Relatively speaking the Earth is massive, and it is not possible to view it as a whole body while standing on its surface. Satellites have made it possible for us to look at the Earth in an entirely different perspective. They give us the opportunity to view the world we live in on a large scale. Believe it or not, sea level is not changing at the same rate worldwide. Some places experience little to no change or even a loss in sea level, while in other parts of the world the sea level rises almost ten millimeters in a year. Using satellites, NASA is able to measure and come up with an average sea level, as well as map the changes in sea level over time.
Figure 1. Artist Model of the Jason-3 Satellite. (Presented by NASA, 2016)
Scientists have actually been measuring sea level since the early 20th century, but that kind of isolated sample can be impacted by local factors that do not affect sea levels at other parts of the world. Satellites allow scientists to view the sea level at each end of the globe without having to worry about local variances.
This satellite was launched on August 10, 1992 as a joint effort between the American and French space agencies (NASA and CNES). The satellite covers 95% of the earth's ice free oceans every 10 days. With this satellite they were able to measure sea levels, have continuous data on ocean topography, monitor effects of climate change and how seasons affect currents. In addition they were able to get data on ocean circulation, map heat stored in the upper ocean, and provide global maps of tides. This was the first satellite to measure sea levels to within 5 cm, which was unprecedented in terms of accuracy.
The TOPEX/Poseidon used the following sensors/instruments:
1) NASA dual-frequency altimeter and CNES single-frequency solid-state altimeter
2) NASA microwave radiometer
3) NASA GPS receiver
4) NASA laser retroreflector array
5) CNES DORIS Doppler
Figure 2. TOPEX/Poseidon measurment system model. (Modified from Wikimedia Commons)
The Jason-1 was launched on December 7, 2001 and was used as a follow-up for the TOPEX/Poseidon mission. With this satellite now orbiting earth they were able to continue taking measurements of the ocean surface topography after the TOPEX/Poseidon satellite had ran its course. This mission had a goal of bettering the accuracy of sea level measurements to within 4.2 cm as the requirement, and within 2.5 cm as the goal.
Figure 3. Jason-1 Full Sea Surface Map. (NASA, 2013)
The Jason-1 used the following sensors/instruments, which are updated versions of the instruments used in the TOPEX/Poseidon mission:
The Jason-2 was launched on June 20, 2008. It was a follow-up mission of the Jason-1 mission. The Jason-2 was launched into the same orbit as the Jason-1 satellite and flew in tandem on the opposite side of the globe in order to provide twice as many measurements, giving observations for each location every five days instead of every ten days. The mission had a mandatory measurement accuracy of 3.3 cm (this is what Jason-1 had), with a goal of reaching accuracy of less than 2.5 cm. It was found that on average the sea levels have risen 3 mm per year since 1993. They were able to figure this due to the continuous observations collected from the TOPEX/Poseidon, Jason-1, and OSTM/Jason-2 satellites.
Figure 4. Continuous observations collected from Jason-1 to Jason-2, showing minimal change in sea surface level.
This satellite used further updated sensors/instruments, including:
Radar Altimeters emit pulses at several frequencies to measure distance between the surface of the ocean and the satellite. Electrons in the atmosphere can affect the return time on the pulses, which in turn can effect accuracy, which is why several frequencies are used. The difference between frequencies can be used to determine the amount of error due to electrons in the atmosphere.
Figure 5. The Altimeter that was used on the Jason-1 Satellite missions.
It measured sea level, wave heights and wind speed
Jason 3 is the 4th mission in U.S. satellite missions that measure sea level, and was launched January 17, 2016 from the Vandenberg Air Force Base in California. It was put in orbit with the help from the SpaceX Falcon 9 rocket, and will continue missions previously started by TOPEX/Poseidon that measured ocean surface topography, as well as missions launched by Jason-1 and Jason-2. The Jason-3 satellites'primary instrument of use is a radar altimeter. This radar altimeter is extremely accurate and can measure variations within 3.3 centimeters. Sea level can be measured on a global basis or even by regions, also these measurments aquired from the satellite will provide circulation patterns in the ocean and give climate implications of global warming. By using an radar altimeter instrument, every ten days the ice free oceans are being monitored and previous missions have shown that the sea-level is rising on average 3 mm a year, and in the past 23 years has risen a total of 70 mm(NESDIS, 2016) .
Figure 6 Ocean map comparison of Jaons-2 missions and Jason-3.
From this map we can see that data collected from Jason-2 closely resembles the new data from Jason-3 (Presented by NASA, 2016).
Data will be collected on a continual basis so that changes can be measured in the ocean surface topography, primairly for agencies and scientisit to use. Jason-3 missions go international working with the NOAA, Eumetsat, NASAm and CNES.Just after ten days Jason 3 was able to create its first sea surface map, which corresponds with the previous Jason 2 data. Data collection starts when the satellite reaches an orbit of 1,336 km. Not only will Jason 3 be able to map the sea surface, but with its extremely accurate radar system its measurements can be used for various commercial and operational application like;
(Kawasaki and Srinivasan, 2016)
Figure 7 Expected Mission Milesontes for Jason-3 (From the NOAA Satelite and Information Service Webpage, 2016).
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