# Initial Assessment of the LEO Based Navigation Signal Augmentation System from Luojia-1A Satellite

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

^{−10}. The pseudorange and carrier phase measurement noise evaluated from the geometry-free combination is about 3.3 m and 1.8 cm. Overall, the Luojia-1A navigation augmentation system is capable of providing useable LEO navigation augmentation signals with the empirical user equivalent ranging error (UERE) no worse than 3.6 m, which can be integrated with existing GNSS to improve the real-time navigation performance.

## 1. Introduction

## 2. The Principle of a LEO Navigation Augmentation System

^{−12}[44]. The daily stability of the Beidou satellite reaches 10

^{−13}~10

^{−14}[45,46]. These high-performance atomic clocks are often very large, heavy, and expensive, so they are not suitable for most micro/nano-satellite platforms. An alternative solution for the navigation system is equipping atomic clocks on the ground facilities rather than the space segment, such as the Chinese Area Positioning System (CAPS), but this method is only suitable for these GEO and inclined geo-synchronized orbit (IGSO) satellites [47,48]. Due to cost/size/weight constraints, the Luojia-1A satellite has to employ an oven-controlled oscillator (OCXO) rather than the high-performance atomic clock. The temperature control system of the satellite platform is used to ensure the temperature condition of the onboard OCXO. The short-term stable oscillator generates low phase noise, short-term stable impulses in order to maintain a sufficiently high-quality carrier phase generation. The oscillator is disciplined by the GPS/Beidou signals in-orbit, which calibrates the long-term frequency drift and maintains the frequency accuracy. With the in-orbit discipline technique, the time system of the Luojia-1A satellite is aligned to the GPS time or Beidou time at the level of tens of nanoseconds, which is satisfactory from the performance and cost perspectives.

## 3. Ground Assessment of the Navigation Augmentation Signals

^{−10}. Applying a high-quality oscillator at both the transmitter and receiver side is to ensure the clock frequencies are stable over the experiment period. For the ground ranging signals, the atmosphere delay is negligible, so the ranging observations can be expressed as:

## 4. In-Orbit Performance Evaluation Results

#### 4.1. In-Orbit Evaluation of the Orbit and Clock

^{−10}. The clock assessment results are presented in the left panel of Figure 7. The figure indicates that the clock equipped on Luojia-1A presents different characteristics with the navigation satellite clocks. The in-orbit stability of the oscillator reaches 3 × 10

^{−10}, which is achieved at $\tau =70$ s. The frequency drift results in a quadratic form in the clock bias. Fortunately, the frequency drift does not affect the short-term clock stability and the signals tracking.

^{−10}. Since the relativistic effect on the oscillator is not compensated in Luojia-1A, so the frequency drift is likely to be caused by the relativistic effect. The impact of the relativistic effect (sum of general and special relativistic) for Luojia-1A frequencies is around 2.5 × 10

^{−10}, so whether there is remaining frequency drift need to be further verified. The figure indicates that the time series is fairly stable, so the frequency drift can be treated as a constant, which can be calibrated in the data processing stage.

#### 4.2. Navigation Augmentation Signal Noise Evaluation

_{1}and P

_{2}are independent, then the standard deviation of the pseudorange measurement is 3.3 m (1σ). The observation noise presents strong correlation as the elevation angle The GF combination of the carrier phase measurement is presented in the lower panel of Figure 11, which indicates that the standard deviation is 1.8 cm. Due to large acceleration in the line-of-sight direction (up to 77 m/s

^{2}) between the LEO and receiver, it is difficult to balance the sensitivity and the tracking loop precision. We still attempt to optimize the tracking loop parameters of our ground receiver to obtain better accuracy.

