The Global Positioning System for the Geosciences Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications (1997) / Chapter Skim
Currently Skimming:
8 Remote Sensing of the Atmosphere
8 Remote Sensing of the Atmosphere
Pages 110-163
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 110... ...
8 Remote Sensing of the Almosphere
|
From page 111... ...
One of those new technologies, using radio occultation techniques in connection with the Global Positioning System, is the subject of this essay. The last section of this paper addresses the potential usefulness of atmospheric refractivity inferred from these techniques in operational numerical weather prediction.
|
From page 112... ...
This requires the development of a forward model to go from model variables (bending angles or index of refraction in the case of the radio~ccultation GPS data)
|
From page 113... ...
GPS/MET data will influence that only indirectly in the extra tropics, and not at all in the tropics. It does appear possible that GPS/hIET observations based on radio occultation techniques can improve the description of the distribution of moisture.
|
From page 114... ...
1) , atmospheric soundings are retrieved from observations obtained when the radio path between a GPS satellite and a GPS receiver in low Earth orbit (LEO)
|
From page 115... ...
19931. For operational numerical weather prediction, it is possible to assimilate directly refractivity measurements into the model.
|
From page 116... ...
These signals have dominant vertical wavelengths of about 10 km and horizontal wavelengths of about 1000 km." Fig. 4 shows a comparison of a GPS/MET temperature retrieval with a retrieval from the MLS and The Global Positioning System for the Geosciences the global analysis of temperature from the National Centers from Environmental Prediction (NCEP)
|
From page 117... ...
The awed temperature retrievals refer to temperatures computed from (2) Utah water vapor pressure obtained from the NCEP analysis.
|
From page 118... ...
118 The Global Positioning System Jor the Geosciences 25 r DrY GPS tem erasure -NCEP temperature ~ . _ Ave i ~ ~ 0 ~ '~ o -10 -8 -6 ~ -2 0 2 4 6 8 ~T' °C ^^ Wat GPS temperature- NCEP temperature ~v- _ E 215 -- _ 10 u- .
|
From page 119... ...
9 on a global basis would be extremely useful for research and operational purposes. ASSIMILATION OF REFRACTIVITY DATA IN A NUMERICAL WEATHER FORECAST MODEL One of the greatest potential applications of GPS~IET data is in operational numerical weather .., ~ prediction.
|
From page 120... ...
To investigate the potential impact of refractivity data on subsequent model forecasts, we degraded the control model data at 120011TC 3 January 1989 (me) and then assimilated simulated refractivity data from the control simulation over a 6-h period from 1200 to 1800 llTC 3 January on the region shown in Fig.
|
From page 121... ...
Because obtaining accurate refractivity observations in the lower troposphere on a regular basis is still problematic, as discussed above, we performed Em. 5 in which refractivity data are assumed available only above 3 km.
|
From page 122... ...
Accurate temperature profiles in the upper troposphere and lower stratosphere distributed uniformly over the Earth would be useful in operational numerical weather prediction, global and regional climate change studies and in studies of atmospheric dynamics and chemistry. ACKNOWLEDGMENTS We thank Bob Corell, Dick Greenfield, Jay Fein and Mike Mayhew of the National Science Foundation for their support of the GPS/MET project.
|
From page 123... ...
Sokolovskiy, F Solheim, climate change using the global positioning system.
|
From page 124... ...
in the dual-frequency carrier phase observations is obtained by the LEO GPS receiver. As the occulted GPS satellite sets below the horizon as viewed from the LEO satellite, the signal path descends through successively denser layers of the atmosphere, and the delay increases to approximately 1 km at the Earth's surface.
|
From page 125... ...
These observations show promise for operational use within the new National Space Weather Program.4 PRELIMINARY RESULTS FROM MICROLAB-1 Temperature profiles retrieved from several thousand GPS/MET "soundings" have been compared to radiosonde data, profiles from other satellite remote sensing instruments and analysis obtained from operational weather prediction centers, such as the National Centers for Environmental Prediction (NCEP)
|
From page 126... ...
3) Combine the precise GPS orbits, precise LEO orbits, low rate fiducial observations, high rate LEO and high rate fiducial observations to form high level meteorological data products.
|
From page 127... ...
However, if it is assumed that meteorological data must be delivered no later than 2 hours after the observation, and the LEO observations can take up to 100 minutes to flow through the LEO-to-central store and foreword communications network, then it follows that predicted orbits, propagated from the most recent observations practical, will be required for both the GPS and LEO satellites. Since GPS orbits require relatively more computing time, but degrade more slowly, the GPS orbits could be re-computed on a 12 hour cycle and propagated ahead roughly 24-36 hours from the end of the data arc.
|
From page 128... ...
