Very long baseline interferometry (VLBI) measures the difference between arrival times of a radio wave at two or more different stations (time delay). The radio wave arrives to stations from a great distance and for this reason plane wave front considered in calculation. Very high distance of the radio source from station causes signal’s flux density to be so low therefore received signals strength in VLBI is at least 6 times weaker than signals emitted by GPS satellites (MacDoran et al. 1983) and to detect signals, radio telescopes must have a larger collection area and a higher sampling and data recording rate (Schuh and Böhm 2013). One of the features of VLBI technique is that it can measures Earth rotation and orientation as well as precise station positions and their velocities without involving the gravity field of the Earth (Campbell et al. 2000).
Antenna resolution is directly related to the wavelength per antenna diameter ratio; so to improve resolution shorter wavelength (if possible) or antennas with high diameter must be used but increasing diameter has its own problems such as decreasing of slewing speed, more vulnerability during earthquakes and etc. Therefore, two stations that were relatively far apart were used, which were connected electrically each other by cable. Connected stations share a standard frequency, and signal correlation perform in real time (Sovers et al. 1998). With the passage of time and the need for higher resolution in studying of radio sources, the distance between stations became more and more, but increased cost of cable to connect two stations and also difficulty of cross from natural obstacles such as mountains or sea caused collecting data by stations that are not directly connected to each other. High stable atomic clocks are used in each station and signals that received to each station recorded on magnetic tapes and by using special techniques, time delay is obtained. In 1967, several groups used two-station interferometers for example Bare et al. 1967, so the mid-1960s can be considered the origin of the VLBI technique in the modern sense. Later, due to the greater advantages of computer disks over magnetic tapes, from the MARK IV system onwards computer disks were used to store data (Heinkelmann 2013).
Geodesists, astronomers and scientists of Earth and space related sciences are using VLBI. Geodesists are using VLBI to measure distances with very high accuracy and consequently to obtain station coordinates and their changes. Given that celestial bodies are the best estimation of an inertial system, VLBI has too attracted the attention of geodesies due to its ability to determine the coordinates of celestial bodies. Main goal of astronomers in using this technique is to obtain high-resolution radio source maps because two stations act as the corners of a very large telescope. The reasons that astronomers interest to the VLBI technique are 1. due to longer wavelength, the fluctuation of the incoming signal path length through the atmosphere at radio frequencies is much smaller than that at light frequencies, 2. Coherent radio waves are much easier to obtain than coherent light signals, so that radio telescopes can operate much closer to the theoretical resolution limit than optical ones (Kellermann 1972).
Accuracy of products increased over time; the improved delay uncertainty is obtained through the increased data acquisition record rate and a much wider spanned bandwidth (Niell et al. 2018). Todays, 1 mm accuracy is followed for station coordinates. Conditions that can be considered in order to reach 1 mm accuracy include reducing both the random and systematic components of the delay observable itself, increasing the number of observations per unit time, increasing the number of antennas and their geographic distribution, reducing susceptibility to external radio frequency interference (RFI) and developing improved observing strategies (Petrachenko et al. 2009).
Mathematical basic of VLBI is simple. Consider two stations that spaced apart. Radio wave that reaches to first station will reach to second station in 𝜏 seconds (time delay). In Eq. (1) mathematical relation for calculating 𝜏 is expressed.
$${\tau }=\frac{-b.{S}_{0}}{c} \left(1\right)$$
\(c\) is the speed of wave and considered equal to the speed of light in vacuum; \(b\) is baseline vector that connect station1 to station2 and \({S}_{0}\) is the unit vector that describes the wave direction. In Fig. 1 basic concept of VLBI is showed. Received signals at two stations after recording are sent to special centers called correlators. In these centers, cross-correlation function is used to obtain 𝜏. This function gives 𝜏 where it reaches maximum value. For more information on how correlators works see Whitney (2000).
Some of the important products of VLBI data analysis are: 1) Earth Orientation Parameters (EOPs): One of the most important products of VLBI are EOPs; these parameters are indispensable for positioning and VLBI is the only technique that provides the full set of EOPs (Schuh and Böhm 2013). EOPs are motion of the celestial pole in the celestial reference system ([\(dX\), \(dY\)]), rotation of the Earth around the axis associated with the pole (DUT1) and polar motion ([\({x}_{pol}\),\({y}_{pol}\)]) (Petit and Luzum 2010). 2) Reference Frames: VLBI technique with determination of the coordinates of celestial bodies is exclusively capable of determining the Celestial Reference Frame (CRF) at radio frequency (Fey et al., 2015). VLBI due to its ability in determination coordinates of stations and their speed with high accuracy, along with other satellite techniques plays an important role in the realization of the Terrestrial Reference Frame (TRF) (Altamimi et al. 2016). 3) Testing the theory of relativity: By analyzing observations of sources close to the Sun, it is possible to obtain the parameter of light deflection (\(\gamma\)) which determines the curvature of space due to massive bodies. Shapiro (1967) used VLBI to study the light deflection for first time. Using the data of more than 20 years, \(\gamma =0.99992\pm 0.00012\) was obtained by Lambert & Le Poncin-Lafitte (2011). 4) Determining the motion parameters of tectonic plates: If stations are located on different tectonic plates, relative changes of the antennas to each other can be calculated by the antennas coordinate time series. Jagoda and Rutkowska (2020) have analyzed the motion parameters of six large plates using VLBI technique. 5) Love and Shida numbers: The Love number (h) and the Shida number (l) are dimensionless parameters, which characterize the ability of the Earth to react to tide-generating forces. If the Earth would be a completely rigid body, h and l would be equal to zero and there would be no tidal deformation of the surface (Spicakova et al. 2009). Krásná et al. (2013) used 27 years data to obtain these numbers. 6) Ionosphere study: Observations in different bands allow us to study the ionosphere. Also since any atmospheric disturbance affects the distribution of the ionization and because of its extreme sensitivity to atmospheric changes, the ionosphere can be used as a sensor of atmospheric events (Hobiger et al. 2003). 7) Troposphere study: VLBI is able to provide time-series of Zenith wet delay (ZWD), which can be transformed to precipitable water (PW), i.e., high-quality information about the amount of water vapor above the VLBI stations. Water vapor in the atmosphere is an important storage and energy transfer medium of the global water household, influencing Earth’s climate in many ways (Heinkelmann et al. 2007). VLBI data can also be used to verify models. For example, it has been shown that the current IERS model for high frequency EOPs variations needs to be updated (Nilsson et al. 2019).
In this study the improvement of geodetic parameters estimation evaluated by adding new stations to Legacy-1 and Legacy-2 networks that used in CONT17 campaign. The added station is located in the Middle East since there is no VLBI station in this region. VLBI observations are generated artificially by simulation and Baseline Length Repeatability (BLR) is used as selection criteria for optimal location of new stations. We show that by adding a new station located in Saudi Arabia, the mean value of BLR in the Legacy-1 network improved by about 3.3%. Also adding a new one located in Oman, the mean value of BLR in the Legacy-2 improved by about 7.4%.