CN110824510A - Method for increasing number of sea surface reflection signals received by GNSS-R height measurement satellite - Google Patents
Method for increasing number of sea surface reflection signals received by GNSS-R height measurement satellite Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
- G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
- G01S19/13—Receivers
- G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
- G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
- G01S19/13—Receivers
- G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
- G01S19/30—Acquisition or tracking or demodulation of signals transmitted by the system code related
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
- G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
- G01S19/13—Receivers
- G01S19/35—Constructional details or hardware or software details of the signal processing chain
- G01S19/37—Hardware or software details of the signal processing chain
Abstract
The invention discloses a method for increasing the quantity of sea surface reflection signals received by a GNSS-R altimetry satellite, which comprises the following steps: acquiring antenna parameter combinations of N groups of antenna gains and antenna pointing angles; acquiring position information of all the mirror reflection points, and determining the satellite altitude angle of each mirror reflection point; under the condition of each group of antenna parameter combination, screening available specular reflection points according to the specular reflection points to obtain the number of the available specular reflection points corresponding to each group of antenna parameter combination; determining a group of antenna parameter combinations with the largest number of available specular reflection points as an antenna optimal parameter combination; and according to the antenna optimal parameter combination, parameter configuration is carried out on the GNSS-R height measurement satellite so as to improve the quantity of sea surface reflection signals received by the GNSS-R height measurement satellite. The method and the device improve the quantity of the GNSS-R altimetry satellites for receiving the sea surface reflection signals.
Description
Technical Field
The invention belongs to the technical field of satellite height measurement, and particularly relates to a method for increasing the quantity of sea surface reflection signals received by a GNSS-R height measurement satellite.
Background
The global sea surface height data can be used for inverting an ocean gravity field, acquiring submarine topography and establishing an ocean tide model, and plays an important role in research in the fields of geodesy, geophysics, ocean dynamics and the like. At present, sea surface height measurement data can be obtained through a ship survey station, a tide station and a satellite altimeter. However, the ship survey and tidal station sampling efficiency is low and cannot reach global ocean coverage; although the satellite altimeter can acquire high-precision sea surface altitude data in a global range, the spatial resolution of the satellite altimeter cannot meet the requirement of medium-scale observation.
The GNSS-R height measurement satellite is used as a novel double-base microwave remote sensing technology, has the advantages of multiple signal sources, low cost, wide coverage, simultaneous measurement of multiple reflection points and the like, can be well applied to sea surface height measurement, and is expected to effectively make up for the defects of a conventional measurement method. At present, the feasibility of the GNSS-R sea surface height measurement technology is verified, and the key for realizing the application is further improving and playing the advantages of the high-spatial resolution observation capability. The GNSS-R sea surface height measurement data with higher spatial resolution can effectively improve the spatial resolution of an inverted marine physical model, so that errors caused by numerical value difference in model application are reduced, the point location information precision is improved, and the method has important significance for finely researching marine motion.
The underwater gravity matching navigation based on the ocean gravity field model is an important means of current underwater autonomous navigation and has the characteristics of high precision, long endurance, concealment and the like. The underwater gravity matching navigation can be matched with global gravity field model information according to the real-time gravity measurement data to obtain position information, so that the positioning error accumulated by inertial navigation along with time is corrected, and the underwater navigation positioning precision is improved. The establishment of the global gravity field model with high precision and high spatial resolution is the basis for applying the gravity passive navigation technology to actual underwater navigation.
Compared with the measurement of a satellite radar altimeter, the GNSS-R altimetry satellite can perform sea surface altitude detection with higher density, so that the observation space resolution is higher. The number of sea surface reflection signals received by the satellite-borne downward-looking antenna is a main factor influencing the GNSS-R sea surface height measurement spatial resolution. Because the energy of the reflected signals reaching the satellite-borne GNSS-R altimetry satellite receiver is weak, the downward-looking antenna is required to have high gain, and meanwhile, in order to ensure the detection spatial resolution, the downward-looking antenna also has large half-power beam width, and the antenna gain and the half-power beam width are in inverse proportion. Furthermore, antenna pointing also affects the antenna coverage area. Therefore, the downward-looking antenna parameters need to be optimally designed, which directly determines the capability of acquiring, tracking and utilizing sea surface reflection signals, and finally influences the spatial resolution of GNSS-R sea surface altimetry.
Disclosure of Invention
The technical problem of the invention is solved: the method overcomes the defects of the prior art, provides a method for increasing the quantity of sea surface reflection signals received by the GNSS-R altimetry satellite, and aims to increase the quantity of the sea surface reflection signals received by the GNSS-R altimetry satellite.
In order to solve the technical problem, the invention discloses a method for increasing the number of sea surface reflection signals received by a GNSS-R altimetry satellite, which comprises the following steps:
acquiring antenna parameter combinations of N groups of antenna gains and antenna pointing angles; wherein N is more than 250000;
acquiring position information of all specular reflection points calculated and output by a GPS satellite ephemeris and a TDS-1 satellite ephemeris within 24 hours, and determining the satellite altitude of each specular reflection point;
under the condition of each group of antenna parameter combination, according to the satellite altitude angle of each mirror reflection point, screening available mirror reflection points of all the mirror reflection points which are output by the calculation of the GPS satellite ephemeris and the TDS-1 satellite ephemeris within 24 hours to obtain the number of the available mirror reflection points which are respectively corresponding to each group of antenna parameter combination;
determining a group of antenna parameter combinations with the largest number of available specular reflection points as an antenna optimal parameter combination;
and according to the antenna optimal parameter combination, performing parameter configuration on the GNSS-R height measurement satellite so as to improve the quantity of sea surface reflection signals received by the GNSS-R height measurement satellite.