#### 4.3. Handling Error Sources of the Ranging Measurements

- The satellite hardware delay bias. Since the time system of the Luojia-1A satellite is aligned to the GPS time. The onboard GNSS receiver can only provide the time of receiving, rather than transmitting. The time consumption of baseband processing, position/velocity/timing computation, the ranging signal generation and the circuit delays all contribute to the hardware delays. Although some of the delays are compensated based on laboratory calibration, it still varies due to the complex space environment and temperature variation. Therefore, how to calibrate the hardware biases of the transmitter is challenging.
- The relativistic effect. Due to low orbit height, the relativistic effect calculation method for the medium earth orbiters and GEOs is not directly applicable to the LEOs since the Earth cannot be treated as a mass point in this case. For the LEO, the J2 term of the gravity field has to be considered during the relativistic correction calculation [67,68]. The average term of relativistic effect is not compensated in the ground test, so a new relativistic model should be developed for LEO navigation augmentation satellites.
- The satellite phase center offset. For the GNSS satellites, the PCO refers to the phase center offset with respect to the mass center. For Luojia-1A satellite, it refers to the offset between the −z receiving antenna and the transmitting antenna installed on the +z side. The offset for Luojia-1A is a few decimeters. The transmitting beam angle of Luojia-1A satellite is also not the same as the GNSS. For the MEOs, the beam width is 14.6° to cover the earth [69]. While the beam width has to be up to 60° for Luojia-1A satellite to achieves maximum service coverage. Hence the satellite PCV also has different characteristics.
- The ionosphere delay of the ranging signals. Luojia-1A satellite is moving in the F2 layer of the ionosphere, while most GNSS satellites are located above the ionosphere. Hence the empirical ionosphere delay model based on GPS observations, such as the Klobuchar model may not work well for the Luojia-1A ranging signals.

## 5. Conclusions and Future Work

^{−10}and clock drift is observed during the in-orbit test. As a low-cost oscillator, its stability meets the expectation. The precision of the observed pseudorange measurement is assessed with the time differenced approach, and the precision of the navigation augmentation signals reaches 3.3 m for the H1 signal. With the dual-frequency measurement, the precision of the geometry-free combination of the pseudorange and carrier phase measurement reaches 1.8 cm. However, the pseudorange measurements suffer from severe multipath impact, which needs to be improved in the future. Finally, the correctness of the navigation augmentation is validated by calculating the residuals from the empirical models. After removing the error sources, the residuals of the augmentation signals are stable over a long period. After correcting the error sources with the empirical models, the precision of the pseudorange residuals is around 3.6 m. However, the remaining pseudorange residuals still present elevation-dependent biases, which need to be further studied. The precision of the carrier phase is limited by the clock phase noise of the ground receiver, which attained about 22 cm accuracy in the current stage. With a higher quality clock, centimeter-level accuracy carrier phase measurements can be expected. Generally, the assessment results verified that the methodology of the Luojia-1A satellite’s navigation augmentation system is practically feasible and the augmentation signals from Luojia-1A is useable. However, there is still a great deal of work to do in the future, including improving ground receiver performance, modeling the time synchronization errors, refining the error source models, and carrying out more experiments to investigate the benefit of combining GNSS/LEO for positioning and navigation applications. With better understanding of the LEO navigation augmentation signals, higher precision and better performance can be expected.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

