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FAQ
1. Where is the BDS ICD downloaded?
A: At present, the authorities have released four interface control files of B1I, B1C, B2a and B3I. The China satellite navigation system management office is the competent department of the BeiDou satellite navigation system in China. Its portal website is www.beidou.gov.cn, the only official website of the BeiDou satellite navigation system, and the only official distribution channel of the BeiDou satellite navigation system interface control document (ICD).
As a professional research institute under the China satellite navigation system management office, we also provide backup and download functions on the website, but all the relevant information should be based on the official website of the "BeiDou satellite navigation system".
2. According to the BDS ICD document, the BDS-3 basic space constellation will contain 24 MEO satellites, 3 IGSO satellites and 3 GEO satellites, but the space constellations of the current operation have 5 IGSO satellites and 5 GEO. Will the number of those satellites be transferred to other uses?
A: BDS-2 has been providing formal operational services since the end of 2012. On-orbit satellites include five geostationary orbit (GEO) satellites, four medium-earth orbit (MEO) satellites, and five inclined geosynchronous orbit (IGSO) satellites.
According to the plan, the BeiDou system space constellation will gradually transition from BDS-2 to BDS-3. The final basic space constellation will include 3 GEO satellites, 24 MEO satellites and 3 IGSO satellites.
The original BDS-2 satellites (including the additional IGSO satellites and GEO satellites) will not be withdrawn immediately or transferred to other uses. Instead, they will be gradually retired as their life expectancy expires. In addition, 30 satellites are just the basic operating constellations of BDS-3. In the future, more satellites may be launched according to actual needs. The number of satellites actually operating in orbit may exceed 30.
3. Does the calculation of the near-point angle Ek use a time that is not relativistically correct or has the relativistic effect been corrected?
A:  It is suggested that Ek should use the time that the relativistic effect has been corrected. The specific number of iterations you can make depends on the actual situation of the project.
4. Is ionospheric model parameters, ephemeris parameters, and clock bias parameters updated at the same time?
A: According to the description of the BDS ICD document, the ionospheric model parameters updated by BeiDou navigation satellite are the same as ephemeris parameters.
For the same open service signal, the ionospheric parameters of the satellites are the same.
When the ephemeris parameter is updated, the ephemeris reference time toe will also change.
The clock error parameter and the ephemeris parameter update cycle coincide and will be updated together.
5. The reason for rotating the -5 degree around the X axis in the formula of GEO satellite coordinates?
A: In order to avoid the singularity of Kepler orbit parameters in calculating the position of the satellite at i=0, it is necessary to rotate the -5 degree around the X axis when carrying out the coordinate transformation.
For a detailed description of the issue, you can refer to the twenty-fourth edition of the journal "astronomy progress", volume first, "analysis of the accuracy of the mid and high orbit satellite ephemeris".
6. What is the error of BeiDou system's orbit signal at present and in the future?
A: According to the public service specification (1 edition) of the BeiDou satellite navigation system (1 edition) published on the BeiDou official network in December 27, 2013, the spatial signal precision (including track and satellite clock difference) is better than 2.5 meters (confidence level 95%).
7. How does the value of parameter SOW in D1 and D2 navigation messages change?
A: For D1 navigation message, the second time of the week's count (SOW) refers to the corresponding time of the first rising edge of the synchronization head. Since each sub frame in the D1 navigation message lasts 6 seconds, the SOW value of the two successive subframes is 6 seconds, and the count is incremented at intervals of 6 seconds.
For the D2 navigation message, the second time corresponding to the internal second count (SOW) is the corresponding time of the first pulse rising edge of the current main frame neutron frame 1 synchronization head. Since the length of each main frame in the D2 navigation message is 3s, the SOW value of the two consecutive main frames is 3 seconds difference and is incremented by 3 seconds interval. The SOW values of different sub frames in the same frame are the same.
8. Is the strength of the BeiDou satellite navigation system B1I and B3I the same?
A: Due to the different heights of different satellites, the bandwidths of different frequency signals are also different, resulting in different spatial transmission loss of satellite signals. The design of the BeiDou satellite signal transmission power takes this into consideration and ensures that the landing level of each signal tends to be uniform for user convenience.
When the satellite elevation angle is greater than 5 degrees and the receiver's right-hand circularly polarized antenna near the earth's surface is 0 dB in gain, the minimum guaranteed level of the I-branch of the navigation signal transmitted by the satellite to the receiver antenna output is -163 dBW. Due to the influence of the actual environment, the signal power of the B1I and B3I signals arriving at the receiver antenna will be slightly different, but the difference is not significant.
9. Are the ephemeris parameters and clock error parameters updated simultaneously in the navigation message of B1I and B3I signals? Is there a clear correspondence between the two?
A: Under normal circumstances, the BeiDou ephemeris parameters and clock error parameters are updated at the same time, the update cycle is 1 hours, and updated at BDT. A non integral point update will be performed only when an exception occurs and a new navigation message is injected. It can be judged by toe or TOC. The ephemeris parameters and clock difference parameters in the BeiDou navigation message have a corresponding relationship. They can be combined with toe, AODE, TOC and AODC, and refer to the related contents of the BDS ICD file.
10. According to the B2a signal ICD, the navigation message type 11 does not broadcast the IODE parameter. How can we tell which group ephemeris II is in the information type 11?
A: According to the definition of the "BDS-SIS-ICD-B2a-1.0" file 6.2.3 section, the broadcasting order of the B-CNAV2 message type 11 is strictly constrained: "the broadcasting order of the B-CNAV2 information type can be dynamically adjusted, but the information type 10 and the information type 11 remain on the back broadcast." That is to say, the information type 11 will continue to be broadcast continuously with the information type 10 to ensure that the two persons constitute the same set of ephemeris, and the set of ephemeris is identified by the IODE value in the ephemeris type 10.
11. According to the latest BDS ICD, we know that the B2I signal will be replaced by the B2a signal in the future. Can you tell me if it is possible for the schedule of the project and which satellites are currently transmitting the B2a and B1C signals?
A: According to the related definition of BDS ICD, BDS-2 satellite broadcasting B1I, B2I and B3I signals, BDS-3 satellite will retain the broadcast B1I and B3I signals, while the BDS-3 MEO/IGSO satellite will also broadcast B1C and B2a signals. Therefore, the B2I signal will be replaced by the B2a signal gradually with the decommissioning of the BDS-2, which requires a process. The B1C and B2a signals are a new generation of navigation signals, which will be broadcast on the BDS-3 MEO/IGSO satellite, and will be gradually put into use in the future with the launch of the BDS-3 satellite.
12. Can you further explain the BCH coding scheme for B1C signals?
A: Subframe 1 consists of 14 bits before encoding, 6 bits high for BCH (21, 6) encoding, and the lower 8 bits for BCH (51, 8) encoding. The encoded length is 72 symbols. Among them, the BCH(21,6) code uses 6 shift registers in Figure 6-2 (see section 6.2.2 of “BDS-SIS-ICD-B1C-1.0”), during the first 6 clock cycles, gate 1 Off, gate 2 is opened, the upper 6 bits of sub-frame 1 are input to the register from high to low, and the purpose is to set the 6-stage shift register to the initial value; after that, gate 1 is opened, gate 2 is closed, and 6 shift registers are performed. 21 shift operations to generate and output 21-bit code. BCH (51,8) uses 8-bit shift register, similar to the method, the lower 8 bits of sub-frame 1 use BCH (51,8) encoding to generate 51-bit code. Finally, the encoded upper 21 bits and lower 51 bits are combined to form 72 bits of subframe 1 encoded by BCH.

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