RU2313103C1 - Method for single-frequency determination of delay of satellite navigation system signals in ionosphere - Google Patents

Abstract

FIELD: satellite navigation.

SUBSTANCE: the invention can be used for determination of ionosphere delay of propagation of signals of global satellite navigation systems with the aid of the user's navigation equipment operating on a single frequency, and makes it possible to determine the delay of signals of the satellite navigation system due to evaluation of the electric concentration in the ionosphere.

EFFECT: provided evaluation of the electron concentration in the real time conditions with a high precision and low extent of computations.

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Description

The present invention relates to the field of satellite navigation and can be used to determine the ionospheric delay propagation of signals from global navigation satellite systems (GNSS) using consumer navigation equipment operating on a single frequency.

All communications between surface consumers and navigation spacecraft (NSC) are made through the Earth’s atmosphere, including the troposphere and ionosphere. The atmosphere has a pronounced frequency selectivity. The inhomogeneous distribution of dielectric constant causes a curvature of the path of propagation of radio waves - refraction. As a result, at the operating frequency of the spacecraft (1.6 GHz), the error in determining the pseudorange is from 5 to 50 meters. The main influence on the delay of the radio signal in the ionosphere is made by the electron concentration, which constantly changes depending on time (hour of the day, season, phase of the solar activity cycle), geographical coordinates, altitude, and solar activity.

A known method for determining the delay of signals in the ionosphere at one frequency [1], which consists in measuring the difference between the pseudoranges measured by the rangefinder code and the phase of the carrier frequency, is based on the fact that in the ionosphere the propagation speed of the modulating signal is less than the speed of light. The difference between the pseudorange measurements by the code delay and the phase of the carrier frequency is equal to twice the ionospheric delay of the signal and can be used to determine it.

The magnitude of the signal delay and the initial ambiguity of phase measurements in this method are estimated using the Kalman filter. The dimensionality of the state vector used in the filter depends on the number of navigation spacecraft and the approximating polynomial.

The disadvantage of this method is the large dimension of the state vector, which entails a large amount of calculations and, as a consequence, an increase in the calculation time. When phase ambiguity is eliminated, the time of the first countdown significantly increases and the stability of the system decreases.

A known method for determining the delay of GNSS signals in the ionosphere at a single frequency [2, p. 83], taken as a prototype, based on the use of a simplified model of the integrated electron concentration of the ionosphere. The algorithm uses an approximation of the variation in the electron concentration of the positive half-wave in the daytime and a constant value at night. The initial data for calculating ionospheric corrections are approximate coordinates of the consumer navigation equipment (NAP), elevation, azimuth of navigation spacecraft, time and model coefficients. The coefficients of the model are transmitted in the navigation message of the GPS system (Global Position System) and updated every 6 days [3].

The disadvantage of this method is the low accuracy of determining the delay of signals in the ionosphere, due to outdated information, the inability to assess local changes in electron concentration. In addition, the algorithm poorly reproduces the behavior of the ionosphere at equatorial latitudes due to the limited possibility of cubic approximation. Using the algorithm at high latitudes is not advisable, because the integral electron concentration is lower than at middle and low latitudes, and its variations in time are significant. Model coefficients are not transmitted as part of the GNSS GLONASS Russian system. The above disadvantages significantly narrow the applicability of this system, especially for Russian users.

The basis of the invention is the task of providing the possibility of estimating the electron concentration in the ionosphere in real time, with high accuracy and with a small amount of computing operations, both for domestic consumers and consumers of foreign systems.

