Space Environment Laboratory

325 Broadway, Boulder, CO 80303-3326

(303) 497-5127 SE-11

Navigation

avigation, the science of finding

one’s way on Earth, has become

ever more exacting as we endeavor

to travel faster with more precision.

In ancient times, the stars provided markers

sufficient for a traveler to determine at least an

approximate position. But as journeys moved

upward from sea level, then far away from

Earth’s surface, the necessity for three-dimensional positioning was born.

For twentieth century travel, latitude, longitude, and altitude information in real time are

necessary. To help meet more stringent requirements, radio-wave systems such as Loran-C

and Omega were developed. These terrestrialbased systems use large transmitter antennas to

send low-frequency (LF) and very-low-frequency (VLF) radio waves, along the ground

and off the reflective layer provided by the ionosphere, to vast distances over land and sea (see

Figure 1). The ionosphere is formed by ultraviolet radiation from the Sun impinging on the

upper atmosphere and photo-ionizing the atmospheric constituents.

Figure 1.— The paths taken by a radio wave transmitted by a terrestrial navigation system.


2 

Figure 2.—Ground stations monitor the information from the satellites and upload changes as needed to ensure

that the navigation information is accurate.

More recently, space-based systems—the

Global Positioning System (GPS) and others—

have become the pre-eminent methods of navigation (see Figure 2). Satellites afford greater

coverage of the globe than do land-based systems; they provide a constant source of guidance for the longest terrestrial journey. When at

least four satellites are in view, a user can obtain

an accurate, three-dimensional position. At

times when more than four satellites are visible

from one location, the navigator enjoys an even

Figure 3. The constellation of navigation satellites assures that four satellites are always in view.

greater level of confidence in the computed

position (see Figure 3).

The thread common to both terrestrial and satellite-borne systems is the ionosphere; Omega

navigation requires it, Loran tries to work

around it, and GPS is hindered by it. Unlike the

low-frequency radio transmissions used by terrestrial systems, GPS uses radio signals that

pass through the ionosphere (see Figure 4). The

ionosphere, lying above 50 km out to a few

Earth radii, is neither homogeneous in structure

nor constant over time. Solar and geomagnetic

activity affect the character of the ionosphere

and, consequently, the proper function of navigation systems. (Refer to Space Environment

Topics SE-10 for more on radio-wave propagation.) Since both solar and geomagnetic activFrequencies (hertz)

   	      

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Figure 4. The frequencies of the most common

navigation signals.

OMEGA signals Loran-C signals GPS signals

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3 

ity vary over the course of the 11-year solar

cycle, the modern-day navigator will experience conditions in the solar maximum epoch

different from those in the years near solar minimum. The specific circumstances (i.e., the system being used, the user’s location, the phase of

the solar cycle, the month of the year, and the

time of day) will impact the accuracy of the location determined by the navigator.

Solar Flares

Solar flares, the impulsive release of large

amounts of radiant energy, disturb the ionosphere on the sunlit side of Earth. These flares

are usually associated with the strong magnetic

fields that manifest themselves in sunspots, so

their occurrence rate increases and decreases in

step with the sunspot cycle (see Figure 5). Navigators find it more difficult to use Loran-C and

Omega during daylight hours in the maximum

phase of the solar cycle; conversely, fewer

problems are experienced during solar minimum. These flare effects, which are due to solar

x-rays, are not a problem for GPS users. GPS

signals are generally immune to ionospheric

changes in response to large infusions of solar

x-rays.

Geomagnetic Storms

The geomagnetic field is affected by solar stimuli

whose frequency and intensity differ according

to the phase of the solar cycle. Geomagnetic

storms usually result in ionospheric storms and,

hence, they affect navigation systems. Unlike solar x-rays, which impact only the sunlit hemisphere of Earth, geomagnetic storms are ubiquitous. However, the ionospheric response to these

storms is dependent on latitude; so conditions,

nearer the equator or nearer the pole, vary for the

navigator. Paradoxically, a quiet, undisturbed

geomagnetic field does not necessarily dictate an

undisturbed equatorial ionosphere; this underscores the great variability in the environment: a

geomagnetic storm is sometimes a foe for a navigator and sometimes an ally.

