PPP/PPP-RTK open formats: Overview, comparison, and proposal for an interoperable message

2.1 PPP/PPP-RTK services viewed from the OSI model

A generic structure of open PPP/PPP-RTK services is represented in Figure 2 based on the OSI seven-layer model from top to bottom: application, presentation, session, transport, network, data-link, and physical. Some of the layers are grouped for the sake of simplicity.

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FIGURE 2

Structure of PPP/PPP-RTK services based on OSI layers model

2.2 Definition of corrections for PPP, PPP-AR, and PPP-RTK

Figure 3 shows the definitions of multiple range error sources in GNSS positioning and positioning methods such as PPP/PPP-AR, RTK, and PPP-RTK. The range error sources include satellite signal-in-space (SIS) errors such as orbit, clock and signal bias (phase and code), atmospheric delays such as regional or local dependent troposphere and ionospheric delays, and receiver errors such as multi-path and receiver bias (code and phase).

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FIGURE 3

Definition of range error sources and positioning methods

The conventional RTK provides code and phase correction to correct the satellite SIS error and the atmospheric delay. PPP provides a centimeter-to-decimeter level positioning solution by compensating for the satellite SIS errors such as orbit, clock, and signal code bias. PPP-AR provides a centimeter level positioning solution by correcting signal phase bias to resolve the integer ambiguity of the carrier phase.

In PPP/PPP-AR positioning, the ionospheric delay is eliminated by constructing the ionospheric-free observable combination, and the tropospheric delay is estimated at the receiver. PPP-RTK is able to provide position solutions at centimeter level with fast convergence time similar to RTK by providing atmospheric correction for each sub-network.

The required corrections for PPP, PPP-AR, and PPP-RTK are shown in Table 2. The error of satellite orbits and clocks in the broadcast ephemeris and the code bias originated by the group delay for each satellite signal is provided in PPP to correct the SIS error. The phase bias of each satellite signal originated by the phase-clock correction is also provided in PPP-AR for the ambiguity resolution (Laurichesse, 2014). The atmospheric corrections including slant total electron content (STEC) and tropospheric delay are also provided by PPP-RTK for faster convergence.

The atmospheric corrections in PPP-RTK are defined on the grid points in the service area. Figure 4 shows an example for QZSS CLAS with 212 grid points (Cabinet Office, 2021).

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FIGURE 4

The definition of grid points for QZSS CLAS

2.3 RTCM SSR and IGS SSR

The SSR format standardization started in 2007 at the RTCM SC-104. The standardization process consists of four stages as follows:

• Stage 1: The satellite orbit, clock corrections, and code biases for PPP

• Stage 2: The phase bias and the VTEC for PPP-AR and single-frequency PPP

• Stage 3: The atmospheric corrections for PPP-RTK

• Stage 4: Compression

Stage 1 was completed in 2011 for GPS and GLONASS. However, Stage 1 for other GNSSs and Stage 2 and beyond are still not completed.

RTCM SSR defines the orbit correction, clock correction, code bias, phase bias, and user range accuracy (URA) messages for each GNSS as shown in Table 3. The vertical total electron content (VTEC) proposed message includes the spherical harmonic term of the VTEC model.

The orbit correction includes the position and velocity term in range, along-track, and cross-track coordinates. The clock correction is represented as a second-order polynomial representing the bias, drift, and drift-rate terms. The high-rate clock correction includes only the bias term. The message types in parentheses are currently in the proposal phase and not completed yet.

The International GNSS Service (IGS) has published an open standard for their real-time service based on RTCM SSR (IGS, 2020). IGS SSR messages define the orbit correction, clock correction, code bias, phase bias, and URA for each GNSS and VTEC based on the content of the RTCM SSR Stages 1 and 2. The messages are identified in an RTCM-3-compatible format by a subtype of MT4076, as shown in Table 4.

2.4 CSSR

CSSR has been proposed in demand of an effective open format for PPP/PPP-RTK that is lighter than conventional SSR such as RTCM SSR and IGS SSR. CSSR is defined as a proprietary message of MT4073 in an RTCM-3-compatible format (Mitsubishi Electric, 2019), with these features:

• Compact representation optimized for the satellite-based narrow-band service

• Atmospheric corrections to support PPP-RTK

• Quality indicators to support the protection level estimation in the user equipment

CSSR supports multi-GNSS including modernized signals, and satellites and signal groups can be effectively selected using satellite and signal masks. CSSR is about 70% more bandwidth-effective than RTCM SSR for global PPP.