The invention has the following advantages:
the method for improving the number of the sea surface reflection signals received by the GNSS-R altimetry satellite comprehensively considers the observation capability analysis of various antenna parameters on the satellite-borne downward-looking antenna of the GNSS-R altimetry satellite, can obtain the parameter solution when the observation capability of the GNSS-R altimetry satellite is optimal, provides a reference basis for parameter configuration of the GNSS-R altimetry satellite, and improves the working efficiency and reliability of the system.
Drawings
FIG. 1 is a flowchart illustrating steps of a method for increasing the amount of sea surface reflection signals received by GNSS-R altimetry satellites in accordance with an embodiment of the present invention;
FIG. 2 is a graphical representation of the relationship between Brewster's angle and Fresnel reflection in an embodiment of the present invention;
fig. 3 is a schematic diagram illustrating a geometric relationship between an antenna pointing angle and a half-power beam width when θ is 0 ° in an embodiment of the present invention;
fig. 4 is a schematic diagram illustrating a geometrical relationship between the pointing angle of the antenna and the half-power beam width when θ < γ/2 according to an embodiment of the present invention;
FIG. 5 is a schematic diagram illustrating a geometrical relationship between the pointing angle of the antenna and the half-power beam width when θ > γ/2 according to an embodiment of the present invention;
FIG. 6 is a schematic diagram illustrating a relative position geometry of a GNSS-R altimetric satellite and a GNSS satellite according to an embodiment of the present invention;
FIG. 7 is a schematic diagram of a comparison of model data and TDS-1 observed data in an embodiment of the present invention;
FIG. 8 is a schematic diagram illustrating a relationship between a minimum satellite elevation angle and a satellite antenna gain that satisfies a signal-to-noise ratio requirement according to an embodiment of the present invention;
FIG. 9 is a schematic diagram illustrating a relationship between the amount of sea surface reflection signals received by a GNSS-R altimetry satellite under four signal channels, the satellite antenna gain, and the beam pointing angle according to an embodiment of the present invention;
fig. 10 is a schematic diagram illustrating the relationship between the number of available specular reflection points and the antenna gain and beam pointing angle under a comprehensive sea condition according to an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the embodiments of the present invention will be described in detail with reference to the accompanying drawings.
The invention provides a method for increasing the number of sea surface reflection signals received by a GNSS-R height measurement satellite based on an electromagnetic scattering theory and a GNSS-R working principle, comprehensively considers the influence of parameters such as gain, half-power beam width, pointing angle and the like, and uses the utilization rate of the reflection signals to represent the observation capability of a downward-looking receiving antenna, thereby determining the optimal combination of antenna parameters, increasing the number of sea surface reflection signals received by the GNSS-R height measurement satellite and further increasing the spatial resolution.
Example 1
Referring to fig. 1, in this embodiment, the method for increasing the number of sea surface reflection signals received by GNSS-R altimetry satellites includes:
In this embodiment, the antenna parameter combinations are: any combination of various antenna gains and various antenna pointing angles. Wherein N is more than 250000.
In this embodiment, the data sources used are mainly as follows:
TDS-1 satellite data
TDS-1 satellites are launched in 2014 at 7, 8 and are technical verification satellites designed in the United kingdom. The satellite carried 8 test loads including SGR-ReSI. After the on-orbit operation, rich GNSS-R observation data are obtained through the SGR-ReSI. The data products are classified into three levels of L0, L1 and L2 according to the data processing method. The L0-grade product mainly comprises original sampling data, the L1-grade product mainly comprises DDM (data mining model) acquisition data, satellite orbits, mirror reflection point positions and other information, and the L2-grade product mainly comprises results of mean square slope, sea surface wind speed, wind direction and the like. This example mainly uses the following TDS-1 data:
(1) DDM signal to noise ratio data. This data is recorded in the TDS-1L1 grade product's metadata.nc file, used in this example to verify the accuracy of GSNASNRM.
(2) TDS-1 satellite ephemeris. Because threshold value screening has been carried out on the aspects of channel number, signal-to-noise ratio, altitude angle and the like in TDS-1 related products, all available specular reflection point information in a certain time cannot be comprehensively reflected, the specular reflection point information is reacquired in the embodiment, and in order to avoid errors caused by orbit simulation, the embodiment uses coordinate information of a TDS-1 satellite in an L1 product.
(3) And (4) wind speed information. In order to count the weight of different wind speeds in a certain time, the present embodiment obtains the wind speed information inverted according to the TDS-1 observation data according to the "L2 _ fdi.nc" file.
In the embodiment, TDS-1 observation data of 24 hours in total is mainly used for research of 2018-03-31-20:00: 00-2018-04-01-20: 00: 00.
