History of Radio Occultation
The radio occultation technique was first used on the Mariner IV mission for the flyby around Mars in 1964 [1].
It measured Mars's atmosphere and ionosphere. It was also used in the Pioneer and Voyager missions. At this
point, all planets in our solar system has data collected using radio occultation.
Physical Principles of Radio Occultation
Radio occultation relies on the signal from the transmitter to take multiple paths to the receiver. Some of
these paths will only go through space and be the "baseline" for the measurements. The rest of the signals
will go through a section of the atmosphere of the planet. The signal will be refracted and the difference
in phase between these signals will indicate how much the signal was refracted. After adding in some data,
such as clock correction and the position in the orbit, the bending angles can be calculated of the refraction [2].
Current Uses
Scientists are now finding use for this data collection method on Earth. Using the existing GPS satellites
in orbit, they can detect GPS signals at different points using much more quickly orbiting LEO satellites.
This is going to be used in research to study long term climate change. They will be used to determine the
temperature and water vapor concentration of the different levels of atmosphere.
NASA is also using RO in the New Horizons mission to analyze the atmosphere of Pluto.
Why Radio Occultation?
This project plans to combine the two current uses of RO: the characterization of other planet's atmospheres during flybys as well as monitor the change in climate over time. While RO has been used in other space missions before, there has never been a project of this scope, both in the percentage of the planet analyzed as well as the period of time that the measurements are taken. Much of Jupiter is still a mystery and by analyzing the refraction of RF signals off of Jupiter's atmosphere, we will be able to map out a clearer picture of what makes up the clouds that are Jupiter's trademark.
Proposed Sensor Constellation
There were three proposed solutions on how to arrange our sensors for this system.
1. Use many transmitters on a higher orbit and few receivers on a lower orbit. This is similar to how the GPS system on Earth is set up.
2. Use few transmitters on a higher orbit and many receivers on a lower orbit. This is essentially an inverse of solution 1.
3. Use only receivers on a low orbit and transmit a high powered signal from Earth. This is being used in the New Horizons mission.
While solution one works well for Earth as solar power is much more effective in Earth orbit, it is very costly on the power budget to transmit a signal with a high uptime. This means that the transmitter orbiters would likely be more costly than the receiver orbiters. For this project, reducing the complexity on most of the parts is important, so solution one was eliminated.
Solution three was considered because it would remove most of the high energy costs from the orbit around Jupiter and place it these costs on the systems on Earth. Getting a clear enough path from Earth to Jupiter over the course of this long mission is not guaranteed and it would require multiple earth stations (like NASA's Deep Space Network) if an uptime of transmission greater than one-third is desired.
With solution two, we trade off "unlimited" energy for more direct access to the planet.
Signal Generation and Capture
The signal generation and capture are all going to take place within the orbit of Jupiter, something that is not normally done in RO measurements in space missions. The Mother Ship will transmit a signal towards Jupiter at varying locations that the smaller Cubesats orbiting around Jupiter at a much faster rate will pick up. This system will use the antenna already provided by the communications link between the Mother Ship and the Cubesats. This means that the frequencies and hardware are shared between transmitting the occultation signal and relaying the collected data back to the Mother Ship.
The data collected by the receivers is then sent back to the Mother Ship. This includes timing information and the differences in phase in received signals. This data can be analyzed on Earth to provide information on Jupiter's climate.
References:
[1] Wikipedia. (2014, July 18). *Mariner 4* [Online]. Available: http://en.wikipedia.org/wiki/Mariner_4
[2] L. Cucurull. (2009, July). *Global Positioning System (GPS) Radio Occultation (RO) Data Assimilation* [Online PDF]. Available: http://www.jcsda.noaa.gov/documents/meetings/2009summercoll/Cucurull_GPS.pdf
[3] EUMETSAT. (2014). *The Radio Occultation Method* [Online]. Available http://www.romsaf.org/radio_occultation.php
[4] Wikipedia. (2013, Oct. 26). *GNSS radio occultation* [Online]. Available: http://en.wikipedia.org/wiki/GNSS_radio_occultation
[5] G. L. Tyler, *et al.* (2007, Aug. 1). *THE NEW HORIZONS RADIO SCIENCE EXPERIMENT
(REX)* (Submitted Draft 8jan07) [Online PDF]. Available: http://www.boulder.swri.edu/pkb/ssr/ssr-rex.pdf
The radio occultation technique was first used on the Mariner IV mission for the flyby around Mars in 1964 [1].