Although the primary target application of CSSR is QZSS CLAS, it has influenced other satellite-based and ground-based correction services. For example, Galileo HAS has taken into account CSSR fields as the baseline of the HAS message. Mobile communication applications in 3GPP also use CSSR in Rel.16 of LTE/5G standard (3GPP, 2020).

CSSR defines the messages shown in Table 5. To improve the effectiveness, the orbit includes only the position term, and the clock correction includes only the bias term. Whereas standalone messages define the format of individual states, composite messages improve efficiency by combining multiple types of states. CSSR includes the atmospheric correction including the slant TEC (STEC) and the tropospheric delay to support PPP-RTK.

The ionospheric delay (STEC) is represented as a two-layer model which consists of a functional part and a residual part. The tropospheric delay consists of a functional part containing a hydro-static term and a residual part containing a non-hydrostatic (wet) term.

The functional part is represented by a low-order function of the approximate user position, and the function type and coefficients are transmitted in the atmospheric correction message (Subtype 8 or 12). The residual part of atmospheric correction is defined on a grid, where the values should be defined by interpolating the values on the surrounding grid points at the user position. The residual part of STEC could be a large part of the stream in PPP-RTK service, the multiple set of parameters with different bit length and resolution are defined, and the parameters can be independently selected per satellite based on the condition of STEC irregularities to minimize the required data rate for the transmission of STEC residuals.

The grid coordinates for atmospheric corrections can be defined in the interface control document as in QZSS CLAS, or included in the data stream using the grid information message (Mitsubishi Electric, 2020). The grid information message is defined as a type of service information message, with a message structure compatible with 3GPP LPP (3GPP, 2020).

The stochastic estimation of errors in the correction messages is provided in CSSR messages. The signal-in-space errors are provided in the URA message (Subtype 7) while the errors in the atmospheric correction are provided in the atmospheric correction message (Subtype 8 or 12). An extension for correction message authentication using Subtype 13 is also proposed to prevent spoofing attacks (Hirokawa & Fujita, 2019).

The range of states is optimized for a centimeter-level positioning service. The service information is defined to send slowly changing data such as the service operation information, grid definition, and coordinate transformation parameters (Hirokawa, 2019). An open-source implementation of a CSSR-to-OSR converter is available in Motooka et al. (2019).

Satellites and signals are selected using a mask message in Subtype 1 as shown in Figure 5. The selected satellites in each GNSS are identified with the satellite mask. The selected signals for each GNSS are identified with the signal mask, and the selected signals for each satellite are identified with the signal mask and the cell mask.

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FIGURE 5

Concept of the satellite and signal selection in CSSR

For each GNSS, each of the 40 bits in the satellite mask define the enabled PRN number. The scheme is similar to RTCM multiple signal messages (MSM). The atmospheric correction including ionospheric delay and tropospheric delay is provided in a local area, defined as a subset of the whole service area. The specific area is identified by the area-ID, and the group of satellites for the specific area is selected by the network SV mask as a subset of the satellite mask.

As an example, in Figure 4, 10 satellites (7 GPS, 3 QZSS) were selected by the satellite mask, five signals (GPS: L1C/A, L2C, L5; QZSS: L1C/A, L5) were selected by the signal mask, and a total of 20 signals for all 10 satellites were selected by cell mask. Eight local satellites were selected for a specific area as a subset of the selected satellites using the network SV mask.

The cell mask is useful to reduce the amount of data for GNSS including multiple satellite generations and signals for GPS. Figure 6 shows the required data rate for the signal bias of 31 GPS satellites with and without the cell mask. The code bias is transmitted for PPP and both code and phase biases are transmitted for PPP-AR.

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FIGURE 6

Required data rate (BPS) for the signal bias of 31 GPS satellites including code and phase bias for two to five signals (L1 C/A, L2 P(Y), L2C, L5C, L1C) with a 30-second update rate

The GPS constellation currently includes 9 Block IIR, 7 Block IIR-M, 12 Block IIF, and 3 Block III satellites, at the time of writing this article. L1 C/A and L2 P(Y) will be available for all generations until the late 2020s. L2C, L5, and L1C are available from Blocks IIR-M, IIF, and III, respectively. The code and phase bias require 10 bits and 17 bits for each signal, respectively.