GNSS precise ephemeris
The calculation of the position of the specular reflection point requires orbit information of a GNSS satellite, and the embodiment uses a final precise orbit ephemeris of the GNSS satellite issued by igs (international GNSS service).
It should be noted that, in this embodiment, the observation time interval is set to be 15s, the TDS-1L1b sampling interval is 1s, and the GNSS satellite ephemeris data interval is 15min, so that the time resolutions of the two satellite coordinates need to be unified to 15 s. The TDS-1 satellite coordinates with corresponding resolution can be obtained by extracting the original information every 15s, and the GNSS satellite coordinates with the interval of 15s can be obtained by performing Chebyshev polynomial fitting on the precise ephemeris. Since TDS-1 can only receive GPS reflected signals, this embodiment uses only the GPS satellite information in the GNSS ephemeris.
And 103, under the condition of each group of antenna parameter combination, screening available mirror reflection points of all the mirror reflection points calculated and output by the GPS satellite ephemeris and the TDS-1 satellite ephemeris within 24 hours according to the satellite altitude angle of each mirror reflection point, so as to obtain the number of the available mirror reflection points corresponding to each group of antenna parameter combination.
In this embodiment, the observation capability of the GNSS-R altimetric satellite downward-looking antenna at epoch time is represented by the number of specular reflection points available for receiving the reflection signal. Wherein the available conditions of the specular reflection point include: the signal polarization mode is consistent, the SNR requirement is met, and the signal polarization mode is within the working range of the antenna. Whether the three conditions are met can be judged through the satellite altitude angle of the mirror point.
Preferably, an alternative specific procedure for screening the available specular reflection points may be as follows:
and a substep 1031, performing a primary screening on the specular reflection points according to the comparison result of the satellite height angle and the brewster angle of the specular reflection points to obtain the primarily screened specular reflection points.
In this embodiment, the GNSS satellite signals belong to right-hand circularly polarized electromagnetic waves, but the polarization mode may change after sea surface reflection, and the change depends on the signal characteristics, the ocean temperature and salinity state, and the satellite altitude. The brewster angle is the critical angle for the change in signal characteristics when electromagnetic waves are reflected at the sea surface. According to the electromagnetic field theory, when the satellite height angle is larger than the Brewster angle, the polarization mode of the GNSS satellite signal is changed from right-hand circular polarization to left-hand circular polarization. At present, the lower view antenna of the GNSS-R height measurement satellite can only receive left-handed circularly polarized signals, so that available mirror reflection points are screened according to the Brewster angle.
Preferably, the procedure of one screening may be as follows:
(a) determining Brewster's angle ζ
Brewster's angle ζ is primarily in terms of the Fresnel reflection coefficient
The obtained Fresnel reflection coefficient is determined by a signal polarization mode, a seawater dielectric constant epsilon and a local incidence angle delta, and the expression is as the following formula (1):
where the subscript LR denotes left-hand circular polarization → right-hand circular polarization, and the subscript RL denotes right-hand circular polarization → left-hand circular polarization.
As shown in fig. 2, the satellite height angle corresponding to the zero point in equation (1) is the brewster angle ζ. Taking a GPS L1 wave band as an example, assuming that the temperature of seawater is 25 ℃ at normal temperature, the salinity is 35 per mill, and epsilon is 70.53+65.68 i; the brewster angle ζ under this condition is 5.9227 °.
(b) Reserving specular reflection points satisfying the following formula (2) as the specular reflection points after the primary screening:
α＞ζ···(2)
in this embodiment, the reflected signal is likely to be received by the GNSS-R altimetric satellite down-looking antenna only when the satellite altitude α for the mirror point is greater than the Brewster angle ζ.
And a substep 1032 of performing secondary screening on the primarily screened specular reflection points according to the satellite altitude of the specular reflection points and by combining a GNSS-R satellite-borne downward-looking antenna signal-to-noise ratio model GSNASNRM, so as to obtain secondarily screened specular reflection points.
In this embodiment, the antenna gain is a determining factor affecting the power strength of the satellite-borne GNSS-R altimetry satellite receiving the sea surface GNSS satellite reflected signal. However, the received signal power does not completely describe the clarity of the signal, and it is still necessary to obtain the strength of the signal relative to the noise. Therefore, the present embodiment uses the signal-to-noise ratio to measure a signal quality: only when the signal-to-noise ratio of the GNSS satellite reflected signal meets the requirement, the GNSS satellite reflected signal can be used for sea surface height measurement, and therefore, the specular reflection points which do not meet the signal-to-noise ratio threshold condition need to be removed.
Preferably, the procedure of the secondary screening may be as follows:
substituting the satellite altitude corresponding to the specular reflection point after primary screening into a GNSS-R satellite-borne downward antenna signal-to-noise ratio model GSNASNRM to solve to obtain the signal-to-noise ratio corresponding to the specular reflection point after primary screening; and then, taking the specular reflection point with the signal-to-noise ratio larger than the signal-to-noise ratio threshold (0dB) in the specular reflection point after the primary screening as the specular reflection point after the secondary screening.
