News FAQ FTP Site map


What is the ionosphere?
What are the ionospheric scintillations?
What are the GPS signals and observables?
What are the effects of the ionosphere on the GPS signals?
What are the effects of the ionospheric scintillations on the GPS signals?


What is the ionosphere?

The ionosphere is a complex part of the atmosphere, lying from about 60 km of altitude up to several hundreds of kilometres. The ionising radiations of the sun and energetic particles transported by the solar wind produce concentration of free electrons especially in the 250-400 km high layer known as the F-region. This phenomenon results in changes in the refractive index of the medium. Radio-electric waves over 100 MHz that cross the ionosphere are then refracted and delayed. In the L-band, which corresponds to the GPS frequencies, the delay may reach several tens of meters.

The ionosphere presents pronounced geographical variations with maximum electron concentration on both sides of the magnetic equator during the day. The ionisation level follows the 11 year solar cycle and varies according to the local time and the season. Strong ionospheric perturbations may occur, in particular during geomagnetic storms.

What are the ionospheric scintillations?

The Total Electron Content (TEC) is the key parameter to characterize the ionosphere influence on Earth-space radio links. Moreover, the E (80-110 km) and F (>100 km) layers may be affected by strong irregularities. They are separated into two classes. One is the "travelling disturbances", another is the "ionospheric scintillations". The latest are random irregularities, localized in space and time, which. generate very complex scattering and dispersion of the radio-signals.

Both the equatorial regions and the high latitude regions are affected by scintillation but with different signatures. In the high latitude regions i.e. the auroral regions (65-75° latitude), the sources of scintillations are mainly the storms and sub-storms induced particles that precipitate between 100 and 150 km. This phenomenon is responsible for the occurrence of aurorae. Geomagnetic storms are in general more frequent and severe several years after the solar cycle maximum. The last maximum was during the 2000-2001 period.

In the equatorial regions, plasma irregularities are often observed after about 21h local time. Those irregularities cause intense scintillations after sunset with maximum during the solar max and the equinoxes. The electron density change during scintillation events is estimated to be at a level of 10% in the worst case.


What are the GPS signals and observables?

(formalism from S. Schaer S. - Mapping and predicting the Earth's Ionosphere using the Global Positioning System. Geodätisch-geophysikalische Arbeiten in der Schweiz, Schweizerische Geodätische Kommission, vol. 59, 1999)


The GPS signals are generated onboard spacecrafts by a high-quality oscillator. They are two coherent L-Band carriers L1 at 1575.42 MHz (19 mm wavelength) and L2 at 1227.60 MHz (244 mm wavelength). Two pseudo-random noise (PRN) codes are generated. One is the C/A or coarse/acquisition modulated on L1 available for the Standard Service. Another one is the precision code (P-code or Y-code if encrypted) restricted to military applications. A navigation message contains the ephemerides and the clocks of the satellites, the GPS time, the system status.

Pseudo-range or code observations

The C/A-, P-, or Y-code , transmitted by satellite k at time tk and registered by receiver i at time ti lead to the signal propagation delay. This may be transformed into distance - also called "pseudo-distance" and expressed as follows (in meter):


are the receiver and satellite clocks offsets with respect to the GPS system time,
is the tropospheric delay,
is the ionospheric delay,
are the satellite and receiver hardware delays, expressed in unit of time,
is a random error (or residual).

 estimated uncertainties are 3 meters for C/A code and 0.3 m for P-code.

Phase observations

The carrier phase observation after multiplication by the signal wavelength at each frequency is expressed in meter as:


is the corresponding wavelength and
is the constant bias (initial carrier phase ambiguity ), expressed in cycles

Estimated uncertainties of are very low around 2 mm.

Linear combination of code and phase

The linear combination of codes or/and phases is at the basis of any GPS data processing. For dual-frequency geodetic GPS receivers, a simplified frequency-dependent notation is adopted .

