Observe the laser light emitted by the satellites (ICESAT2, DAQI1)
Unlike normal satellites that shine by reflecting sunlight, satellites that orbit while emitting their own light are also interesting subjects to observe.
■ICESat-2
The Earth observation satellite ICESAT-2, launched by NASA in September 2018, flies while constantly shining a green laser with a wavelength of 532 nm toward the Earth's surface. It continues to measure the altitude of the entire Earth using a so-called LIDAR, which measures distance from the time difference between the light sent out and the light returned. Since it continues to conduct observations in about 90 days, it is possible to grasp, for example, changes in the thickness of ice in the polar regions, so it can be said that this project is connected to themes related to global warming. On the evening of 2022-09-16, Daichi Fujii's meteor observation camera accidentally captured the tip of the beam of laser light emitted by this satellite moving while illuminating the clouds in the sky, which became a hot topic.
Specifically, a laser beam from a common light source is split into six beams by a diffractive optical element, and three pairs of strong and weak beams scan the ground. The ratio of strong and weak optical power is 4:1, the distance between the strong and weak beams is 2.5 km, the distance between the pairs is 3.3 km, and the strong and weak beams do not pass through exactly the same position, but at positions 90 m apart. The light source is driven at 10 kHz, so it is visually perceived as almost continuous light emission.
The diameter of the beam on the ground is about 17m. When I was hit directly by the beam on a hazy night, I could feel the surroundings glowing green. This was probably beginner's luck, but I have not been hit directly since.
The ICESat-2 official website has an overview and explanatory video.
https://icesat-2.gsfc.nasa.gov/science/specs
https://icesat-2.gsfc.nasa.gov/multimedia
The period of ICESAT-2 is 94.2 minutes, so the speed at which the laser spot moves on the surface of the Earth is
6378000*2Pi/(94.2*60)=7090m/s=25525km/h
This is about 100 times faster than the Shinkansen. Since it emits a 10 kHz pulse, a 17 m diameter beam is emitted every 70.9 cm of travel. It is emitted without interruption along the direction of travel.
On a clear, cloudless night, if you observe from a position where the strong and weak pair passes, you can see with the naked eye two strong and weak beams, 2.5 km apart in the direction of travel, passing by quickly with a time difference of about 0.35 seconds. In particular, in hazy weather conditions, the laser beam undergoes Mie scattering along the way, making the path of the green light easier to see. This phenomenon is called the Tyndall effect, named after its discoverer. The same principle is used in smoke-filled concert halls and "Angel's Ladder," where sunlight reaches the ground through gaps in the clouds.
So, what should you do if you want to see this? The most reliable way is to take a position where the center pair of the three pairs passes by. The beam is not exactly directly below the satellite, but the angle is adjusted slightly for each observation cycle. (I heard this from someone inside.) However, if we amateurs just want to watch it for fun, it's probably okay to think of it as being almost directly below.
The URL to set in Stellarium to display ICESAT-2 is
https://celestrak.org/NORAD/elements/gp.php?CATNR=43613
For information on how to view satellite positions and movements in Stellarium, please see the post below.
https://apnea3.blogspot.com/2023/08/starlink.html
Now we know how it looks from our current location, but the problem is where is the observation point where this satellite can be seen at the zenith. It is possible to find this with Stellarium, but it is time-consuming and troublesome. There is a site that can meet this purpose, so let's start with this one.
https://www.n2yo.com/passes/?s=43613
If there is a visible passage within the next 10 days, it will be displayed here, but at the moment there happen to be no visible passages at all, so nothing is displayed by default.
To view passes where no light hits the satellite, press the "All passes" button and a list of passes visible above the horizon from your set location will be displayed.
Look at the "Max Altitude" column in the center of this table. Rows where the LocalTime is daytime are not visible, so they are excluded. Among the rows where this time is night, look for rows with the largest possible EL (elevation above the ground). The larger this is, the closer the distance from your location to the observation location, so less movement is required. 90 degrees is ideal, meaning that the laser beam will pass while illuminating your location. This table is based on Kurashiki City, but we can see that it will rise to 87 degrees at 23:04 on 11/10/2023. With this, the satellite will be visible at the zenith with just a little movement.
