Since the last decades of the past
century, a more and more growing of experimental evidences supports the
“hypothesis” that most of the observed warming of the Earth surface, over the
last 50 years, is likely to have been due to the increase of greenhouse gas
concentrations of anthropic nature.
The atmospheric concentration of carbon dioxide, that is the most important anthropogenic greenhouse gas, has increased from a pre-industrial era value of about 280 ppm (parts per million) to 379 ppm in 2005 and mainly it is due to fossil fuel use. This fact explains why the increasing of temperature of industrial countries is larger than not-industrial countries.
Source: IPCC 2007 report. Comparison of north America and south America changes in surface temperature (black line) with results simulated by climate models using natural and anthropogenic forcings (in red are represented the results by models using both natural and anthropogenic forcings, while in blue are represented the results by models using only natural forcings).
The third IPCC (International Panel on
Climate Change) report emitted on
February 2007 states that: “ Warming of the climate system is unequivocal, as is
now evident from observations of increases in global average air and ocean
temperatures, widespread melting of snow and ice, and rising global average sea
level” and “Attribution studies have established anthropogenic contributions to
all of these changes”.
In particular, it is confirmed one of the main results of the previous IPCC 2001 Report, that is the experimental evidence of the recent increasing of the atmospheric temperature is correlated to an increasing of the content of water vapour in the Troposphere and a decreasing of the Stratospheric temperature (see Figures 1 and 2).
Source: IPCC 2007 report. Comparison of observed continental- and global-scale changes in surface temperature (black line) with results simulated by climate models using natural and anthropogenic forcings(in red are represented the results by models using both natural and anthropogenic forcings, while in blue are represented the results by models using only natural forcings).
Due to the difficulties to measures the physical characteristics of the atmosphere in a global, precise, and affordable way it is necessary to develop innovative ideas, new techniques and instruments able to measure temperature, pressure and humidity of our atmosphere. The Radio Occultation Technique, developed over the last 35 years for planetary purposes, is a very powerful tool also for sounding the structure of atmosphere of our planet. This technique will provide accurate measurements of the atmospheric refractive indexes from which it is possible to derive atmospheric vertical profiles of temperature, pressure and humidity, as well as profiles of electron content in the ionosphere. The application of the Radio Occultation technique to sound the Earth's atmosphere needs the presence of transmitting sources like the satellites of the GPS (Global Positioning System) constellation.
One of the most
recent and promising atmospheric remote sensing technique is the Radio
Occultation applied to GPS measurements (hereafter GPS RO). Its importance has
been increasing during the last decade, since the first related mission
highlighted its possibilities.
Similarly to what happen to the Petit Prince of Antoine de Saint Exupery, where in figure 3 is looking up to the wonderful “deformations” of the sunlight during a sunset, the Radio Occultation technique exploits the correspondent phase and amplitude deformations affecting the GPS signal due to its propagation through the terrestrial atmosphere, during the rising or setting of the GPS satellite above the limb.
Fig. 3 – (Courtesy of J. Wickert) Le Petit Prince (Antoine de Saint Exupery) anticipating the GPS - Radio Occultation principle.
The Radio Occultation (RO) is a
technique for sounding the structure of planetary atmospheres developed over the
last 35 years at Jet Propulsion Laboratory and at the Stanford University. To
sound the planetary atmosphere, the technique is based on deriving the
refractivity index from radio signals perturbations transmitted by a space
probes while a satellite is moving around the planet. The application of the
Radio Occultation technique to sound the Earth's atmosphere is strongly linked
to the GPS constellation development. The GPS Radio Occultation observations
were used for the first time to study the Earth's atmosphere in 1995 during the
Global Positioning System/Meteorology (GPS/MET) mission. This mission provided
an active sounding of the atmosphere under all-weather conditions and with
relatively high vertical resolution.
A GPS Radio Occultation occurs when a transmitting GPS satellite, setting or rising behind the Earth's limb, is viewed by a LEO satellite (figure 4).
Fig. 4 . Principles of RO measurement
The relative motion between the GPS and LEO (Low-Earth Orbiter) satellite provides a vertical scanning of the atmosphere. In the geometrical optics approximation and considering the spherical symmetry, a ray passing through the atmosphere is refracted and delayed due to the vertical gradient of density. As the satellites geometry changes (figure 5), the radio waves sample subsequently layers of the atmosphere. By the phase variation of the GPS signal, these limb sounding measurements are used to retrieve a vertical profile of bending angle.
