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skysat

WorldView-4 and SkySat launches

The next couple of days will see two significant launches for satellite imaging. If all goes well, the first launch will be four SkySat satellites owned by Google’s Terra Bella. They are expected to launch with a Peruvian reconnaissance satellite aboard a European Vega rocket. See the count down clock and more launch details here. The launch takes place from ZLV, Kourou, French Guiana. Next will be WorldView-4. It is a DigitalGlobe satellite with similar specifications to WorldView-3, the current leader in high resolution commercial satellite imagery. According to spaceflightnow.com the launch is scheduled to take place from SLC-3E, Vandenberg Air Force Base, California.

[ Update: We didn’t realise at the time of writing that PerúSAT-1 which is being launched together with the SkySat satellites is also an Earth observation satellite with a resolution of 70 cm per pixel panchromatic and 2 m per pixel for colour. Learn more about it here]

We already looked at WorldView-4 last month, so today we are focusing on the SkySat satellites. Those being launched today are SkySats 4 through 7. SkySat 1 was launched in November 2013, SkySat-2 in July 2014 and SkySat-3 in June 2016. We have seen imagery from them a number of times, including imagery of the damage caused by Italy’s recent earthquake, a poppy display at the Tower of London and the Burning Man festival. We also once came across a SkySat image in the Sahara, which has since disappeared from Google Earth.

The SkySat satellites have an imagery resolution of about 90 cm per pixel. This is not as good as WorldView-4’s 30 cm per pixel, but is better than Planet Lab’s Dove satellites, which have a resolution of 3-5 m per pixel. It is also better resolution than the best imagery Google Earth currently has for some locations, so we hope Google considers using Terra Bella imagery to fill in the gaps in Google Earth.

Unlike SkySats 1 and 2, SkySat-3 has propulsion, which gives it greater flexibility in capturing images. Presumably 4 thorough 7 also have propulsion. Read more about the differences between SkySat’s 1 and 2 and SkySat-3 here.


The SkySat satellites being prepared for launch as tweeted by CNES. Image credit ESA-CNES-ARIANESPACE.

Italy’s earthquake

As of this writing, Google has not updated ‘historical imagery’ for almost three months. Up until mid-June they had been updating it almost weekly. As a result, there have been a lot of events over the past few months that we know were captured by DigitalGlobe but we cannot access the imagery in Google Earth. One such event was the deadly earthquake that struck central Italy on August 24th, 2016.

Google has provided an image of the region from one of Terra Bella’s SkySat satellites. It can be viewed in Google Earth using this KML file. They SkySat satellites are not as high resolution as most Google Earth satellite imagery, but in this case, some of the affected regions in Italy do not have high resolution satellite imagery – all they have is SPOT imagery, which is lower resolution than the SkySat imagery. Despite the relatively low resolution, we can see some of the effects of the earthquake in the imagery.

before
after

Before image: CNES/Spot Image. After image: Google / Terra Bella.
Amatrice, Italy. 1: The location of most of the damaged buildings. 2 & 3: Tents set up after the disaster.

We also saw tents in Grisciano in the Terra Bella image.

Also worth looking at is the Copernicus Emergency Management Service. They gathered satellite imagery of the affected region almost immediately after the earthquake and within a day or two had arranged aerial imagery too. The results can be seen on this page. There are maps of the affected towns, including grading the amount of damage down to building level. For example, you can see the map for Amatrice in this PDF.

Sentinel-1A and Sentinel-1B are radar satellites that are particularly good at detecting changes in terrain. You can see deformation maps in this article that uses images from before and after the event to detect how much the ground had moved after the earthquake.

Sun-synchronous orbit

In yesterday’s post we suggested that the reason for the near polar orbit of most imaging satellites was to improve coverage. After a bit more research it turns out to be more interesting than that. Apparently there is a special orbit called Sun-synchronous orbit, which is designed such that the satellite always crosses a given latitude on the ‘day’ side of the Earth at the same time of day. This has two benefits:

  • if two neighbouring images, or images of the same location are captured on consecutive passes of the satellite, they will have the same lighting (both the overall brightness as well as the length and direction of shadows), which makes it much easier to stitch them together in the case of side by side images or comparing for changes in the case of images of the same location.
  • the orbit can be positioned such that the satellite is always overhead as close to noon as possible (or whatever time is considered the best for imaging) for the latitudes of greatest interest. A non Sun-synchronous near polar orbit would result in some days when the satellite is orbiting in a plane at 90 degrees to the direction of the sun, which would make it constantly sunrise or sunset.

