=Paper=
{{Paper
|id=Vol-3919/short22
|storemode=property
|title=Seamless Real-Time Precise Point Positioning in Areas without Internet and with Internet
|pdfUrl=https://ceur-ws.org/Vol-3919/short22.pdf
|volume=Vol-3919
|authors=Xiaohan Wang,Zhetao Zhang,Hao Wang,Xinle Pei
|dblpUrl=https://dblp.org/rec/conf/ipin/WangZWP24
}}
==Seamless Real-Time Precise Point Positioning in Areas without Internet and with Internet==
https://ceur-ws.org/Vol-3919/short22.pdf
Seamless Real-Time Precise Point Positioning in Areas
without Internet and with Internet
Xiaohan Wang1, Zhetao Zhang1,∗, Hao Wang1 and Xinle Pei1
1
School of Earth Sciences and Engineering, Hohai University, Nanjing, 211100, China
Abstract
To reduce the cost of real-time high-precision positioning for users, international GNSS service (IGS) and
BDS-3 launched the real-time precise point positioning (PPP) service in 2013 and 2020, respectively. IGS
provides the services over the internet, while BDS-3 uses the B2b signal from the geostationary earth
orbit satellites to offer the service to China and its surrounding areas. IGS state space representation (SSR)
and PPP-B2b employ different means of communication and have pros and cons regarding positioning
performance and available range. Combining their advantages, this paper proposes a seamless real-time
PPP mode to cope with complex communication conditions. Specifically, the mode automatically switches
between SSR and B2b messages based on communication availability. When no enhanced message is
available, PPP is performed using the broadcast ephemeris (BRDC). First, static positioning experiments
are carried out using SSR messages, B2b messages, and BRDC to evaluate their positioning performance.
The results indicate that positioning using SSR messages is more accurate and has a shorter convergence
time than B2b. Afterward, different communication conditions are simulated, and the performance of the
proposed method is verified using a kinematic dataset. Compared to the traditional mode that relies solely
on SSR or B2b messages, the seamless PPP mode shows improvements in 3D root mean square error by
20.65% and 10.15%, respectively.
Keywords
GNSS, PPP, real-time, communication condition, SSR message, B2b message 1
1. Introduction
As the demand for high-precision positioning grows, positioning technologies based on global
navigation satellite systems (GNSS) are becoming more diverse, such as real-time kinematic (RTK)
positioning and precise point positioning (PPP) [1]. In recent years, PPP has made significant
progress, evolving towards real-time PPP and kinematic PPP applications [2]. To realize real-time
PPP, the international GNSS service (IGS) began to provide real-time service (RTS) in 2013, which
broadcasts real-time satellite orbit and clock corrections based on state space representation (SSR).
At present, some IGS analysis centers provide the products over the internet, such as centre
national d’etudes spatiales (CNE), Wuhan university (WHU), etc. Elsobeiey and Al-Harbi described
in detail how to use the SSR products [3]. Some scholars compared the performance of SSR
products generated by different analysis centers [4, 5, 6]. Li et al. evaluated real-time SSR products,
which indicates that the best-quality products are provided by CNE and WHU [6]. Hadas and Bosy
analyzed the accuracy of satellite orbit and clock corrections for GPS and GLONASS and proposed
a method for predicting corrections [7]. Overall, IGS provides real-time products with orbit
accuracy generally better than 5 cm and clock accuracy better than 0.2 ns, which can satisfy most
PPP users' positioning needs [4, 5, 6, 7, 8].
The SSR products provided by IGS analysis centers have been thoroughly researched and well
applied, but this service is completely dependent on the internet, severely limiting PPP availability.
To alleviate this situation, BDS-3, QZSS, and Galileo have launched satellite-based real-time PPP
services [9]. In 2020, BDS-3 started to broadcast satellite correction information in and around
Proceedings of the Work-in-Progress Papers at the 14th International Conference on Indoor Positioning and Indoor
Navigation (IPIN-WiP 2024), October 14 - 17, 2024, Hong kong, China
∗
Corresponding author.
xiaohan_wang@hhu.edu.cn (X. Wang); ztzhang@hhu.edu.cn (Z. Zhang); wang_hao@hhu.edu.cn (H. Wang);
xinlepei@hhu.edu.cn (X. Pei)
0000-0002-0565-2038 (Z. Zhang)
© 2024 Copyright for this paper by its authors. Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0).