And a substep 1033 of determining a maximum satellite altitude angle threshold and a minimum satellite altitude angle threshold corresponding to the current antenna parameter combination, and performing three-time screening on the mirror reflection points subjected to the secondary screening according to the maximum satellite altitude angle threshold and the minimum satellite altitude angle threshold to obtain all available mirror reflection points corresponding to the current antenna parameter combination.
In this embodiment, a GNSS satellite reflected signal can only be captured by the antenna when the signal is within the operating range of the GNSS-R altimetric satellite downward-looking antenna. The antenna operating range is mainly determined by the antenna half-power beam width and the antenna pointing angle. As mentioned above, the antenna parameter combination includes the antenna pointing angle θ and the antenna gain G, and the antenna gain G and the antenna half-power beam width satisfy the following relationship:
where γ denotes a half-power beamwidth of the horizontal plane,
representing the half-power beamwidth of the vertical plane, η representing a dimensionless efficiency factor, η having a value in the range of [0, 1%]For a well designed antenna, the value of η may be close to 1.
Preferably, the procedure for the three screens may be as follows:
firstly, according to the current antenna parameter combination, determining an antenna pointing angle theta and a half-power beam width gamma of a horizontal plane corresponding to the current antenna parameter combination;
then, as shown in FIGS. 3-5, a satellite elevation angle minimum threshold α is determined based on the relationship between the antenna pointing angle θ and the half-power beamwidth γ of the horizontal planeminAnd satellite altitude maximum threshold αmax：
When theta is 0 deg., αmin＝90°-γ/2，αmax＝90°
When theta is<At gamma/2, αmin＝90°-(θ+γ/2)，αmax＝90°
When theta is>At gamma/2, αmin＝90°-(θ+γ/2)，αmax＝90°-(θ-γ/2)
Finally, the mirror reflection points in the mirror reflection points after the secondary screening areIs greater than the satellite altitude minimum threshold αminα being smaller than the maximum threshold value of the satellite altitudemaxThe specular reflection points of (1) are used as the specular reflection points after the three-time screening.
Therefore, the number of the available specular reflection points corresponding to each antenna parameter combination in each final group can be obtained through the three screening of the substeps 1031 to 1033.
In this embodiment, the satellite-borne GNSS-R downward-looking antenna corresponding to the antenna optimal parameter combination has the optimal observation capability, that is, the maximum number of received sea surface reflection signals.
Example 2
In this embodiment, the procedure for establishing the GNSS-R satellite-borne downward antenna signal-to-noise ratio model GSNASNRM may be as follows:
referring to fig. 6, according to the relative positions of the GNSS-R altimetry satellite and the GNSS satellite, and combining with the geometric theorem (in fig. 4, the point P is the specular reflection point, and the point O is the geocentric), the following formula (3) can be obtained:
wherein R isTPRepresenting the distance, R, of a GNSS satellite to a specular reflection pointPRRepresenting the distance from the GNSS-R altimetric satellite to the specular reflection point, RERepresents the radius of the earth, HTRepresenting the altitude, H, of a GNSS satelliteRRepresenting the altitude of the GNSS-R altimetric satellite.
Solving equation (3) to obtain:
in order to more accurately evaluate the performance of the antenna for receiving signals, the signal-to-noise ratio (SNR) needs to be calculated by combining the influence of antenna thermal noise:
wherein, YS(tau, f) represents a scattered signal power calculation function, tau represents the time delay of the GNSS satellite sea surface scattered signal, f represents the Doppler frequency of the GNSS satellite sea surface scattered signal, PNRepresenting the thermal noise power, P, of the antennaTRepresenting the GNSS satellite signal emission power, λ representing the GNSS satellite signal wavelength, TiRepresenting the coherent integration time, ASRepresenting the blazed area, p the position vector of the scattering point of the sea surface, GT(p) denotes the GNSS satellite transmit antenna gain, GR(rho) represents the gain of a GNSS-R altimetry satellite receiving antenna, delta tau (rho) represents the difference between the time delay of a scattered signal component of a sea surface scattering point and tau, delta f (rho) represents the difference between the Doppler frequency shift value of the scattered signal component of the sea surface scattering point and f, lambda represents a GNSS satellite pseudo-random code autocorrelation function, S represents a Doppler spectrum function, and sigma represents a Doppler spectrum function0(p) represents the scattering coefficient at the sea surface reflection point, RTP(p) represents the distance function of the GNSS satellite to the specular reflection point, RPR(ρ) represents a distance function from the GNSS-R altimetric satellite to the specular reflection point, K represents the Boltzmann constant, and T representsTemperature ofRepresenting equivalent noise temperature, BiRepresenting the signal bandwidth.
And finally, substituting the formula (4) into the formula (5) to obtain the GNSS-R satellite-borne downward antenna signal-to-noise ratio model GSNASNRM.
It should be noted that, in this embodiment, the scattered signal power calculation function is expressed by using a Z-V (Zavorotny-Voronovich) model:
it can be seen that the reflected signal power depends mainly on the position of the specular reflection point, the downward looking antenna gain, sea state, etc. Wherein σ0(ρ) is determined by the GNSS satellite signal frequency and sea state, which is the GNSS satellite signalAn important characterization of scattering intensity at the sea surface.
