A proposed navigation concept requires that a user measure the time-delay that satellite-emitted signals experience in traversing the distance between satellite and user. Simultaneous measurement of the propagation time from four different satellites permits the user to determine his position and clock bias if satellite ephemerides and signal propagation velocity are known. A pulse propagating through the ionosphere is slowed down somewhat, giving an apparent range that is larger than the equivalent free space range. The difference between the apparent range and the true range, or the free space velocity and the true velocity, is the quantity of interest. This quantity is directly proportional to the total electron content along the path of the propagating signal. Thus, if the total electron content is known, or is measured, a perfect correction to ranging could be performed. Faraday polarization measurements are continuously being taken at Fort Monmouth, N. J., using beacon emissions of the ATS-3 (137.35 MHz) satellite. Day-to-day variability of the diurnal variation of total electron content values is present with differences of up to 50% or more not being uncommon. In addition, superposed on the overall diurnal variation are smaller scale variations of approximately 5 to 10% of the total content which are attributed to ionospheric density irregularities

Gorman, F. J., Jr.

Soicher, H.

English

NASA Technical Reports Server

IONOSPHERIC EFFECTS ON ONE-WAY TIMING SIGNALS 
H. Soicher 
F. J. Gorman, Jr. 
U . S. Army Electronics Command, Fort Monmouth 
ABSTRACT 
A proposed navigation concept requires that a user measure the time-delay that 
satellite-emitted signals experience in traversing the distance between satellite 
and user. Simultaneous measurement of the propagation time from four differ- 
ent satellites permits the user to determine his position and clock bias if satel- 
lite ephemerides and signal propaption velocity a r e  known. A pulse propagating 
through the ionosphere is slowed down somewhat, giving an apparent range that 
is larger than the equivalent free space range. The difference between the a p  
parent range and the true range, o r  the free space velocity and the true velocity, 
i s  the quantity of interest. This quantity i s  directly proportional to the total 
electron content along the path of the propagating signal. Thus, if the total elec- 
tron content i s  known, o r  is measured, a perfect correction to ranging could be 
performed. 
Faraday polarization measurements a re  continuously being taken at Fort Mon- 
mouth, N. i , , using beacon emissions of the ATS-3 (137.35 MHz) satellite, 
which i s  in a geostationary orbit. The polarization data yield electron content 
values up to -1500km since the measurement technique is based on the Faraday 
effect which i s  weighted by the qeomagnetic field. Day-to-day variability of the 
diurnal variation of total electrol: content values is present with differences of 
up to 50% o r  more not being uncommon. In addition, superposed on the overall 
diurnal variation a r e  smaller scale variations of - 5 to  10% of the total content 
which a r e  attributed to ionospheric density irregularities. 
Future experiments planned for the emissions of the forthcoming ATS-F will 
permit Faraday rotation, dispersive phase, and dispersive group delay measure- 
ments. The latter two will give the integrated electron density to geostationary 
altitudes while the first  will give the density integrated to - 1500 km. The dif- 
ference-referred to as  the exospheric content-will yield the currently unknown 
propagation time delay from ground to geostationary altitudes. 
A space-based radio nu: igation system could provide military and civilian users 
with precise three-dimensional position and velocity data. 
https://ntrs.nasa.gov/search.jsp?R=19750003226 2020-03-23T03:04:46+00:00Z
The navigation signals contain data euch a s  satellite identity, real-time ephem- 
erides information, system time and other data. To determine his poeition and 
velocity, the user cross-correlates the coded time signal received from the 