The satellite orbits through the polar regions, so it may pass from south to north or from north to south. You can see the direction of the pass by looking at the Start and End columns.
Clicking the "Map and details" link in the rightmost column will display a map with the satellite's ground orbit overlaid. This map can be enlarged to the level of a residential map, allowing for precise confirmation of the observation location. Since the laser light can be received anywhere along this line, you can select suitable observation locations by examining the local conditions, visibility, and whether there are parking spaces available on this map or the corresponding Google Map or Google Street View. It is a good idea to look for observation locations in remote locations, taking into account the weather when the satellite passes by.
Once you have determined the exact observation location, press and hold the corresponding point on Google Maps to find the coordinates. Bring up the Stellarium observation location setting screen, paste the coordinates of this location into the "City Name" field, and register the location. The name will be
ice-20231110-Kurashiki-34°35'01.5"N 133°47'12.4"E (The coordinates are a sample.)
Then, enter something like this, select and copy the coordinate portion of the string, paste it into the settings field on the left, and save it with the "Add to list" button. This will then allow you to call up the search later by simply entering "ice-", which is very convenient.
Once the exact coordinates are determined, check with Stellarium whether the satellite really passes through the zenith. For some reason, in most cases the satellite's orbit is not at the zenith. (I'm writing this on November 7th.)
While I was writing this article, there were changes in the orbital elements. The following two figures show the orbits that have changed from the above. The top shows a point on the forecast line on the N2YO site, and the bottom shows the orbit of ICESAT-2 where only the longitude was corrected by minus 30.7 seconds of angle so that the orbit passes exactly through the zenith.
So, on the other hand, when I searched for "34°35'01.5"N 133°46'41.7"E" on Google Maps, the icon was displayed 774m further west than the N2YO forecast. This means that it is dangerous to trust the N2YO forecast as it is.
Since the source of the orbital elements used by the N2YO site is unknown, comparisons are not possible, but there may be differences due to the frequency and timing of updates, or differences in the algorithm. In this example, there are several days until the actual passage, so there is likely to be a significant margin of error. Orbital elements fluctuate due to various factors. When the density of the upper atmosphere changes due to solar activity, the resistance of flight also changes. In addition, orbits and attitudes may be changed artificially for satellite operations. In any case, it will be necessary to check the ground orbit to reflect the most recent changes as much as possible. It is necessary to be flexible at all times when searching for candidate sites.
There is also a mobile app that can forecast the passage of ICESAT-2, which is also introduced on NASA's ICESAT-2 website.
https://play.google.com/store/apps/details?id=gov.nasa.gsfc.icesat2.icesat_2&hl=en_US&gl=US&pli=1
But the bottom line is that this is currently useless.
The position and time are inaccurate. Since it orbits, it may happen that it passes through a close orbit, but even in that case, the time is off by several tens of minutes. I asked someone inside, and they said that for one cycle (about 90 days), the RGT (KML of the reference ground orbit) is calculated one month before the cycle begins. This means that there can be a time difference of up to four months. As mentioned above, the orbit of a satellite is constantly changing, and the orbital elements must be constantly corrected based on the observation results. Even an amateur can understand this much, so it is a bit poor that the app introduced and released by NASA has such specifications. The purpose of ICESAT-2 is to obtain accurate attitude and position information at that time in combination with LIDAR measurements, so it is not a problem if the pre-calculated RGT is different from the actual one...
■ ICESAT-2 observation points
The location varies greatly depending on the weather. If the sky is clear and the atmosphere is clear, the scattering of the beams shown above will be difficult to see. Also, only the pair of beams with weak and strong beams will be visible, and another pair 3.3 km to the east and west will be almost invisible.
If there is haze or PM2.5 that causes the Tyndall effect, you can see the scattered light shown above. If particles are present at high altitudes, you may be able to see the scattered light caused by the stronger beam of the adjacent pair.
The situation is different if there are thin clouds in the sky. The thin clouds act as a screen, projecting the end of the beam as a point of light. Since this moves at the satellite's flight speed, it is seen as a line. In this case, the light is attenuated by the thin clouds, so the scattered light due to the Tyndall effect will be difficult to see.
