solarcycle24[1]

7/29/2019 SolarCycle24[1]

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Copyright 2009 C&C Tec hnologies, Inc. All rights reserved

Solar Cycle 24 Rev2.1

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Scintillating Performance

C-Nav and Solar Cycle 24

GNSS and its satellite-based augmentation services are in for another bumpy ride as

we sail towards the next solar maximum. Forecasting these events is a bit like

predicting climate change: everyone agrees its happening but no can agree on the

outcome. The optimists forecast nothing too spectacular; the pessimists predict

something much more exciting.

The late start for the current solar cycle - number 24 since counting began in 1755 -

suggests that the solar maximum will now occur in May 20131. However, the GNSS

community will be feeling the impacts long before the maximum arrives and for long

afterwards. This short article explores the cause of these cyclical events, their effect

on GNSS and how the consequences can be mitigated and the risks managed.

Space weather

Every eleven years or so, the Sun erupts in a paroxysm of energy. These cyclical

events are caused by polarity changes in the Suns magnetic field and are

manifested by an increase in sunspots. The number of sunspots is an indication of the

severity of the situation, but not the whole story.

0

50

100

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Noofsunspots

Sunspot Cycle number

Monthly averaged sunspot count (1755 to present)

Figure 1 The sunspot count since records began

The Sun is constantly sending out a stream of charged particles, the solar wind,ejec ted from its upper atmosphere. These particles, interacting with Earths

magnetosphere, create geomagnetic storms that produce the spectacle of the

1 Solar Cycle 24 Prediction Update released May 8, 2009, NOAA/ Space Weather Prediction

Center


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aurora and, when particularly severe, can be powerful enough to knock out power

grids and terminate radio traffic. An increase in the number and severity of these

solar magnetic storms is also assoc iated with the sunspot count.

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10.00

20.00

30.00

40.00

50.00

60.00

70.00

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100.00

110.00

Jan-2009

May-2009

Sep-2009

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May-2010

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Jan-2012

May-2012

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Jan-2014

May-2014

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Jan-2015

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Sunspotnumbers

Stat istic source: NOAA, Space Weather Prediction Center (8 May 2009)

high low mean

Figure 2 Cycle 24 prediction of sunspot counts for solar maxima

Experts are divided on the severity of the next solar cycle. Estimates vary from

average monthly sunspot counts of about 90 spots up to 140. Everyone agrees that

the cycle has started the first Cycle 24 spot was spotted in January 2008. The date

for the solar maximum likewise varies the low c ount is assoc iated with a more

prolonged event, a higher count with a shorter one.

Ionospheric effects

The ionosphere is that upper atmospheric zone ionized by the Suns radiation. The

height of the ionosphere varies, ranging from 50 to 300 kilometers. The ionosphere is

divided into the F-Region, E-Layer and D-layer. The layers are denser during the day

than at night when the D-Layer all but disappears.

For the GNSS community, the presence of charged particles in the ionosphere (the

Total Electron Count or TEC) has a major impac t on the propagation of radio signals,particularly the frequency-dependent signal delay. Dual frequency receiver

equipment can measure and remove this delay from pseudorange and carrier

phase measurements whereas single frequency receivers cannot. Single frequency

receivers must rely on an ionospheric model to estimate or predict the signal delay.

As solar activity increases, the ionosphere will become increasingly more active,


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Copyright 2009 C&C Technologies, Inc. All rights reserved


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dynamic, irregular and unpredictable. This will lead to a decrease in single frequency

receiver navigation accuracy as the errors of the modeled/predicted ionosphere

depart from the real-world situation.

Solar maximum affects all areas of the globe but it is essentially an issue for singlefrequency receivers because dual frequency receivers are able to better manage

the effects.