The required data rate is calculated for a 30-second update rate using the definition of CSSR (Mitsubishi Electric, 2019). The signal bias data rate can be reduced considerably (by 34.2%) with the cell mask. For GNSS with a unique satellite type like Galileo, the cell mask can be disabled by the cell-mask availability flag.

The Issue of Data State-Space Representation (IOD SSR) identifies the generation of the mask and the generation of ephemeris. If the selected set of satellites has changed or the ephemeris has been updated, then the IOD SSR should be incremented. The consistency between orbit, clock, bias, and atmospheric corrections is implicitly guaranteed based on RTCM SSR, and the system requires consistency to be maintained by applying the conditional update.

The group of satellites with ionospheric delay is represented as the network satellite mask which is defined as the subset of the satellite mask. The group of satellites and signals is identified as IOD SSR.

2.5 Galileo HAS

Galileo HAS defines for the moment a unified message type (MT1) based on CSSR fields. It includes mask, orbit, clock, code bias, phase bias, and URA as defined in Table 6. The availability of each component is defined by header flags. The group of satellites is selected by the satellite mask, and the signals are selected using the signal mask and the cell mask as defined in CSSR. The mask is identified by the mask ID, and the generation of ephemeris is identified by an IOD Set ID, which defines a single ID for the set of satellite IODs corrected in a given message.

The value range of phase bias and the resolution of orbit correction is slightly reduced to support the slow data rate of the Galileo HAS signal data channel (E6-B, 448 bps). Another feature of Galileo HAS is the clock subset section, which allows it to update the clocks of some satellites more frequently, in order to cope with the different performance of different satellite clocks. Notice also that the Galileo HAS Interface Control Document (ICD) in the public domain is only a testing version (European Commission et al., 2020) and parameters and ranges may slightly evolve in the near future.

The Galileo HAS message is very flexible but its performance has been reported according to the following configuration (Borio et al., 2020): two MT1 message types, MT1-orb and MT1-clk. The update interval of MT1-orb message was 120 s, including mask, orbit, code bias, and a URA section (which nevertheless will not be transmitted in the initial HAS service). The update interval of the MT1-clk message including clock was 10 s. MT1-orb and MT1-clk are 15 pages (6,360 bits) and two pages (848 bits) long, respectively, and contain correction data for up to 60 satellites with multiple signals.

2.6 Long-Term Evolution (LTE) Positioning Protocol (LPP)

Based on the high demand for high-accuracy GNSS positioning in mobile communications, 3GPP, the international standardization group for LTE/5G mobile communications, defined the message structures to support the high-accuracy GNSS positioning in LPP, as shown in Table 7. The satellite part of corrections such as orbit, clock, and code bias based on RTCM SSR are included in Rel.15 (Release 15 of the standard) for PPP support. The atmospheric correction, phase bias, and URA based on CSSR are added for PPP-RTK support in Rel.16 (3GPP, 2020).

Based on the use case of mobile communication users, GNSS-SSR-CorrectionPoints and the structure of the message to define the grid position are defined. The coordinates of the grid are defined by reference points and the relative coordinates of each grid point. Two types of definitions for the relative coordinates, equally spaced grid points with bitmask, and the list of grid coordinates are included. The equally spaced grid points are highly effective to cover a large area, and the bitmask can be used to represent the complex shape of land and coast area.

2.7 SPARTN

Safe position augmentation for real-time navigation (SPARTN) is an industry-driven open format that has been designed for wide and global area broadcasts. Its requirements for modern positioning systems include a combination of low bandwidth, accuracy, availability, reliability, and integrity for the safety of life applications (SAPCORDA, 2020; Vana et al., 2019).

While the majority of contents in SPARTN is compatible with RTCM SSR and CSSR, there are several differences in the parameters and the structures. The message structure of SPARTN is independent, as shown in Table 8. The satellite correction part is defined as a unified message Type 0 for orbit, clock, and bias (OCB) including the mask, orbit, clock, code bias, phase bias, and URA.

The OCB message is defined for each GNSS. Currently, subtypes for GPS and GLONASS are defined, and subtypes for other GNSSs are planned for the upcoming version. The satellite group and the signals are selected by bitmask as in CSSR, whereas the signal mask is included for each satellite but the cell mask is not included in the specification. The mask information is included in the header of the OCB message; therefore, it should be included in all OCB messages.

Atmospheric correction is defined as a unified message named the High-Precision Atmospheric Correction (HPAC). It includes the functional model part represented as polynomials and the residual part for the ionospheric delay (STEC) and tropospheric delay. The value range, affecting the bit length, can be selected per satellite to minimize the required data rate.

