The effects of the ionospheric retardation of satellite-emitted timing signals was presented. The retardation at the navigation frequencies, which is proportional to the total ionospheric electron content (TEC), was determined by Faraday polarization measurements of VHF emissions of a geostationary satellite. The polarization data yielded TEC up to approximately 1200 km only, since the measurement technique is based on the Faraday effect which is weighted by the terrestrial magnetic field

Gorman, F. J., Jr.

Soicher, H.

English

NASA Technical Reports Server

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PRECEDING PAGE BLA.NK NOT FfLuIED 
PLASMASPHERIC EFFECTS ON ONE-WAY SATELLITE TIMING SIGNALS 
F. J. Gorman, Jr. and H. Soicher 
Communications/Automatic Data Processing Laboratory 
U. S. Army Electronics Command 
ABSTRACT 
At the 1973 PTTI Planning Meeting, the effects 
of the ionospheric retardation of satellite- 
emitted timing signals was presented. The 
retardation at the navigation frequencies, 
which is proportional to the total ionospheric 
electron content (TEC), was determined by Far- 
aday polarization measurements of VHF emissions 
of a geostationary satellite. 
tion data yielded TEC up to ~1200 km only, 
since the measurement technique is based on 
the Faraday effect which is weighted by the 
terrestrial magnetic field. 
The polariza- 
The radio beacon experiment aboard the recently 
launched geostationary ATS-6 offers the unique 
opportunity to determine TEC, or equivalently, 
the signal propagation delay, to geostationary 
altitudes. The beacon package provides the 
opportunity to conduct two relevant experi- 
ments. The first utilizes the Faraday rota- 
tion technique for determination of TEC. The 
second utilizes the dispersive group delay . 
technique which is independent of the magnetic 
field and determines TEC to geostationary al- 
titudes. The difference between the values of 
the integrated electron content obtained by 
the two techniques yields the content from 
$1200 km to geostationary altitudes, i.e., the 
plasmaspheric electron content. 
Faraday rotation and' dispersive group delay 
observations have been conducted at Fort Mon- 
mouth since the launch of ATS-6 in late'May, 
1974. The plasmaspheric content exhibited 
diurnal as well as day-to-day variations. Its 
absolute magnit de varied from 1 t o  $8 TEC 
content was nearly always above 50% of the 
ionospheric content and on occasion exceeded 
100%. During the day, this ratio averaged ~ 3 5 % .  
units e/m Y ). A t  night the plasmaspheric 
441 
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INTRODUCTION 
At the 1973 Precise Time and Time Interval (PTTI) Symposium, 
the effects of the ionospheric total electron content onthe 
accuracy of transionospheric satellite navigation systems 
were discussed [I]. It was shown that in traversing the 
ionosphere, a propagating navigation signal i s  slowed by an 
amount which is directly proportional to the total electron 
content (TEC) along its path. This gives an apparent range 
that is larger than the equivalent free-space range. If 
the TEC is known, or is measured, a correction to the rang- 
ing may be applied. The TEC 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 used only one frequency. 
In such a case, the ionospheric time-delay will have to be 
determined through empirical modeling techniques based on 
existing and future global electron content data, and will 
be transmitted to the user for correction via the navigating 
signal. 
Most TEC data to date has been obtained by the Faraday pol- 
arization rotation technique, The Faraday rotation is a 
terrestrial-magnetic-field-dependent phenomenon, and its 
magnitude is heavily weighted near the earth. It is there- 
fore considered to provide electron content values at alti- 
tudes below %1200 km. The navigation system will utilize 
satellites at considerably higher altitudes. The navigat- 
ing signal will be slowed by free electrons in the iono- 
sphere as well as in the plasmasphere (i.e:, at altitudes 
>1200 km). The corrections for TEC supplied by a model 
based on the Faraday technique will not adequately compen- 
sate for the total signal delay since the free electrons in 
the plasmasphere will not be accounted for. The radio bea- 
con experiment (RBE) aboard the geostationary Applied Tech- 
nology Satellite (ATS-6 launched in May 1974) permits the 
accumulation of TEC data which includes the plasmaspheric 
content. The technique utilized is the dispersive-group- 
delay technique which is independent of the terrestrial- 