If the thin clouds are high up, there is a chance that the adjacent beam pairs will also come into view, which can lead to some interesting sights. In fact, I have witnessed with the naked eye six points of light, as shown in the schematic diagram, moving at high speed on top of thin clouds. The photograph I took at that time captured pairs of strong and weak light trails, so I was able to estimate that the cloud height was about 300m. There were thin clouds moving in the background, with the stars visible through them, but even at this low height it is possible to see all six points at the same time. If you go higher you will be able to enjoy the view at a more leisurely pace.
Here is another example where we were able to capture the beam trajectory.
2023-11-10 23:04:54 Yorishimacho, Asakuchi City, Okayama Prefecture α7SIII + EF50mmF1.4 120fps video
On this day, the cloud height was quite high, and the central and western pairs were captured within the same angle of view, and it was estimated that they were about 9 km from here. Depending on the cloud conditions, if you have a bright lens of about 35mm to 20mm, you may be able to capture the trajectories of the beam ends of all six beams in the three pairs.
When shooting, I don't recommend taking time-lapse photos. The laser light only passes for an instant, so if it shines between frames, nothing will be captured. Video shooting is essential. Especially when there are clouds in the sky, it's better to increase the frame rate as much as the sensitivity allows in order to capture the movement of the end of the beam reflected in the clouds, so that you can see the movement. I've only had a few experiences, so I'm still in the trial and error stage, but I make some discoveries (and failures and regrets) every time.
Taking photos is fun, but we highly recommend looking up at the zenith with your naked eyes at the moment of the passage. Once you have decided on the coordinates of your observation location, you can use Stellarium to check the time of the satellite's passage at those coordinates, so there is almost no chance of an error, and you should be able to get the timing right by
holding . If the atmosphere is transparent, the beam will be particularly sharp, so unless you hold it directly below with great precision, it may not be visible to the naked eye.
For more information about ICESAT-2, please see the following:
https://icesat-2.gsfc.nasa.gov/
You can also get more detailed information for a fee from the following website:
https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11151/111510C/ICESat-2-mission-overview-and-early-performance/10.1117/12.2534938.short?SSO=1
This is the video that the image above was based on. The playback speed is set to 1/4. First, a weak beam is seen, followed by a strong beam, and then a faint, but scattered light from what appears to be a strong beam from the eastern pair. I took the photo on the forecast line on the N2YO site. The beam was seen to the right (west), confirming that the prediction by Stellarium was correct.
2023-11-02 23:21:48 JST Moon age 19.6 Elongation 60 degrees
EOS6D 50mmF1.4 Video 1920x1080 30fps
Photographed in Miki City, Hyogo Prefecture
The image has been brightened slightly using image processing.
■2024-05-10 Suspended due to CME
Since May, when an intense CME occurred that was so strong that low-latitude auroras were visible even in Japan, ICESat-2 has been affected and has been in safe hold, suspending observations. I set out to photograph the zenith passage under clear, favorable conditions, but I couldn't see any laser light, and when I checked the official website, I found the following notice. I went out to observe without noticing the notice from @NASA_ICE, which I follow, so my trip was wasted.
After that, the following article was published on NASA's blog, but the schedule has not been updated as of June 25th, and there is no confirmed information. It may have already started observations and equipment tests are being conducted. It will pass near Hinase, Bizen City, Okayama Prefecture on June 28th, so I will check it out if it is clear.
2024-07-02 Recovery confirmed!
Even though the announced date of 6/17 had passed, there was still no announcement of recovery from the ICESat-2 official website or from NASA, so I couldn't wait any longer and went out to check the light emission when it passed over the nearby city of Kasaoka on 7/2. The rain had just stopped and the sky was covered in clouds, but as if in time for the satellite's passage, a thin part of the clouds moved to the zenith, allowing me to capture the green laser light with my naked eye and camera. Vega in the constellation Lyra is nearby, but it is hidden by the clouds and cannot be seen at all. Since I could see it from directly below the correct orbit, I was able to confirm that ATLAS was operating and emitting laser light at NadirPoint.