In this embodiment, the magnitude of the received power at the specular reflection point is mainly studied, and therefore, only the signal power of the GNSS satellite signal forward scattered at the specular reflection point is considered, so that the following formula (6) is selected to perform σ0Resolution of (ρ):
q＝(q⊥，qz)＝k(n-m)···(7)
wherein q represents a scattering vector, q⊥And q iszRespectively representing the horizontal and z-axis components of the scattering vector q, P (-q)⊥/qz) Representing the joint probability density function of sea surface inclination, m representing the incident wave vector, n representing the scattered wave vector, and k representing the wave number.
Further, P (-q)⊥/qz) The sea scattering coefficient of the GNSS satellite signal is determined, and if the sea obeys Gaussian distribution, under two dimensions, the following steps are carried out:
wherein Q isxRepresenting the component of the slope of the oblique direction downwind, QyRepresenting the component of the slope of the diagonal upwind, mssxRepresents the slope of the x-direction in an oblique direction, mssyRepresents the slope of the y-direction in an oblique direction, bx,yRepresenting the correlation coefficient between the two orthorhombic slope components in the x-direction and the y-direction.
The position of the specular reflection point is set as the origin, the direction of the GNSS-R satellite motion is set as the y-axis, and the horizontal direction is orthogonal to the x-axis.
Example 3
On the basis of the embodiment, the invention also provides a verification and simulation result of the method for improving the quantity of the sea surface reflection signals received by the GNSS-R altimetry satellite.
Received GNSS reflected signal to noise ratio
The present embodiment verifies the reliability of GSNASNRM using TDS-1 satellite observation data, so that the parameter settings are as consistent as possible with TDS-1 when calculating SNR. The orbit height of the GNSS-R altimetry satellite is set to be 635km, the downward-looking antenna is set to point to the nadir direction, the gain is 13.3dBi, the frequency of a received signal is set to be 1575.42MHz of an L1 waveband, the wavelength is 0.19m, the temperature of the antenna is set to be 300K, and the noise bandwidth is set to be 1000Hz as pseudo code de-spreading processing is carried out on TDS-1 observation data. As shown in FIG. 7, the relation between the signal-to-noise ratio and the altitude angle under different sea surface wind speeds obtained based on GSNASNRM under the current parameter setting is given, and the actually measured SNR data of TDS-1 in 24 hours of 2018-03-31-21:00: 00-2018-04-01-21: 00:00 is also given.
As can be seen from fig. 7:
(1) TDS-1 has threshold value set at both height angle of 45 deg and SNR of-10 dB to ensure the quality of observed data. The SNR data obtained according to the GSNASNNRM is consistent with the change trend of TDS-1 observation results, and the numerical value is also in the same order. Although the sea surface obtained according to the wave spectrum has errors with the real sea condition, the relation between the satellite height angle and the SNR can still be well reflected.
(2) As the satellite altitude increases, the signal-to-noise ratio rises. This is because the GNSS signal propagation path length is reduced with the increase in satellite altitude, which in turn reduces the power loss due to signal propagation; meanwhile, the higher the satellite altitude angle is, the stronger the scattering ability of the signal on the sea surface is. When the satellite altitude is 0-10 degrees, the rising trend of the signal-to-noise ratio is obvious, and the change gradually tends to be gentle along with the gradual increase of the altitude, which shows that the SNR is sensitive to the response of the change of the satellite altitude in a lower satellite altitude interval, and the SNR is less influenced by the change of the altitude in a middle and high altitude interval. This can provide reference for parameter setting of future GNSS-R satellite-borne multi-channel antennas.
Minimum satellite elevation angle that meets signal-to-noise ratio requirements
If the GNSS satellite reflected signal power is lower than the antenna thermal noise power, the signal will be submerged in noise and cannot be better captured and tracked by the antenna. Therefore, the present embodiment sets the SNR threshold to 0 dB. Obtaining the minimum satellite elevation angle meeting the signal-to-noise ratio requirement is a key for screening available specular reflection points, and can link antenna gain with antenna observation capability, and fig. 8 is a relation between the minimum satellite elevation angle and the antenna gain under different sea surface wind speeds.
As can be seen from fig. 8, as the antenna gain increases, the minimum satellite height angle decreases. This is because an increase in gain increases the power at which weak signals arrive at the antenna to meet the signal-to-noise ratio requirement, which decreases the minimum satellite elevation. Furthermore, the effect of wind speed on the minimum satellite altitude is also significant. As wind speed increases, the minimum satellite altitude increases. The reason is that the sea surface is rougher due to the rise of the wind speed, so that the proportion of components of GNSS satellite reflected signals in the specular reflection direction is reduced, and finally, the signal strength is weaker and cannot be captured by a satellite-borne GNSS-R downward-looking antenna. It is noted that at different wind speeds, it is only possible to receive a reflected signal that meets the threshold condition when the antenna gain meets certain requirements.