- Li, X.; Ge, M.; Dai, X.; Ren, X.; Fritsche, M.; Wickert, J.; Schuh, H. Accuracy and reliability of multi-GNSS real-time precise positioning: GPS, GLONASS, BeiDou, and Galileo. J. Geod.
**2015**, 89, 607–635. [Google Scholar] [CrossRef][Green Version] - Liu, T.; Yuan, Y.; Zhang, B.; Wang, N.; Tan, B.; Chen, Y. Multi-GNSS precise point positioning (MGPPP) using raw observations. J. Geod.
**2017**, 91, 253–268. [Google Scholar] [CrossRef] - Choy, S.; Bisnath, S.; Rizos, C. Uncovering common misconceptions in GNSS Precise Point Positioning and its future prospect. GPS Solut.
**2017**, 21, 13–22. [Google Scholar] [CrossRef] - Collins, J.P. Isolating and estimating undifferenced GPS integer ambiguities. In Proceedings of the 2008 National Technical Meeting of the Institute of Navigation, San Diego, CA, USA, 28–30 January 2008; pp. 720–732. [Google Scholar]
- Ge, M.; Gendt, G.; Rothacher, M.; Shi, C.; Liu, J. Resolution of GPS carrier-phase ambiguities in Precise Point Positioning (PPP) with daily observations. J. Geod.
**2008**, 82, 389–399. [Google Scholar] [CrossRef] - Laurichesse, D.; Mercier, F.; Berthias, J.P. Real time precise GPS constellation orbits and clocks estimation using zero-difference integer ambiguity fixing. In Proceedings of the 2009 International Technical Meeting of the Institute of Navigation (ION GNSS 2009), Savannah, GA, USA, 22–25 September 2009; pp. 664–672. [Google Scholar]
- Geng, J.; Meng, X.; Dodson, A.H.; Ge, M.; Teferle, F.N. Rapid re-convergences to ambiguity-fixed solutions in precise point positioning. J. Geod.
**2010**, 84, 705–714. [Google Scholar] [CrossRef][Green Version] - Zhang, X.; Li, X. Instantaneous re-initialization in real-time kinematic PPP with cycle slip fixing. GPS Solut.
**2011**, 16, 315–327. [Google Scholar] [CrossRef] - Banville, S. Improved Convergence for GNSS Precise Point Positioning. Ph.D. Thesis, University of New Brunswick, Saint John, NB, Canada, 2014. [Google Scholar]
- Kerner, S.M. Chinese Unicorn Team Hacks GPS at DefCon. Available online: http://www.eweek.com/security/chinese-unicorn-team-hacks-gps-at-defcon (accessed on 30 September 2018).
- Kube, C. Russia Has Figured out How to Jam U.S. Drones in Syria, Officials Say. Available online: https://www.nbcnews.com/news/military/russia-has-figured-out-how-jam-u-s-drones-syria-n863931 (accessed on 30 September 2018).
- Yang, Y. Concepts of Comprehensive PNT and Related Key Technologies. Acta Geod. Cartogr. Sin.
**2016**, 45, 505–510. [Google Scholar] - Parkinson, B.W. Assured PNT for Our Future: PTA. GPS World
**2014**, 9, 24–31. [Google Scholar] - Choy, S.; Harima, K. Satellite delivery of high-accuracy GNSS precise point positioning service: An overview for Australia. J. Spat. Sci.
**2018**, 1–12. [Google Scholar] [CrossRef] - Choy, S.; Kuckartz, J.; Dempster, A.G.; Rizos, C.; Higgins, M. GNSS satellite-based augmentation systems for Australia. GPS Solut.
**2016**, 21, 835–848. [Google Scholar] [CrossRef] - Shi, C.; Lou, Y.; Song, W.; Gu, S.; Geng, C.; Yi, W.; Liu, Y. A wide area real-time differential GPS prototype system in China and result analysis. Surv. Rev.
**2011**, 43, 351–360. [Google Scholar] - Shi, C.; Zheng, F.; Lou, Y.; Gu, S.; Zhang, W.; Dai, X.; Li, X.; Guo, H.; Gong, X. National BDS Augmentation Service System (NBASS) of China: Progress and Assessment. Remote Sens.
**2017**, 9, 837. [Google Scholar][Green Version] - Zhang, Y.; Chen, J.; Yang, S.; Chen, Q. Initial Assessment of BDS Zone Correction. In Proceedings of the China Satellite Navigation Conference (CSNC 2017), Shanghai, China, 23–25 May 2017. [Google Scholar]
- Enge, P.K.; Talbot, N.C.; San, J. Method and Reciever Using a Low Earth Orbiting Satellite Signal to Augment the Global Positioning System. U.S. Patent 5,812,961, 22 September 1998. [Google Scholar]
