Analysis of Civil Navigation Message Format for GPS Modernization

Transcription

1 Analysis of Civil Navigation Message Format for GPS Modernization LU Songtao, ZHAO Yun, KOU Yanhong, HUANG Zhigang Electrical Information and Engineering School, Beihang University Beijing, China Abstract: This paper summarizes the characteristics of navigation message design for GPS modernization, and compares the differences about the utilization efficiency, channels coding, flexibility, subframe synchronization and data rate among the NAV, CNAV and CNAV-2 messages format, and then analyzes the compact and flexible navigation message structure design, which will impact the positioning accuracy of GPS receivers and the system update in the future. Finally, we present that the civil navigation message design will meet some challenges in the process of GPS modernization. The main idea of this paper is that the navigation data format in the trend will be more compact and flexible in the future. Keyword: GPS modernization; CNAV; message format; flexibility 1 Introduction The Global Positioning System (GPS) satellites have been transmitting navigation signals with NAV message format since the test program was started in the late 1970s. The Block IIR-M series is an initial upgrade of the navigation signals and message format, enabling the first broadcast of a transitional civil navigation (CNAV) message on the already transmitted second civil signal (L2C), which is an upgraded version of the original NAV navigation message and contains higher precision representation and more accurate data than the NAV data. The next generation of GPS system, GPS III, is going to have more navigation signals and message types than IIF [1, 2]. Navigation message structure design will play a more important role in navigation system design. Navigation message not only affects the accuracy of positioning, but also relates to system efficient operation. Many scholars focus on t the new navigation message study [3, 4], but rare of them to analyse the message format, which affects the performance of GNSS receiver greatly. This paper describes the design of the data format for the navigation messages. Then it analyses some improvements and advantages of new navigation message data format, which provides flexibility of format, control and content in GPS modernization, and compares their differences with NAV. 2 Navigation Message for GPS Modernization The proposed CNAV data message design replaces the use of frames and subframes, as in the current NAV data message, with a packetized Message-based communications protocol. The decision to dispense with a periodically repeating fixed format was motivated by the need to improve the ability of the system to accommodate new data contents, including ephemeris, almanac and differential correction etc. Control of the CNAV data message content has also been dramatically improved. Each operational satellite may transmit different data message content on L1, L2 and L5 channels, and at different data rates. Similarly, the data message content from different space satellites may differ. This flexibility permits system operators to configure the Space Segment (SS) in a variety of spacedivision or frequency-division modes to respond to a wide range of operational needs and circumstances. Instead of the current structure with frame/subframe architecture of data repeating in a fixed pattern, the CNAV structure which is used on L2C and L5 has individual message packets which can be broadcast in a flexible order with variable repeat cycles, and CNAV-2 structure which is used on L1C employs three different Table 1. Message modernization process Year Satellite L5 L2C L1/L1C 80s~90s Block IIA/R NAV NAV 1 st launch 2009 Block IIF CNAV CNAV NAV 1 st launch 2014 Block III CNAV CNAV CNAV-2 335

