Temperature gradient 
Each region of the atmosphere is defined according to the Temperature gradient sign.
Two principals regions will be considered in this presentation.
Laboratorio de Técnicas Satelitales**
Departamento de Física, FACET, UNT
CONICET
It is convenient to define several regions of the upper atmosphere on the basis of the vertical temperature as shown in Figure 1.
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Figure 1: Several regions of the upper atmosphere.
Temperature gradient Each region of the atmosphere is defined according to the Temperature gradient sign. Two principals regions will be considered in this presentation. |
TROPOSPHERE  |
THERMOSPHERE  |
| Region nearest to the earth's surface.
Characteristics:
10 km for high latitudes 17 km for low latitudes |
Above the mesopause, there exist the thermosphere which
contains the ionospheric E and F regions.
Characteristics:
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Figure 2: Methods of ionospheric investigation.
1) Vertical (oblicua) sounder 2) Topside sounding satellite. 3) Incoherent bakscatter radar. 4) Partial reflection method. 5) Riometer. 6) Doppler method with rocket (and satellites). 7)-8) Probe techniques aboard spacecrafts. 9) Faraday rotation method with satellite. 10) Time delay and change of phase of transionospheric signals of Global Positioning System (GPS) for determination of TEC. |
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Figure 3 which is taken from ( J. Labrecque, 1999) shows a radio occultation mission of SAC-C that uses the DoD GPS to probe the Earth's Atmosphere.
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| Figure 3: The GPS/MET Mission with SAC-C |
In this mission concept the navigation signals from Global Positioning System (GPS) satellite that is being by the Earth's limb are observed by GPS flight receiver on board a Low Earth Orbiter (LEO = SAC-C) satellite.
As the received signals deeper layers of the atmosphere, its amplitude and phase are progressively altered through atmospheric refraction and possibly interference arising from spatial irregularities in refractivity.
The objetive of instrumentation is to record all of the direct , refracted and Earth reflected GPS signal as received by the low earth orbiting SAC-C satellite.
The scientific objective is the determination of ionospheric and atmospheric structure and study of GPS signals reflected from the Earth's surface.
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As the SAC-C orbits the Earth, the received navigation signal from a distant and setting (rising) GPS satellite passes through successively deeper(higher) layers of the Earth's atmosphere.
The GPS signal is both bent and retarded, causing a delay in arrival at SAC-C (LEO, Figure 4). At the required sampling rate, the SAC-C records the dualband carrier phase measurements, the C/A and P-code group delay measurements, and the signals strength measurements made by flight receiver; these are transmitted from time to time to the ground at designated telemetry sites.
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Figure 4: The ray path geometry for SAC-C/GPS satellite combination. |
Highly accurate ephemerides of SAC-C (Figure 5) and GPS satellites (Figure 6a, 6b) referenced to ground-based terrestrial frame are produced along with the carrier phase and amplitude profiles arising from the occulted GPS satellites.
The radio occultation technique takes advantage of the extremely precise phase and amplitude measurements of the GPS navigation signals that pass through the Earth's atmosphere to provide retrievals of the vertical refractivity profile.
The nominal instrumental accuracy of L1 carrier phase measurement for the TurboRogue, which uses both C/A and P-codes to obtain in-phase and quadrature carrier phase estimates every 20 ms, is around 0.1 mm at 1 Hz sample rate.
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Figure 5: SAC-C satellite. |
Therefore, a detectable effect will be observed in the carrier phase residuals from GPS signal passing through the mesopause as high as 90 km.
The TurboRogue also measures the amplitude of signal to an accuracy of about 0.3% at a 1 Hz sampling rate at nominal SNR conditions.
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| Figure 6: a) Constelation of satellites GPS. b) SV3 satellite. | |
The earth's ionosphere, which extends from about an 80 km altitude upward, acts as a lens that overlays the neutral atmosphere. In an occultation geometry, the signal must pass through the ionosphere on its way into and out of the neutral atmosphere below a 100 km altitude.
The dispersive nature of the ionosphere causes the two GPS signals to travel at different speeds (Figure 7) . A simple linear combination of L1 and L2 signals can be formed to substract out most of linear combination, however, assumes that the two GPS signals are traveling along exactly that the same paths.
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Figure7: Geometry of radio occultation |
Moreover, it also ignorates higher order terms in the expansion of ionospheric index refraction. These residual ionospheric effects, if left uncalibrated, act as an error source that maps into neutral atmospheric profiles errors.
