# How does Hayabusa-2's “Communication Simulator” on its website work? What does it show?

The pinned tweet on Hayabusa-2's Twitter account leads to http://haya2now.jp/en.html which is a kind of control panel readout of the spacecraft's situation.

I clicked open communications simulator and some other things and got this screen. What does it mean? How can I interpret these dots? What is it telling me?

## 2 Answers

It's a teaching tool to help beginners understand the realities of communicating with deep-space spacecraft.

1. Hayabusa-2 has two High Gain antennas. The one shown on the left in this view is for X-band (7-8 GHz) which is currently the work-horse of deep space communications. Hayabusa-2 also has a complementary Ka-band high gain antenna on the right, but currently only a few of the Deep Space Network antennas can receive Ka-band. The Japanese ground stations shown can not use Ka-band. According to this answer Ka-band has some sensitivity to weather and water vapor, so it can not be reliably used at all times and all elevations above the horizion, so even though it provides a higher gain and data rate, X-band will still be used frequently for command and general spacecraft data links that don't require it.

There's more about Hayabusa-2's systems including communications here and also at 1 and 2.

2. Currently the one-way light time is almost 20 minutes. The illustration shows that you can send several commands before the first one is received, and you may still need to wait to find out the results of the first command even after the third command has been acknowledged and perhaps even responded to. The 2nd image below shows the round trip is nearly 40 minutes. Unfortunately the main page does not show the actual distance, but a quick check of Horizons shows about 356 million km, which implies a one-way light time of 1186 seconds or 19.8 minutes.

3. The main page shows six ground stations used with Hayabusa-2. Only the DSN stations can use Ka-band. See table below.

4. Visibility of Hayabusa-2 during communications is also shown. In the example shown below, it's clear that by the time the return signal is received in this case Hayabusa-2 has set below the horizon and the return signal would not be received. Thus planning and scheduling is critical to ground station management and successful spacecraft communication scheduling.

stations mentioned:

Agency  earth station locations       X-band  Ka-band
------  ---------------------------   ------  -------
JAXA    Usuda, Uchinoura                 X
NASA    Goldstone, Canberra, Madrid      X       X
ESA     Malargüe                         X


It is also pointed out that a ground station can consist of a complex of dish antennas each executing a different task:

Source

Currently there seems to be a bug showing the azimuth of Hayabusa-2 at the Goldstone location constantly moving much faster than is physically possible:

update: it appears this may have been fixed this morning (UTC +8)

The pannel also reflects times when actual communication is happening between Hayabusa-2 and ground stations in the real world. Here JAXA's Usuda Deep Space Center is communicating with Hayabusa-2, something that you can not see on the DSN Now page of course.

• Wow! Thanks for answering the question I forgot to ask forever ago when I played with this page and understood next to nothing about the context. – Magic Octopus Urn Dec 18 '18 at 17:52

It's just a very simple visualization of light lag - the 'dots' represent 'messages', and travel from the ground station (bar on the bottom) to the probe (bar below the drawing) at such speed that their progress corresponds to actual light lag - time for messages from the ground to reach the probe. Once each reaches it, a message back is sent and begins the travel 'downwards'.

It's just a nice educational tool to help visualizing how long actually it takes for communication to reach the probe, and for the response to arrive. It's practical usability is marginal (after a loooong wait you'll get the current round-trip time.)

• @uhoh: Size? I didn't notice that. Position - the vertical space represents the space between Earth and the probe, the position is just the distance traveled. Their color is just visual distinction. The probe image blinks the HGA discs upon reception of the message. Haya2now is mostly a fancy educational visualization, with little actually useful telemetry. – SF. Dec 15 '18 at 15:17
• @uhoh: because it reverses direction upon reaching the probe? Because the time to reach the probe roughly corresponds to the light lag which I know Hayabusa2 has? Because it closely corresponds to the main page visualisation of distance between Earth, Hayabusa2 and Ryugu? – SF. Dec 15 '18 at 16:26
• The visualisation doesn't make any significant distinction between the differences in the two HGAs, treating them quite equally. Light speed is constant so the height corresponds to distance the same as time. I suggest you just patiently wait until the dot reaches the probe, read out the time, then check Earth-Haya2 distance in light seconds using Nasa's Eyes and just compare the two values, if you need verification. – SF. Dec 15 '18 at 16:30