Ionospheric bias corrections associated with radio tracking of spacecraft depend on the following measuring techniques for integrated electron content: (1) Faraday rotation measurements from an earth synchronous satellite; (2) ranging measurements at two frequencies; and (3) group and phase velocity measurements obtained from tracking data. The extraction of the integrated electron content directly from tracking data is achieved by comparison of range-rate measurements based on Doppler shift with differentiated range measurements based on tone delay. This method is most desirable because the measured corrections pertain directly to the spacecraft whose orbit is being determined and can be used in near earth as well as deep space tracking data

Schmid, P. E.

English

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TECHNOLOGYACCOMPLISHMENTS,1970 3
RADIO WAVE PROPAGATION EXPERIMENTS
TO PROBE THE IONOSPHERE
P. E. Schmid
The Earth's ionosphere biases all measurements associated with radio
tracking of spacecraft because the velocity of radio waves in the ionosphere
differs from that in free space. The integrated effect of this velocity
difference is directly proportional to the total number of electrons in a unit
cross-section tube along the ray path connecting spacecraft and tracking
station. All tracking measurement ionospheric biases can be shown to be
directly proportional to this "integrated electron content" and inversely
proportional to the square of transmission frequency. This paper briefly
discusses three techniques which have proved highly successful in measuring
integrated electron content.
The three techniques are
1. Faraday rotation measurements from an Earth synchronous satellite.
2. Ranging measurements at two frequencies.
3. Group and phase velocity measurements obtained from tracking
data.
The third scheme (see Reference 1) is the most desirable because
measured corrections pertain directly to the spacecraft whose orbit or
trajectory is being determined. Similar work has been performed for deep
space applications at the Jet Propulsion Laboratory (JPL) where group and
phase velocity differences through ionized media are used in their so-called
"charged particle calibrations" (Reference 2).
Such ionospheric corrections are of special importance in the case of
VHF tracking. From the standpoint of spacecraft and ground antenna
simplicity and the related ease of signal acquisition (because of antenna
beamwidth relationships), 136 to 148 MHz is still a highly desirable
https://ntrs.nasa.gov/search.jsp?R=19720018106 2020-03-17T03:51:26+00:00Z
4 GODDARDSPACEFLIGHTCENTER
frequencyrangefor trackingand subsequentorbit computationof
low-power,low-data-rate,Earth and near-Earth orbiting spacecraft. Such
approved future VHF missions include the Radio Astronomy Explorer B
(RAE B), the Synchronous Meteorological Satellite (SMS), and the continu-
ing Explorer series of spacecraft.
Figure 1 shows Faraday rotation data recorded at Goddard Space night
Center from an Earth synchronous satellite, ATS3, during the
March 7, 1970, solar eclipse. The measured polarization twist is a function
of both the integrated electron content and the Earth's magnetic field. Since
the Earth's magnetic field is well established, the observed Faraday rotation
from ATS3 at 137.350 MHz is directly related to changes in integrated
electron content and hence to changes in ionospheric biases in tracking data.
By the use of ionosphere bottom sounding data from Wallops Island, the
absolute (i.e., total unambiguous) number of polarization rotations was
noted during the eclipse. These data indicated that the content dropped
from 6 × 1017 to 4.5 X 1017 electron/m 2 as the lunar shadow passed
through the ionosphere. This corresponds to a range bias change of
approximately 500 m at 140 MHz.
The lower portion of Figure I shows a normal buildup in ionization
peaking at approximately 3:15 p.m. local time at a level on the order of
1018 electron/m 2. The upper portion of Figure 1 shows the same type of
display with the exception of the abrupt decrease in electron content (phase
reversal at approximately 12 noon), which then recovered to the normal
diurnal cycle at 2:20 p.m. to reach the afternoon peak integrated density at
3:30 p.m. All times are local (i.e., Eastern Standard Time).
Figure 2 shows a comparison of range bias determined by Faraday
rotation from ATS 3 and that determined by ranging to ATS 3 at two
widely separated frequencies. In this case, the frequencies are nominally
140 MHz and 6000 MHz. The range bias is given by
40.3I. 40.3Iv
AR .... (m), (1)
f2 f2 sin E
where
I = total integrated electron content along ray path (electron/m2),
f = _equency (Hz),
TECHNOLOGY ACCOMPLISHMENTS, 1970 5
E = local elevation angle of ray intercept point at 300-km height (i.e.,
height of maximum ionization),
I v = vertical integrated content in column intersecting 300-kin intercept
point (electron/m2).
For elevation angles above 45 deg, E can usually be taken as the
ground-measured elevation angle to the spacecraft. For lower angles, Earth
curvature should be taken into account:
E = arccos cos E' , (2)
where
a = Earth radius,
h = 300 km (or height of maximum electron density if known more
accurately),
E' = elevation angle as measured by tracking station.