CEUR
ceur-ws.org
Workshop ISSN 1613-0073
Proceedings
China through the B2b signal of the geostationary earth orbit (GEO) satellites [10]. In recent years,
evaluating the accuracy of B2b products and the performance of positioning with the products has
become a hot research topic [11, 12]. Studies have shown that the B2b signal provides orbit and
clock corrections with accuracy at the decimeter and centimeter levels, respectively [11]. Ouyang
et al. explored the characteristics of B2b messages and evaluated the positioning performance [12].
B2b messages can achieve centimeter-level and decimeter-level positioning accuracy in static and
kinematic modes, respectively [12, 13]. Moreover, to further enhance the performance of real-time
PPP or realize PPP with ambiguity resolution (PPP-AR), some scholars investigated the use of the
BDS short-message communication (SMC) service to convey more enhanced information [14, 15].
Despite correction products based on the B2b signal effectively alleviating the constraints
imposed by communication conditions on PPP, the RTS provided by IGS analysis centers still offers
advantages, such as update interval and product comprehensiveness. SSR corrections are usually
broadcasted with a 5-s interval, while orbit corrections in B2b messages are broadcasted with a 48-s
interval. [16]. Tao et al. carefully compared the accuracy of the products from the CNE and PPP-
B2b, and the results showed that the B2b message is more accurate for BDS-3 satellites, while the
SSR message provided by CNE is more accurate for GPS satellites [17]. In addition, the PPP-B2b
service currently supports GPS and BDS-3 satellites, while IGS SSR services typically also provide
orbit and clock corrections for GLONASS, Galileo, and BDS-2 satellites.
In summary, both B2b and SSR messages offer distinct advantages and limitations. However,
there is a paucity of research on their combined use. To maximize their respective benefits, this
paper proposes a seamless PPP mode. Firstly, the methods of using SSR and B2b products are
derived, and the seamless PPP mode positioning process is described in detail. Subsequently, static
positioning experiments analyze the effects of SSR messages, B2b messages, and broadcast
ephemeris (BRDC) on the results. Finally, a vehicle dataset is used to validate the performance of
the proposed method under challenging communication conditions.
2. Satellite-based Correction Models under Different Communication
Conditions
2.1. Correction Model Based on SSR Messages Transmitted over the Internet
The real-time SSR products are generated by IGS analysis centers and uploaded to the internet.
These products consist mainly of parameters for calculating satellite orbit and clock corrections.
The satellite orbit corrections are calculated as follows
���� ���� ���
���� = ���� + ��� � − �0 (1)
���� � ���� �0 ���
where ���� , ���� , and ���� represent the satellite orbit corrections in the radial, along-track, and
cross-track directions, ��� , ��� , and ��� the rate of change in the three directions, � and �0 the
current and reference epochs, respectively. Since the corrections provided by SSR and B2b
messages are based on the satellite-fixed coordinate system. To obtain the corrections in the earth
center earth fixed (ECEF) coordinate system, the transformation matrix � is computed as follows
� �� � ��
�= × (2)
� �� � ��
where � and � denote the satellite position and velocity vectors calculated using the BRDC. After
that, the real-time precise position of the satellite calculated using the orbit corrections can be
represented as follows [7]
���� �0 ����
���� = �0 − � ���� (3)
���� � �0 � ���� �
where ���� , ���� , and ���� denote the corrected satellite coordinates, �0 , �0 , and �0 the satellite
coordinates calculated from the BRDC. With the above algorithm, the accuracy of the satellite
position can be significantly improved by utilizing the orbit corrections in the SSR messages, i.e.,
���� , ���� , ���� , ��� , ���, and ��� .
The clock correction parameters in the SSR messages include three fitting coefficients at the
reference epoch, i.e., �0 , �1 , and �2 . The satellite clock correction at the current epoch can be
calculated as follows
�0 + �1 � − �0 + �2 � − �0 2
����� = ������ + (4)
�
where ����� denotes the corrected satellite clock offset, ������ the satellite clock offset calculated
from the BRDC, � the speed of light in a vacuum.
2.2. Correction Model Based on B2b Messages Transmitted over the Satellite
Telegram
Similar to the SSR messages, B2b messages broadcasted by the BDS-3 GEO satellites also provide
correction parameters of satellite orbit and clock offset for BDS-3 and GPS. Furthermore, the B2b
message aligns with the CNAV1 message of BDS-3 and the LNAV message of GPS through the
issue of data (IOD). The satellite position calculated using the BRDC can be corrected as follows [16]
��2� �0 ����
��2� = �0 − � ���� (5)
��2� �0 ����
where ��2� , ��2� , and ��2� denote the corrected satellite coordinates by B2b messages; ���� , ���� ,
and ���� express the satellite position corrections provided by B2b messages in the radial, along-
track, and cross-track directions.