satellite with the same coded time signal generated in his receiver. The relative 
phase, or  equivalently the time displacement between the user's receiver and 
the incoming code, determines the range to the satellite. Simultaneous meas- 
urment of such relative phases from four different satellites permits the user 
to determine his range and his clock bias with respect to the satellite's position 
and clock respectively. In addition to such range measurements, corresponding 
range-rates are  measured by the carrier frequency Doppler shift of each signal 
and the correeponding motion of the particular satellite as described by the 
ephemerides data. 
The range from user to the various satellites is, of course, not the correct geo- 
metric range. A propagating navigation signal i s  slowed down by the ionosphere 
by an amount proportional to the total electron content along its path. The elec- 
tron content may be measured in real time, provided the user has dual-frequency 
capabilities. However, substantial reduction in the cost of user equipment could 
be realized if the navigation system uses only one frequency. In such a case, the 
ionospheric time delay will have to be determined through empirical modelling 
techniques, based on existing and future global electron content data, and will 
be transmitted to the user for correction via the navigating signal. To calculate 
the true range, one must determine the group velocity of the signal along the 
path. 
The transit time of a transionospheric signal from a satellite to a ground observer 
ie: 
where ds is an element of distance along the signal's path and vg  and ng are  the 
group velocity of the signal and the group refraction index respectively, c is the 
speed of light, and 0 and S are the observer and satellite position respectively. 
In the high-frequency approximation, the group index of refraction is: 
where N is the electron density per meter cubed, and f is the operating frequency 
in Hz. Equation 1 becomes: 
The first term in Equation 3 represents the free space signal transit time, while 
the second term represents the signal's excess time delay in the ionosphere. 
The excess time delay is  inversely proportional to the frequency squared so ?&at 
in the L band (the proposed navigation frequency is in the 1600MHz band) it i p  
o ery small. However, the precision required by the system is such that the 
ionospheric time-delay has w be accounted for. 
The variation of the time delay d t h  electron content for 1.6 GHz, 400 MHz, and 
150 MHz is shown in Figure 1. At 1.6 GHz, the excess delay times are  0.524 nano- 
seconds and 52.4 nanoseconds for a total content of 10 l6 el/m and 10 l%l/m respec- 
tively, which are the lower and upper bounds of the normal vertical ionospheric 
electron content. For an oblique signal path, the total content increases by an 
amount proportional to the additional path length (assuming spherical stratifica- 
tion of the ionospheric density distribution). At low elevations, i. e. below lo0 ,  
the slant total content exceeds the vertical content (i. e. elevation angle = 90') 
by a factor of 3; while at 40' elevation, the slant content exceeds the vertical 
content by a factor of 1.5. Thus, for the vertical content at the upper bound and 
a low-elevation signal path, the total time delay introduced by the ionosphere is 
-150 nanoseconds at 1.6 GHz. 
THE FARADAY ROTATION 
In the high-frequency and quasi-longitudinal approximations, the two magneto- 
ionic modes are  nearly circularly polarized in opposite senses, thus a plane 
polarized wave traversing the ionosphere may be regarded as  the vector sum of 
the ordinary and extraordinary components. Since the two components travel 
at different phase velocities, the plane of polarization rotates continually along 
the signal's path. The total rotation from the signal source to the observer is 
related to the total electron content by the expression: 
k S k S 
a =7; [ BcosO Nds = p  I: (BcosBsec~)Ndh. 
where k = 2.36 x B is  the local magnetic field flux density in gammas, O 
is  the angle between the radio wave normal and the magnetic field direction, and 