The problem is solved in that in the method of single-frequency determination of the delay of the signals of the navigation satellite system in the ionosphere, based on the assessment of the electron concentration in the ionosphere, according to the invention, the temperature, humidity and pressure of the environment are measured in the navigation equipment of the consumer, the integral index of the refractive index of the signal in the troposphere is calculated from the formula:

where: i = 1 ... N is the number of signals received by the navigation equipment of the consumer from the navigation spacecraft;

ν _n = ν ₀ e ^{-b / h} ;

b = 136 m; h = 8 km; h ₁ = 25 km; h ₂ = 3 km;

P is the air pressure;

T is the air temperature;

e - water vapor pressure in millibars, calculated by the formula

where: J is relative humidity;

P _p (T) is the saturated vapor pressure in millibars,

and evaluate the electronic concentration in real time according to the formula:

where: H ₀ = 200 km;

H ₁ = 325 km;

H ₂ = 32.5 km;

высота источника излучения;

height of the radiation source;

R _e is the radius of the Earth;

α _i - elevation angle of the i-th spacecraft relative to the NAP;

R _i is the distance from the i-th satellite to the radiation receiver;

c is the speed of light in vacuum;

f _i is the signal frequency of the i-th spacecraft;

τ _ci is the measured propagation time of the signal from the i-th satellite to the NAP;

where: x, y, z - coordinates of the NAP;

x _ci , y _ci , z _ci - coordinates of the i-th satellite,

for subsequent time instants, the propagation delay of the signal in the ionosphere is calculated by the formula:

for the i-th NKA.

To describe the altitude concentration profile, various model dependences are used that convey the most characteristic structural features of the ionosphere. Currently, the most commonly used parabolic-exponential model and biexponential model. From a comparison of the graphs of these models presented in [4], we can conclude that both models lead to approximately the same results. For example, consider a biexponential model. The correction to the group path for the biexponential model of the electron concentration profile for an arbitrary elevation angle [4] is

The distance from the NKA to the receiver can be determined by the formula

Since the reference generator located on the navigation equipment of the consumer is not synchronized with the reference generator located on the navigation spacecraft, there is a discrepancy in the time scale between the NAP and the spacecraft, respectively, the distance between the ith spacecraft and the spacecraft is determined by the formula

where τ _ci is the propagation time of the signal from the i-th satellite to the NAP;

C is the speed of light:

Δt is the discrepancy between the time scales between the NAP and the navigation system.

The angle of the spacecraft relative to the NAP can be determined by the formula

The delay in the propagation of a signal from a spacecraft to a NAP depends on the geometric distance and on the propagation delay of the signal in the ionosphere and troposphere. The equation for determining the delay will be

- задержка распространения сигнала от i-го НКА до НАП в тропосфере.Where:

- delay in signal propagation from the i-th spacecraft to the NAP in the troposphere.

Let us consider the influence of the troposphere on the refraction of the emitted signal from a satellite. In the radio frequency range, the refractive index depends on the parameters of the troposphere [4]

ν ₀ = 77.6P / T + 3.73 · 10 ⁵ e / T ² .

This ratio gives an error of less than 0.5% for frequencies up to 100 GHz [4]. The dependence of water vapor pressure on temperature is a tabular value and can be taken, for example, in [5]. In calculating the radiophysical characteristics of the signal propagating in the troposphere, we will use the exponential model of the altitude profile of the refractive index, so that

ν _n = ν ₀ e ^{-b / h} ,

From this expression, we can estimate ν _n and calculate the integral index of refractive index [4] for the i-th satellite

In the flat earth approximation for the i-th spacecraft, the propagation delay of the signal can be determined

Finally, the expression for calculating the propagation time of the signal from the i-th spacecraft to the NAP is determined

Then the expression for determining the propagation delay of the signal from the ith SCA to the NAP in the ionosphere will have the form

The expression for determining the electron concentration in the ionosphere can be determined by the formula

To calculate the delay time of the propagation of a signal in the ionosphere, it is sufficient to carry out a simultaneous measurement of navigational parameters from 5 NCA.

As experimental data show [2], the variation of the ionosphere during the day can be up to five times. If minor inhomogeneities are not taken into account, then over a certain time interval the electron concentration can be considered constant and the same for all the spacecraft, which allows us to further consider the value of N _{m as a} constant over a certain time interval.