The top line of N2YO is the pass this time.
Check the visibility and time from the candidate location found on Google Maps with Stellarium
The site after the rain
The timing of ICESat-2's zenith passage and the thin clouds passing over the zenith coincided.
The light was visible to the naked eye. (EOS6D+50mmF1.4 30fps video)
SONY α7sIII+14mmF1.8 120fps video 0.25x faster
■ Posting to X (formerly Twitter)
Inspired by Daichi Fujii's tweet, I traveled all the way from Okayama to Suzuka in search of conditions where I could photograph laser light.
■DAQI-1
Besides ICESAT-2, another satellite that shines a laser on the Earth's surface is DAQI-1, operated by China. Although its laser output seems to be weaker than ICESAT-2's, it is still capable of taking images. In January 2023, it
became a hot topic when scattered light from a laser emitted by this satellite was accidentally captured on a high-sensitivity live camera on Mauna Kea.
https://youtu.be/vn_PMiND4Yw
Looking at the video footage, it appears that the satellite is emitting radiation intermittently at about 20Hz.
Information about the satellite is provided below.
https://space.skyrocket.de/doc_sdat/daqi-1.htm
The URL to set in Stellarium to check the orbit is
https://celestrak.org/NORAD/elements/gp.php?CATNR=52257
The passage forecast from the N2YO website is
https://www.n2yo.com/passes/?s=52257
.
2023-11-01 01:51:40 Taken in Kurashiki City
Borg77mm+Reducer 合成焦点距離330mm
A 4K resolution central HD cut from an α7SIII video.
Videos shot at 120fps cannot be displayed as is, so they are slowed down four times.
In the comparatively bright composite image, the period from 30' before the zenith passage until the zenith passage appears relatively bright.
If we assume that the beam spread from an altitude of 700 km to the ground is a perfect circle, we can calculate the diameter:
2*Tan(0.5deg/2)*700＝6.1km
If you look up from within a band of about 6 km width centered on the moving line of NadirPoint, you will likely be able to see the spillover of the beam being emitted. (However, this is not the case if there is an offset to the left or right of the direction of travel.) This is merely a spillover, and there is no data available on the diameter of the strong beam in the center (the beam pillars standing at intervals of about 338 m) that spreads out. (In the case of ICESAT-2, the diameter is about 17 m.)
■Is it possible to capture beam scattering light like that seen in Hawaii?
Last night (early morning 2023-1-28 HST) was cloudy on Maunakea. But our Subaru-Asahi Star Camera captured quite an interesting view -- green laser lights coming from the sky! It was only a second or less -- but our keen viewers did not miss the event!
According to the video caption, there were clouds on the day. The beam is much weaker than ICESAT-2, so I don't think it would be possible to capture scattered light on a clear day, but I think it would be possible if there was a new moon, no light pollution, and thin clouds or haze. The Subaru-Asahi Starry Sky Live Camera is a combination of SONY α7S III + FE 24mm F1.4 GM. (I'm jealous of the bright lens...)
I searched on YouTube and found a video in Fukushima Prefecture showing the DAQI-1 beam being scattered by thin clouds. Only the areas where the beam passes through the patchy clouds are lit up. It seems possible to capture this if the weather conditions are right in a location with little artificial light.
Fukushima Takikawa Live 2022/12/20 Mysterious green light at 1:30 (Thought to have been resolved on 1/29, but new information on 2/13!)
https://youtu.be/Zg3EIKyw4Ns?si=_uNLWpQyrmsXEgng
The clipping
https://youtu.be/QhhmxJsZzQU?si=9v4JVuBxTwpaKyaV&t=24
I found someone else who took some great photos. It seems that the DAQI-1 laser light was captured by chance while observing the Lyrid meteor shower in Kuwana City, Mie Prefecture. The equipment was a combination of UFOCapture with motion detection function and Nikon Z6 + 24mm F1.4, so the angle of view is the same as Mauna Kea. There seemed to be layered clouds, and dots of light were projected onto the clouds in response to the pulsed light emission, and some scattered light on the line in the direction of the beam was also captured. The photo was taken on 2024-04-23 03:39:18-19. Because the existence of this video was noticed late, the orbital elements have changed considerably, and it is now impossible to simulate it with Stellarium.