The satellite clock correction parameters provided by B2b messages are ��2� . Afterward, the
satellite clock offset can be corrected by
��2�
���2� = ������ − (6)
�
where ���2� denote the corrected satellite clock offset by B2b messages. The algorithm described
above enhances the accuracy and convergence time of real-time PPP in and around China by
utilizing the orbit and clock corrections provided in the B2b messages, i.e., ���� , ���� , ���� , and
��2� .
2.3. Correction Model Combining SSR and B2b Messages
In this section, a seamless PPP mode is proposed to enhance real-time PPP positioning performance
in challenging communication environments. Based on current communication conditions, this
mode integrates SSR and B2b messages for real-time PPP. Figure 1 shows the flowchart of the
seamless PPP mode. The steps filled with blue, yellow, and green denote the input, judgment, and,
processing respectively. The entire process can be divided into two main parts: determining
communication conditions and using enhanced messages, which are inside the red and purple
dotted boxes, respectively. The specific steps are as follows.
Upon receiving real-time GNSS data, the satellite positions and clock offsets are calculated
through the BRDC. For instance, the ping command and ConnectivityManager can be used on
Linux and Android devices respectively to obtain real-time network connection status. If internet
connectivity is stable, the latest SSR messages are obtained, and the delay time is usually no more
than the update interval, i.e., 5 s. After that, the satellite-end errors are corrected by (1)-(4) at the
current epoch. This process is generally used in densely populated areas with well-developed
infrastructure, such as cities.
When the device cannot connect to the network or SSR messages are frequently interrupted,
satellite telegraph communication serves as an alternative. Here, it refers to various satellite-based
communication services, such as PPP-B2b, BDS-3 SMC, Galileo high accuracy service (HAS), etc.
Considering the cost and availability of these services, only PPP-B2b is used in this paper. Since
PPP-B2b currently provides corrections only for GPS and BDS-3 satellites, if the receiver tracks
these satellites, B2b messages are received and decoded, and the correction program is run
according to (5) and (6); otherwise, satellite positions and clock offsets calculated from the BRDC
are used for positioning. This positioning procedure is usually used in the wild or where
communication facilities are inadequate.
In the worst case, SSR and B2b messages are unavailable, PPP can only be performed using the
satellite position and clock offset calculated from the BRDC, i.e., �0 �0 �0 T and ������ . Under
these circumstances, a well-observed environment is required to achieve convergence of the
positioning results, and the results will be worse for shaded scenes. This flow is suitable for areas
where neither ground communication base stations nor the B2b signal is available. However, other
communication services can compensate if possible, such as BDS-3 SMC, HAS, Starlink, etc. As the
user moves between these scenarios, every epoch reassesses the communication conditions to
execute the optimal processing flow. The SSR messages, B2b messages, and BRDC seamlessly
switch in real time and finally get seamless PPP solutions.
Figure 1: Flowchart of the seamless PPP mode.
3. Measurement Campaign
Representative static and kinematic experiments were conducted in this study. The GPS and BDS-3
dual-frequency observations (GPS: L1, L2 and BDS-3: B1I, B3I) were collected with a sampling
interval of 1 s. For the static experiment, to evaluate the effect of different ephemerides on
positioning accuracy and convergence time, a 12-h static dataset on DOY 151, 2023, is used.
For the kinematic experiment, to verify the feasibility of the seamless PPP mode under harsh
communication conditions, a 1-s dual-frequency vehicle dataset was collected from P5
manufactured by CHCNAV on DOY 251, 2023. The duration is 5 min and 7 s, and the mean velocity
is approximately 27.18 km/h. The kinematic experiment site is located in the urban area of Nanjing,
China. As illustrated in Figure 3, the experimental area comprises both open and blocked scenes.
The yellow boxes indicate locations where the signal is heavily obstructed and reflected by trees
and high-rise buildings.
The kinematic dataset is divided equally into three segments to simulate challenging
communication conditions. They can be categorized as both internet and satellite telegraph at 0-
102 epochs, only satellite telegraph at 103-204 epochs, and neither internet nor satellite telegraph at
205-306 epochs. Table 1 describes the ephemerides chosen for use by four methods. Method A uses
ephemeris without any enhanced products, i.e., BRDC; methods B and C use improved ephemeris
by SSR and B2b messages, respectively; and method D seamlessly switches between SSR and B2b
messages depending on the communication conditions, i.e., the seamless PPP mode. In addition,
BRDC, SSR messages, and B2b messages were saved in advance. In this study, the SSR messages
used are generated by the CNE.
