Guidance Material on Collection of Ionospheric Scintillation

Guidance Material on Collection of Ionospheric Scintillation
ISTF/3
Appendix B to the Report
Guidance Material on collection of Ionospheric Scintillation data
at strategic locations in the Low-latitude Region
(Ver. 1)
1.
Background
The second meeting of Ionospheric Studies Task Force (ISTF/2) noted the limited
scintillation monitoring facilities established in the region, and decided to develop a guidance
material on collection of scintillation data at strategic locations (Action Item 1). This document
has been developed to address the AI-1 of ISTF/2.
There are two types of ionospheric scintillations in GPS measurements, amplitude
and phase scintillations. Amplitude scintillation refers to rapid fluctuation in signal intensity (or
carrier-to-noise ratio, C/N0) measured by a receiver, while phase scintillation refers to rapid
fluctuation in the carrier-phase measurements. Levels of amplitude and phase scintillations are
commonly represented by the standard deviations of amplitude and phase, respectively S4 and σφ,
in a certain time period (typically 1 min). The ways of estimating the S4 and σφ indices are given
in Appendices A.1 and A.2.
For the amplitude scintillation, rapid sampling of C/N0 is necessary, while rapid
carrier-phase measurements are required for the phase scintillation. Furthermore, GPS receivers
for phase scintillation measurements need to be equipped with a highly stable clock (oscillator)
such as OCXO (oven-controlled crystal oscillator) to distinguish the phase fluctuations due to
ionospheric scintillation and clock (oscillator) noise.
Both types of ionospheric scintillations are caused by plasma irregularities in the
ionosphere. In the low-latitude regions where the background electron density is high and plasma
drift velocity is relatively slow, the amplitude scintillation is dominant. In this guidance, therefore,
the amplitude scintillation is focused on.
2.
Receiver performance
2.1 Receiving frequency
Since only the GPS L1 (1.57542 GHz) is currently used, GPS L1 single-frequency receivers
satisfying other performance requirements in this section are acceptable. In addition to GPS,
however, receivers should be capable of GLONASS and SBAS GEO satellites for wide
coverage of the sky.
For the use of the L5 frequency in the future, receivers capable of tracking L1 and L5
signals would be a good choice.
If a receiver could track L2 frequency, it could be used to measure ionospheric delays
(or ionospheric total electron contents (TECs)).
2.2 Receiver clock
Since the amplitude scintillation is of interest, a highly stable clock is not necessary, but a
standard clock such as TCXO (temperature compensated crystal oscillator) is enough.
2.3 Sampling rate
The amplitude scintillation is caused by the Fresnel diffraction due to the ionospheric
irregularities. The typical scale size causing the Fresnel diffraction (DF) is described as
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DF  2h
(1)
where λ is the wavelength of the radio wave (0.19 m for the GPS L1 frequency) and h the
height of the irregularities (typically 300-400 km). Thus, the typical scale size is 300-400 m.
The amplitude
will fluctuate at the Fresnel frequency
f F  V / DF
(2)
where V is the drift velocity of the irregularities. Since the drift velocity of plasma
irregularities (V) is typically 100-200 m/s, the amplitude will fluctuate at 0.25-0.67 Hz.

