The theoretical basis and avaliable accuracy verifications were reviewed and compared for ionospheric correction procedures based on a global ionsopheric model driven by solar flux, and a technique in which measured electron content (using Faraday rotation measurements) for one path is mapped into corrections for a hemisphere. For these two techniques, RMS errors for correcting satellite altimeters data (at 14 GHz) are estimated to be 12 cm and 3 cm, respectively. On the basis of global accuracy and reliability after implementation, the solar flux model is recommended

Nesterczuk, G.

English

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https://ntrs.nasa.gov/search.jsp?R=19820004574 2020-03-21T11:38:00+00:00Z
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Ionospheric Ropoft Correction Modeling
for Satellite Altimeters
(N>S#-C8-156881) IO10SP8BHIC P209AoBPI0)r
	 582-12447
COBBBCtION BOBBLING FOR SAULL122 LLURZIBBS
(BG 8 G Vashingtoa Analytical Services)
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VALCEIVO
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November 1981
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National Aeronautics and
Space Administration
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Wallops Island. Virginia 23337
AC 804 824-3411
I
nTABLE OF CONTENTS
Soction	 page
1.0 INTRODUCTION	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 1
.' 2.0 IONOSPHERIC MODELS	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 2
2.1	 NASA Mode l	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 2
2.2	 JPL Model	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 7
3.0 MODEL RECOMMENDATION .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 .	 8
4.0 IMPLEMENTATION AND CHECKOUT OF NASA MODEL	 . . .	 9
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iii
NASA Contractor Report 156841
Ionospheric Propagation Correction Modeling
fvSatellite Altimeters
G. Nesterczuk
EG&G Washington Analytical Services Center, Inc.
6801 Kenilworth Avenue
Riverdale, MO 20840
Prepared Under Contract No. NAS6.3075
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Section 1.0
INTRODUCTION
Propagation delay
 through the ionosphere is one of the error sources
for which a correction must be made for future satellite altimeters, with
satellite to mean sea surface height accuracies required at the few centi-
meter level. The maximum correction amounts to approximately 15 cm. at
14 GHz. Due to the large variations between daytime and nighttime ionospheric
electron densities, a correction would be required even for precision pro-
filing missions for which absolute height accuracy is not required.
Several approaches are possible, including measuring the correction
through the use of a dual frequency altimeter, and the use of various types
of ionospheric modeling. This report considers the two modeling techniques
which are considered most promising for application to near real-time
altimetry processing, with a recommendation made as to the technique giving
superior accuracy on a global scale.
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Section 2.0
IONOSPHERIC MODELS
Two types of ionospheric models have been proposed as a basis
for making height corrections for satellite altimeter data. The basic
characteristics of these two approaches are summarized below.
2.1 NASA Model
The "NASA" model is an empirical worldwide model of the ionosphere,
and exists in several different versions. What will be referred tc here
as the NASA model [1] is presently .i n3rt of the operational Goddard
Trajectory and Determination System (GTDS) at Goddard Space Flight Center.
An earlier version is also in operation at the Naval Surface Weapons Center
at Dahlgren, Virginia.
The NASA model assumes a profile shape as shown in Figure 1. This
profile includes a number of key parameters, with the maximum electron
density and height of the maximum density among the most critical. Typical
global maps of height of maximum density (H max ) and critical frequency
(corresponding to maximum electron density) are shown in Figures 2 and 3.
All parameters have latitude, longitude and diurnal dependence, which
are driven by daily solar flux (f 10,7 ) values (plus averaged values over the
previous year) and a set of M3000 coefficients available from NOAA/Boulder.
A typical global map of total electron content, to which the ionospheric
propagation correction is proportional, is shown in Figure 4.
With the input of daily flux values, this technique gives corrections
which are accurate to within approximately 20-255 [2]. Since the largest
ionospheric propagation correction (at 14 GHz) is —15 cm. for a maximum
solar activity period, the NASA model errors could reach 4 cm. For less than
peak solar activity, the error would be considerably smaller and the RSS
error would probably be on the order of 1-2 cm. even for maximum solar