X ie the angle between the wave normal and the vertical. Since B decreases 
inversely with the cube of the geocentric distance, ancl since the electron den- 
sity decreases exponentially with altitude above F2 max ( 4 0 0  km), the integral 
is heavily weighted near the earth and is considered to provide electron content 
values below - 1500 km. 
The term M = B cos 8 sec x in Equation 4 may be taken out of the integral sign 
and replaced by its value at a "meanH ionospheric altitude. Equation 4 then 
becomes: 
= '  M J N dh. 
f 2  
The correct conversion of polarization rotation data to electron content data de- 
pends on the altitude chosen ior the calculation of i/?. A method for arriving at 
such an altitude is shown in Figure 2. The height distribution of the electron 
density was obtained by converting the topside and bottomside ionograms recorded 
at close geographic proximity to Fort Monmouth into electron density profiles. 
Polarization-rotation data on 40 MHz was obtained from an overhead orbit of the 
S-66 satellite (nominal altitude = 100 km). The vertical component of the geo- 
magnetic field (curve B) was derived from the spikes in the topside ionogram 
which measure the gyrofrequency at the satellite altitude. At lcrwer altitudes, 
the field intensity was calculated from the reference at tha- satellite using the 
magnetic d t ~ l e  approximation. Numerical, successive integration of the pro- 
duct N * B produced the curve of the polarization angle rotation Z a which ends 
with the total polarization angle observed at the ground (llmeasuredv in Figure 
2). The next procedure was integration of the bottomside and topside profiles 
to obtain j N dh. Next, the magnetic field component, R, was obtained from 
Equation 5 by inserting the measured total polarization angle Z a, and j N dh of 
the integrated profiles. The figure  indicate^ that for the above example cor- 
responds to a local field value about 60km above h,, . Figure 2 indicates fur- 
ther that 90% of the total polarization-angle rotation took place below 550 km. 
THE DATA 
The need for an empirical model of the global distribution of the ionospheric 
electron content for prediction purposes ha8 focussed attention on the availability 
of such data on a global basis and on future requirements. The advent of beacon 
emissions from geostationary satellites has clearly facilitated data gathering at 
wide geographic areas, with the diurnal variation of content being obtained with- 
out contamination by satellite orbital changes. With low flying satellites, months 
of data from a particular station are necessary to obtain a diurnal variation. The 
L 1 I I 
-30 .40 .SO 
B GAUSS 
Figure 2. CalcuMion of the Fama%y Polarization Rotation Angle Za as a Func- 
tion of Altitude Using the Actually Meascred Electron Density Profile N(h), and the 
Geomagnetic Field Component B Obtained from Gyrofrequency Spikes in the 
Topside Io11ogram. 
day-to-day as well a s  the geographic and seasanal variability of the ionosphere 
fs superimposed on the diurnal variation thus obtained. 
Polarization rotation measurements are  performed continuously at  Fort 
Munmouth, N. J. (40. 2S0N, 74. 02S0W) utilizing b e a m  emissions of the ATS-3 
(Xi7.35 MHz). The subionospheric point (below 350 km along the path from Fort 
Moomouth to ATS-3) is located at  37. 1°N, 73.6OW. For the purpose of this re- 
port, data obeemtions for time periods between the 123rd and 178th days of 
1973, i. e. , May 3 to June 27, 1973, a re  presented. The data have been norma- 
lized to the vertfcal direction and, hence, represent the vertical total electron 
content. Representative d i u n d  variations are shown in Figures 3 through 5. 
The following obsematioae are made with respect to the absolute values of the 
content (and, equivalently, of the ionospheric time delay): 
1. l%e diurnal variation a s  well as the day-to-day variability of the total 
vertical content is evident. The lower and upper limits of the content 
are -0.2 x 10 17 and - 3.0 x 10 1 7 respectively and correspondingly the 
ionospheric time delay is -1 nanosecond and 15.7 nanoseconds. Differ- 
ences of up to 100% can be observed in the figures. 
2. For the reported data, buildup of ionospheric ionization started daily at 
-0400 EST. For the time of year considered, the dawn buildup begins 
wken solar illumination is  at -100km (solar zenith angle = -98"). T o p  
side ionospheric densities have been reported to decrease with increasing 
solar illumination until about ground sunrise. 1 The discrepancy between 
topside density and total content could be explained by thermally induced 
particle fluxes from the topside ionosphere to the bottomside ionosphere 
during ionospheric sunrise and by the quicker buildup of the bottomside 
ionosphel e. 
3. The buildup phase of the diurnal variation is smooth, and its time slope 
is nearly constant. On most days, the sustained buildup phase ends prior 
to noontime, although the diurnal maximum is not reached until later. 
4. The maximum of the content is variable in absolute value a s  well as  in 
the time of occurrence. Such maxima usually occur after 1600 EST, and 
sometimes much later. 
5. In terms of absolute values of the electron content, magnetic activity does 
not have a consistent effect on the content. Figures 3 through 5 show 
cases when magnetically disturbed days show aepressed content values 
with respect to quiet days, and vice versa. Even data taken on consecu- 
tive disturbed days have totally different characters. 
DAYS 123-129,1973 
1 I I I 1 I I I 
00 04 00 * TIME (EST) 16 26 24 
Figure 3. Total Integrated Electron Content Murnal Variation for the Days 123 Through 
129, 1973. (QQ, Q, D refer to Magnetically Extremely Quiet, Quiet, or Dlaturbed 
Days, Respectively). 