According to the sampling theory, the sampling rate of the amplitude should be at
least twice as fast as the Fresnel frequency, 0.5-1.33 Hz. Considering that the spectrum of
amplitude fluctuation contains higher frequency components, the sampling rate should be
much higher than the Fresnel frequency. It is common to sample the amplitude at 20 Hz or
more. It should also be noted that the default sampling rate of the amplitude by the widely
used GSV4004B receiver is 50 Hz.
The raw amplitude measurements at high sampling rates can be recorded. However,
it would take a lot of file size. Therefore, the raw amplitude measurements could be
discarded after calculating and recording scintillation intensity, although the raw amplitude
measurements data would still be useful for future re-analysis and irregularity drift
measurements with closely spaced scintillation receivers.
If the ionospheric delay is desired to be derived, both the pseudo-range and
carrier-phase need to be sampled. However, the sampling rates of them do not have to be the
same as the amplitudes, but can be much slower than that of the amplitude. The typical
sampling rate for the ionospheric delay measurements is 1 sec. (For GBAS, the minimum
sampling rate of a ground subsystem is 2 Hz, though)
TEC measurements can also be used to derive another index of ionospheric
irregularities: the rate of TEC Index (ROTI). ROTI is defined as the standard deviation of
rate of TEC in a certain time period, typically 5 min [2]. ROTI is another indicator of
ionospheric irregularities that can be derived from standard low sampling rate dual-frequency
receiver measurements. The way of estimating ROTI is given in Appendix A.3.
2.4 Multi-path effect avoidance
The measured amplitude often fluctuates at low elevation angles due to
multi-path effects and result in artificial enhancements in the scintillation level. There are
two ways to eliminate the multi-path effects. One is simply to set a higher elevation mask
such as 30º. However, it would have a drawback of loosing data at low elevation angles
where the path length in the ionosphere is long and more scintillation is expected.
Alternatively, the standard deviation of the code-carrier divergence (sigma-CCD)
can be utilized. The code-carrier divergence is the difference between the rates of change in
pseudo-range and carrier-phase measurements. When there is no multi-path and ionospheric
effects, the rates of change in pseudo-range and carrier-phase changes will be the same,
except for ambient and receiver internal noises.
The multi-path signal generally accompanies much larger sigma-CCD than
ionospheric scintillation signal, which can be used to distinguish between scintillation
enhancements by multi-path and ionospheric irregularities [1]. If a sigma-CCD value
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calculated for the same period as the S4 index exceeds a certain limit, the signal is likely to
be affected by multi-path effects. To do this, the pseudo-range and carrier-phase need to be
sampled at a certain rate, such as 1 Hz. Sigma-CCD can be calculated afterwards if the
pseudo-range and carrier-phase are recorded, while some receivers such as GSV4004B can
calculate sigma-CCD internally and record it. The way of deriving the sigma-CCD is given
in Appendix A.4.
2.5 Other useful measurements
The satellite azimuth and elevation angles are not essential, but will make
post-analysis easier. The sampling rates of the azimuth and elevation angles can be as low as
those of the pseudo-range and carrier-phase.
2.6 Summary
The receiver should be able to track at least GPS L1 frequency signals.
Tracking capability of GLONASS and SBAS GEO satellites are very useful.
The receiver do not have to be equipped with a highly stable clock (oscillator)
as long as it is used for the amplitude scintillation measurements, which is the case in the low
latitude regions.
The most important value to be recorded for the low latitude ionospheric scintillation is
the amplitudes (C/N0) of the signal for each satellite. The sampling rate should be much
higher than the Fresnel frequency and typically 20 Hz or more. Once the scintillation
intensity is calculated and recorded, the raw amplitude measurements data can be discarded,
unless future re-analysis or irregularity drift velocity measurements are not planned.
The pseudo-range and carrier-phase can be recorded at relatively low rates than the
amplitude, such as 1 Hz. They are not mandatory, but useful to distinguish between
ionospheric scintillation and multi-path signals.
The satellite azimuth and elevation angles are not essential, but will make post-analysis
easier, if recorded together.
3.
Antenna
¥
3.1 Antenna frequency
Antenna should be able to track signals corresponding to the signals which are desired to be
tracked by the receiver.
As described in 2.1, an antenna capable of tracking L1 and L5 signals is a good choice.
3.2 Antenna type
To avoid the multi-path effect as much as possible, a choke-ring antenna or others with
equivalent multi-path-resistant performance are preferable. Simple antennas could be used,
but with a drawback of lower data availability especially at low elevation angles.
3.3 Antenna environment
Antennas should be located at places with the open sky without obstacles that may
shadow satellites down to the elevation angle as low as possible.
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The antenna site should be free from obstacles as wide as possible to avoid multi-path
effects. Practically, it is very difficult to find ideal antenna locations. Therefore, the
sigma-CCD filtering is very useful to enhance data availability under the multi-path
conditions.
For example, installation of receiver in localizer building is recommended owing to its
strategic location which is usually free from multipath. However, seaside runways are at
disadvantage. We have observed more multipath effects in scintillation at sites surrounded by
ocean.
3.4 Antenna separation
When more than one scintillation receivers are available, one can consider operating
them with spaced antenna groups. There are some advantages of spaced antenna
measurements.
With a few 100 m to a few kilometers antenna separation and the high frequency C/N0
data recording, drift velocity of ionospheric irregularities causing scintillation can be derived
from the time lag between the two C/N0 variation patterns. It could potentially used to
predict propagation of the ionospheric irregularities.
Spaced antenna measurements with separation of several tens to a hundred kilometers,
which is comparable to the typical scale size of plasma bubbles contribute to increase the
number of points probed by the GNSS signals to achieve spatially denser observations.
Multiple receiver measurements at multiple locations that share common sky () would
be useful for redundant measurements against receiver failure. The radius of the area of the
ionosphere that can be probed at an altitude of 350 km is about 560 and 1600 km for
elevation mask angles of 30 and 5º, respectively.
4.
Cables
A cable between an antenna and a receiver should be selected so that the signal level at the
front-end of the receiver is in the range suitable for each receiver.
The expected signal level at the front-end of the receiver (Pin) is given by
Pin  Pnom  Gant  G pa  Lcable  Gila
(3)
where Pnom is the nominal signal level at the antenna, Gant the antenna gain, Gpa the gain
of the preamplifier,
Lcable the cable loss, and Gila the gain of an in-line amplifier. The nominal