Strictly speaking, the propagation of GPS signal through the atmosphere obeys Maxwell's equations in which the propagation medium is characterized by a three-dimensional spatial distribution of complex and dispersive refractive index. Here, it is convenient to assume that refractive index is real (i.e., zero absorption) and to assume that the signals are monocromatic; both of these assumption are largely valid for GPS signals.
The Appleton-Hartree formula for the ionospheric index of refraction (Budden, 1995) and expanding in power of inverse carrier frequency can be written as:
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(1) |
where
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(2) |
where 
is the number density of electrons; 
and 
are the electron charge and mass, respectively; 
is the
permitivity of the free space; 
, 
and 
are the plasma, giro, and carrier frequencies, respectively; 
is the angle
between the Earth's magnetic field 
and the direction of propagation of the wavefront 
; 
is the refractivity of
the neutral atmosphere.
Classical atmospheric parameters of interest can be derived from the refractive index profile through the following sequence of steps. To simplify the explanation, the process will first be described for the case of dry air. Then, the effect of moisture will be considered.
First, as the index of refraction, 
, is close to unity in the terrestrial atmosphere, it is convenient to define the
refractivity
:
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(3) |
For dry air, can be expressed as:
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(4) |
where 
is the pressure in millbars and 
is the temperature in Kelvins. Furthermore, the equation of state for dry air
takes the form:
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(5) |
where 
is the air density in kg/m*3. Equations (4) and (5) show that is directly proportional to for dry air, so
that
can be obtained easily from 
. Next , 
can be obtained from by integrating the equation
of hydrostatic equilibrium:
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(6) |
In summary, vertical profiles of , , and can be obtained from in a direct and simple manner.
The total refractive bending angle, , shown in Figure 7 is greatly exaggerated. For the Earth's atmosphere, the maximum
bending angle is on the order of 0.02 radians (1 deg ). To place this in perspective, the phase shift measurements made
with the Voyager spacecraft demonstrated that could be measured with an accuracy approaching 
radians. With
comparable performance from a space-borne GPS receiver, the refractive bending caused by the terrestrial atmosphere
could be to resolved to about 1ppm.
It is this type of precision in the radio measurements that leads to the expectation of obtaining high precision vertical profiles of N, , P, and T in regions of the atmosphere where the air is dry.
The procedure described above must be modified to account for the presence of water vapor . When the effect of the water vapor is included, the expresion for the refractivity becomes:
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(7) |
where 
is the vapor pressure of water in millibars. The "dry term" from (4) has been suplemented by the contribution
from water vapor ( the "wet term") which can be substancial in the lowest scale height of the atmosphere above the earth's
surface. The moist term also exhibits considerable variations with location an time. The separate contributions to N by the
dry and moist terms cannot be distinguished uniquely through occultation measurements with the current capabilities of
the GPS satellites.
This introduces an ambiguity into the profiles of , P, and T; the effects of water vapor at variable and uncertain concentrations are indistinguishable from the effects of background variations in temperature and pressure. At altitudes above 8-10 km, this ambiguity is not a significant problem as the contribution to the refractive index by water vapor is usually much less than 2% . Similarly, the contribution of moisture to refractive index is negligible throughout the polar atmosphere during winter. In the lower troposphere, the water vapor limitations can be overcome by one of several means, such as use of auxiliar methods for estimation of water vapor content ( e.g., microwave radiometry).
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The basic observable for each occultation is the phase change between the transmitter and the receiver as the signal propagates through the ionosphere and the neutral atmosphere (Figure 1).
A GPS phase measurement can be modeled as
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(8) |
where
is the geometrical range,
Btrans and Brec are clock biases for the transmitter and the receiver, respectively.
neutral and iono are the delays due to the neutral atmosphere and the ionosphere,
b is a phase ambiguity.
In addition to the occulting GPS and LEO satellites, other measurements, taken from a network of ground receivers tracking GPS and from the LEO tracking other GPS satellites, are used to obtain precise orbit and clock solutions of the satellites.
The details of how the GPS/MET signal is calibrated in order to isolate the atmospheric effects on the occulting signals are given elsewhere [Hajj et al., 1995].
This ionospheric Doppler shift can be used to derive the bending of the signal, , as a function of the asymptote miss distance, a, (Figure 7) by assuming spherically symmetric atmosphere in the locality of the occultation.