It measured Mars's atmosphere and ionosphere. It was also used in the Pioneer and Voyager missions. At this
point, all planets in our solar system has data collected using radio occultation.
Physical Principles of Radio Occultation
Radio occultation relies on the signal from the transmitter to take multiple paths to the receiver. Some of
these paths will only go through space and be the "baseline" for the measurements. The rest of the signals
will go through a section of the atmosphere of the planet. The signal will be refracted and the difference
in phase between these signals will indicate how much the signal was refracted. After adding in some data,
such as clock correction and the position in the orbit, the bending angles can be calculated of the refraction [2].
Current Uses
Scientists are now finding use for this data collection method on Earth. Using the existing GPS satellites
in orbit, they can detect GPS signals at different points using much more quickly orbiting LEO satellites.
This is going to be used in research to study long term climate change. They will be used to determine the
temperature and water vapor concentration of the different levels of atmosphere.
NASA is also using RO in the New Horizons mission to analyze the atmosphere of Pluto.
Why Radio Occultation?
This project plans to combine the two current uses of RO: the characterization of other planet's atmospheres during flybys as well as monitor the change in climate over time. While RO has been used in other space missions before, there has never been a project of this scope, both in the percentage of the planet analyzed as well as the period of time that the measurements are taken. Much of Jupiter is still a mystery and by analyzing the refraction of RF signals off of Jupiter's atmosphere, we will be able to map out a clearer picture of what makes up the clouds that are Jupiter's trademark.
Proposed Sensor Constellation
There were three proposed solutions on how to arrange our sensors for this system.
1. Use many transmitters on a higher orbit and few receivers on a lower orbit. This is similar to how the GPS system on Earth is set up.
2. Use few transmitters on a higher orbit and many receivers on a lower orbit. This is essentially an inverse of solution 1.
3. Use only receivers on a low orbit and transmit a high powered signal from Earth. This is being used in the New Horizons mission.
While solution one works well for Earth as solar power is much more effective in Earth orbit, it is very costly on the power budget to transmit a signal with a high uptime. This means that the transmitter orbiters would likely be more costly than the receiver orbiters. For this project, reducing the complexity on most of the parts is important, so solution one was eliminated.
Solution three was considered because it would remove most of the high energy costs from the orbit around Jupiter and place it these costs on the systems on Earth. Getting a clear enough path from Earth to Jupiter over the course of this long mission is not guaranteed and it would require multiple earth stations (like NASA's Deep Space Network) if an uptime of transmission greater than one-third is desired.
With solution two, we trade off "unlimited" energy for more direct access to the planet.
Signal Generation and Capture
The signal generation and capture are all going to take place within the orbit of Jupiter, something that is not normally done in RO measurements in space missions. The Mother Ship will transmit a signal towards Jupiter at varying locations that the smaller Cubesats orbiting around Jupiter at a much faster rate will pick up. This system will use the antenna already provided by the communications link between the Mother Ship and the Cubesats. This means that the frequencies and hardware are shared between transmitting the occultation signal and relaying the collected data back to the Mother Ship.
The data collected by the receivers is then sent back to the Mother Ship. This includes timing information and the differences in phase in received signals. This data can be analyzed on Earth to provide information on Jupiter's climate.
References:
[1] Wikipedia. (2014, July 18). *Mariner 4* [Online]. Available: http://en.wikipedia.org/wiki/Mariner_4
[2] L. Cucurull. (2009, July). *Global Positioning System (GPS) Radio Occultation (RO) Data Assimilation* [Online PDF]. Available: http://www.jcsda.noaa.gov/documents/meetings/2009summercoll/Cucurull_GPS.pdf
[3] EUMETSAT. (2014). *The Radio Occultation Method* [Online]. Available http://www.romsaf.org/radio_occultation.php
[4] Wikipedia. (2013, Oct. 26). *GNSS radio occultation* [Online]. Available: http://en.wikipedia.org/wiki/GNSS_radio_occultation
[5] G. L. Tyler, *et al.* (2007, Aug. 1). *THE NEW HORIZONS RADIO SCIENCE EXPERIMENT
(REX)* (Submitted Draft 8jan07) [Online PDF]. Available: http://www.boulder.swri.edu/pkb/ssr/ssr-rex.pdf