GPS operations anywhere on Earth are affected

by the changes in total electron content (TEC) of

the ionosphere along the path to the satellite during large magnetic storms. Large increases and

decreases in the bulk plasma TEC directly influence the accuracy of single-frequency GPS.

Dual-frequency GPS receivers actually measure

the effect of the ionosphere on the GPS signals

and can better adjust to these difficult circumstances.

On a smaller scale, irregularities in TEC that

produce scintillations occur in varying

amounts, depending on latitude. For example,

the equatorial region, (the latitude zone that

spans 15-20 degrees either side of the magnetic

equator) is the site of some of the greatest ionoFigure 5.— The sunspot cycle suggests the cyclical nature of the sun’s activity. The cycle corresponds

with some, but not all, types of disturbances to navigation systems.







	











	





		

Number of sunspots
4 


	
 	

	
	

	
	


 	


 	

	 

	
 

 
 	 


spheric irregularities, even when magnetic

storms do not occur (see Figure 6). Seemingly

unpredictable episodes of density enhancements in the upper ionosphere can occur in the

evening hours there and can cause radio waves

to be misdirected. These scintillations make

GPS operations difficult, and they can affect

both dual- and single-frequency GPS receivers.

Solar Energetic Particles

Occasionally, but more often during the years

near solar maximum, the Sun ejects large quantities of energetic protons and electrons. These

energetic particle events persist for a few days

at a time; they can affect both ground-based and

space-based systems, but in different ways. Loran-C and Omega signal propagation is degraded, especially at polar latitudes, due to the

ionosphere’s response to the addition of these

solar particles (primarily protons). GPS and all

other satellites must contend with the detrimental effects energetic particles have on the onboard systems.

Navigation has advanced dramatically from the

days when the stars in the sky marked the way.

Now, it is the behavior of our nearest star that

tempers the function of navigation systems. ______________________________________________________________________________

Systems affected by the space environment

Loran-C

Phase and amplitude shifts due to skywave

interference at the limits of coverage area.

GPS

Carrier loss-of-lock due to ionospheric

density fluctuations with solar or

geomagnetic activity.

Omega

Phase anomalies due to varying ionospheric

reflection heights, associated with solar or

geomagnetic activity.

The Space Environment Services Center

monitors and forecasts these phenomena:

Solar flares of sufficient energy to disrupt

navigation systems,

Geomagnetic storms that may affect ionospheric

conditions,

Solar proton events that can perturb the ionosphere

locally and globally.

Written by Joe Kunches, 1995

References

N. Cohen and K. Davies, Radio-Wave Propagation,

Space Environment Laboratory, SE–10, 4 pp.,

1994.

K. Davies, Ionospheric Radio, P. Peregrinus, London,

580 pp., 1990.

G. Hefley, “The development of LORAN-C

navigation and timing,” National Bureau of

Standards Monograph, no. 129, 118 pp., 1972.

M. Jackson, R. Reilinger, M. Bevis, C. Rocken, B.

Stephens, and J. Stowell, “Impact of ionospheric

effects on GPS campaigns in southern California

and the South Pacific,” EOS, p. 1049, October

24, 1989.

J. Klobuchar, “Ionospheric effects on GPS,” GPS

World, vol. 2, no. 4, p. 48, April 1991.

A.L. Vampola, “Solar cycle effects on trapped

energetic particles,” J. Spacecraft and Rockets,

no. 26 (6), p. 422, 1989.

L. Wanninger, “Effects of the Equatorial Ionosphere

on GPS,” GPS World, vol. 4, no. 7, p. 48, July

1993.