A Sun-synchronous orbit has a particular inclination depending on the altitude of the satellite. The lower the altitude, the closer the orbit is to the north-south direction. For a table of altitude vs. maximum latitude, and other technical details, see Wikipedia.

As far as we can tell almost all imaging satellites are in Sun-synchronous orbit, including SkySat-1 and SkySat-2. They also all have altitudes in the 500-900 km range, meaning they should all have similarly tilted orbits with a maximum latitude of around 82 degrees.

Thank you to GEB readers franksvalli2 and Vasilis for letting us know that the mystery image in yesterday’s post is almost certainly from one of the SkySat satellites. See this PDF file for details on those satellites, including their sensor arrangements which creates the distinctive ‘Y’ pattern. Also thank you to GEB reader Daniel Plant for bringing our attention to TeLEOS 1, which, as you can see here has a very different orbit.

We used our circle drawing tool to estimate the orbit of the satellite that took yesterday’s image and it came remarkably close to the expected 82 degree maximum latitude.

Although DigitalGlobe imagery in Google Earth is typically in both vertical and horizontal stripes, we believe that the imagery is actually captured by a satellite following a Sun-synchronous orbit very similar to the Sky-Sat orbits. We believe that the almost perfect north-south or east-west alignment of the DigitalGlobe strips is for some reason other than orbit. We have noted in the past that they line up with degrees of latitude and longitude.

For more interesting reading about orbits see this article from NASA.

Google Earth Imagery Update: Strange image in the Sahara

Google has recently pushed out another imagery update. We created maps for March, April and May imagery, but did not find any significant changes in the March and April maps since the last update a week ago. There is, however, quite a lot of new May imagery.


May imagery. Red: Recently added imagery. Blue: imagery as of May 29th.

To find the locations in Google Earth download this KML file.

We haven’t been able to find any major events captured in the new imagery, but we did come across a strange image in the Sahara. It is in the south of Algeria and covers an area that has not previously been imaged with high resolution imagery.

We can see nothing of particular interest in the imagery, with half of the area being obscured by clouds, and no distinguishable features on the ground. Although it is hard to judge resolution, we think it is lower resolution than the nearby DigitalGlobe image. At first sight it appears to consist of three parallel strips, but the clouds all line up which would not be the case if it was three consecutive passes of a satellite, so we suspect it is all one image or three images captured in one pass. It also has no attribution (the NASA attribution is for the very low resolution background image).

Or first guess is a low altitude, relatively low resolution satellite, such as are used by Google’s own Terra Bella (formerly Skybox Imaging) and Planet Labs.

The strips are at a different angle from most satellite imagery which tends to be nearly aligned in a north-south direction. Strips of other alignments do exist but we believe they are typically for newly launched satellites that have not yet moved to a polar orbit. Near polar orbits tend to be preferred as it provides greater coverage. We do not know if this is the case for companies like Planet Labs which has large numbers of satellites.

If any of our readers know anything more about the origin of this imagery, please let us know in the comments.

Find it in Google Earth with this KML file.

Google’s Skybox Imaging renamed Terra Bella

Google has just announced that Skybox Imaging has been renamed Terra Bella. The new name is intended to indicate a change of focus from just a satellite imaging company to pioneering the search for patterns of change in the physical world.

Google acquired Skybox Imaging back in June 2014. We have not seen Skybox imagery in Google Earth, mainly because it is lower resolution that than offered by Google’s usual imagery suppliers. Despite the lower resolution, Skybox has produced some interesting products over the years. We saw a gif animation of the Burning Man festival and in the same post you can see the first HD resolution video of Earth from space. We also had a look at Skybox’s image of the November 2014 Poppy display at the Tower of London. We love the Google Chrome extension “Earth View” and we noticed that it includes a few images from Skybox Imaging. In October 2014 Skybox announced the Skybox for Good programme, which works with non-profits to provide fresh satellite imagery where they need it.

Terra Bella plans to launch a lot more satellites in the coming years and we hope to see great things from them in the future.


The Burning Man gif animation.


SkySat-1 Video of Mount Ontake, Japan on October 16, 2014. Mt. Ontake erupted on September 27, 2014.

Be sure to visit the new Terra Bella website for more examples of their products. The new Terra Bella blog can be found here.

As of this writing, the old domain, skybox.com seems to be inaccessible, making any links to the old Skybox Imaging blog no-longer valid.

Thank you to GEB reader Eric for letting us know that the Terra Bella name is derived from the street where they have their offices (Terra Bella Ave, Mountain View, California, USA).