The relationship between the direction of the signal's propagation and ionospheric (fig.8), Doppler shift, f, is then given by:
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(9) |
where
f is the operating frequency,
c is the speed of light,
vt and vr are the transmitter and receiver velocity respectively,
kt and kr are the unit vectors in the direction of the transmitted and received signal respectively,
k is the unit vector in the direction of the straight line connecting the trasmitter to the receiver.
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Figure 8: Geometry showing the direct line of sight between transmiter and receiver and the asymptotes of transmitted and received signals. |
Assuming spherical symmetry introduces the extra constraint
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(10) |
where
rt and rr are the coordinates of the trasmitter and the receiver respectively
n is the index of refraction at the specified coordinate.
Eqs. (9) and (10) can be solved simultaneously in order to estimate the total atmospheric bending. Even though solving Eqs. (9) and (10) ideally requires knowledge of n at the satellites locations, in the ionosphere n is nearly equal to 1 and no significant error is introduced by using this approximation in Eqs. (10).
The spherical symmetry assumption can also be used to relate the signal's bending to the medium's index of refraction, n , via the relation [Born and Wolf, 1980] :
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(11) |
where
a' = n r
r is the radius of the tangent point [Figure 7].
This integral equation can then be inverted by using an Abel integral transform given by [ see Tricomi, 1985 ]
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(12) |
The upper limit of the integral in Eq. (12a) requires knowledge of the bending as a function of a all the way up to the top of the ionosphere.
The GPS is above most of the ionosphere ; however, this is not true of the GPS / MET instrument, at 7110 km radius (740 km altitude). For GPS/MET geometry and a tangent height around 300 km, we have
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In order to obtain (a) for a > 7110, an exponential extrapolation (a) based on information from a < 7110 is used. In order to avoid dealing with the singularity at the lower boundary of the integral, equation (12a) is rewritten as
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(12b) |
where 
is an intermediate value between a and and is normally chosen to be slightly larger than a Eqs. (9)-(12) with
(1)-(7) constitute the essence of the radio occultation profiling technique as it applies to the ionosphere and atmosphere.
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Data were taken on May 4- 5, 1995 by Hajj et al., 1998 . The L1 and L2 phase measurements for the occulted link were recorded every 10 seconds when the tangent point was above ~ 120 km altitude and every 20 ms ( 50 Hz rate ) when the tangent point was below that height. Therefore, results presented will reflect a rather coarse vertical resolution (~20- 30 km) above ~ 120 km and of order 1.5 km, below ~ 120 km. The bending of the signal locally is in the direction of the refractivity gradient.
In a general and approximate sense, the gradient of refractivity in the ionosphere is pointing upward above the F2 peak and downward below that peak. Therefore, the GPS signals will generally bend upward and downward above and below the F2 peak respectively.
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Figure 9: Bending induced by the ionosphere and neutral atmosphere on L1 signals for 61 globally occultations on May 4, 1995. |
Examining the bending (Figure 9) of the GPS L1 signal for 61 GPS / MET occultations that took place on May 4, 1995, we observe the following features:
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The most striking feature of these data is how sharp the signature is around the E ( or sporadic E ) layer. Even though determination of the magnitude of the E-peak electron density might be obscured due to the overlaying layers and the assumption of spherical symmetry, the height of sharp E-layers can be determined very accurately.
The bending derived from the time derivative of phase provides one piece of information on the location of sharp layers in the ionosphere.
Bending for the L2 signal is a factor of 1.65 ( = (154 / 120) 2), the square of the ratio of L1 to L2 frequencies) larger than for L1 (see Eq. (1)).
This dispersive nature of the ionosphere causes the L1 and L2 signals to travel slightly different paths and therefore sample different regions (as indicated by the solid and dashed lines in Figure 7). This causes the tangent points of the two links to be at different heights in the atmosphere at a specific time.
With the Abel inversion technique, the electron density profile and the height of the tangent point at a particular instant during the occultation an be solved for Figure 10 shows an example of an electron density retrieval obtained from GPS / MET for an occultation taking place near - 6 N latitude and 228 E longitude around 20: 04 UT of May 4, 1995 ( the corresponding local time is 11 : 01 hs ).
Also shown on the figure is the separation between the L1 and L2 tangent points as a function of altitude. In the neighborthood of the F2 peak, the relative position of the two signals change due to changing direction of bending.
