Signal propagation through the troposphere
Space environmental effects on satellite communication can be considered as related to space segment, ground segment and on the signals propagating through the earth’s lower and upper atmosphere. The atmospheric structure has significant influence on signal propagation. This has definite influence on the data processing methodologies. The specific applications with which are concerned here are essentially related to navigation and positioning. Troposphere and ionosphere are the two regions that have different properties of signal propagation. The structure of atmosphere can be described through concentric layers of atmospheric domains with different physical and chemical properties. The signal propagation results in ionospheric errors, tropospheric errors and multipath errors apart from the clock errors. In addition, the error in the signal can be significantly increased depending upon the geometry of the satellite used to determine a position. This can be estimated through the parameter PDOP – (Position dilution of precision). The Dilution of precision factors (DOP) explain how geometry effects to yield position accuracy and scale ranging accuracy. The optimum geometry accuracy for four GPS satellites is achieved when three satellites are equally spaced on the horizon and one directly at zenith point of observation station. The aspects related to GPS accuracies shall be discussed in subsequent Class Room Sessions.
The most significant error occurs when the satellites signal goes through the earth atmosphere. This is a blanket of electrically charged particles located between 130 and 195 km above the earth. These particles affect the speed of light and so affect the speed of the GPS radio signals.
Specific features as given below characterize the broad categories of lower and upper atmosphere.
The troposphere is the lower part of the earth’s atmosphere extending from the earth’s surface to 40 km above the earth. The signal propagation depends mainly on the water vapor content and on the temperature of the atmospheric layers. The troposphere is the gaseous atmosphere and the temperature decreases with height by 6o to 7o C per km. The horizontal temperature gradients vary possibly at the rate of 1o to 5o C per 100 km depending upon the latitudes.
In the troposphere region air pressure, temperature, and water vapor pressure influence the index of refraction. The atmosphere can be thought of as mixture of two ideal gases, dry air and water vapor. The dry part contributes 90% of tropospheric refraction. The distribution of water vapor cannot be accurately predicted. Fortunately it comprises only 10% of the tropospheric refraction. The conditions are extremely dynamic in this zone. The index of refraction influences the propagation and it is greater than 1 and decreases to 1 (at upper limit of the troposphere) with increase in height. The delay caused by the troposphere (zenith delay) depends on the refractive index of the atmosphere. The troposphere is a non-dispersive medium for radio waves up to about 15 GHz. Tropospheric refraction is thus identical for both GPS carriers, L1 (1575.42 MHz) and L2 (1227.60 MHz). The troposphere delay reaches a value of about 2 meters in the zenith direction and varies inversely with the sine of the elevation angle of the signal up to about 27 meters at angles of 5 degrees. The lower the elevation angle of the signal, the more it is adversely effected because it must travel a longer path through the troposphere. There are several models to calculate the tropospheric delays. Prominent among them is the modified Hopefield model. The propagation delay caused by the troposphere is nearly identical for the total spectrum of visible light and for the radio frequency domain. Due to the wet component, the absorption is much greater for visible light. When high accuracy is required as in the case of geodynamic modeling, attempts can be made to measure the water vapor content directly along the signal propagation path with water vapor radiometer. An earlier example of this is the use of dual frequency microwave water vapor radiometer developed for geodetic applications at ETH- Zurich- The instrument operates at 23.8 and 31.5 Ghz and is capable of automatically tracking space targets like GPS satellites. The accuracy estimate for the determination of the signal path delay is 1 to 3 cm.