The Geographic Area Definition (GAD) message is defined to inform about the coordinates of the grid in the stream. GAD includes the reference point and the reference coordinates represented by the equally spaced grid points, whereas the bitmask is not supported in the GAD.

The Basic-Precision Atmospheric Correction (BPAC) message, including the coordinates of the grid and VTEC correction, is defined to support the single-frequency users. The tropospheric delay correction is not included in BPAC.

The Encryption and Authentication Support (EAS) message is defined to support the encrypted data stream and the grouped subscription for commercial correction services. The inconsistency for the ephemeris update can be explicitly avoided by including IODE continuity (3 bits) in the clock correction.

2.8 BeiDou PPP

BDS has provided a satellite-based PPP service for China and its neighboring areas since August 2020. The service defines multiple message types with a fixed length of 486 bits as shown in Table 9 (CSNO, 2020a; Liu et al., 2020). Currently, it supports corrections for GPS and BeiDou-3 satellites. The update intervals for clock and other corrections are 6 seconds and 48 seconds, respectively.

The definition of a satellite group for the clock correction message (Type 4) and the URA message (Type 5) is based on the mask message (Type 1). The mask message identified by IODP (Issue of Data, PRN mask) defines the selected group of satellites based on a 255-bit bitmask assigned for all GNSSs.

The index in full bitmask is used to select a group of satellites for the orbit correction message (Type 2) and the code bias message (Type 3). The signal mask and the cell mask are not used for the selection of signals. Two types of combined clock and orbit messages (Type 6 and 7) are also defined. The consistency between the orbit and the other corrections including the clock and the code bias messages is explicitly identified using the IOD of the correction (3 bits).

BDS also provides a ground-based augmentation service including a wide-area augmentation service, a network RTK service based on RTCM MSM, and a post-processing service for China and its neighboring areas (CSNO, 2020b). The wide-area augmentation service includes a single-frequency pseudorange augmentation service, a single-frequency carrier-phase augmentation service, and a dual-frequency carrier-phase augmentation service.

It uses an RTCM-3 format with some extensions as shown in Table 10. The message types surrounded by parentheses are non-standard messages in RTCM 3. The format for the single-frequency pseudorange augmentation service is MT1331, which includes ionospheric grid point (IGP) VTEC corrections.

The format for the single-frequency carrier-phase augmentation service is defined in MT1330, which is similar to the SSR Ionosphere VTEC Spherical Harmonics (MT1264) in RTCM 3. However, MT1330 supports a single layer only.

The dual-frequency carrier-phase augmentation service supports GPS and BeiDou using RTCM SSR whereas the special message types (MT1302, MT1303) are used for Bei-Dou support. It uses the combined orbit and clock messages to ensure the time consistency between the orbit correction and the clock correction.

2.9 Summary of characteristics of open formats

The structure and characteristics of the aforementioned open message formats (RTCM SSR, IGS SSR, CSSR, GAL, 3GPP LPP, SPARTN, and BDS-PPP) are summarized in Table 11. The values in parentheses indicate that a parameter is under definition and will be implemented in future versions.

The message framing of all formats except BDS-PPP uses variable-length encoding to have the flexibility for the number and type of satellites and signals. The message types of RTCM SSR, IGS SSR, and SPARTN are defined for each GNSS, whereas CSSR, GAL (Galileo HAS), and BDS-PPP define the unified message types for several GNSSs.

All formats support multiple GNSSs (G: GPS, R: GLONASS, E: Galileo, J: QZSS, S: SBAS, C: BDS), and there is room for extensions for new GNSSs. They also support corrections for orbit and clock, URA, and code bias for PPP.

All formats except BDS-PPP support phase biases for PPP-AR. CSSR, 3GPP LPP, and SPARTN support atmospheric corrections for PPP-RTK. RTCM SSR, IGS SSR, and SPARTN support VTEC (vertical TEC, total electron content) for single-frequency PPP.

The satellite mask included in CSSR, GAL, SPARTN, and BDS-PPP improves throughput efficiency. CSSR, GAL, and SPARTN also support signal masks for signal selection for each GNSS effectively. CSSR and GAL also support a cell mask for effective signal selection per satellite. CSSR, 3GPP LPP, and SPARTN define the message to include coordinates of the grid.

The issue of data (IOD) to identify the mask is defined in GAL, SPARTN, and BDS-PPP, and the issue of data to identify the consistency of corrections against the generation of ephemeris is included in GAL, SPARTN, and BDS-PPP. In CSSR, the mask and the consistency of ephemeris change are identified by IOD SSR.