A practical system will require real-time 30 second fiducial data from 20-30 fiducial sites, plus "high rate fiducial data" from approximately 15 select sites. In addition, predicted GPS and LEO satellite orbit solutions unroll be needed, updated at frequencies sufficient to keep overall orbit error below approximately 0.5 m at the time of use.
|
From page 129... ...
NASA Global Water Vapor Data Set Project (Nvap) has blended TOYS, SSM/I and radiosonde data into a five year (1988-1992)
|
From page 130... ...
Additionally, use of the TAO buoy array in the equatorial Pacific Ocean could be explored as potential platforms for GPS receivers. Another oceanic region where current satellite observations of precipitable water are inadequate is The Global Positioning System for the Geosciences theArctic Ocean.
|
From page 131... ...
The synthesis of the disparate data and algorithm output is implemented via a fuzzy logic algorithm. As the use of filzy logic algorithms have been limited in the atmospheric sciences, a detailed introduction is given below.
|
From page 132... ...
. The confidence values will range between zero and one, with DATA INGEST Diagnostic Algorithms The Global Positioning System for the Geosciences zero implying no-confidence (perhaps missing data)
|
From page 133... ...
A simple and robust technique for performing this synthesis task is a fuzzy logic algorithm. The Integration Algorithm will ingest the venous detection, diagnosis, forecast, and confidence grids from the algorithm modules and via the fi~zy logic machinery, produce composite "ridded values for each We of hazardous weather phenomena.
|
From page 134... ...
The use of inference rules is not incorporated in the current application, the synthesis of different data types is handled through the next step, composition. Composition The third step in building fuzzy logic algorithms is composihon wherein the interest values from a number of different data types are combined in a systematic fashion.
|
From page 135... ...
The parallel magnitude grid method would separate the event detection and magnitude estimation tasks. That is, use the fuzzy logic machinery to find the location of the events.
|
From page 136... ...
The outputs from the individual algorithm modules are synthesized using a fuzzy logic algorithm. Fuzzy logic algorithms are well suited to this type of problem, wherein a number of disparate data sources must be combined in a simple and efficient manner.
|
From page 137... ...
Ideally, this instrument package should be interfaced directly to a GPS receiver, and incorporate the pressure and temperature data directly into the GPS data stream. A means of transferring both the GPS and surface meteorological data to a central processing facility in near real time.
|
From page 138... ...
Precise GPS orbits, obtained using data from a global network of GPS receivers, were used in estimation of the total zenith tropospheric delays. Data from elevation angles as low as 7.0° were processed to estimate the total zenith tropospheric delays from the GPS data at the JPL site.
|
From page 139... ...
The 3 days of measurements shown here illustrate the effect of different elevation angle cutoffs when processing the GPS data to estimate precipitable water vapor. The results obtained using a 7° elevation angle cutoiT exhibited smaller differences with the WVR results.
|
From page 140... ...
The GPS based PWV estimates described in the previous section required the use of precise GPS orbits obtained by processing data from a global network of~30 GPS receivers. Because of the time required to collect and process the data used to provide these precise orbits, it is not practical to use them as the basis for a GPS-based system capable of providing near real time PWV estimates.
|
From page 141... ...
The quantity plotted on the vertical axis is the difference between the PWV estimates obtained using precise orbits and those obtained using predicted orbits. The data used for this plot were recorded at the JPL and Pietown sites on Oct.
|
From page 142... ...
If orbits are not adjusted, the receiver position should be estimated along with the total zenith delay.2 These are in addition to the instrumental accuracy requirements discussed earlier in the section describing comparisons with WVR measurements. SUMMARY In this paper we have presented the requirements for a ground-based system for measurement of precipitable water vapor in near real time using the Global Positioning System.
|
From page 143... ...
Rocken, and R.H. Ware, 1994, GPS Meteorology: Mapping Zenith Wet Delays onto Precipitable Water, J
|
From page 144... ...
The vertical difference between these antennas is strongly influenced by the change in the multipath environment. Phase center motion as a function of elevation angle is apparent in the antenna height difference as a function of elevation cutoff angle (Figure 21.
|
From page 145... ...
· .o B'o.o ~ ~ ·~.0 · .0 ~ ~ 0.0 ·. .0 0 20 40 SO 80 elevation angle I: '1~ 0 20 40 so So · Ie v ~ t i o n ~ n 9 I ~ FIGURE 3 Comer phase residual reduction by 85cm diameter choke ring antenna backplane attached to a Dorne Margolin choke ring antenna.
|
From page 146... ...
