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The amazing things about Google Earth


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WorldView-4

DigtialGlobe’s WorldView-4 is expected to be launched next month, on September 15, 2016. According to Wikipedia, WorldView-4 started life as GeoEye-2 back in 2007. However, DigitalGlobe bought GeoEye in early 2013. At the time, DigitalGlobe had a very similar satellite, WorldView-3, being prepared for launch and it was capable of seeing more wavelengths, most notably in the shortwave infrared. So, WorldView-3 was launched in August 2015 and GeoEye-2 was put on hold.

WorldView-4 isn’t so much a successor to WorldView-3 but more of a companion satellite and is being launched to increase capacity in line with greater demand for satellite imagery. A comparison of the advertised stats for the two satellites shows that WorldView-4 covers less spectrum, but is otherwise almost identical in capability to WorldView-3.

Read more about the development of WorldView-4 and its transport to the launch site here. One interesting fact pointed out is that one of the technical difficulties with imaging satellites, which typically travel at about 7.5 m per second, is to capture imagery without blurring. It helps, of course, to be over 600 km from the subject, but even then, as we saw with ‘rainbow planes’, the movement of the satellite can have a significant effect. Telescopes used for astronomy can have long shutter times, but imaging satellites do not have that luxury.

WorldView-4 has a 1.1 m mirror and weighs nearly two and a half tons. Compare that with Plant Lab’s dove satellites, which are 10x10x30 cm and weigh 4kg. The key difference is resolution, with WorldView-4 capable of 30 cm per pixel and the doves around 3 to 5 m per pixel.

Planet Lab’s satellites are based on the CubeSat concept, which aims to standardise small satellites for cost savings. Incidentally, the WorldView-4 launch will include seven CubeSats (not for imaging) as part of a ride share agreement with DigitalGlobe.


image credit: DigitalGlobe.

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.

Saab and DigitalGlobe working on 3D globe

Saab and DigitalGlobe have announced a new joint venture called Vricon. It will be a Google Earth like product featuring 3D models similar to the 3D mesh that Google is rolling out. The key difference is that Vricon is using DigitalGlobe’s satellite imagery to create the 3D, whereas Google uses aerial imagery that must be specially captured for the areas they wish to map. This means that although Vricon’s imagery will necessarily be of lower resolution, it can have far greater reach and can be produced from the already existing DigitalGlobe archives. Also of note is that this is a commercial product targeted at enterprises and governments, whereas Google Earth is a free product.

Learn more about it and how it is produced in this article from Time magazine.

To see sample 3D imagery see the various demonstration videos at Vricon’s home page. For a tour of various locations look under “Capabilities->Explore Vricon 3D data”.

Vricon
New York as seen in a Vricon promotional video.

Google Earth
The same location as seen in Google Earth.

As you can see above, the Vricon 3D model struggles with the gaps between buildings. Google’s 3D models often have similar problems but it is more pronounced in the Vricon model due to the lower resolution. Also of note in the New York images above, the Vricon model seems to have completely failed with the new World Trade Centre towers. This is likely due to the fact that they use satellite imagery from a range of dates, including during the World Trade Centre construction, which would have confused the algorithms used to create the 3D.

Vricon - Beijing
Beijing, China, as seen in a Vricon promotional video.

One of the locations featured in the promotional videos, is Beijing, China. Google has not yet released any 3D for China – possibly due to the difficulties in getting clearance from the Chinese government. However, with satellite imagery no such clearance is necessary. So maybe Google should consider using a similar technique to provide lower resolution 3D in areas where they cannot gather aerial imagery. The only problem is that Google does not have access to the full archive of DigitalGlobe satellite imagery, and now that DigitalGlobe has the commercial Vricon product, they might be reluctant to allow Google to use their imagery to create a free version. There are, however, other satellite imagery providers that might be willing to work with Google. DigitalGlobe, however, has the highest resolution satellite imagery available coming from their WorldView-3 satellite.

Oil Slick from Oil Platform Explosion in Google Earth

Most oil platforms can not be seen in Google Earth. This is because Google and its imagery providers do not bother with imagery of the oceans far from the coast unless there is something of particular interest. Back in 2006 Google Sightseeing was able to find a few oil platforms in Google Earth’s imagery, which they showcase in this post.

In March we told you about DigitalGlobe’s First Look program and its public map showing where imagery has been captured of particular events. One such event was an explosion on an oil rig in the Gulf of Mexico on April 1st, 2015. DigitalGlobe captured imagery of the location on April 5th, which shows an oil slick coming from the platform.

https://www.gearthblog.com/wp-content/uploads/2015/05/OilSlick.jpg

To find the location of the oil rig above in Google Earth download this KML file.

Several images from before and after the event have been added to Google Earth in the area and a number of other oil rigs can be seen in the imagery. The imagery can only be seen in ‘historical imagery’. Many of the oil rigs show plumes of smoke but these are normal gas flares. If you turn on the ‘Earth City Lights’ layer (found in Gallery->NASA), you will notice a very bright area where the above oil rigs are due to the gas flares.

If you turn on the ‘photos’ layer you can find a number of pictures of oil rigs in the region.

Another look at DigitalGlobe’s FirstLook coverage

Last week we had a look at Digital Globe’s FirstLook service and some imagery of the damage done by Cyclone Pam to Vanuatu.

We have been going through some of the locations from the DigitalGlobe FirstLook Coverage map and finding them in Google Earth. Some locations we have covered in the past such as:

There are, however, a few locations that we have not covered before that have relevant imagery in Google Earth and you can see the events in question. The ones we have found so far that you may find interesting are:

A train derailment in India
A train derailment in India.

Volcanic Eruption in Gifu and Nagano Prefecture, Japan
A volcanic eruption in Gifu and Nagano Prefecture, Japan.

The 2014 Glastonbury Festival
The 2014 Glastonbury Festival.

May Day celebrations in Moscow
May Day celebrations in Moscow.

To find the above locations, and a few more interesting ones in Google Earth, download this KML file. You may need to move the ‘historical imagery’ slider to find the correct image(s). Remember that sometimes the event is captured on more than one date.