activity, since nearly half of an average pass would be in nighttime for
which the correction error is near zero.
2
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2.2 JPL Faraday Rotation Model
The Faraday Rotation or "JPL" model has been used at the Jet Propulsion
Laboratory, Pasadena, California. It is based on measured Faraday rotation
measurements from one or more ground stations to a satellite. These measure-
ments can then be converted to integrated election density measurements along
the slant range from the ground station to the satellite (outside the ionosphere).
These measurements are then extrapolated [3] to other parts of the world as
functions of time, spacecraft, geomagnetic latitude, and local solar zenith
angle. In the implementation of this model for Seasat altimeter data cor-
rections [3], two ground stations were used, Armidale, Australia for the
southeren hemisphere and Goldstone, California for the northern hemisphere
(both stations using the ATS 1 satellite.
Based on various comparisons made during Seasat data evaluation, including
comparisons with corrections computed from the dual-frequency Geoceiver data,
the accuracy of the Faraday Rotation method has been estimated [3] to be
3 cm. The corrections are, of course, very good near the measured ionospheric
path but can have errors on the order of 50% at geographical locations far
from the measurements.
L
7
Section 3.0
MODEL RECOMr1ENDATION
Of the models discussed in the previous section, the NASA solar flux
model* is recou.mended for the following reasons:
1. Since the ionosphere is a highly variable and inhomogeneous medium,
it does not extrapolate well beyond about 800 km. [4]. Thus, a limited
set of Faraday rotation measurements is not sufficient to provide cor-
rections on a global basis with as high degree of accuracy as can be
obtained from the global empirical model.
2. Should something happen to the Faraday rotation data flow, the
extrapolation technique then degrades rapidly. The global model, even
without solar flux updates, degrades much more slowly.
* The author would like to acknowledge that he is hardly an unbiased
observer with regard to ionospheric models. having been deeply involved in
the devel)pment of what is referred to here as the NASA ionospheric model.
It is believed, however, that the arguments and evidence in support of the
empirical modeling approach are overwhelming.
8
i.
Section 4.0
IMPLEMENTATION AND CHECKOUT OF NASA M00EL
The NASA model in the GTDS program already contains the NOM GSM
coefficient set. These coefficients consist of 36 sets of coefficients,
each covering a 10 day time span, and have had seasonal and solar flux
dependency ramoved. They are thus permanent coefficients and need not be
periodically replaced. In operatio3, the data set is stored on disk,, with
only a limited set read into core as needed.
The model is currently operated as a subroutine and its implementation
as a part of another system would require the removal of some linkage
with GTOS. But the subroutine package is largely self-contained and the
required modifications should be relatively minor.
Model maintenance requires that daily solar flux values be continually
provided. As previously indicated, a set of values for the previous year is
required.
Although the NASA model presently makes corrections for non-vertical
paths and for angle as well as ranging measurements, there would be minimal
improvement in efficiency or reduction in core requirements by making the
program correct only altimeter data. It is thus recommended that the
program be used basically as is (and as a black box). Extensive checkout
of a transferred program could be made through comparisons of intermediate
printout of the various critical parameters, as well as the computed
corrections, and comparison with computed quantities on the base computer.
9
REFERENCES
I	
1.	 Nesterczuk, G., J. G. Kozelsky, R. @. Bent, S. K. Llewellyn,
"Evaluation, Modification and Improvement of the Ionospheric
Refraction Correction Algorithm in the GTOS", Report /76M110F4,
Atlantic Science Corporation, Riverdale, MD. August 1976.
i
t	
2.	 Nesterczuk, G., J. G. Kozelsky, and J. A. Behuncik, "Line-of-Sight
Electron Density Gradients as Deduced from an Empirical Ionospheric
Model Symposium Proceedings on the Geophysical Use of Sate llite
Beacon Observations, pp. 706-716. Boston University, June 1976.
3. Lorell. J., E. Colquitt. and R. J. Anderle. "Al f lmeter Height
Correction for Ionosphere". JGR, in cress 1981.
4. Llewellyn, S.K., G. Nesterczuk. R. B. Bent, "Evaluation of the Faraday
Rotation Technique for Use in Real Time Ionospheric Modeling".
Atlantic Science Corporation Report prepared for NASA/GSFC under
Contract No. NASS-22353. February 1976.
10
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Ionospheric propagation correction modeling for satellite altimeters

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