2 Figure 1. Comparison of structure for CNAV and CNAV-2 Note: RA=Reduced, DC=Differential Correction formats for each subframe with different length, including subframe 1 Time of Interval (TOI), compact subframe 2 and flexible subframe 3 which is commutated over multiple pages. The third civil signal will be added centred at the L5 frequency ( MHz), as part of the GPS modernization program. L5 and L2C will also carry CNAV messages and will broadcast by GPS IIF and III satellites. The CNAV structure, as defined in Interface Specification Document [5, 6], allows up to 63 different message types, of which 15 types have already been defined. The type of information (Time, Status, Ephemeris, and ) is still transmitted using the new CNAV format, and added some new types of information (EOP, differential correction, group delay correction) to the CNAV message content, it features a new pseudo-packetized format made up of 300-bit message packets. In CNAV, two of every four packets are ephemeris data and at least one of every four packets will include clock data, but the design allows for a wide variety of packets to be transmitted. The structure of the navigation message for L2C, CNAV, is basically same as that of the L5 signal. It is more compact and more flexible than that of the current NAV message, and allows the Control Segment to specify the sequence and timing of each message component. Since the data rate of the L2C signal is 25bps, each subframe requires 12 seconds to be transmitted. CNAV-2 [7] is broadcasted at L1C, each frame consists of 9 bits of TOI data-subframe 1600 bits of nonvariable clock and ephemeris data with cyclic redundancy check (CRC)-Subframe 2, and 274 bits of variable data with CRC-Subframe 3. The content of Subframe 3 nominally varies from one frame to the next and each Subframe 3 is also identified by a page number. The content of Subframe 2 is nominally invariant over a period of multiple frames lasting nominally 2 hours, allowing Subframe 2 symbols to be combined over multiple messages for demodulation at lower values of carrier power to noise density ratios (C/N0). 3 Advantages 3.1 Utilization efficiency With the comparison the message utilization efficiency between NAV, CNAV and CNAV-2, the reserved bits affect the message efficiency significantly, which is important to accuracy and time to access the clock and ephemeris data (CED). Utilization efficiency: Utilization Efficiency(%) Utilization Efficiency(%) % n m k n n 99.33% NAV 99.33% 63.39% 99.04% Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 5 Figure 2. Utilization efficiency of NAV Ephem eris Ephem eris Clock IONO Group Delay CNAV Type 10 Type 11 Type 30 Type 34 Type 37 Type 12 Type Clock Clock CDC DC Reduced Midi Figure 3. Utilization efficiency and part of content for message packets CNAV 336

3 100 CNAV-2 Table 2. Channel coding comparison Utilization Efficiency(%) Clock Ephemeris ITOW UTC GGTO EOP Reduced Sigal Phase Coding NAV CNAV Galileo CNAV-2 Hamming Yes No No No Convolution No Yes Yes No CRC No Yes Yes Yes Interleaving No No Yes Yes IONO Midi DC Subframe 2Page 1 Page 2 Page 3 Page 4 Page 5 Page 7 LDPC No No No Yes BCH No No No Yes Figure 4. Utilization efficiency and part of content for subframe and pages of CNAV-2 k Defined bits. n Total bits. m undefined bits (NAV including reserved bits and reserved bits for system use). The Figures 2 4 compute the utilization efficiency for three kinds of message format respectively and show that there is much redundancy at subframe 1 and 4 in NAV which wastes a lot of communication sources during broadcasting navigation data, however, the arrangement of parameters in CNAV and CNAV-2 is more compact than NAV, especially the content of ephemeris and clock data. And the average of the utilization efficiency among message packets is relative high for CNAV, but part of packets in CNAV-2 is still low now, because the CNAV-2 messages have not been designed completely yet. 3.2 Channel Coding Channel coding bits is another reason to determine the message efficiency. Each subframe in NAV is divided into 10 words of 30 bits each. Six bits in each word for parity lead to separate many ephemeris parameters with low coding efficiency. The CNAV and CNAV-2 format provides the improved error control schemes, including a modern parity algorithm and forward error control (FEC), such as convolutional encoding, CRC, interleaving and LDPC etc, which enhance the efficiency and improve performance in fading channels. And FEC with a rate 1/2 convolution code in CNAV, so while the navigation message is 25bps/50sps, a 50bps/100sps signal is transmitted. The L1C signal provides the first navigation signal using modern advanced FEC-LDPC (Low Density Parity Check). These codes whose parity-check matrix is very sparse approach the Shannon capacity limit of a communication system and operate at very low symbol signal-to-noise (SNR) ratios. LDPC added with interleaving in CNAV-2 will enhance the quality of message data and performance under the challenging channel such as may be applied in urban environments [8] and indoor circumstances. CRC in the last twenty four bits of message packets or subframe provides protection against burst as well as random errors with a probability of undetected error for all channel bit error probabilities 0.5. The L1C signal also employs a robust TOI encoding scheme, BCH (51, 8) linear code with a large minimum Hamming distance, to permit identification of the L1C data frames, which ensure the low bit-error probability in various fading channels with repetition of the fixed message [9]. In addition, sensitivity can be improved through code combining over identical TOI words received from different satellites. 3.3 Flexibility The flexible data format for navigation message design is the main way for GPS to interoperate with the other GNSS, while providing considerable flexibility to accommodate growth and changes in GPS operational needs. Galileo and GPS III, with similar timescales and aims, could be interoperable at every level. Due to GPS had leapt forward and was going to improve even more, transmission formats have now been mutually agreed and the systems will be compatible, both able to work 337