6. The sustained decay of ionization with the setting of the sun starts, in 
most cases, from the maximum absolute value of the content. As during 
the buildup period, the decay with time is smooth and has a constant 
slope. The decay elope i s  sharper than the buildup slope. 
7. At night the content continues to decline until i t  reaches an absolute mini- 
mun, from which the buildup phase starts at  sunrise. The minimum may 
be reached just before sunrise or  it may be sustained an hour or  so before 
sunrise. 
8. Smaller scale variations are  superimposed on the overall diurnal varia- 
tion of the electron content as  is  evident from Figures 3 through 5. These 
variations are  caused by ionospheric ionization irregularities along the 
signals1 path. The irregularitieu are  finite in extent and are Imown to be 
moving. Some of the irregularities are  caused by ionospherically travel- 
ing internal gravity waves. Their finite size means that two relatively 
closely-spaced ground stations monitoring the same satellite may observe 
different vertically normalized electron content values since one's path 
to the satellite goes through the irregularity while the other's does not. 
The traveling ionospheric disturbance which cannot be predicted, except possibly 
statistically, constitutes a natural limit to the accuracy of any real-time predic- 
tion of the total integrated electron content. To ascertah the extent of this limit 
and its possible diurnal variation, the following procedure was used. Each day 
was divided inta 6 four-hour intervals and the maximum deviation from the am- 
bient content at  any of the intervals was recorded. The hounds of the deviation 
ranged from no deviation to an -20% deviation. The deviations were then aver- 
aged for all the days and plotted in Figure 6. The following conclusions may be 
drawn: 
a. Electron content deviations occur, on the average, at all diurnal periods 
with a maximum of 5.4% and a minimum of 2.8%. 
b. On the average, the deviations are smallest during the sustained iono- 
spheric buildup period (0400-0800 EST) and during the sustained iono- 
spheric decay period (1600-2000 EST). 
c. The greatest deviations occur during the day between the buildup and the 
decay. Nighttime deviations are also large. 
d. From the viewpoint of prediction schemes, the daytime deviations pose 
a greater problem to accuracy, since the daytime content i s  larger than 
the nighttime content by a factor of 3 or  so. 
00-04 0 4 - 0 8  08-12 12- 16 16-20 2 0 - 2 4  
TIME INTERVAL (EST 1 
Figure 6. Ionization Irregularity in Percent of Ambient Content 
for Six Daily Time Periods 
FUTURE EXPERIMENTS 
The forthcoming launch of the geostationary ATS-F will offer a unique and ex- 
cellent opportunity to study the dim.ml behavior of the total electron content at 
various geographic locations. Our group will perform two experiments. In one 
experiment, the polarization rotation of the 140 MHz beacon wiIl be measured. 
This will not be different from the experimenter carried out to date utilizing other 
geostationary satellites. In a second experiment, continuous dispersive group 
delay meaeurementa will be made on 140 MHz and 360 MHz, both of which will 
be complemented with k1 MHz sideband emissions. With such a modulation on 
both frequencies, the distance between peaks of the modulation envelope is 300 
meters in f ree  space and very little different in the ionosphere. The modulation 
envelope travels at the group velocity and is retarded in phase by bP/300 cycles 
at the modulation frequency where 
A t  night (assuming J N ds = 10t7el/m *), AY - 200 meters for the 140 MHz fre- 
quency and AP - 30 meters for the 360 MHz frequency, i. e. , phase reduction is 
<1 cycle for both frequencies. The modulation phase difference is thus obtained 
unambiguously and yields the absolute value of the total electron content. Fo: 
other times of the day when the total content is larger, the absolute valuz c~**dd 
easily be derived from the nighttime values since the total content varies  ont tin- 
uously with time. 
Since the phase effect is independent of the terrestrial magnetic field, it gives 
the true electron content along the entire propagation path, as  opposed to the 
content given by the polarization rotation effect which pertains only to a signal 
path portion of 1500 km from the earth's surface. Tiie difference between simul- 
taneous polarization and group-delay data will yield the currently unknown ex* 
spheric electron content, i. e., the content between 1500km and synchronous 
altitudes. 
SUMMARY 
With stringent precision requirements, a navigation system which measures 
signal propagation ti me between high-altitude satellites and users at or near the 
earth's surface must take into accouat the excess time delay of the signal owing 
to the existence of free electrons along its path, The excess signal time delay 
is proportional to the total electron content along the propagation path. The up- 
per bound for non-anomalous ionospheric conditions is -150 nanoseconds for a 
highly oblique propagation path at a frequency near 1600 MHz. 
For dual-frequency user-capability, the excess time delay may be measured in 
real-time and a correction applied to the apparent range. For single-frequency 
user-capability, the ionospheric time delay must be arrived at through empirical 
global prediction techniques based on eAsting and future electron content data, 
The pertinent information will be transmitted to the user via the navigating sig- 
nal. To date no truly global coverage of the total electron content exists. Most 
existing data are  based on Faraday rotation measurements of the electron con- 
tent in the earthle immediate vicinity only (i. e., up to -1500km). Future ex- 
periments will yield content values up to geostationary altitudes. 
A limit to the accuracy of the prediction is set by superposition of the variations 
of ionization irregularities on the diurnal variation of the total electron content. 
The greatest average percent variation was observed during the day between the 
buildup and decay phases of the diurnal variation and amounted to -5.4% of the 
ambient total electron content. Since the content is a t  its highest during the day, 
such perc 3ntage deviations cause relatively high absolute time delay deviations. 
Of course the complexity of the ionospheric processes and their interrelationships 
with many other geophysical phenomena will introduce additional uncertaintiee 
into the predictions. 
ACKNOWLEDGMENT 
The investigation reported in this paper has been supported by the U.S. Army 
Satellite Communications Agency, Fort Monmouth, N. J. 
REFERENCES 
1. H. Soicher, '*The Topside Ionosphere at Mid-Latitudes During Local Sun- 
rise," Journal of Atmospheric and Terrestrial Physics, Vol. 35, pp. 657- 
668, 1973. 
2. K. C. Yeh, "A Study of the Dyanmics of Travelling Ionospheric Disturbances, lT 
Space Research XII, Akademic Verlag, p. 1179, 1972. 
3. "The ATS-F and -G Data Book," NASA Goddard Space Flight C e ~ t e r ,  1971. 
QUESTION AND ANSWER PERIOD 
MR. EASTON: 
Any question8 ? No questions ? 
(No response. ) 
MR. EASTON: 
Thank you very nuch, Mr. Soicher. 


Ionospheric effects on one-way timing signals

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