1=1575.42 MHz
2=1227.60 MHz

where .

The difference of two codes (P2-P1) or phases (L2-L1) quasi-simultaneously acquired almost completely eliminates the geometric terms and associated errors (satellite orbit, clock offsets, troposphere). It is called the "geometry free linear combination" and classically used for ionospheric investigations. (P2-P1) gives the absolute level of ionospheric correction with the code uncertainty while the ambiguities remain on (L2-L1) phase difference.


What are the effects of the ionosphere on the GPS signals?

Effects description

When a radio-signal propagates across the ionosphere, it is both bent and retarded. The ionospheric path delay results from the integration of the refractive index which is < 1 and varies as a function of the electron content N and the signal frequency .

The ionospheric correction of the GPS observations may be related to the Total Electronic Content (TEC) as follows:

where (>0 for code and <0 for phase),
is the line-of-sight or slant Total Electronic Content in 1016 electrons per square meter (TECU unit), and is the frequency.

The ionospheric path delay is about 0.162m/TECU at frequency 1.

For absolute TEC mapping using the ground-based GPS data, a mapping function is required to provide the TEC along the vertical. A so-called single-layer model is used. All free electrons are assumed to be contained in a shell of infinitesimal thickness at altitude H. The altitude of this idealized layer is usually set to 400 kilometers approximately corresponding to the altitude of maximum electron density.

The relation between the vertical VTEC or TEC(0) and the slant TEC(z) if z is the satellite zenith distance at the receiver's location (figure 2) is:


z' being the satellite's zenith distance at the point where the single layer is pierced.

Ionosphere at Yellowknife (Canada- AOA Benchmark receiver)

Quiet magnetic period (July 19th, 2004)

The Geometry free linear combination is applied on both the code observations (P2-P1) and the phase observations (L2-L1). The latest is biased due to the ambiguities but the two terms should indicate the same tendency versus time (about 1 m after 1 hour). In this case, it is clear that the code differences are much less precised than the phase differences. This is reinforced by the low elevation of the satellite pass affected by a lower budget link and possibly some tropospheric residual error (not very likely at high latitudes). It is also notable that the (L2-L1) is affected by a cycle slip. The time differences of two consecutives terms plotted as D(L2-L1) constitutes an efficient tool to isolate the cycle slips.

Magnetic storm (October 29th, 2003)

In that case, the ionospheric signals are significant in the order of a one meter within a few minutes. The level of (P2-P1) noise is reasonable probably due to a high satellite elevation.


What are the effects of the ionospheric scintillations on the GPS signals?

The most significant effects include the signal power attenuation, the loss of lock (L2 tracking mainly) and the phase fluctuations. Significant impacts are observed on GPS availability, on precise positioning, navigation, determination and transmission of ionospheric dual frequency corrections.

Scintillations at Tromsoe (Norway)

At Tromsoe, two GPS receivers are operating. The figure shows the same variability of D(L2-L1) -time difference of the phase geometry free - and of the signal to noise count during the same satellite pass. It should be a phenomena related to the propagation environment. The two receivers operate with a 1 second repetition rate. The high amplitude of the fluctuations help to suspect the occurrence of ionospheric scintillations.

Scintillations at Douala (Cameroun)

ESA provided calibration data from an ISM (Ionospheric Scintillation Monitor) located at Douala (Cameroun). It is a Novatel material specially adapted to track signal during the signal and calculate their intensity. The classical S4 scintillation index is available every minute and also the C/No. The nearest GPS permanent station is Libreville (gabon) about 400 km South. Even if the repetition rate is 30 s, the empirical scintillation index has been calculated based on high frequency fluctuations of the D(L2-L1) differences. A two days period of quiet magnetic activity has been examined and the plots are given versus local time. It clearly appears that between 19h up to 22h local time, perturbations appear in the two equipments. For Libreville, the scintillation indicator is not yet normalized.

P being the amplitude of the signal,


Copyright © 2003-2005