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Figure 10: Electron density retrieved from occultation and corresponding amount of L1 and L2 signal vertical separation at tangent point. |
Above the F2 peak, since the bending is generally upward, the L2 tangent point will always be lower than the L1 tangent point. The situation reverses when the signal tangent point is below the F2 peak.
For this particular profile, the maximum separation is of order 300 meters; this scales linearly with the amount of bending a signal experiences. Therefore, one can expect separations that are two orders of magnitude smaller ( as seen with the bending ) or one order of magnitude larger during solar-max day-time.
A large separation of the two signals can be a limiting error for neutral atmospheric retrievals at altitudes above ~ 40 km [Kursinski et al., 1997] unless higher order corrections are applied to calibrate for the ionosphere.
Figure 11 shows the flight receiver signal - to - noise ratio of the L1 and L2 signals for four different occultations, where time = 0 corresponds to the start of high - rate data at about 120 km altitude for each occultation. The gradual decrease of SNR starting at about 30- 40 seconds is due to significant atmospheric bending starting at about the tropopause. As the signal approaches the surface, it bends significantly ( up to -1).
A good fraction of them ( see Fig. 11 a, b and c) show one or several sharp changes in SNR which can be attributed to sharp layers (e.g. sporadic E ) at the bottom of the ionosphere.
That these scintillations are caused by the ionosphere and not the neutral atmosphere can be seen from the fact that the L2 SNR fluctuation is larger than that of L1, consistent with its lower carrier frequency.
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Figure 11: Instrumental signal - to - noise ratio as a function of time for L1 and L2 signals for differents occultations. |
The electron density profiles obtained with the Abel inversion corresponding to the occultations of Figure 11 are shown in Figure 12.
Figures 12 a, b and c respectively show one, several and two sharp layers at the bottom of the ionosphere.
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Figure 12: GPS profiles of electron density corresponding to the occultations . |
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Due to the antenna field - of - view ( ± 30) and memory limitations on board the satellite, only 100 - 200 occultations per day are collected from the GPS / MET ( SAC-C 500 occultations per day).
The coverage obtained during 20 days of the mission is shown in Figure 13a where each occultation is represented by one point and Figure 13b GPS/MET coverage in sun - fixed coordinates in 24 hours. At low latitude, the occultations sample roughly the same local time and same latitude every orbital revolution.
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| Figure 13a: GPS / MET occultations of Fig.11 indicating the ability of GPS occultations to resolve sharp layers in the ionosphere. |
By contrast, the GPS/MET coverage for one day in sun-fixed coordinate is shown in Fig. 13b where each line corresponds to one occultation intersecting the ionospheric shell between 100- 400 km altitude.
Since the coverage in Fig. 13b is shown as a function of sun-fixed longitude (0 sun-fixed longitude corresponds to noon local time), and the occultations are scattered along the LEO orbit, the occultations are concentrated along the ground track of the GPS / MET satellite.
The width of the spread of occultation around the SAC-C (LEO) track is determined by the width of the field - of - view of the receiving antenna and the distance to the limb.
For a 740 km altitude satellite the limb is about 3000 km away from the satellite. This implies that the tangent point of an occultation falls within radius from the satellite trajectory during that occultation, setting an upper limit on the width of the spread of occultations around the SAC-C track to be ~ ± 27 equatorial degrees.
The reccurrence of occultations at nearby local times and latitudes is illustrated by showing the retrievals of four equatorial occultations appearing at consecutive orbital revolutions, each taking place near noon local time. ( These four occultations cross the circle in the middle of Fig. 10b)
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Figure 13b: GPS / MET coverage in sun-fixed coordinates in 24 hours, May 4, 1995. At low latitude, the occultations sample roughly the same local time and same latitude every orbital revolution. |
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For comparisons, profiles from the Ionospheric Parameters Model (PIM) [Daniels et al. 1995] derived with input parameters suitable for the same day are also shown (fig. 14a, b).
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Figure14: Examples of electron profiles obtained from GPS /MET and PIM for May 4, 1995. a) low latitude profiles b) high latitude profiles. |
Some of the main features to observe are :
(1) The ability to observe the E - F1 - and F2 layers that are characteristic of mid-and low latitude day-time ionosphere.