"Photo diary of sky and clouds"
https://kokoten.raindrop.jp/kumonikki_sub_2023_04/web20230423/kishou20230423.html
As far as we know at this time, there are no images or videos that capture the scattering of DAQI-1's laser light.
2023-01-28 Mauna Kea, Hawaii
2022-12-20 Takikawa Valley, Fukushima Prefecture
2023-04-23 Kuwana City, Mie Prefecture
There are only three cases. There are quite a few examples on the Internet, including my own, of the diffuse light of a beam reaching directly, but I would like to capture the end of the beam projected onto a cloud layer and the beam scattering due to the Tyndall effect.
I'll also simulate what the beam will look like when I take the photos myself.
The period of DAQI-1 is 98.8 minutes, so the speed of movement of the Nadir point is
6378000*2Pi/(98.8*60)=6760m/s
Dividing the Daqi-1 pulse frequency of 20Hz gives the beam spacing.
6760/20=338m.
Let's draw the situation where pillars of light stand at this interval on Google Earth. You can calculate the coordinates yourself, but it's a bit of a hassle, so I gave the following prompt to the AI ??"perplexity" to create a KML file.
"Create a KML file that shows 22 vertical lines stretching from sea level to 5,000 meters, spaced 338 meters apart from north to south. The surface coordinates of the northernmost line should be 35°23'20.03"N 138°43'46.58"E. Calculate the surface coordinates of the remaining 21 lines and show them as KML without omissions."
This exchange can be reproduced using the URL below, so you can run it again by changing the parameters.
https://www.perplexity.ai/search/5000338m22KML-35232003N-138434658E21KML-Ai.nLspgToyTvGlETpXfzw#0
Click the icon in the upper right corner to copy the answer. Paste it into a text editor and save it with the extension kml. Open it in Google Earth, change the height in the properties from "fixed to ground" to "above sea level" and you will see a line that reflects the height. You can also freely change the color, thickness and transparency on Google Earth.
In this example, to give you a sense of scale, I have drawn the beam passing north and south over Mt. Fuji as seen from the Subaru Line with a horizontal angle of view of a 35mm lens. The beam moves about this much in one second.
By reading the coordinates of the Nadir points at two convenient times using N2YO or stellarium and having the above AI perform interpolation calculations, a precise simulation of each passage is possible.
Trying to take a photo of an actual train passing by
The DAQI-1 NadirPoint was forecast to pass through Bizen City, Okayama Prefecture on 2024-06-29. According to the SCW, it is forecast to be cloudy rather than clear. There was a good observation location directly below, but I tried shooting from a location about 2km west of there, looking up. I created a KML and plotted it on Google Earth with HFOV set to 14mm to check the angle of view. This makes it easier to set the azimuth and elevation angle in conjunction with the silhouette of the mountain at the site. When I left Kurashiki, the clouds were thin and patchy, so I was a little hopeful, but by the time I arrived at the site, the entire sky was covered with thick clouds, and even the moon low in the sky was completely invisible. I took a 14mm F1.8 + 120fps video and a 50mm F1.4 + 10-second exposure still image, but it was not visible in either. I was able to confirm the work flow this time, so I'm happy. In the future, if it is not clear, I would like to aim to shoot beam scattered light in this pattern.
Orbit of the candidate observation site directly below
Looking down from 11km above the ground
A simulation of the view from "Osafune Beautiful Forest" 2km away (HFOV 14mm equivalent)
Check local images using StreetView
The scene immediately after it passed on the day. The thick clouds blocked the photo, so nothing could be captured.
HKIR EOS6D+50mmF1.4 10-second exposure with center of pass
↓
I wasn't sure about the image, so I processed it to get the image below.
HKIR EOS6D+50mmF1.4 10-second exposure with center of pass
Image processing with PixInsight (DynamicBackgroundExtraction+CurvesTransformation)
As expected, the clouds were too thick and the laser light could not reach us.
