The ionosphe re, an area of the at mo sphe re which extends fr om ~80 to ~1000 km, can significantly affect the propagation of radio frequency (R! signals which pass through it or are reflected "y it [Cannon, 1##$a% Cannon, 1##$"&' The effects are aried "ut include a"sorption, refraction, retardation and scintillation' )t frequencies a"oe ~1 *+, the lower D region causes a"sorption and the higher E and F regions cause a ariety of other effects' These effects, which inclu de refra ction , signal grou p delay , signal phase adance, puls e "road enin g and arada y rot ation of the polari sat ion ector, all follo w an in erse power law and are generally only significant up to a frequency of ~- .+' /elow ~1 *+ radio systems "ounce their signals from the tenuous D region% consequently although the height of the layer is important for system operation, a"sorption is not an issue' igure 1' ystems affected "y the ionosphere The dierse set of affected systems (igure 1! include groundground high frequency (+! communications, groundspace communications, .2 (.lo"al 2aigation atellite ystems!, such as .3 (.lo"al 3osit ioning ystem! and .al ileo 4 particularly si nglefrequency sys tems, + oerthehorion radars, satellit e altimeters and space"ased radars [Goodman and Aarons, 1##0&' + communications and radar systems rely on the ionosphere for their operation "ut also hae to contend with its effects' *ost other systems are degraded "y the ionosphere and its aria"ility "ut for certain specialist applications this aria"ility can "e exploited' 5oss of phase lock and range errors in .2 are examples of such deleterious effects' 6f the ionospheric enironment were isotropic and sta"le in time, it would "e relatiely easy to determine its effects on the propagation of R waes' 7nfortunately, this is not the case' The spatial scales of inhomogeneities ary from thousands of kilometres to tur"ulence with scale sies of a less than a metre' 5ikewise, the temporal scales ary oer many orders of magnitude from many years (solar cycle effects on ionospheric propagation! to hours or een minutes (the scale of weather phenomena!' )s a consequence of this aria"ility, timely and relia"le strategies are required to "oth specify and accurately forecast the enironment and to assess the attendant imp act on the operational pe rforma nce of the R systems' These strategies can "e used to automatically apply corrections to a systems operating parameters or, ia a decision aid, adise the user on a course of action that will improe a systems functionality [Cannon et al ', 1##&' Satellite Altimetry EQUATORIAL F-REGION ANOMALIE S GPS and Galileo Ground Based Sa!e Sur"eillan!e AURORAL#TROUG$ IRREGULARITI ES REGION Geo-lo!ation NIG$T %A& Sa!e Based Sur"eillan!e $F ' o m m u n i ! a t i o n s 7+ )T9:* $F OT$R MAGNETI' EQUATOR

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The ionosphere, an area of the atmosphere which extends from ~80 to ~1000 km, can

significantly affect the propagation of radio frequency (R! signals which pass through it or are

reflected "y it [Cannon, 1##$a% Cannon, 1##$"&' The effects are aried "ut include a"sorption,

refraction, retardation and scintillation' )t frequencies a"oe ~1 *+, the lower D region causes

a"sorption and the higher E and F regions cause a ariety of other effects' These effects, which

include refraction, signal group delay, signal phase adance, pulse "roadening and araday

rotation of the polarisation ector, all follow an inerse power law and are generally only

significant up to a frequency of ~- .+' /elow ~1 *+ radio systems "ounce their signals from

the tenuous D region% consequently although the height of the layer is important for system

operation, a"sorption is not an issue'igure 1' ystems affected "y the ionosphere

The dierse set of affected systems (igure 1! include groundground high frequency (+!

communications, groundspace communications, .2 (.lo"al 2aigation atellite ystems!,

such as .3 (.lo"al 3ositioning ystem! and .alileo 4 particularly singlefrequency systems,

+ oerthehorion radars, satellite altimeters and space"ased radars [Goodman and Aarons,

1##0&' + communications and radar systems rely on the ionosphere for their operation "ut also

hae to contend with its effects' *ost other systems are degraded "y the ionosphere and its

aria"ility "ut for certain specialist applications this aria"ility can "e exploited' 5oss of phase

lock and range errors in .2 are examples of such deleterious effects'

6f the ionospheric enironment were isotropic and sta"le in time, it would "e relatiely easy to

determine its effects on the propagation of R waes' 7nfortunately, this is not the case' The

spatial scales of inhomogeneities ary from thousands of kilometres to tur"ulence with scale siesof a less than a metre' 5ikewise, the temporal scales ary oer many orders of magnitude from

many years (solar cycle effects on ionospheric propagation! to hours or een minutes (the scale of weather phenomena!' )s a consequence of this aria"ility, timely and relia"le strategies are

required to "oth specify and accurately forecast the enironment and to assess the attendant

impact on the operational performance of the R systems' These strategies can "e used to

automatically apply corrections to a systems operating parameters or, ia a decision aid, adise

the user on a course of action that will improe a systems functionality [Cannon et al ', 1##&'

Satellite Altimetry

EQUATORIAL

F-REGION

ANOMALIES

GPS and

Galileo

Ground BasedSa!e Sur"eillan!e

AURORAL#TROUG$