The assigned bit length and the resolution for corrections such as orbit, clock, and signal bias are also shown in Table 11. The resolution of code (orbit+clock+code bias) and phase (orbit+clock+phase bias) for all formats are 2 cm (or less) and 2 mm (or less), respectively; they are sufficiently small to realize the positioning accuracy of PPP/PPP-RTK service.

3.1 Global PPP case

The parameters of the global PPP service are defined in Table 12. It supports GPS, Galileo, and BeiDou with corrections for two signals per satellite. The number of satellites for GPS, Galileo, and BeiDou are 32, 24, and 28, respectively. The target data rate is 500 bps, and the update intervals for clock and other states are 10 s and 30 s, respectively.

Figure 7 shows the required data rate (bps) for the global PPP service for RTCM SSR, IGS SSR, 3GPP LPP, CSSR, GAL, SPARTN, and BDS PPP. The required data rate for each format was calculated based on the bit assignment defined in the ICDs and the parameters shown in Table 11.

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FIGURE 7

Required data rate (bps) to transmit a global PPP service: 32 GPS + 24 Galileo + 28 BDS; two signals corrected per satellite; 10-second clock update rate; 30-second mask, orbit, bias, and URA update rate

The required data rate for CSSR (360.9 bps) is considerably smaller than that of RTCM SSR (846.3 bps) and IGS SSR (839.7 bps). The required data rate for LPP (452.6 bps) is smaller than that of RTCM SSR. This is because the orbit velocity term, clock drift, and drift-rate term are optional, and have been excluded.

The current data rate of GAL (346.1 bps) is slightly less (4.1%) than the original CSSR because of the unified structure and the reduced dynamic range of orbit correction. The data rate of SPARTN (487.5 bps) is larger than CSSR because it defines the message per GNSS; the unified OCB message includes the mask in the header part which should be included in all OCB messages.

The data rate of BDS PPP (599.4 bps) is larger than CSSR and SPARTN because the signal mask is not applied and fixed-length framing (486 bits) is used. The IOD correction is included in the orbit correction and the clock correction to ensure the consistency.

Figure 8 shows the required data rate (bps) of a global PPP-AR service for RTCM SSR, IGS SSR, LPP, CSSR, GAL, and SPARTN. The phase bias is included to support PPP-AR and 56 satellites of GPS and Galileo are selected, as shown in Table 13.

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FIGURE 8

Required data rate (BPS) for a global PPP-AR service (GPS + Galileo)

BDS-PPP is excluded because it doesn’t support the phase bias. The required data rate for each format was calculated based on the bit assignment defined in the ICD for each format and the parameters shown in Table 12.

The required data rate for CSSR (306.6 bps) is considerably smaller than that of RTCM SSR (738.7 bps) and IGS SSR (740.0 bps). The data rate of GAL (285.1 bps) is less (7.0%) than the original CSSR because of the reduced dynamic range of phase bias. The data rate of LPP (551.1 bps) is smaller than that of RTCM SSR because the orbit velocity term, clock drift, and drift-rate are options and the phase bias format is based on CSSR.

3.2 Nationwide PPP-RTK case

The parameters for a nationwide PPP-RTK service are defined in Table 14. The selected parameters for the total 240 grids are based on QZSS CLAS (Cabinet Office, 2021). It supports GPS (12 SVs), Galileo (10 SVs), and QZSS (3 SVs) with three signals per satellite. The target data rate is 2,000 bps and the update intervals for clock and other states are 5 s and 30 s, respectively.

The number of areas for the local atmospheric correction and the number of grid points per area are 12 and 20, respectively. For the functional part of the troposphere and the ionosphere (STEC), the linear and the bi-linear model are applied respectively. For the residual part of STEC, the minimum bit length (4 bits) is applied in CSSR and SPARTN by assuming a modest ionosphere condition.

Figure 8 shows the required data rate (bps) of the nationwide PPP-RTK service for 3GPP LPP, CSSR, and SPARTN. The required data rate for each format was calculated based on the bit assignment defined in the ICD for each format and the parameters shown in Table 13. The other formats do not support PPP-RTK.

The required data rate for LPP, CSSR, and SPARTN are 2,944 bps, 1,686 bps, and 1,809 bps, respectively. The data rate of LPP is considerably larger than that of CSSR because the orbit, clock, and code bias are based on RTCM SSR, and the bit length of the residual part of STEC is fixed as 7 bits.

