Alternatively transmitting with two linear orthogonal polarization (nominally horizontal and vertical) with a switching rate around 900 Hz (note that 933 Hz was used for a beacon at 50 GHz [1]). The receiver has two sub-circuits (namely H and V circuits) that will receive the two polarized signals separately. In one-half cycle, the co-polar signals (transmitted and received with horizontal polarization) and the cross-polar signals (not transmitted but received with horizontal polarization) are received in the H-circuit and the V-circuit respectively. The received signals will allow the measurements of atmospheric attenuation, and the XPD measurements (the same cross-polarization discrimination) of the vertical polarization. In the other half-cycle, the co-polar attenuation for the vertical polarization and its associated XPD can be measured. With rapid switching between the two orthorgonal polarization senses, the cross-polarization isolation, XPI can also be measured. [2, 3]
Figure 1. Illustration of XPD and XPI measurements.
A radiometer integrated with the beacon receiver is included in the design to measure noise. In satellite propagation experiments, radiometers are used to measure the brightness temperature of the atmosphere, which is directly related to the path loss. Then the attenuation values can be used as a reference for the beacon measurements [5].
Radiometer data: measurements of a sequence of samples (at 1 Hz) of a voltage, Vrad, which carries the measurements of the brightness temperature.
Beacon data: measurements of a sequence of complex numbers (I+jQ) that represent the amplitude and phase of the four measured signals (ac, bc, ax, bx) sampled around 18 Hz as they reach the digital beacon receiver.
The measurements will be processed through several phases to determine the desired parameters. Both raw data and referenced data are inspected considering different factors such as spurious data due to occasional interference, excess noise due to sun/moon sources, and signal loss during eclipse. The calculations will also identify propagation effects such as rain fades and scintillation.
To calculate the atmospheric attenuation, Ar, from radiometer data, the following equations are used. [1]
Equation 1. Calculation of brightness temperature.
k is the Boltzmann constant (1.38 x 10-23 J/K).
B is the noise bandwidth of the receiver in Hz.
g is the total receiver gain.
Tb is the brightness temperature.
Tssp is the noise temperature of the antenna's sidelobes in K.
Trx is the receiver noise temperature in K.
η is the sky-coupling coefficient.
Equation 2. Attenuation as a function of the brightness temperature.
Tmr is an effective medium temperature (which can be estimated from the surface temperature).
To is the cosmic background temperature (2.73K).
Note that Eq. 2 is valid for low attenuation and without scattering. The equation is utilized here for the investigation of gas and cloud effects. Then, Ar can be used as a reference for the co-polar beacon measurements.
The reference signal, R, is obtained by adding the co-polar signal level, SL = 10 log(I2 +Q2), to the atmosphere attenuation, Ar, that is obtained in the radiometer measurements. Recall that I and Q are the real and imaginary part representing the magnitude and phase of the measured signals (ac, bc, ax, bx) sent from the beacon. The reference signal is calculated as follows.
Equation 3. Calculation of Reference Signal.
<SL> and <Ar> are the average values so the rapid fluctuations due to scintillation and others are eliminated.
Next, the atmospheric attenuation, Ab, measured by the beacon receiver is calculated as follows.
Equation 4. Calculation of Atmospheric Attenuation.
Note that R in Eq. 4 is obtained with averaged values of SL and Ar in wideband radiometer sampled at 1 Hz, while SL and Ab in Eq. 4 are the individual measurements performed by the small band beacon receiver sampled around 18 Hz. The value Ab is obtained for each polarization to give the four signals (ac, bc, ax, bx) from which XPI and XPD can be calculated.
References
[1] J. Riera, K. Al-Ansari, et. al. "Low-cost Millimeter-Wave Beacon Receiver Including Total-Power Radiometer: Design, Implementation and Measurement Calibration," IEEE Antennas and Propagation Magazine, February, 2002, pp. 45-54.
[2] D. C. Cox, and H. W. Arnold, "Comparison of Measured Cross-Polarization Isolation and Discrimination for Rain and Ice on a 19 GHz Space-Earth Path," Radio Science, Vol. 19, March-April 1984, pp. 617-628.
[3] P. N. Kumar, "Depolarization of 19 GHz Signals," Comsat Technical Review, Vol. 12, 1982, pp. 271-293.
[4] T. Pratt, C. Bostian, J. Allnutt, Satellite Communications, 2nd Ed. John Wiley & Sons, 2002.
[5] W. L. Stutzman, F. Haidara, P. W. Remaklus, "Correction of Satellite Beacon Propagation Data using Radiometer Measurements," IEEE Proceedings on Microwave, Antennas and Propagation, Vol. 14,1 February 1994, pp.62-64.
[6] F. Barbaliscia et. al., OPEX, Second Workshop of the Olympus Propagation Experimenters, 5, (Reference Book on Data Processing), Noordwijk, The Netherlands, ESA/ESTEC, 1994.