way that could be unpredictable.

This excellent answer describes the use of conical scanning to determine the size and direction of the offset between a radio source's actual position and the current pointing direction of an antenna.

You can think of this as a form of dithering, to sense the local gradient of signal strength with respect to pointing offset.

The GIF below shows a spinning secondary reflector of a Cassegrain dish, and in this answer I mention how even the largest 70 meter Deep Space Network dish uses CONSCAN (https://deepspace.jpl.nasa.gov/dsndocs/810-005/302/302C.pdf section 2.6.1 page 17) to "zero-in" on a target even if it's far away and thus not changing so fast in position in the sky.

below: GIF from here:

enter image description here

below: GIF from here:

enter image description here

below: From here:

enter image description here

uhoh
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