(2) The ability to observe the evolution of the ionosphere at the same local time and latitude every ~ 100 minutes ( the GPS / MET orbital period ) for mid - and low - latitude occultations.
(3) Except for the far-left profile shown in 14a, the PIM reproduces F2 peak densities and heights that are in reasonable agreement with the GPS / MET retrieval.
(4) Comparisons with the PIM is generally better below the F2- peak than at the top - side.
(5) The ability of both the model and the retrievals to reproduce the sharp bottom of the ionosphere.
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Fig. 15a shows a GPS / MET profile obtain on May 5, 1995, at about 0320 UT, with tangent points coordinates about 41.9N and 282.3E.
In Fig. 15b the same GPS / MET profile is compared to an ISR profile obtained with a 320 microsecond pulse mode about 20 minutes after the occultation. Millstone Hill is located at 42.6N and 288.5E, which is about 6east of the occultation location. The general agreement is fairly good.
Discrepancies between the ISR and the occultation can be ascribed to several factors, including the spatial separation between the occultation and the ISR measurements, error introduced by the spherical symmetry assumption when doing the GPS/MET retrieval, and the lower vertical resolution of the ISR measurements.
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Figure 15: Comparison of an electron density profile obtained from GPS / MET on May 5, 1995, 03 20 UT, with nearby measurements from Millstone Hill ISR a) :GPS / MET vs two ISR measurements with 640 µs pulse mode at 0321 UT and 0340 UT. b) GPS / MET vs.ISR measurements with 640 µs pulse mode at 0341 UT. |
A more extensive comparison of NmF2 derived from f0F2 ionosonde measurements and GPS / MET profiles has been performed, with results shown in Fig. 16.
The middle line in Fig. 16 corresponds to perfect agreement between these two measurements of NmF2 . The upper an lower lines on the figure correspond to +20 % and - 20% deviation of GPS/MET derived NmF2 from the ionosonde NmF2 respectively.
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Figure 16: A scatter plot of NmF2 derived from ionosonde of foF2 and GPS / MET electron density profiles, showing the degree of correlation between the two. The middle line corresponds to perfect correlation |
Differences in these two measurements are due to
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GPS occultations have been shown to provide a new and complementary vantage point over ground based measurements for probing the ionosphere.
Another approach, which is appropriate for ground - based or uncalibrated space - based measurements, would be to process the combination L1 - L2 ; this directly isolates the ionospheric delay at the cost of higher data noise.
Processing L1, L2 also ignores the differential bending of the two frequencies in the ionosphere. For the period analyzed (near solar minimum), bending in the ionosphere is on the order of 0.01 deg or less, with occasional stronger bending (up to 0.03 deg) occurring near sporadic E layers ; this amount of bending implies a separation between the L1 and L2 signals of several hundred meters near the tangent point.
The strong vertical refractivity gradient at sporadic E layers causes strong scintillation and relatively large bending, which makes this technique suitable for detecting the heights of the layers with very high vertical resolution. The amplitude of the electron density at these heights, however, might be masked by the overlaying dominant F- region.
With the assumption of spherical simmetry used in the Abel transform, the peak electron density is overestimated or underestimated at the tangent point, depending on whether the ionosphere at that point is at a relative minimum or a relative maximum, respectively.
Linear (or higher odd) power gradients in the horizontal distribution do not influence the retrievals when spherical symmetry is a assumed, simply because these terms cancel when integrated across an occultation link ; only even terms in the gradient survive an appear as errors in the retrievals.
Hajj et al. [1994] have shown that a significant improvements can be made to the spherical symmetry assumption by making use of global ground maps of vertically integrated TEC measurements such as those computed by Mannucci et al. [1997]
The idea introduced there was to impose a horizontal gradient at each layer identical to that of the TEC map, and then solve for a scale factor for each layer. In this manner, each occultation is processed individually, but without assuming a spherically symmetric ionosphere.
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When a signal trasmitted by the Global Positioning System (GPS) and received by a low-Earth orbiter (LEO) passes through the Earth's atmosphere, its phase and amplitude are affected ways that are characteristic of the index of refraction of the propagating medium.
In the lower troposhere, where water vapor contribution to refractivity is appreciable, independent knowledge of the temperature can be used to solve for water vapor abundance.
The basic observable for each occultation is the phase change between the trasmitter and the receiver as the signal descends through the neutral atmosphere.