- Zhang, X.; Zhao, C.; Wang, Q.; Ma, Y. Carrier Phase Differential Positioning augmented by LEO satellites. Sci. Surv. Mapp.
**2017**, 42, 13–18. [Google Scholar] - Zhao, J.; Yu, X.; Feng, S.; Deng, L. Design and Performance Analysis of LEO Satellites Enhanced COMPASS System. Telecommun. Eng.
**2013**, 53, 131–135. [Google Scholar] - Joerger, M.; Gratton, L.; Pervan, B.; Cohen, C.E. Analysis of Iridium Augmented GPS for floating carrier phase positioning. J. Inst. Navig.
**2010**, 57, 137–160. [Google Scholar] [CrossRef] - Tian, S.; Li, G.; Chang, J.; Lv, J.; Dai, W. Receiver autonomous integrity monitoring in Iridium-augmented GPS. J. PLA Unv. Sci. Technol.
**2013**, 3, 237–241. [Google Scholar] - Ge, H.; Li, B.; Ge, M.; Zang, N.; Nie, L.; Shen, Y.; Schuh, H. Initial Assessment of Precise Point Positioning with LEO Enhanced Global Navigation Satellite Systems (LeGNSS). Remote Sens.
**2018**, 10, 984. [Google Scholar] [CrossRef] - Reid, T.G.R. Orbital Diversity for Global Navigation Satellite Systems. Ph.D. Thesis, Standford University, Stanford, CA, USA, 2017. [Google Scholar]
- Yang, Y.; Xu, T.; Xue, S. Progresses and Prospects in Developing Marine Geodetic Datum and Marine Navigation of China. Acta Geod. Cartogr. Sin.
**2017**, 46, 1–8. [Google Scholar] - Reid, T.G.R.; Neish, A.M.; Walter, T.F.; Enge, P.K. Leveraging Commercial Broadband LEO Constellations for Navigation. In Proceedings of the 29th International Technical Meeting of the Satellite Division of the Institute of Navigation (ION GNSS+ 2016), Portland, OR, USA, 12–16 September 2016. [Google Scholar]
- Reid, T.G.R.; Walter, T.; Enge, P.K.; Sakai, T. Orbital representations for the next generation of satellite-based augmentation systems. GPS Solut.
**2015**, 20, 737–750. [Google Scholar] [CrossRef] - Xie, X.; Geng, T.; Zhao, Q.; Liu, X.; Zhang, Q.; Liu, J. Design and validation of broadcast ephemeris for low Earth orbit satellites. GPS Solut.
**2018**, 22, 54. [Google Scholar] [CrossRef] - Fang, S. Model Design of Broadcast ephemerides for LEO Augmentation Navigation Satellites. Master’s Thesis, Information Engineering University, Zhengzhou, China, 2017. [Google Scholar]
- Rabinowitz, M. A Differential Carrier Phase Navigation System Combining GPS with LEO for Rapid Resolution of Integer Cycle Ambiguities. Ph.D. Thesis, University of Standford, Stanford, CA, USA, 2000. [Google Scholar]
- Satelles. Satelles Time and Location White Paper; Satelles: Redwood, CA, USA, 2016. [Google Scholar]
- Li, D.; Shen, X.; Gong, J.; Zhang, J.; Lu, J. On Construction of China’s Space Information Network. Geomat. Inf. Sci. Wuhan Univ.
**2015**, 40, 711–715. [Google Scholar] - Li, D.; Shen, X.; Li, D.; Li, S. On Civil-Military Integrated Space-Based Real-Time Information Service System. Geomat. Inf. Sci. Wuhan Univ.
**2017**, 42, 1501–1505. [Google Scholar] - Montillet, J.-P.; Roberts, G.W.; Hancock, C.; Meng, X.; Ogundipe, O.; Barnes, J. Deploying a Locata network to enable precise positioning in urban canyons. J. Geod.
**2009**, 83, 91–103. [Google Scholar] [CrossRef] - Jiang, W.; Li, Y.; Rizos, C. Locata-based precise point positioning for kinematic maritime applications. GPS Solut.