Figure 9 also shows that the residual term of atmospheric correction consumes approximately 50% of the stream for CSSR and SPARTN. If the residual term could be excluded for the medium grade application having 10 cm to 30 cm positioning accuracy, the required data rate could be reduced considerably (1,059.0 bps for LPP, 816.1 bps for CSSR, and 940.3 bps for SPARTN).

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FIGURE 9

Required data rate (bps) for nationwide PPP-RTK service (GPS + Galileo + QZSS)

A generic estimation of data rate for nationwide PPP-RTK is shown in Figure 10. In this figure, the number of satellites and number of grid points are selected as the parameter. The service area S [km²] can be approximated by the following equation:

1

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FIGURE 10

Required data rate (bps) using CSSR for nationwide PPP-RTK service: the real field performance of CSSR applied for nationwide PPP-RTK is already shown in Hirokawa et al. (2019) with a positioning accuracy (95%) of 6 cm (horizontal) and 12 cm (vertical).

where N_grid is the total number of grid points, b is the interval of grid [km], and c is a ratio between short and long side of service area (c = 1 if the service area is square, 0<c<1 for others).

In Equation (1), and approximate the number of grid points for the long and short side, while and approximate the length of the long and short side, respectively. As an example, for QZSS CLAS with N_grid = 164 excluding the grid points nearby the small islands, b = 60 km and c = 0.11, while the service area is estimated as 439,705 km².

4.1 Satellite grouping

Multiple satellite groups can support satellites with a high-quality onboard atomic clock and satellites with less-accurate clocks simultaneously and optimally. Figure 11 shows the Allan variance of clock correction for GPS, GLONASS, Galileo, and BeiDou satellites on August 22, 2020, using the real-time stream of RETICLE by DLR (Hauschild, 2018).

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FIGURE 11

Allan variance of clock correction (RETICLE/DLR, August 22, 2020)

The Allan deviation of the clock correction of GPS Block II-F using Rubidium (Rb) clock, Block III satellites, and Galileo satellites are at the level of 1 cm at 100 s of averaging time, which has higher stability than that of other satellites. This suggests that the update interval could be longer to reduce the effective data rate.

Satellites can be grouped using sub-masks as shown in Figure 12 using CSSR. In the mask message (ST1) with extension, the sub-mask is defined only if the sub-mask availability flag is set to one (1). The sub-mask is identified by sub-mask ID. The sub-mask is defined per GNSS. It determines the subset of satellite in the previously defined satellite mask.

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FIGURE 12

Grouping of satellites using sub-mask

If it is applied for the global PPP-AR service defined in Table 12, the satellite sub-mask defines the group of GPS satellites equipped with lower stability of onboard atomic clock, which is assumed to be 18 GPS satellites in this case.

For example, as per Figure 9, clock corrections for satellites with highly stable atomic clocks are sent every 30 seconds, and clock corrections for the 18 GPS satellites with less stable atomic clocks are sent every 10 seconds. This approach, compared to refreshing all clock data every 10 seconds, reduces the required data rate for clock corrections from 87.7 bps to 49.7 bps.

4.2 Nationwide PPP-RTK service using IGSO/GEO satellite constellations

For a nationwide PPP-RTK service as defined in Table 13, a maximum of 25 satellites can be corrected at a data rate of 1,700 bps. If multiple satellites broadcasting the correction data can be tracked by the user receiver in a specific area, the effective data rate can be increased by mixing the multiple channels of these satellites.

For the proposed format based on CSSR, two or more different satellite groups can be supported simultaneously using the network SV mask in Figure 3. Figure 13 shows the number of visible GNSS satellites in Tokyo on August 22, 2020, with a 15-degree elevation mask. Up to 37 satellites are visible if GPS, Galileo, QZSS, GLONASS, and BeiDou-3 are supported.

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FIGURE 13

Number of visible satellites in Tokyo (August 22, 2020)

Figure 14 shows the elevation angle of QZSS satellites in Tokyo in which multiple QZSS satellites are visible with a 40-degree elevation angle. If 25 satellites with the highest elevation are assigned for the data channel of the highest elevation QZSS satellite, and the remaining satellites are assigned for the data channel of lower elevation QZSS satellites, the correction data for all visible satellites shown in Figure 12 can be sent.

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FIGURE 14

Elevation of QZSS satellites in Tokyo (August 22, 2020)