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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.

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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.

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