Number of received reflected signals
The number of received reflected signals is equivalent to the number of available specular reflection points. The specular reflection point data is an important database studied in this embodiment, and the information is obtained by TDS-1 satellite and GNSS satellite space coordinates. The present invention calculates the position and height of 106150 specular reflection points within 24 hours using GF-NPRRSCCM method, where the difference between the obtained height and TDS-1L1b related data is below 3.6 ". In the embodiment, the observation capability of the satellite-borne GNSS-R downward-looking antenna is represented by using the utilization rate of the reflection signals, and the number of available mirror reflection points needs to be acquired. As shown in fig. 9, the relationship between the number of available specular reflection points of the four-signal channel receiver and the antenna gain and the beam pointing angle at different wind speeds is shown.
As can be seen from fig. 9:
(1) under different wind speed conditions, the number of available mirror reflection points has peak points along with the gradual increase of the antenna gain and the pointing angle. This shows that the reasonable design of the antenna gain and the pointing angle can optimize the amount of received GNSS-R sea surface altimetric signals.
(2) When the pointing angle is fixed, the number of available specular reflection points is increased along with the increase of the antenna gain, and is gradually reduced after reaching the peak value. This is because initially increasing the antenna gain raises the weak signal power, causing more reflected signals to be captured by the antenna, which increases the number of signals more than the number of signals that decrease as the antenna operating range decreases (half beam width decreases) due to the increased antenna gain. The number of available specular reflection points peaks when the two are equal. As the antenna gain continues to increase, the number of signals increasing due to power boost is progressively less than the number of signals decreasing due to the decreasing antenna coverage area, and the number of available specular reflection points is progressively decreased.
(3) When the antenna gain is low, the number of available specular reflection points gradually decreases with increasing pointing angle. This is because the increase of the pointing angle makes the signal propagation path longer, which causes more power loss, and thus part of the signal cannot be captured by the low gain antenna. When the antenna gain is high, the number of available specular reflection points gradually decreases as the beam pointing angle increases to reach a peak value. This is because the increase of the pointing angle increases the coverage area of the antenna, and although the increase of the pointing angle may lengthen the signal propagation path and further cause more power loss, resulting in that part of the signal cannot be captured, the increase of the pointing angle also causes more available GNSS reflected signals to be received by the high-gain antenna, thereby increasing the number of finally available specular reflection points. When the number of the available specular reflection points reaches the peak value, compared with the number of GNSS reflected signals increased by increasing the angle, the number of signals which cannot meet the signal-to-noise ratio requirement of the receiving antenna is more due to the lengthened signal propagation path, and finally the number of the available specular reflection points is reduced. Therefore, optimal combination of antenna gain and beam pointing angle is key to improving the downward looking antenna observation capability.
Antenna parameter combination optimization
There are multiple GNSS satellite reflections on the sea surface at epoch time, but not all of the reflections can be used by GNSS-R altimetry satellites for altimetry. In order to better reflect the use condition of the reflected signal, the observation capability of the antenna is evaluated and optimized and analyzed by utilizing the rate of the reflected signal. The reflected signal utilization is the ratio of the number of available specular reflection points to the total number of specular reflection points.
With the increase of the wind speed, the antenna gain corresponding to the peak value is gradually increased, and the number of the received signals and the utilization rate of the reflected signals are gradually reduced. Compared with the wind speed of 1m/s, the gain value of the antenna corresponding to the wind speed of 23m/s is improved by 37.41%, and the utilization rate of the reflected signal is reduced by 13.28%. This is a difficulty with current GNSS-R observation data acquisition. Sea conditions are complex and variable, and GNSS satellite reflected signals cannot be acquired and tracked easily. When sea conditions are severe, the number of reflected signals captured by the downward-looking antenna of the GNSS-R altimetry satellite can be increased only by needing higher antenna gain, so that higher power consumption is caused, and the corresponding increase of the volume, weight and cost of the satellite is inevitably caused. Therefore, modeling and optimally designing antenna parameters according to the actual requirements of the satellite observation capability are of great importance.
For comprehensive evaluation to obtain the optimal parameter combination, the present embodiment performs weighted average on the number of available specular reflection points obtained at different wind speeds: a
Wherein N iscpxThe number of available specular reflection points in the comprehensive sea condition is shown, w represents the number of wind speed samples, and in the embodiment, w is 12, and N is setiRepresenting the number of available specular reflection points, p, at different wind speedsiShowing the weight occupied by different wind speeds.
Based on U10(U10 ═ 2i-1, i ═ 1,2,3, …,12) data in L2-level data of TDS-1, the number of reflection events in the range of ± 1m/s of the division point was counted, and N was obtainedi. As shown in fig. 10, the relationship between the number of available specular reflection points and the antenna gain and the pointing angle under the integrated sea conditions is obtained according to equation (9).
As can be seen from fig. 10, when the apparent antenna gain reaches 20.88dBi and the pointing angle is 32.76 °, the number of available specular reflection points reaches 19276 in total, and the utilization rate of the reflected signal reaches 18.16%.
In the embodiment, only the GPS satellite signals are used as the signal source, and if all GNSS satellite signals are used simultaneously to perform GNSS-R sea surface height measurement, the antenna gain and beam angle corresponding to the peak are expected to be reduced. This is due to the increased number of GNSS system satellites available which can provide more available reflected signals in the higher satellite altitude range (45).