One attraction of this antenna is its simple ground plane, in contrast to expensive and heavy choke rings. For 30 AOA Cal L ~ FIGURE 7 L1 Gain of Dorne Margolin choke ring antenna as a function of azimuth and elevation angles nitl~out (top)
|
From page 148... ...
comprised of seventeen digital ionospheric sounders giving bottomside electron density profiles or from the two Defense Meteorological Satellite Program (DMSP) satellites in sun-synchronous orbits at 840 km.
|
From page 149... ...
· Ne, Ni, Te, Ti, V at DMSP (SSlES) 840 Km Allitucie · Energetic Particle Fluxes / Auroral Boundaries From SSU/4; Significant Specification Improvement Using Future DMSP UV Data · Outputs Provider · Ion and Electron Density Profiles From 90-1600 Km Altitude · Profiles for Every 2° Latitude From 90°N to 90°S Latitudes · Profiles Every 5° Longitude From 0° to 360° · Specification of Total-Electron-Content (TEC)
|
From page 150... ...
During the daytime, and eastward~irected electric field, The Global Positioning System for the Geosciences E, generated around 120 km altitude (ionospheric E region) causes EM drift which is upward and away from the magnetic equator.
|
From page 151... ...
151 o lo -- -- 30 - ~~ 20 >, The Near Real-time (GIRT) network of 24 GPS dual frequency receivers run by JPL for NASA are displayed in Figure 7.
|
From page 152... ...
NRT GIobal Network n.t, n ~o 60 _ ~ ~e~ FIGURE 7 The near real-time global network. o.~< Aukia tIta~ Isl ~cMutd' 60 120 180
|
From page 153... ...
~] ~;c ' - n7~cE FIGURE 8 Observed TEC values from the dual-frequency altimeter on the TOPEX/Poseidon satellite.
|
From page 154... ...
The occultation of the GPS satellites by the Earth allows the GPS~Iet LEO dual frequency sensor to measure height profiles of TEC. These TEC profiles can be converted to electron density profiles if certain assumptions are made about the horizontal homogeneity of the ionosphere.
|
From page 155... ...
The Thermospheric Forecast Model Cam currently being validated will be coupled with IFbI to form the Coupled Ionosphere Thermosphere Forecast Model (CITED which will be a completely self-consistent, coupled model prodding 12 hour forecasts of the neutral winds, temperature, densities and ion and electron density profiles. This uphill be especially important during geomagnetic storms.
|
From page 156... ...
Forecast and SpeciFicatior Services , ~ ~Technology Transition Fee ~ ack ~ and Integration . ~ Observe- | Models | Education | FIGURE 13 Program elements of the National Space Weather Program.
|
From page 157... ...
As fully deployed, WAAS will be composed of a 157 nationwide reference stations for GPS data collection and a master station responsible for data crunching . Because of the widely distributed network, the GPS error components, mainly orbital and ionospheric error become observable and even the system integrity can be monitored at real time.
|
From page 158... ...
the estimation of satellite orbit and clock errors to reduce DGPS errors, 2) ionospheric delay modeling for WAAS L1 single frequency users and 3)
|
From page 159... ...
Under this circumstance, the wide area L1 ionospheric delay model must be transmitted to the single frequency WAAS users who constitute the main service volume, in spite of the quick development of the dual frequency cross-correlation receiver technologies. The generation of ionospheric delay model and provision of real-time ionospheric correction integrity monitoring are therefore the main goal of this research.
|
From page 160... ...
and (2) , the interfrequency biases has to be separated from the ionospheric delay.
|
From page 161... ...
By using Inky the weighting function w is a function of distance from IPP to the desired grid point. The current implementation of the ionospheric delay model in Stanford WAAS conforms to the RTCA-SC159 Working Group 2 (WG2)
|
From page 162... ...
It will not only provide the background of The Global Positioning System for the Geosciences how to choose the cut-off distance for choosing IPP's to create and ionosphere correction, using figure 9, but also provide important messages about the study of ionosphere integrity monitoring. To attack this problem, the ionospheric variation from the IPP's geo-magnetic longitude and latitude has to be taken out or modeled first in order to reveal the correlation function associate with distance alone.
|
From page 163... ...
FIGURE 10 Correlation coefficients of the ionospheric delay vs. IPP separation distance using daytime data (preliminary result)
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.