magnetic-field and hence yields the integrated electron con- 
tent from observer to satellite. 
THE FARADAY AND DISPERSIVE-GROUP DELAY METHODS 
The Faraday polarization rotation technique has long been 
used in measurement of TEC. 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 
442 
be regarded as the vector sun of the ordinary and extraor- 
dinary components. Since these two components travel at 
different phase velocities, the plane of polarization ro- 
tates continually along the signal's path. The total rota- 
tion from the signal source to the observer is related to 
the total electron content by the expression: 
B C O S ~ N ~ S  - (BcosesecX)Ndh, (1) k r S  a = -  2 '  f2 j o  f Jo 
where k = 2.36 x lo-', B is the local magnetic field flux 
density in gammas, 8 is the angle between the radio wave 
normal and the magnetic field direction, and x is the angle 
between the wave normal and the vertical. Since B decreases 
inversely with the cube of the geocentric distance, and 
since the electron density decreases exponentially with al- 
titude above F max (Q 300 km), the integral is heavily 
weighted near $he earth and is considered to provide elec- 
tron content values at altitudes below $1200 km. 
The term M = Bcosesecx in E q .  (1) may be taken out of the 
integral sign and replaced by its value at a "mean" iono- 
spheric altitude (420 km). Equation (1) then becomes: 
3 
(2 )  
a t - p * ,  k r k -  Ndh = - M NI, 
f2 ' Ionosphere f2 
where NI is the ionospheric total electron content measured 
by the Faraday rotation technique. At Fort Monmouth, where 
the numerical value for a is 56292 y, for NI = 10l6 e/m2 de- 
fined as l TEC unit, we calculate a = 38.83' (for f = 140 
MHz) . 
Using the dispersive-group-delay technique, the phase of 
the modulation envelope between a carrier and its sideband 
is compared at two frequencies (nominally f = 140 and 360 
MHz with sideband displacements of Af  = +lMHz). Since the 
phase is insensitive to the earth's magnetic-field, this 
technique yields the number of electrons along the entire 
path from satellite to observer (NT). 
The time delay of a signal propagated through a medium con- 
taining free electrons with a group velocity v is: g 
443 
where l.~ is the group refractive index and c is the veloc- 
ity of light in vacuum. For two signals at frequencies 
fl and f2, the differential time delay is: 
8 
If the two signals are modulated by a sideband separated by 
an equal Af, then the modulation time delay Atm is equal 
to the differential group time delay, i.e., 
At = At, 
360 Af C 
where A$ is the differential modulation phase shift in 
degrees. It follows that 
-1 
f I Ndh = NT - - A$ ,I.- c(secX) (L-- 
J Total 360 Af 40.3 f; 
At Fort Monmouth for NT = 
30.55' is measured. 
a phase difference of 
THE DATA 
The variation of the total electron contents measured by 
the Faraday and group delay techniques is shown in Fig. 1 
at 15-minute intervals for the time period 1600 EDT on 
3 July to 0800 EDT on 8 July. The temporal variations of 
NI and NT were nearly parallel with most density variations 
444 
observed on both curves. 
Prominent during the time period covered were the large in- 
creases of total electron content in response to two large 
solar flares (see Fig. 1). Between 0945 EDT and 1000 EDT 
on 4 July, NI increased by l ~ 1 . 5  x 3.016 e/m2, while NT 
increased by ~2 x 10l6 e/m2. Since the content values in 
Fig. 1 are given for every 15 minutes, the full increase 
of NI and NT is not indicated there. 
NI increased by l ~ 3 . 3  x 10l6 e/m2 in 3 minutes and then de- 
cayed to its value at 1000 EDT. 
increased by approximately the same amount. On S July 
between 1730 and 1745, N increased by $1.9 x while 
NT increased by ~ 2 . 3  x lof6. The rapid increases started 
at 1740 EDT with NI increasing by ~2.1 x 1OI6 in 6 minutes; 
NT increased similarly. 
The equivalent signal-delay time at 1.6 GHz (in the naviga- 
tion frequency band)corresponding t o  the vertical electron 
content distribution was always below 15 nanoseconds for 
1 the time period reported. On different days, the time de- 
I lay varied by as much as 60%. Between maximum and minimum 
’ on any one day, the largest factor was 12. Superimposed 
Starting at 1~0953 EDT, 
At the same time, NT 
on the normal diurnal variations of the content were quasi- 
sinusoidal variations which usually occur near the time 
period of maximum content of the daily cycle. These varia- 
tions are caused by ionospheric irregularities. 
The dispersive-group-delay technique measures the total 
electron content from observer to satellite, whereas the 
Faraday technique yields the content only in the vicinity 