Estimating the electronic concentration using one of the NKA, one can correct the measured pseudorange and obtain more accurate coordinates of the consumer navigation equipment according to the algorithm described in [2].

The algorithm for calculating the delay of signals in a navigation satellite system is presented in figure 1. At the beginning of the algorithm, the number of radio-visible spacecraft (N) is checked, if the number of radio-visible spacecraft is at least four, then the temperature, humidity and pressure of the environment are measured, as well as the signal propagation time from the ith spacecraft to the spacecraft (τ _ci ). Based on four spacecraft, the coordinates of the NAP are estimated (5). Using the measured environmental parameters, the signal propagation delay from the ith SCA to the NAP in the troposphere is determined (6). The calculation of the electron concentration is carried out if N> 4, and the electron concentration is determined from the remaining measured τ _ci , which did not participate in the calculation of the NAP coordinates, (3). If N> 5, then find the average value of the electron concentration. Based on the obtained value of the electron concentration, the propagation delay of the signal in the ionosphere is calculated. This algorithm can be implemented at the software level using a standard computing module, for example, FC-303 from FASTWEL, or on a similar computer used in a PC.

Thus, the proposed method for estimating the electron concentration in the ionosphere allows real-time, with high accuracy and with a small amount of computational operations to determine the delay of the signals of the satellite navigation system in the ionosphere.

Information sources

1. Nisner P., Trethewey V. GPS Ionospheric Determinathions Using LI Only // Proceeding of the 5 ^th International conference on "Differential Satellite Navigathion System". Additional Volume, St. Peterburg, Russia, May, 1996.

2. V.S.Shebshaevich, P.P.Dmitriev and others. Network satellite radio navigation systems. - M.: “Radio and Communications”, 1993. - 408 p.

3. ICD-GPS-200, Revision C, U.S. Government, October 10, 1993.

4. R.V. Bakitko, M. B. Vasiliev, A. S. Vinitsky. Interplanetary spacecraft radio systems. - M: "Radio and communications", 1993

5. S.Z. Sapozhnikov, E.L. Kitanin. Technical thermodynamics and heat transfer. St. Petersburg, publishing house SPbSPU, 2003

Claims (1)

The method of single-frequency determination of the delay of the signals of a navigation satellite system in the ionosphere, based on the assessment of the electron concentration in the ionosphere, characterized in that the temperature, humidity and ambient pressure are measured in the navigation equipment of the consumer, and the integral index of the refractive index of the signal in the troposphere is calculated by the formula

where i = 1 ... N is the number of signals received by the navigation equipment of the consumer from the navigation spacecraft; ν _n = ν ₀ e ^{-b / h} ; ν ₀ = 77.6P / T + 3.73 · 10 ⁵ e / T ² ; b = 136 m; h = 8 km; h ₁ = 25 km; h ₂ = 3 km; P is the air pressure; T is the air temperature; e - water vapor pressure in millibars, calculated by the formula e = JP _P (T), where J is the relative humidity; P _P (T) is the saturated vapor pressure in millibars, and the electron concentration in real time is estimated by the formula

where H ₀ = 200 km; H ₁ = 325 km; H ₂ = 32.5 km;

- the height of the radiation source; R _e is the radius of the Earth; α _i - elevation angle of the i-th navigation spacecraft (HKA) relative to the navigation equipment of the consumer (NAP); R _i is the distance from the i-th HKA to the radiation receiver;

c is the speed of light in vacuum; f _i - signal frequency of the i-th HKA: τ _ci is the measured propagation time of the signal from the i-th HKA to NAP;

where x, y, z are the coordinates of the NAP; x _ci , y _ci z _ci - coordinates of the i-th HKA, and for subsequent time instants, the propagation delay of the signal in the ionosphere is calculated by the formula

for the i-th NKA.