Table 1
Descriptions of Different Methods Based on the Simulated Communication Conditions
Ephemeris used at different epochs
Method
0-102 103-204 205-306
A BRDC
B SSR BRDC
C B2b BRDC
D SSR B2b BRDC
4. Experimental Tested and Result Analysis
4.1. Experiments with Static Datasets
The 12-h high-end receiver dataset was used, collected in a slightly obstructed (southeast)
observation circumstance. The number of available satellites with dual frequency observations
(GPS: L1, L2 and BDS-3: B1I, B3I) is approximately 15-20, and their geometric dilution precision
(GDOP) is stabilized at 1.5 to 2 when the elevation mask is set to 10 degrees.
Figure 2 shows the static PPP positioning errors in east-west (E-W), north-south (N-S), and up-
down (U-D) directions using the BRDC, SSR messages, and B2b messages, respectively. It can be
seen that the positioning results of the U-D and E-W directions converge slowly, and its
positioning error curves fluctuate greatly, especially for the BRDC. It can be clearly seen that the
positioning errors and convergence time using SSR and B2b messages are significantly better than
those using BRDC. Comparing Figure 2 (b) and (c), it can be noticed that the convergence time
using SSR messages is shorter than those using B2b messages. This is mainly due to the higher
update frequency of SSR messages transmitted via the internet. The satellite orbit and clock
corrections in the SSR messages by CNE are updated with intervals of 5 s. However, in the B2b
messages, they are updated with intervals of 48 s and 6 s.
Figure 2 (a) shows that the positioning errors using BRDC have long-term fluctuations at the
decimeter level in the U-D direction. Therefore, we defined the convergence criteria of static PPP to
be that the positioning errors in the E-W and N-S directions are continuously better than 0.1 m for
at least 600 epochs. In order to accurately assess the impact of the three ephemerides on
positioning results, Table 2 lists the root mean square error (RMSE) values of all static tests and the
horizontal, i.e., E-W and N-S directions, convergence time. It can be seen that the convergence time
using BRDC is up to 3.5 h. Compared to the BRDC, the convergence time of the SSR and B2b is
shortened by 58.51 and 44.32%, respectively. Moreover, the use of satellite orbit and clock
corrections has also greatly improved positioning accuracy. Compared with the BRDC, the 3D
RMSE of the SSR and B2b are reduced by 35.78 and 23.48%, respectively.
In short, PPP works best with SSR messages, followed by B2b messages. Furthermore, the SSR
messages additionally provide corrections for GLONASS, Galileo, and BDS-2 satellites, thus
positioning results based on SSR messages are superior when multi-GNSS observations are used.
As a result, when both SSR and B2b messages are available, SSR messages are preferred.
Figure 2: Static PPP positioning errors in E (blue), N (green), and U (red) directions by using the
BRDC (a), SSR messages (b), and B2b messages (c), respectively.
Table 2
Static PPP Positioning Accuracy and Convergence Time Using the Three Ephemeris
RMSE (m) Convergence
Ephemeris 3D (m) time (min)
E-W N-S U-D
BRDC 0.361 0.117 0.498 0.626 211.083
SSR 0.115 0.028 0.384 0.402 87.583
B2b 0.193 0.090 0.428 0.479 117.533
4.2. Experiments with Kinematic Datasets
Considering the distribution of ground communication base stations, the communication
conditions are related to the user's location, so the seamless PPP mode is mainly applied to
kinematic scenarios. In this section, the performance of the seamless PPP mode is carefully
analyzed and verified. Based on the four methods listed in Table 1, a seamless PPP experiment
with/without the internet is carried out. Figure 3 illustrates the scenario of the experiment site. It
can be seen that there are dense buildings and trees around the trajectory, and signals are easily
obscured and reflected.
The number of available satellites and the processing of satellite-end errors directly affect the
accuracy of PPP. Figure 4 presents the number of satellites corrected using enhanced messages for
methods B, C, and D at 0-204 epochs. The receiver is unable to acquire any enhanced messages
after 204 epochs, thus the number of satellites corrected by the three methods is 0. Similarly, the
internet is unavailable after epoch 102, so the number of satellites corrected by method B is also 0.