Fig. 5 . RO event geometry
A GPS Radio Occultation measurement
represents a nearly instantaneous image of the atmospheric physical state. A GPS
receiver on board of a LEO satellite, forward and backward viewing, provides
about 500 globally distributed occultations at a very low cost and with a
relatively uniform coverage every day (figure 6). A GPS occultation event takes
about 1-2 minutes and results in about 4000 measurements for each profile, which
typically starts at 100 km and ends near the Earth's surface.
Figu. 6 . Number of occultation events occurred during 24h for a LEO satellite with i=98 and H=600km.
During a GPS Radio Occultation event, the phase variation is
function of: the satellite clock drift errors, the Doppler shift due to the
relative motion between the transmitter and the receiver and the additional
propagation delay due to the reduction of the light velocity in the atmospheric
medium respect to the vacuum. By the double differences technique it is possible
to delete the clock errors. Knowing precise positions and velocities of the GPS
and LEO satellites it is possible to calculate the expected Doppler shift in the
vacuum. The atmospheric contribution in the shift Doppler (excess Doppler) of
the received signal is obtained subtracting from the observed shift the clock
errors shift and the expected shift in the vacuum. For more details see the
Bibliography, for instance Vespe (2004).
The main advantages of the Radio Occultation technique are:
• Global coverage
• Atmospheric profiles (T, H, p) with very high vertical resolution
• Very high Temperature measurements accuracy
• Low-cost data
• Continuously operation
The Italian Space Agency (ASI) is strongly involved in the
Earth Observation activities. To contribute to a better knowledge of the Climate
Change, ASI, supported by a scientific community, is promoting and funding
several scientific and technological activities related to the study of
Atmosphere using the Radio Occultation Technique.
From the space, this kind of measurements are usually made using IR and MW sounders, which have some well known limitations in vertical resolution and temperature sensitivity. While the use of GPS Radio Occultation technique is able to investigate deeply and widely for retrieving physical parameters of Earth atmosphere.
The mainly scientific applications of the Radio Occultation data are in the field of:
• ionospheric studies
• solid earth physics
The core of the ASI activities was the development of the ROSA (Radio Occultation Sounder for Atmosphere) instrument. ROSA, a state of art instrument, is able to measure temperature, pressure and humidity profiles of the atmosphere with very high vertical resolution ( minor/equal to 200 m in the troposphere and minor/equal to 1000m in the Stratosphere) and high temperature accuracy.
ROSA is also the name of the ASI Mission that covers the overall Scientific and Technological activities related to the Space, Ground and User segments for the use of the Radio Occultation techniques.
The main activities of this mission are:
• development and manufacturing of a state of art Atmospheric Sounder for the Radio Occultation of the GNSS satellites, this instrument is called ROSA
• installation of similar ROSA instruments on several space missions
• developments of a G/S based on the ASI Multi-Missions Center
• development of an advanced RO data processors
• implementation of an open data policy
Up to now, ASI accepted the flight opportunities to install ROSA on the following space missions
Fig. 7 . Launch schedule of the missions on board of which ROSA is installed
Products obtained by processing Radio Occultation data can significantly contribute to many research fields. In particular:
• Climatology: the long-term stability of the ROSA instrument may assure the continuous monitoring of the atmosphere over long-time period, giving climatological significance to the observations themselves;
• Meteorology: global weather forecasting, with expected improvements due to meteorological data acquisition in area where ground based data are not available (oceans, polar regions, deserts). The occultation data may contribute to the global moisture distribution knowledge, to tropopause monitoring and to lower stratosphere temperatures profiling. Assimilation of RO data in NWP (Numerical Weather Prediction)
• Space Weather: ionosphere physics, with expected improvements in Space Weather research and Space Weather forecasting. Dynamical aspects of the ionospheric plasma can be monitored through scintillations measurements, while the continuous global electron density monitoring may assure the data base needed for predictive models testing;
• Space Geodesy and Geophysics: Earth's gravitational field studies, for the improvement in solid Earth Sciences. Indeed, orbits determination may be improve through a better knowledge of the atmosphere itself.
Emerging applications related to the use of GPS receiver in a bistatic configurations may permits, through scatterometric and altimetric measurements, the study of sea state and of terrain properties.