4 Table 3. Rate at different frequencies Message L5 L2 L1C NAV 50bps/50sps 50bps/50sps CNAV 50bps/100sps 25bps/50sps CNAV-2 50bps/100sps without causing interference to each other. Although there have been two independent constellations, but for the end-users they would have made every Galileo satellite as a GPS and every GPS satellite as a Galileo, a super-system with 60 satellites. There is a packet that contains a GPS-to-GNSS time offset. This allows for interoperability with other global time-transfer systems, such as Galileo and GLONASS, both of which are supported. L1C message format design allows the CED to be invariant over a known period of time to as CED interval which could be as short at five minutes or as long as lasting up to 2 hours. Within this CED does not change. The one of benefit is the ability for receiver to perform code combining to read the unchanging CED under stressed conditions and circumstances. The other benefit of the repeated invariant CED occurs when the receiver does not need to read CED, having received it out-ofband (OOB) or previously read it from the signal-inspace during the same CED interval. For example, the extra bandwidth enables the extraction of a packet for differential correction, which can be used to correct the L1 NAV clock and ionospheric data etc. The different message data rates at different frequencies could satisfy the receiver to keep tradeoffs between the robustness and quick access to CED. And flexible CNAV will provide more timely ephemeris and clock update. Every packet contains an alert flag, to be set if the satellite data can not be trusted, this means users will know within 6 seconds if a satellite is no longer usable, and indicate the User Range Accuracy (URA) and/or User Differential Range Accuracy (UDRA) may be worse than indicated in the respective message types. As civilian applications grow and new message types are defined, this is valuable for navigation service notifications including rapid notification of constellation health and configuration changes. Figure 5. CNAV and CNAV-2 message structure for synchronization 3.4 Subframe Synchronization The L5/L2C CNAV message types will be incrementally in over time, 300-bit long message including the preamble, satellite pseudorandom noise (PRN) number, message type ID, GPS time of week and a cyclic redundancy check parity block. The GPS week number is now represented as 13-bits, or 8192 weeks, and only repeats every years, meaning the next return to zero won't occur until the year This is longer compared to the L1 NAV message's use of a 10-bit week number, which returns to zero every 19.6 years. As previously mentioned, the GPS L5/L2C CNAV format significantly differs from that of GPS L1 NAV. This needs receiver to optimize algorithm for new subframe synchronization. The message configuration is highlighted in Figure 5. The features of interest for the synchronization are: 1) the preamble, 2) the PRN number, 3) the Z-count, and 4) the cyclic redundancy check. The preamble used on L5/L2C and L1 C/A are similar. Once it is detected in the packets stream, the synchronization algorithm checks that PRN number corresponds to the PRN of the satellite being tracked, if the examination of CRC is correct, the Z-count is increasing by one from subframe to the next. If any of these checks failed, the algorithm will be reset back to preamble detection mode. Once synchronization 338