Statistical analysis of the results in Figure 4 reveals that the mean values of the number of satellites
corrected by methods B, C, and D are 5.04, 9.47, and 9.67, respectively. Since SSR messages have
better completeness than B2b messages, the seamless PPP mode, i.e., method D, has more satellites
with corrected errors than method C.
An analysis combining Figures 3 and 4 finds that the number of available satellites plummets to
5-8 in the two heavily obscured segments, i.e., the area inside the yellow rectangle in Figure 3,
which causes a drastic reduction in the accuracy of kinematic PPP. However, the number of
corrected satellites remained above 10 most often.
Figure 3: Description of kinematic experimental scenarios and trajectory of method A. The yellow
boxes denote areas with severe obstruction.
Figure 4: Number of corrected satellites for the methods B (grey), C (blue), and D (pink).
To obtain the high-precision reference values, the tight integration of short-baseline RTK and
inertial navigation system is applied. The reference station is fixed on the roof of a nearby building
with an open sky, and the baseline length is within 1 km throughout the kinematic experiment. To
verify the performance of the seamless PPP mode in terms of positioning, Figure 5 (a), (b), (c), and
(d) show the positioning results of the methods A, B, C, and D, respectively. The positioning errors
in the E-W, N-S, and U-D directions are denoted by blue, green, and red points, respectively. The
re-convergences of the positioning results for all four methods can be attributed to complex
environments, such as canyon or tree-shaded environments, especially in the U-D direction. A
closer look at Figure 5 finds that the positioning results fluctuate dramatically at 55-85 and 222-251
epochs. Combining Figure 5 with Figures 4 and 3 reveals that at these times, the vehicle is heavily
shaded, resulting in a reduction in the number of available satellites. Since methods A and B use
BRDC for a long time, the results shown in Figure 5 (a) and Figure 5 (b) are less accurate and
contain slight bias. Method D fully uses the communication network to obtain the satellite-based
corrections, and the horizontal positioning accuracy reaches 1.40 m. According to the
communication conditions, methods A, B, C, and D all use BRDC at 204-306 epochs, which caused
significant fluctuations in the positioning accuracy in this part.
Table 3 shows the RMSE values of the four methods and the 3D RMSE improvement rates of
methods B, C, and D compared to method A. Due to the complexity of the obscuration situations
and communication conditions, the positioning accuracy in this experiment is slightly lower than
the kinematic PPP in normal scenes. The 3D RMSE values for methods A, B, C, and D are
calculated, which are 3.51, 3.06, 2.70, and 2.43 m, respectively. It can be seen that the proposed
method D has the most significant 3D RMSE improvement compared to method A, i.e., the BRDC-
only method, which is 30.88%. In addition, method D showed an improvement of 20.65 and 10.15%
compared to methods B and C with only SSR or B2b messages, respectively. The above results
show that the proposed method D fully utilizes the communication network to improve positioning
accuracy under complex communication conditions.
Figure 5: Kinematic PPP positioning errors in E (blue), N (green), and U (red) directions by using
the methods A (a), B (b), C (c), and D (d), respectively.
Table 3
Kinematic PPP Positioning Accuracy and Improvement Rates Using the Four Methods
RMSE (m) Improvement
Method 3D (m) rate (%)
E-W N-S U-D
A 1.531 1.854 2.555 3.509 N/A
B 1.275 1.309 2.450 3.056 12.885
C 1.122 1.133 2.178 2.699 23.063
D 0.978 1.010 1.976 2.425 30.878
5. Conclusion
This paper proposes a seamless PPP mode to alleviate the limitations of communication conditions.
Specifically, SSR messages are used in areas covered by the internet; B2b messages are employed
when the user moves to an area with poor ground communications; and BRDC is employed for
positioning if the communication conditions continue to deteriorate, such as in the wilderness
beyond the scope of PPP-B2b service. The static experiments show that positioning accuracy and
convergence time are superior when using SSR messages compared to BRDC and B2b messages.
Then, the typical kinematic experiments under challenging communication conditions show that
compared with the traditional mode using only SSR or B2b messages, the 3D RMSE of the seamless
real-time PPP mode is improved by 20.65 and 10.15%, respectively.
In future work, real-time PPP based on the BDS-3 SMC service will be used to improve the
BRDC PPP. The PPP-B2b service cannot cover the whole world, so the HAS service will be
integrated into the seamless PPP mode.
Acknowledgments
This study is funded by the National Natural Science Foundation of China (42374014, 42004014),
and State Key Laboratory of Geo-Information Engineering and Key Laboratory of Surveying and
Mapping Science and Geospatial Information Technology of MNR, CASM (2024-01-07).
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