Scintillation

Scintillation is a rapid phase and amplitude fluctuation of radio signals caused by

variability in the ionosphere. Although scintillation is more severe during a solar

maximum, it can occur at almost any time, particularly in the low latitudes (within

15-20 degrees of the geomagnetic equator)2, kicking in shortly after local sunset

and lasting until just after local midnight. Scintillation affects both single and dual

frequency GNSS receivers in that it prevents the receivers from tracking the signals.No radio signals passing through the ionosphere are immune; for GNSS users,

scintillation affects both the GNSS signals and the communications from the

geostationary satellites used to deliver GNSS augmentation corrections.

At its worst, scintillation impacts, often dramatically, the performance of all space-

based communication and navigation system.

Figure 3 C-Nav tracking network and the geomagnetic equatorial region

2 Smita Dubey, Rashmi Wahi and A.K.Gwal. Effect of Ionospheric Scintillation on GPS Receiver

at Equatorial Anomaly Region, Department of Physics Space Science Laboratory, Barkatullah

University


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What to look out for

For the GNSS satellites, ionospheric amplitude and phase scintillation effects on signal

performance can lead to loss of carrier lock. Amplitude scintillation causes cycle

slips, data loss and signal fading as the signal-to-noise ratio drops below a receiversthreshold. It also leads to message errors in satellite communications.

Phase scintillation leads to rapid frequency variations and, during periods of intense

activity, carrier phase observations could be affected.

As the Suns activity increases, the frequency of magnetic storms may similarly

increase. This could lead to large spatial and temporal delays occurring anywhere

around the world3. The regions most affected by ionospheric effects divide into three

broad zones:

The high geomagnetic latitudes, above 65 north and south:

- Relatively low but rapidly fluctuating TEC- Phase scintillation, espec ially during magnetic storms

The mid geomagnetic latitudes between 25 and 65

- Relatively moderate TEC

- Essentially no scintillation

The low geomagnetic latitudes,


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Figure 4 Ionospheric TEC map. These can be downloaded daily. Courtesy J PL/NASA

Precautionary measures

The US GPS is being upgraded with the introduction of additional satellites

broadcasting the L2C, a c ivil access signal on the L2 frequency. This additional signal

in space will help mitigating scintillation effects to some extent.Traditional DGPS systems have an increased failure potential during the solar

maximum because of their reliance on proximity to reference stations. Even if a GPS

receiver remains outside a zone of ionospheric disturbance, its DGPS reference

station may not be so fortunate. The effect will be a reduction in available stations

leading to a decrease in accuracy and redundancy. The latest generations of real-time Precise Point Positioning systems such as C-Nav RTG, which use GNSS tracking

stations, do not suffer from the same spatial decorrelation effects as DGPS and are

inherently more stable.

Communication satellites are a lso vulnerable to upper atmospheric disturbances. The

single point of failure, where a single Land Earth Station (LES) and satellite are used to

deliver corrections, could lead to increased signal outages. By adopting a more

pragmatic approach, a dual delivery system such as C-Navs Net1 and Net2 service

offers spatial separation in both LES and satellites, reducing the likelihood of

correction data flow failure caused by interruptions to satellite communications.

Forward planning will also help mitigate Solar Cycle 24 effects. In particularlyvulnerable regions, plan for contingency by:

a) Maintaining an awareness of the solar cycle among navigation operatives

b) Checking daily the space weather forecast. Daily information and updates on

space weather conditions can be found at http://www.swpc.noaa.gov/ and

www.spaceweather.com


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c) Use the Kp index4 forecast as a mission planning tool in conjunction with the

normal GNSS mission planning packages

d) Schedule position and navigation-critical activities for periods of quiescence

and, if the forecast bodes ill, preferable before the onset of the late afternoon midnight activity period.

C-Nav technology

C-NAV Real-Time GIPSY (RTG) service is a globally correc ted GNSS system employing

a number of proprietary Precise Point Positioning (PPP) algorithms developed by

partner company, NavCom Technology. The solution delivers real-time dual

frequency position at the 10cm level with 20cm height accuracy.