The embodiment calculates the number of available reflection points and the utilization rate of reflection signals according to the downward-looking antenna parameters of the TDS-1 satellite and based on the GSNAOCOM. It is noted that when the antenna gain is 13.3dBi, the antenna gain is based on
The half-power beam width obtained through calculation is 46 degrees, while the half-power beam width of TDS-1 aiming at the L1 wave band is 35 degrees, because the satellite is mainly designed aiming at sea surface anemometry, more attention is paid to time resolution, and the narrower half-power beam width is easier to obtain a reflection signal with a high signal-to-noise ratio, so that more accurate inversion of wind speed information is facilitated, and meanwhile, the actual working efficiency is also considered. Since the half-power beamwidth is calculated according to equation (8) in this embodiment, for the convenience of statistical comparison analysis, the half-power beamwidth is set to 46 ° when calculating the number of available signals under TDS-1 parameter.
Compared with the TDS-1 satellite downward-looking antenna parameter combination, the number of received reflected signals acquired by the optimal parameter combination acquired based on the GSNAOCOM is increased by 5.44 times. This is because higher antenna gain and pointing angle can effectively increase the probability that the antenna will receive the reflected signal. When the same antenna gain and half-power beam width are adopted, the observation capability is optimal when the pointing angle is 20.18 degrees, and is improved by 48.93 percent compared with the observation capability under the TDS-1 parameter combination. The TDS-1 downward view antenna can only receive GPS reflected signals and only has 4 receiving channels, while the current satellite-borne GNSS-R receiver can use a multimode system and the number of the signal channels can reach 16 or more. In order to fully utilize the received signal channel, the downward-looking antenna needs to be optimally designed to ensure that more GNSS reflected signals are captured and tracked. Increasing the signal channel is an effective way to improve the sea surface altimetry resolution of the GNSS-R satellite, and certainly, the higher thermal noise and power consumption caused by the channel increase should be balanced and considered.
Although the present invention has been described with reference to the preferred embodiments, it is not intended to limit the present invention, and those skilled in the art can make variations and modifications of the present invention without departing from the spirit and scope of the present invention by using the methods and technical contents disclosed above.
Those skilled in the art will appreciate that the invention may be practiced without these specific details.
Claims (8)
1. A method for increasing the number of sea surface reflection signals received by a GNSS-R altimetry satellite is characterized by comprising the following steps:
acquiring antenna parameter combinations of N groups of antenna gains and antenna pointing angles; wherein N is more than 250000;
acquiring position information of all specular reflection points calculated and output by a GPS satellite ephemeris and a TDS-1 satellite ephemeris within 24 hours, and determining the satellite altitude of each specular reflection point;
under the condition of each group of antenna parameter combination, according to the satellite altitude angle of each mirror reflection point, screening available mirror reflection points of all the mirror reflection points which are output by the calculation of the GPS satellite ephemeris and the TDS-1 satellite ephemeris within 24 hours to obtain the number of the available mirror reflection points which are respectively corresponding to each group of antenna parameter combination;
determining a group of antenna parameter combinations with the largest number of available specular reflection points as an antenna optimal parameter combination; and according to the antenna optimal parameter combination, parameter configuration is carried out on the GNSS-R height measurement satellite so as to improve the quantity of sea surface reflection signals received by the GNSS-R height measurement satellite.
2. The method of claim 1, wherein the available specular reflection points are selected by the method comprising the steps of:
performing primary screening on the specular reflection points according to the comparison result of the satellite height angle and the Brewster angle of the specular reflection points to obtain the specular reflection points after the primary screening;
performing secondary screening on the mirror reflection points subjected to the primary screening by combining a GNSS-R satellite-borne downward-looking antenna signal-to-noise ratio model GSNASNRM according to the satellite height angles of the mirror reflection points to obtain the mirror reflection points subjected to the secondary screening;
determining a satellite altitude angle maximum threshold and a satellite altitude angle minimum threshold corresponding to the current antenna parameter combination, and carrying out three-time screening on the mirror reflection points after the secondary screening according to the satellite altitude angle maximum threshold and the satellite altitude angle minimum threshold to obtain all available mirror reflection points corresponding to the current antenna parameter combination.
3. The method for increasing the number of sea surface reflection signals received by a GNSS-R altimetry satellite according to claim 2, wherein the step of performing a primary screening on the specular reflection points according to the result of comparing the satellite altitude angle of the specular reflection points with the Brewster angle to obtain the once-screened specular reflection points comprises:
determining a Brewster angle zeta according to the Fresnel reflection coefficient; the expression of the Fresnel reflection coefficient is shown as the following formula (1), and the satellite height angle corresponding to the zero point of the formula (1) is the Brewster angle zeta;
wherein the content of the first and second substances,
represents the fresnel reflection coefficient, the subscript LR represents left hand circular polarization → right hand circular polarization, and the subscript RL represents right hand circular polarization → left hand circular polarization; epsilon represents the dielectric constant of seawater; δ represents the local angle of incidence;
reserving specular reflection points satisfying the following formula (2) as the specular reflection points after the primary screening:
α＞ζ…(2)
where α denotes the satellite elevation angle of the specular reflection point.