of the earth (i.e., up to %1200 km). The difference be- 
tween the two yields the content above %1200 km, which is 
referred to as the plasmaspheric content, Np. 
It follows that 
Np is plotted in Fig. 2 for the same time intervals and for 
the same time period as that of Fig. 1. The plasmaspheric 
content ranged from 1 to ~8 TEC units, or equivalently from 
%0.5 to over 4 nanoseconds for a 1.6 GHz signal. Generally, 
the minimum of the plasmaspheric content occurred near 
ionospheric sunrise while its maximum occurred near iono- 
spheric sunset. During any one day, the diurnal variation 
was not as pronounced as the corresponding variation of the 
total or ionospheric electron contents. The maximum varia- 
tion (by a factor of ~ 3 . 4 )  was observed on 5 July. This is 
compared to a total content variation by a factor of 12 for 
the same day. The day-to-day variability exhibited changes 
of up to 300% during comparable local time periods (e.g., 
on 4 and 7 July during nighttime hours). 
Of great importance to the applicability of global time- 
delay models based on TEC data obtained by the Faraday rote 
tion technique, is the magnitude and variation of the ratio 
of plasmaspheric to the ionospheric time delays, or equiva- 
lently (N /NI). 
as well a{ a day-to-day variability. Between 0100 and 0700 
EDT, the ratio was nearly always above 50% and on occasions 
exceeded 100%. After this time-period, the ratio decreased 
t o  its minimum at ~ 1 1 0 0  EDT, after which time it increased 
with the time of day. From 0700-0100 EDT, the ratio aver- 
age increased from -30 % to ~ 4 0  %. 
This ratio (see Fig. 3) shows a diurnal 
The ratio of the plasmaspheric to the total time-delays due 
to free electrons along the signal's path, or equivalently 
N /N is shown in Fig, 4 .  The diurnal and day-to-day var- 
iEbiTity of this ratio is similar to that of Np/NI. During 
the night the ratio was high, reaching a value up t o  70%. 
During the day, the ratio was lower, averaging from ~ 2 5 %  to
30%. 
CONCLUSIONS 
Preliminary results of the radio beacon experiment of the 
ATS-6 indicate the magnitude and variation of the plasma- 
spheric electron content. 
gation signal due to free electrons in the plasmasphere 
cannot be neglected when it is compared to the delay due to 
the ionosphere. Group path delay prediction models based 
on Faraday data do not adequately compensate for the total 
delay; at night, they may be off by more than 508, and 
during the day by an average of ~ 3 5 % .  This is in addition 
to other prediction errors, i.e., differences between ob- 
served and predicted values of the delay times. Further- 
more, ionospheric (Faraday) prediction models cannot be 
corrected by adding a constant offset to account for the 
plasmaspheric delay, since the plasmaspheric content exhib- 
ited a diurnal and day-to-day variation. Fortunately, the 
highest ratio of plasmaspheric-to-ionospheric delay time 
The group path delay of a navi- 
446 
occurs at night, when the total delay time is relatively 
small. 
The data reported here was taken at a mid-latitude station 
during the quiet phase of the solar cycle. During such a 
phase at such a location, the group delay is generally 
small and modeling schemes yield corrections within the 
accuracy requirements of the proposed navigation systems. 
It remains to be seen if the observed ratios of plasma- 
spheric-to-ionospheric delays will be maintained at other 
geographic locations, and when delay times will be large, 
such as during the maximum phase of the solar cycle. If 
such ratios are maintained during the maximum of the cycle, 
neglecting the plasmaspheric content will cause errors ex- 
ceeding the accuracy requirement of the system. 
REFERENCE 
111 H. Soicher and F. J. Gorman, Jr., "Ionospheric effects 
on one-way satellite timing signals," Proc. Fifth 
Annual PTTI Planning Meeting (Greenbelt, Maryland; 
December 4-6, 1973), NASA Report X-814-74-225, 
December 4-6, 1973. 
4.47 
ORTGIXAL PAGE IS 
OF POOR QUALI!PY 
448 
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It I8 24 ~ ~ ' " ' ' l r l l l ~ l l l ' l l l l '  8 
TIME {EM) 
X Of PLLSYASPHERIC TO IOHOSPHERIC CONTENTS (-) "P 
fORT YOHYOUTH, I J "I 
lirs-6 
Fig. 3. Variation of the ratio of plasmaspheric to 
ionospheric electron contents (in percent) 
at 15-minute intervals for same time period 
as in Fig. 1, 
450 
t I 
Fig. 4. Variation of the ratio of plasmaspheric to 
total electron contents (in percent) at 
15-minute intervals for same time period 
as in Fig. 1. 
451 


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