5 is confirmed, the navigation parameters can be read. In terms of ephemeris and clock parameters, the subframes of interest are 10 and 11, and 30 to 36 respectively. But L1C CNAV-2 has a few differences from CNAV for synchronization. There is no preamble bits in CNAV-2 messages, so the end-user need to decode the BCH (51, 8) firstly by correlation algorithm to extract TOI for subframe synchronization and then utilize ITOW (interval time of week) instead of Z-count to get time information. The receiver can perform code combining of subframe 2 symbols over multiple messages to read clock and ephemeris at progressively lower values of C/N0. 4 Challenges With the rapid development of GPS modernization, data message format may meet some challenges inevitably. 4.1 Rate Data rate will result in the contradiction between the robustness and quick access to CED [10]. One side aspect of viewpoint is to design a more robust data message, which makes data more reliable and enable the receiver could tracking and data demodulation in challenging environments with low effective C/N0. But the other side of design goals needs to quick access to CED, which could increase the capacity for navigation-related message contents such as almanac updates, inter-signal corrections, and possible other used that provide improved accuracy, authentication, integrity and earlier use of a rising satellite. 4.2 Efficiency and Gains To get a low probability of error for TOI, ephemeris and almanac, the error correcting code must have a high Table 4. Comparison of efficiency and coding gain NAV subframe CNAV packet Galileo CNAV-2 subframe2 Bits Symbols Efficiency (%) Coding Gain(dB)* *SNR=10e-6, AWGN channel **log-likelihood decoding ** redundancy. The coding efficiency should be lower than 50% by 1/2 FEC. So, the more coding gain may lead to lower coding efficiency. As is shown in table IV, 1/2 rate LDPC performance is better than 1/2 rate convolutional coding with constraint length 7 on both aspects of efficiency and coding gain. 4.3 Complexity Due to the flexible CNAV design, there is no doubt to increase the complexity of hardware and software. Implementation of the GPS L5 and L1C navigation message decoding (including multiple types of decoding) is more complex than that of GPS L1 C/A as the symbol stream obtained, which needs to be converted to the actual navigation message data stream before any subframe synchronization can be attempted or any ephemeris parameter can be read. Furthermore, there are more additional complexities and computation burden for the next generation operational control segment (OCX) to control and operate efficiently. 5 Conclusions In conclusion, GPS is undergoing continual modernization and optimization, providing ongoing improvements for users worldwide. The powerful new data message format CNAV and CNAV-2 significantly improve key performance measures of the GPS data message, reduce inefficiencies which exist in the current format, and provide the flexibility to manage the GPS signal-inspace data contents to address a wide range of current and future operational needs. CNAV will be the next generation navigation message in a few years. Creative data message design and explicit analysis approach will make a great progress on comprehensive improvement of navigation message format. Various message format designs may satisfy and provide more specialized services for all kinds of consumers. References [1] Col Dave Madden, GPS Program Update to ION GNSS 2008, Global Positioning Systems Wing, 17 September [2] Ollie Luba, Larry Boyd, Art Gower, GPS III System Operations Concepts, IEEE Aerospace and Electronic System Magazine. Volume 20, Issue 1, Jan pp: [3] Chen Nan, Jia Xiaolin, Cui Xianqiang, Some Features of the 339

6 GPS Civil Navigation Message [J], GNSS World of China, 2006, 31(1):1 6. [4] Chen Nan, Performance Evaluation of the Structure of GNSS Navigation Message, Geometrics and Information Science of Wuhan University [J], May 2008, Vol. 33 No. 5. [5] NAVSTAR GPS Joint Program Office (2006), IS-GPS-200D with IRN-200d-001 Navstar GPS Space Segment/Navigation User Interfaces. [6] NAVSTAR GPS Joint Program Office (22 September 2005), IS- GPS-705, INR , Navstar GPS Space Segment/ Navigation User Interfaces. [7] NAVSTAR GPS Joint Program Office (19 April 2006), Draft IS-GPS-800, Navstar GPS Space Segment/User Segment L1C Interfaces. [8] Songtao Lu, Yun Zhao and Zhigang Huang, Geometrically Based Statistical Model for Multipath Propagation of Satellite to the Earth Channel in Urban Environment, the 8 th International Symposium on Antennas, Propagation and EM Theory (ISAPE2008), Kunming, China, November 2 5, 2008, pp [9] John W.Betz, Charles R.Cahn, Philip A.Dafesh etc, L1C Signal Design Options, ION NTM 2006, January 2006, Monterey, CA. [10] P. A. Dafesh, E. L. Vallés J. Hsu and D. J. Sklar etc, Data Message Performance for the Future L1C GPS Signal, ION GNSS 20 th International Technical Meeting of the Satellite Division, 25 28, September 2007, Fort Worth, TX. 340