Figure 2 shows clearly the diurnal variation in electron content at
Rosman, North Carolina, with the typical midafternoon peak occurring at
approximately 2:00 p.m. local time. The difference between Faraday data
and range difference data in this preliminary test is attributed primarily to
the ATS 3 VHF transponder time delay uncertainty. Further tests are being
conducted to improve calibration techniques.
The most desirable way to obtain group and phase velocity informa-
tion, from an operational standpoint, is to extract integrated electron
content information and hence ionospheric bias corrections directly from
the tracking data. Figure 3 shows that this can be achieved by the
comparison of range-rate measurements based on carrier Doppler shift with
differentiated range measurements based on tone delay.
Simply stated, the ionosphere biases range-rate in one direction and
biases the differentiated range an equal amount in the opposite direction.
Thus, the true range rate is halfway between the two curves shown in Figure
3. The reason for this phenomenon is that the Doppler phase is altered by
the phase velocity and the range delay is altered by the group velocity. In a
6 GODDARDSPACEFLIGHTCENTER
dispersivemedialiketheionosphere,thegroupandphasevelocitiesdifferin
apredictablemanner.
ThesametechniqueisusedbyJPLintheirchargedparticlecalibration,
withoneimportantdifference.In ourwork,theanglerateof near-Earth
spacecraftishigh,andanabsolutemeasureofintegratedelectroncontentis
possibleovertheroughly50-secondobservation-timearc.Thedeepspace
angleratesareprimarilyEarthrotation(_7× 10-3 rad/s),andintegration
overlongperiodsof time(hours)areusedto obtainchangesin integrated
electroncontentratherthanabsolutecontent.
The measurability of the electron content can be seen from the
following relation :
[ 1 _ [f2_/,sin 2 E_ [. dR
I v = _--_)_,_ J_c_sEJl_r - k s - -_-_j (electron/m2), (3)
where
Iv, f, and E are as defined previously and
= range-rate from tracking data (m/s),
dR
tx
dt differentiated range from tracking data (m/s),
ks = spacecraft range-rate bias due to spacecraft spin* (m/s),
_7 = rate of change of local elevation angle at ray intercept point at
300-km height (rad/s).
The angle rate E is linked to the tracking station measurements of
elevation E' and elevation rate E' by
/
_7 =_ sin E'
(4)
*If spin stabilized, see Marini, J. W., "The Effect of Satellite Spin on Two-Way
Doppler Range Rate Measurements", Goddard Space Flight Center Document
X-551-69-104, March 1969, Goddard Space Flight Center, Greenbelt, Maryland.
TECHNOLOGY ACCOMPLISHMENTS, 1970 7
As seen in Figure 3, the results show excellent agreement be-
tween the vertical content extracted from Explorer 41 tracking data,
Iv = (2.4 + 0.1)X 1017 electron/m 2, and that based on ESSA foF2 data,
Iv = 2.45 X 1017 electron/m 2. This happened to be during a stable period
(local night) with no solar disturbance. However, daytime estimates based
on average monthly foF2 predictions are often in error by a factor of 2 to 4
throughout the month.
This particular scheme for extracting integrated electron content
directly from tracking data is therefore shown to be both feasible and highly
desirable for corrections to near-Earth as well as deep space tracking data. In
some cases, simultaneous range and range-rate data may not be available or
elevation angle rates may be too small for determining corrections in the
desired observation time. In such cases, measurements involving
synchronous spacecraft such as ATS 1 and ATS 3 can be used to update the
presently used ionospheric profdes, which are based on monthly predictions
that are available three months in advance.
REFERENCES
1. Schmid, P. E., Rangaswamy, S., and Murray, C. W., "Ionospheric
Corrections Based on VHF Range and Range Rate Measurements",
Proceedings of the GEOS-2 Program Review Meeting 22-24 June
1970, Vol. II, November 1970.
2. Mulhall, B. D. "Charged-Particle Calibration System Analysis", JPL
Space Programs Summary 37-64, Vol. II, pp. 13-21, August 31, 1970,
Jet Propulsion Laboratory, Pasadena, California.
8 GODDARD SPACE FLIGHT CENTER
SOLAR LCLIHSL
12N
180'
t j
J
2!
o,L ........
r 2PM
ii
Z_:
_.ua
_]_ _.
6PM
\
f = 137.350 MHz
SOLAR ECLIPSE OF 7 MARCH 1970
8PM
12N 2PM
o i ] !
TIME (EST)_
4PM 6PM 8PM
< .
f 37 350 MHz
TYPICAL DIURNAL REVERSA
(DAIA RECORD OF 25 FEBRUARY 1970)
Figure l-Faraday rotation data from ATS 3,
ROSMAN 13 DECEMBER 1969 ATSR TRACK OF ATS-3
I 1000 _ " ' ' ' ' ' _-= r ' ' ' ' ' ' ' r'-AN= 40.3 Iv
800 _- VHF MINUS C BAND _ _E
_ ATSR RANGING DATA /
,oo
/_m_ BASED ON ROSMAN FARADAY DATA
2001% _ (REDUCED BY STANFORD UNIVERSITY)
7 8 9 10 11 12 13 14 15 16 17
LOCAL HOUR ,,,-
Figure 2-Zenith ionospheric range bias.
TECHNOLOGY ACCOMPLISHMENTS, 1970 9
4700
4696
4692
4688
TANANARIVE, MADAGASCAR 8 OCT 1970 16h lm UT
I l • I I I T-- I
ED RANGE
(GROUP VELOCITY)
4684
4680
t--
rr"
L_J
(_9
Z
r_
4676
4672
4668
4664
4660
4656
RANGERATE-
(PHASEVELOCITY)
1 BASED ON TRACKING DATA
Iv =(24- + 01)×1017 ELECTRONS/M 2
4652 2 BASED ON ESSA fo F2 DATA
I v = 2 45 x 1017 ELECTRONS/M 2
464z
4640
4636
0
TIME (S)
Figure 3-Explorer 41 track by VHF GSFC range and range-rate system.


Radio wave propagation experiments to probe the ionosphere

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