4. The method of claim 3, further comprising:
according to the relative positions of the GNSS-R altimetric satellite and the GNSS satellite, the following formula (3) is determined:
wherein R isTPRepresenting the distance, R, of a GNSS satellite to a specular reflection pointPRRepresenting the distance from the GNSS-R altimetric satellite to the specular reflection point, RERepresents the radius of the earth, HTRepresenting the altitude, H, of a GNSS satelliteRRepresenting the altitude of the GNSS-R altimetric satellite;
solving equation (3) to obtain:
determining an expression for the signal-to-noise ratio that takes into account the thermal noise of the antenna:
wherein, YS(tau, f) represents a scattered signal power calculation function, tau represents the time delay of the GNSS satellite sea surface scattered signal, f represents the Doppler frequency of the GNSS satellite sea surface scattered signal, PNRepresenting the thermal noise power, P, of the antennaTRepresenting the GNSS satellite signal emission power, λ representing the GNSS satellite signal wavelength, TiRepresenting the coherent integration time, ASRepresenting the blazed area, p the position vector of the scattering point of the sea surface, GT(p) denotes the GNSS satellite transmit antenna gain, GR(rho) represents the gain of a GNSS-R altimetry satellite receiving antenna, delta tau (rho) represents the difference between the time delay of a scattering signal component of a sea surface scattering point and tau, and delta f (rho) represents the scattering signal component of the sea surface scattering pointThe difference between the Doppler frequency shift value of the quantity and f, wherein lambda represents the self-correlation function of the pseudo-random code of the GNSS satellite, S represents the Doppler frequency spectrum function, and sigma0(p) represents the scattering coefficient at the sea surface reflection point, RTP(p) represents the distance function of the GNSS satellite to the specular reflection point, RPR(ρ) represents a distance function from the GNSS-R altimetric satellite to the specular reflection point, K represents the Boltzmann constant, and T representsTemperature ofRepresenting equivalent noise temperature, BiRepresents the signal bandwidth;
and (5) substituting the formula (4) into the formula (5) to obtain a GNSS-R combined satellite-borne downward antenna signal-to-noise ratio model GSNASNRM.
5. The method for increasing the number of sea surface reflection signals received by a GNSS-R altimetry satellite according to claim 4, wherein the secondary screening is performed on the mirror reflection points after the primary screening according to the satellite altitude angle of the mirror reflection points and by combining a GNSS-R satellite-borne downward antenna signal-to-noise ratio model GSNANRM to obtain the mirror reflection points after the secondary screening, comprising:
substituting the satellite altitude corresponding to the specular reflection point after primary screening into a GNSS-R satellite-borne downward antenna signal-to-noise ratio model GSNASNRM to solve to obtain the signal-to-noise ratio corresponding to the specular reflection point after primary screening;
and taking the mirror reflection point with the signal-to-noise ratio larger than the signal-to-noise ratio threshold value in the mirror reflection point after the primary screening as the mirror reflection point after the secondary screening.
6. The method of claim 4, wherein the scattering coefficient σ at the sea surface reflection point is calculated by the following equation (6)0(ρ)：
q＝(q⊥，qz)＝k(n-m)…(7)
Wherein q represents a scattering vector, q⊥And q iszWater respectively representing a scattering vector qThe flat component and the z-axis component, P (-q)⊥/qz) Representing the joint probability density function of sea surface inclination, m representing the incident wave vector, n representing the scattered wave vector, and k representing the wave number.
7. The method of claim 6, wherein if the sea surface obeys a Gaussian distribution, in two dimensions, there are:
wherein Q isxRepresenting the component of the slope of the oblique direction downwind, QyRepresenting the component of the slope of the diagonal upwind, mssxRepresents the slope of the x-direction in an oblique direction, mssyRepresents the slope of the y-direction in an oblique direction, bx,yRepresenting the correlation coefficient between the two orthorhombic slope components in the x-direction and the y-direction.
8. The method for increasing the number of sea surface reflection signals received by a GNSS-R altimetry satellite according to claim 2, wherein a maximum satellite altitude angle threshold and a minimum satellite altitude angle threshold corresponding to a current antenna parameter combination are determined, and the mirror reflection points after the secondary screening are subjected to three-time screening according to the maximum satellite altitude angle threshold and the minimum satellite altitude angle threshold to obtain all available mirror reflection points corresponding to the current antenna parameter combination;
according to the current antenna parameter combination, determining an antenna pointing angle theta and a half-power beam width gamma of a horizontal plane corresponding to the current antenna parameter combination;
determining the satellite elevation angle minimum threshold α based on the relationship between the antenna pointing angle θ and the half-power beamwidth γ of the horizontal planeminAnd satellite altitude maximum threshold αmax：
When theta is 0 deg., αmin＝90°-γ/2，αmax＝90°
When theta is<At gamma/2, αmin＝90°-(θ+γ/2)，αmax＝90°
When theta is>At gamma/2, αmin＝90°-(θ+γ/2)，αmax＝90°-(θ-γ/2)
The satellite height angle of the mirror reflection point in the secondarily screened mirror reflection points is larger than the minimum threshold value α of the satellite height angleminα being smaller than the maximum threshold value of the satellite altitudemaxThe specular reflection points of (1) are used as the specular reflection points after the three-time screening.
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