signal level of GPS on the ground is between -153 and -160 dBm for L1 C/A signal [3]. Gant
and Gpa depends on the antenna model. Lcable is determined by the type, thickness of the cable
and proportional to the length.
Cable length should be determined so that Pin is in the range of signal level suitable for
the receiver. When the cable has to be too long to keep Pin in the suitable range, an in-line
amplifier can be inserted between the antenna and the receiver. However, the gain should not
be too high not to exceed the upper limit of the input signal level.
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5.
References
[1]
Van Dierendonck, A. J., and Q. Hua, Measuring ionospheric scintillation effects from GPS signals,
proceedings of ION 57th Annual Meeting, 391-396, 2001;
[2]
Pi, X., A. J. Mannucci, U. J. Lindqwister, and C. M. Ho, Monitoring of global ionospheric
irregularities using the worldwide GPS network, Geophysical Research Letters, 24, 2283-2286,1997;
[3]
ICD-GPS-200 IRN-200C-004, Navstar GPS Space Segment / Navigation User Interfaces. April 2000.
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Appendix B to the Report
Appendix A. Parameter estimation
A.1 Amplitude scintillation index (S4 index)
The amplitude scintillation index (S4 index) is defined as a normalized standard deviation of
C/N0 as given by:
si2  si
S4 
si
2
(4)
2
where < > denotes average , and si is the C/N0 in linear scale (not in dBHz) of the i-th
satellite. The linear-scale C/N0 (si) is related to the C/N0 in dBHz (ci) as:

si  10
0.1C i 
(5)
The period of taking average depends on the time scale of interest. It is common to
calculate S4 every 1 min (i.e., the averaging period of 1 min).

A.2 Phase scintillation index (σφ)
The phase scintillation index (σφ) is basically a standard deviation of carrier-phase
measurements. However, the carrier-phase measurements have a trend associated with the
change of the geometric range. Therefore, the carrier-phase measurements have to be
detrended first.
Defining φ’ as the detrended carrier-phase measurements, σφ can be defined as:
 
2  
2
(6)
A.3 Rate of TEC index
 (ROTI)
ROTI is defined for each satellite as the standard deviation of rate of TEC (ROT) as given
by:
ROTIi  ROTi2  ROTi
2
(7)
where ROTi is the rate of TEC of the i-th satellite in the unit of TEC/min as given by:

ROTi t   TECi t   TECi t   /
where τ is the sampling interval.

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A.4 Sigma-CCD
Sigma-CCD is defined for each satellite as the standard deviation of code-carrier divergence
as given by:
SigmaCCDi  di2  di
2
(9)
where di is the code-carrier divergence of the i-th satellite as given by:

di  i t     i t   i t     i t (10)
where ρi(t) and φi(t) are respectively the pseudo-range and carrier-phase measurements of the
i-th satellite at a time t, and τ is the sampling interval of the pseudo-range and carrier-phase.
 of taking average should be the same as S4 index, and typically 1 min. The
The period
sampling interval of the pseudo-range and carrier-phase is typically 1 sec.
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