The C-NAV infrastructure has been specifically designed to mitigate outages and

atmospheric interference. These precautions include: High levels of redundancy (approx 200%) in GNSS tracking stations

Independence from (DGPS) Reference Station technology

Duplicated systems at each tracking station

Geographically separated Processing Centers

Duplicate processing suites running hot at each Processing Center

Duplicated feeds to Land Earth Station uplink sites

Redundancy in regional coverage (2x Inmarsat satellites per Region) through

Net-1 and Net-2 diversity

4 An index of fluctuations within the Earth's magnetic field ranging from 0 (quiescence) to 9,

where values above 5 indicate a geomagnetic storm.


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Copyright 2009 C&C Technologies, Inc. All rights reserved


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Figure 5 C-Nav Cycle 24 infrastructure precautions

C-Nav receiver technology

The C-Nav range of GNSS receivers has a number of features in-built that will help

mitigate the effects of ionospheric disturbances as Solar Cycle 24 heats up.

Single frequency C-NAV1010

This product utilizes C-Nav algorithms to provide robust, reliable and accurate single-

frequency Position, Velocity and Time (PVT) information augmentable by including

corrections from the Wide Area Augmentation System (WAAS) or the commercial.

The C-NAV1010s remarkable performance is due to a highly accurate proprietary

relative-phase solution known as L1Pnav together with the use of innovative and

proprietary positioning algorithms and RAIM-like outlier detection & removal

methods. The combination of these elements provides for accurate, robust and

reliable positioning performance in the most challenging of operating environments.

Dual frequency C-Nav2050

The C-Nav2050 range of receivers, used in conjunction with the C-Nav subscription

service, provides real-time positioning at the decimeter level, anywhere in the world.

If even higher accuracy is needed, the onboard memory can store the observables

for post-processing at the millimeter level. The C-Nav2050 includes two free-use

WAAS/EGNOS channels which can deliver 0.5 meter accuracy when exploiting thereceivers dual frequency measurements and its enhanced SBAS algorithms.

The C-Nav2050 uses the NCT-2100D GPS Engine, the fourth generation of the

Touchstone ASIC family, incorporating patented interference suppression and

multi-path mitigation solutions along with geodetic quality measurements. The units

compact tri-band antenna provides excellent phase center stability.


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The C-Nav2050s robust design and in-built precautionary feature makes it the ideal

GNSS unit to cope with the effects of the solar maximum as well as providing an

exemplary performance at all times.

Dual frequency C-Nav3050

The C-Nav3050 is the latest addition to the C-Nav range of professional receivers

making it the optimal GNSS unit to cope with the effects of the solar maximum. The

C-Nav3050 is powered by the new Sapphire Engine, providing 66 channel tracking,

including multi-constellation support for GPS, GLONASS and Galileo. It also provides

patented interference rejection and anti-jamming capabilities.

The C-Nav3050 is fully upgradeable allowing customers to upgrade from a single

frequency rec eiver to multi-frequency or anything in between with just a software

bundle upload. The C -Nav3050 features:

L1, L2, L5, G1, & G2 full wavelength carrier phase tracking C/A, P1, P2, L2C, L5, G1 & G2 code tracking

High sensitivity / low signal level tracking

Superior interference suppression (both in-band & out-of-band)

Patented multipath rejection

Further reading

For details of C-Nav and its family of GNSS solutions visit:www.cctechnol.com and

click on the C-Nav link

For a more informed overview of the ionosphere and GNSS visit:

http://www.lima.icao.int/MeetProg/2008/IONOSFERASEMINAR/Ionospheric%20Effect

%20on%20GNSS.pdf

For more information on the Sun and its effects on Earth, try the Space Physics

Textbook, University of Oulu, Finland November 2006 available at

http://www.oulu.fi/~spaceweb/ textbook/content.html

For updates on the predicted progress of Cycle 24

http://www.swpc.noaa.gov/SolarCycle/

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