**2015**, 19, 117–128. [Google Scholar] [CrossRef] - Woodgate, P.; Coppa, I.; Choy, S.; Phinn, S.; Arnold, L.; Duckham, M. The Australian approach to geospatial capabilities; positioning, earth observation, infrastructure and analytics: Issues, trends and perspectives. Geo-Spat. Inf. Sci.
**2017**, 20, 109–125. [Google Scholar] [CrossRef] - Kuang, D.; Bertiger, W.; Desai, S.; Haines, B. Precise Orbit Determination for LEO spacecraft Using GNSS Tracking Data from Multiple Antennas. In Proceedings of the 23rd International Technical Meeting of the Satellite Division of the Institute of Navigation (ION GNSS 2010), Portland, OR, USA, 21–24 September 2010. [Google Scholar]
- Kuang, D.; Bertiger, W.; Desai, S.D.; Haines, B.; Iijima, B.; Meehan, T. Precise orbit determination for COSMIC satellites using GPS data from two on-board antennas. In Proceedings of the IEEE/ION PLANS 2008, Monterey, CA, USA, 6–8 May 2008; pp. 720–730. [Google Scholar]
- Ilcev, S.D. Global Mobile Satellite Communications Applications for Maritime, Land and Aeronautical Applications; Springer: Berlin, Germany, 2018; Volume 2. [Google Scholar]
- Jiao, W. Researches on the Realization of Satellite Navigation Coordinate Reference System; Postdoctoral Report; China Academy of Sciences: Shanghai, China, 2003. [Google Scholar]
- Wang, F.; Gong, X.; Sang, J.; Zhang, X. A Novel Method for Precise Onboard Real-Time Orbit Determination with a Standalone GPS Receiver. Sensors
**2015**, 15, 30403–30418. [Google Scholar] [CrossRef] [PubMed][Green Version] - Wu, Z.; Zhou, S.; Hu, X.; Liu, L.; Shuai, T.; Xie, Y.; Tang, C.; Pan, J.; Zhu, L.; Chang, Z. Performance of the BDS3 experimental satellite passive hydrogen maser. GPS Solut.
**2018**, 22, 43. [Google Scholar] [CrossRef] - Hauschild, A.; Montenbruck, O.; Steigenberger, P. Short-term analysis of GNSS clocks. GPS Solut.
**2013**, 17, 295–307. [Google Scholar] [CrossRef] - Steigenberger, P.; Hugentobler, U.; Hauschild, A.; Montenbruck, O. Orbit and clock analysis of Compass GEO and IGSO satellites. J. Geod.
**2013**, 87, 515–525. [Google Scholar] [CrossRef] - Wang, B.; Lou, Y.; Liu, J.; Zhao, Q.; Su, X. Analysis of BDS satellite clocks in orbit. GPS Solut.
**2016**, 20, 783–794. [Google Scholar] [CrossRef] - Shi, H.; Pei, J. The solutions of navigation observation equations for CAPS. Sci. China Ser. G Phys. Mech. Astron.
**2009**, 52, 434–444. [Google Scholar] [CrossRef] - Zhao, J.; Li, Z.; Ge, J.; Wang, L.; Wang, N.; Zhou, K.; Yuan, H. The First Result of Relative Positioning and Velocity Estimation Based on CAPS. Sensors
**2018**, 18, 1528. [Google Scholar] [CrossRef] [PubMed] - Guo, H. Study on the Analysis Theories and Algorithms of the Time and Frequency Characterization for Atomic Clocks of Navigation Satellites. Ph.D. Thesis, Information Engineering University, Zhengzhou, China, 2006. [Google Scholar]
- Huang, G. Research on Algorithm of Precise Clock Offset and Quality Evaluation of GNSS Satellite Clock. Ph.D. Thesis, Changan University, Xian, China, 2012. [Google Scholar]
- Zhang, S.; Wang, X.; Wang, H.; Yang, J. From Allan Variance to Phase Noise: A New Conversion Approach. J. Meas. Sci. Instrum.
**2011**, 2, 358–363. [Google Scholar] - RIOSDE. Global Navigation Satellite System GLONASS Interface Control Document (Edition 5.1); RIOSDE: Moscow, Russian, 2008. [Google Scholar]
- JPO. Navstar GPS Space Segment/Navigation User Interfaces; JPO: El Segundo, CA, USA, 2000.
- European Union. European GNSS (Galileo) Open Service. Signal in Space. Interface Control Document; European Union: Brussels, Belgium, 2010. [Google Scholar]
