First Light Images

Mosaic image of The Netherlands created using three Sentinel-1 scans in March 2015.
Data Courtesy of Copernicus Sentinel data (2015)/ESA.

Two of the satellites launched on 12th January by the Indian Space Research Organization (ISRO) have released their first images. We wrote about the launch two weeks ago, and wanted to follow up on their initial outputs.

The first is the exciting ICEYE-X1, which is both the world’s first synthetic-aperture radar (SAR) microsatellite and Finland’s first commercial satellite. We currently use Sentinel-1 SAR imagery for some of Pixalytics flooding and water extent mapping products and so are really interested to see what this satellite produces.

One of the key advantages of radar satellites over optical ones is that they can capture images both during day and night, and are not hampered by the presence of clouds.  However, using a different part of the electromagnetic spectrum to optical satellites means that although it is black and white image it’s sometimes easier to distinguish objects within it.

Zoomed in portion of Netherlands mosaic image created using three Sentinel-1 scans in March 2015.
Data Courtesy of Copernicus Sentinel data (2015)/ESA.

For example, the image to the left is a zoomed in portion of Sentinel-1 mosaic of the Netherlands acquired in March 2015 where you can clearly see couple of off-shore windfarms.

Sentinel-1 is a twin satellite constellation and uses a C-Band SAR on board two identical satellites. Over land it captures data in an Interferometric Wide swath mode, which means it takes three scans and then combines them into a single image. Each scan has a width of 250 km and a spatial resolution of 5 m x 20 m, with a six day repeat cycle for an area of land.

In comparison, ICEYE-X1 produced its first image with a spatial resolution of 10 m, and it’s hoped to reduce this down to 3 m. It issued its first image on Monday 15th January, three days after launch, showing part of Alaska, including the Noatak National Preserve, with a ground coverage of approximately 80 km by 40 km. The image can be seen here.

ICEYE-X1 weighs in at under a 100 kg, which is less than a twentieth of Sentinel-1 which weighed in at 2 300kg. This size reduction produces a high reduction in the cost too, with estimates suggesting it only cost ICEYE around a hundredth of the €270 million price of the second Sentinel-1 satellite.

By 2020 ICEYE is hoping to establish a global imaging constellation of six SAT microsatellites that will be able to acquire multiple images of the same location on Earth each day. After this, the company has ambitions of launching 18 SAR-enabled microsatellites to bring reliable high temporal-resolution images which would enable every point on the Earth to be captured eight times a day.

Cartosat-2F also sent its first image on the 15th January. The image, which can be found here, is of the city of Indore, in the Indian state of Madhya Pradesh. The Holkar Stadium is tagged in the centre, a venue which has previously hosted test Cricket. The satellite carries a high resolution multi-spectral imager with 1 m spatial resolution and a swath width of 10 km.

It is the seventh satellite in the Cartosat series which began in 2007, the others are:

  • Cartosat 2 launched on 10th January 2007
  • Cartosat 2A launched on 28th April 2008
  • Cartosat 2B launched on 12th July 2010
  • Cartosat 2C launched on 22nd June 2016
  • Cartosat 2D launched on 15th February 2017
  • Cartosat 2E launched on 23rd June 2017

These two satellites are just at the start of their journey, and it will be interesting to see what amazing images they capture in the future.

Silver Anniversary for Ocean Altimetry Space Mission

Artist rendering of Jason-3 satellite over the Amazon.
Image Courtesy NASA/JPL-Caltech.

August 10th 1992 marked the launch of the TOPEX/Poseidon satellite, the first major oceanographic focussed mission. Twenty five years, and three successor satellites, later the dataset begun by TOPEX/Poseidon is going strong providing sea surface height measurements.

TOPEX/Poseidon was a joint mission between NASA and France’s CNES space agency, with the aim of mapping ocean surface topography to improve our understanding of ocean currents and global climate forecasting. It measured ninety five percent of the world’s ice free oceans within each ten day revisit cycle. The satellite carried two instruments: a single-frequency Ku-band solid-state altimeter and a dual-frequency C- and Ku-band altimeter sending out pulses at 13.6 GHz and 5.3 GHz respectively. The two bands were selected due to atmospheric sensitivity, as the difference between them provides estimates of the ionospheric delay caused by the charged particles in the upper atmosphere that can delay the returned signal. The altimeter sends radio pulses towards the earth and measures the characteristics of the returned echo.

When TOPEX/Poseidon altimetry data is combined with other information from the satellite, it was able to calculate sea surface heights to an accuracy of 4.2 cm. In addition, the strength and shape of the return signal also allow the determination of wave height and wind speed. Despite TOPEX/Poseidon being planned as a three year mission, it was actually active for thirteen years, until January 2006.

The value in the sea level height measurements resulted in a succeeding mission, Jason-1, launched on December 7th 2001. It was put into a co-ordinated orbit with TOPEX/Poseidon and they both took measurements for three years, which allowed both increased data frequency and the opportunity for cross calibration of the instruments. Jason-1 carried a CNES Poseidon-2 Altimeter using the same C- and Ku-bands, and following the same methodology it had the ability to measure sea-surface height to an improved accuracy of 3.3 cm. It made observations for 12 years, and was also overlapped by its successor Jason-2.

Jason-2 was launched on the 20 June 2008. This satellite carried a CNES Poseidon-3 Altimeter with C- and Ku-bands with the intention of measuring sea height to within 2.5cm. With Jason-2, National Oceanic and Atmospheric Administration (NOAA) and the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) took over the management of the data. The satellite is still active, however due to suspected radiation damage its orbit was lowered by 27 km, enabling it to produce an improved, high-resolution estimate of Earth’s average sea surface height, which in turn will help improve the quality of maps of the ocean floor.

Following the established pattern, Jason-3 was launched on the 17th January 2016. It’s carrying a Poseidon-3B radar altimeter, again using the same C and Ku bands and on a ten day revisit cycle.

Together these missions have provided a 25 year dataset on sea surface height, which has been used for applications such as:

  • El Niño and La Niña forecasting
  • Extreme weather forecasting for hurricanes, floods and droughts
  • Ocean circulation modelling for seasons and how this affects climate through by moving heat around the globe
  • Tidal forecasting and showing how this energy plays an important role in mixing water within the oceans
  • Measurement of inland water levels – at Pixalytics we have a product that we have used to measure river levels in the Congo and is part of the work we are doing on our International Partnership Programme work in Uganda.

In the future, the dataset will be taken forward by the Jason Continuity of Service (Jason-CS) on the Sentinel-6 ocean mission which is expected to be launched in 2020.

Overall, altimetry data from this series of missions is a fantastic resource for operational oceanography and inland water applications, and we look forward to its next twenty five years!