After removal of geometrical effects due to the motion of the satellites and proper calibration of the transmitter and receiver clocks. The extra phase change induced by the atmosphere can be isolated. excess atmospheric Doppler shift is then derived.
This extra Doppler shift can be used to derive the atmospheric induced bending, "" as function of the asymptote distance, "a".
Assuming a spherically symmetric atmosphere, the relation between the bending and excess Doppler shift, ƒ , is given by (9).
In the stratosphere and the region of the troposphere where temperature is colder than ~250K, the water vapor term in Eq. (7) is negligible.
In the troposphere, at heigh where the temperature is larger than 250K, the water vapor term in Eq. (7) becomes significant and it is more efficient to solve for water vapor given some independent knowledge of temperature, [Kursinski et al., 1995a].
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Vertical and horizontal resolution of the technique, and refractivity, temperature, pressure, water vapor accuracies as a function of height.
Due to the nature of the measurement, which is a pencil-like beam electromagnetic signal probing the atmosphere, the technique has a much higher vertical and across - beam resolution than horizontal.
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(13) |
where
is the signal's wavelenght, RGPS , RLEO are the distances of tangent point to the GPS and LEO respectively.
In the presence of a medium, due to bending induced on the signal, the Fresnel diameter is ~0.5 km near the surface and approaches 1.5 km above 20 km altitude where bending becomes small. When the signal encounters sharp gradients in refractivity due to water vapor layers near , the surface, the Fresnel diameter shrinks to ~ 200 m.
A horizontal resolution scale is set by the length of the beam inside a layer with a Fresnel diameter thickness. This length is 160-280 km for a Fresnel diameter of 0.5 - 1.5 km.
When a LEO tracking GPS has a 360 field of view of the Earth's horizon, about 750 occultations per LEO per day can be obtained.
In addition, in order to calibrate the clocks of the occulting trasmitter and receiver, one other GPS trasmitter and one ground GPS receiver are required (see Fig. 17 ).
This requirement, in addition to some memory limitations inside the flight receiver, limits the number of occultations to about 100 per day ( 500 for SAC-C). A high inclination LEO provides a set of occultations that covers the globe fairly uniformly.
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Figure 17: Geometry of calibration |
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The main observable used in an occultation geometry is the phase change between the trasmitter and the receiver as the occulting signal descends through the atmosphere.
This phase change is due to:
In order to derive the excess atmospheric Doppler shift, one must remove the contribution of the first two effects.
We show the various steps of processing for a single retrieval in order to understand the basic characteristics of the atmospheric effects on the signal.
After applying the calibration, we obtain the atmospherically induced phase delay (up to a constant bias). Figure 18 shows the L1 delay, Doppler shift and instrumental signal -to-noise ratio for an occultation.
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| Figure 18: a) Left: excess atmospheric phase and Doppler as function of time. b) Right: receiver's signals -to-noise ratio as a funtion of time |
The following features can be observed from these two plots:
The corresponding L1 and L2 bending for the same occultation are shown in Figure 19.
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| Figure 19: Bendings of GPS L1 and L2 signals and ionospheric free bending. |
The L2 bending as a function of asymptote miss distance, 2 (a2), is interpolated to the L1 asymptote miss distance and the following relation is used to calculate the neutral atmosphere's contribution to bending
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(14) |
where the first and second coefficients of Eq. 14 corresponds to ƒ12 / (ƒ12 - ƒ22) and ƒ22 / (ƒ12 - ƒ22) respectively.
Using
temperature is derived in the neutral atmosphere and is shown as a function of pressure in Figure 20.
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| Figure 20: Temperature profile from GPS-MET, radiosonde and NMC stratospheric model |
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Based on theoretical estimations and simulations [Hardy, et al., 1993] atmospheric temperature profiles are expected to be accurate to the sub-Kelvin level between 5-30 km heights.
Initial results of GPS-MET are consistent with these predictions. The GPS radio occultation measurements combine accuracy with the vertical resolution necessary to resolve tropopause structure in a way that is well beyond the capabilities of current space-based atmospheric sounders.
A single orbiting GPS receiver provides up to 500 globally distributed soundings daily. The density of these measurements exceeds that of high vertical resolution radiosonde sounding by several factors in the southern hemisphere.
The coverage, robustness, accuracy, vertical resolution, and insensitivity to cloud inherent to GPS radio occultation suggest that it will have a major contribution to global change and weather prediction programs around the globe.
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