Emerging networks of Global Positioning System (GPS) receivers can be used in the remote sensing of atmospheric water vapor. The time-varying zenith wet delay observed at each GPS receiver in a network can be transformed into an estimate of the precipitable water overlying that receiver. This transformation is achieved by multiplying the zenith wet delay by a factor whose magnitude is a function of certain constants related to the refractivity of moist air and of the weighted mean temperature of the atmosphere. The mean temperature varies in space and time and must be estimated a priori in order to transform an observed zenith wet delay into an estimate of precipitable water. We show that the relative error introduced during this transformation closely approximates the relative error in the predicted mean temperature. Numerical weather models can be used to predict the mean temperature with an rms relative error of less than 1%.

/pdf/gps-meteorology-mapping-zenith-wet-delays-onto-precipitable-1f1pjto6t8.pdf

GPS Meteorology: Mapping Zenith Wet Delays onto Precipitable Water

Recent improvements in microwave techniques have resulted in precise measurements which indicate that the conventional constants K1 = 79°K/mb and K22?=4,800°K in the expression for the refractivity of air, N=(n-1) 106=[K1/T](p+ K2'e/T) should be revised. Various laboratories appear to have arrived at this conclusion independently. In much of radio propagation work the absolute value of the refractive index of the atmosphere is of small moment. However, in some work it is important and it seems highly desirable to decide upon a particular set of constants. Through consideration of the various recent experiments this paper arrives at a relation 77.6 e N = ~ p + 4,810-T T where p=total pressure in millibars e=partial pressure of water vapor in millibars T=absolute temperature=°C+273 This expression is considered to be good to 0.5 per cent in N for frequencies up to 30,000 mc and normally encountered ranges of temperatures, pressure and humidity.

/pdf/the-constants-in-the-equation-for-atmospheric-refractive-1dhfkvs3fq.pdf

The Constants in the Equation for Atmospheric Refractive Index at Radio Frequencies

The atmosphere of Titan: An analysis of the Voyager 1 radio occultation measurements

The Atmosphere of Titan: An Analysis of the Voyager 1 Radio Occultation Measurements.

Bernese GNSS Software Version 5.2

The refractive indices of air and its principal constituents have been measured at a frequency of 24,000 Mc/s. with a precision comparable with that obtained in the optical range. The method is based on the measurement of the resonant frequency of a cavity resonator first when it is filled with the gas and then when it is evacuated. The source of oscillations used is a Pound-stabilized velocity-modulated oscillator, and its frequency is measured by reference to a high harmonic of a quartz standard, with a precision of 1 part in 108. The cavity is provided with a tuning plunger, which changes the resonant frequency through a range of about 10 Mc/s, and is calibrated to an accuracy of within 1 kc/s. Most of the measurements were made by the frequency-change method and the plunger was used in a narrow region only for a precise setting to resonance. It is possible by a larger movement of the plunger to compensate for the whole frequency change due to the removal of the gas and thus to work at a fixed frequency. The following results were obtained for (n - 1)106 where n is the refractive index at 0°C., 760 mm Hg: dry CO2-free air, 288.15 ± 0.1; nitrogen, 294.1 ± 0 1; oxygen, 266.4 ± 0.2; argon, 277.8 ± 0.2; carbon dioxide, 494 ± 1; and the value for water vapour at 20°C., 10 mm. Hg pressure was 60.7 ± 0.1 The dielectric constants can be calculated from the relationship μ = n2, where μ is the magnetic permeability and the dielectric constant, the values of (μ - 1) 106 being taken as 0.4 for air, 1.9 for oxygen and zero for the other gases. Accurate formulae are given for obtaining the refractive index of moist air at different atmospheric conditions, and are reduced to the following simple formula which is applicable for normal atmospheric conditions: (nt,p - 1)106 = (103.49/T)p1 + (177.4/T)p2 + (86.26/T)(1 + 5748/T)p3, where p1, p2, p3 are the partial pressures of dry air, carbon dioxide and water vapour, t is the temperature in degrees c., and T = 273 + t is the absolute temperature. A value of 1.839 ± 0.002 × 10-18 E S.U. was derived for the dipole moment of the water vapour molecule.

https://iopscience.iop.org/article/10.1088/0370-1301/64/10/303/pdf

L Essen

Papers

The Refractive Indices and Dielectric Constants of Air and its Principal Constituents at 24,000 Mc/s