- CSNO. BeiDou Navigation Satellite System Signal in Space Interface Control Document Open Service Signal B2a; Version 1.0; CSNO: Beijing, China, 2017. [Google Scholar]
- JAXA. Quasi-Zenith Satellite System Navigation Service Interface Specification for QZSS; JAXA: Tokyo, Japan, 2012. [Google Scholar]
- Locata Corporation. Locata Signal Interface Control Document; Locata Corporation: Griffith, Australia, 2014. [Google Scholar]
- Montenbruck, O.; Kroes, R. In-flight performance analysis of the CHAMP BlackJack GPS Receiver. GPS Solut.
**2003**, 7, 74–86. [Google Scholar] [CrossRef] - Montenbruck, O.; Hauschild, A.; Andres, Y.; von Engeln, A.; Marquardt, C. (Near-)real-time orbit determination for GNSS radio occultation processing. GPS Solut.
**2012**, 17, 199–209. [Google Scholar] [CrossRef] - Zhao, Y. GNSS Ionospheric Occultation Inversion and Its Application. Ph.D. Thesis, Wuhan University, Wuhan, China, 2011. [Google Scholar]
- Estey, L.H.; Meertens, C.M. TEQC: The Multi-Purpose Toolkit for GPS/GLONASS Data. GPS Solut.
**1999**, 3, 42–49. [Google Scholar] [CrossRef] - Zhao, Q.; Wang, C.; Guo, J.; Yang, G.; Liao, M.; Ma, H.; Liu, J. Enhanced orbit determination for BeiDou satellites with FengYun-3C onboard GNSS data. GPS Solut.
**2017**, 21, 1179–1190. [Google Scholar] [CrossRef][Green Version] - Wang, L.; Feng, Y.; Wang, C. Real-Time Assessment of GNSS Observation Noise with Single Receivers. J. Glob. Position. Syst.
**2013**, 12, 73–82. [Google Scholar] - Goebel, G. Navigation Satellites & GPS. Available online: http://www.faqs.org/docs/air/ttgps.html (accessed on 26 October 2018).
- Admin. Decoding the Russian PARUS (COSMOS) Mavigation Satellites with the RTL-SDR. Available online: https://www.rtl-sdr.com/decoding-russian-parus-cosmos-navigation-satellites-rtl-sdr/ (accessed on 26 October 2018).
- Wang, L. Reliability Control of GNSS Carrier-Phase Integer Ambiguity Resolution. Ph.D. Thesis, Queensland University of Technology, Brisbane, Australia, 2015. [Google Scholar]
- Ashby, N. Relativistic effects on SV clocks due to orbit changes and due to Earths oblateness. In Proceedings of the 33rd Annual Precise Time and Time Interval (PTTI) Meeting, Long Beach, CA, USA, 27–29 November 2001. [Google Scholar]
- Filegel, H.F.; DiEsposti, R.S. GPS and Relativity an engineering overview. Aerosp. Corp.
**1996**, 56, 189–199. [Google Scholar] - Parkinson, B.; Spiker, J.; Axelrad, P.; Enge, P. Global Positioning System Theory and Applications; AAIA: Washington, DC, USA, 1996. [Google Scholar]
- Klobuchar, J.A. Ionospheric time-delay algorithm for single-frequency GPS users. IEEE Trans. Aerosp. Electron. Syst.
**1987**, 325–331. [Google Scholar] [CrossRef] - Saastamoinen, J. Contributions to the theory of atmospheric refraction. Bull. Géod.
**1973**, 14, 94–94. [Google Scholar] [CrossRef]

**Figure 3.**Ground assessment of the Luojia-A ranging measurements: the test scenario (

**left panel**) and measured signal samples (

**right panel**).

**Figure 7.**Hardamard variance of the satellite clock (

**left panel**) and the frequency drift of the satellite clock (

**right panel**).

**Figure 8.**Luojia-1A satellite footprint (

**left panel**) and the setup of the ground receivers on a rooftop structure (

**right panel**).

**Figure 9.**Representative sky plot of the Wuhan station shows the tracks of Luojia-1A and GPS/Beidou satellites over a 10 min pass.

**Figure 11.**The observation noise structure of Luojia-1A pseudorange and carrier phase measurement evaluated via the geometry-free combination.

**Figure 12.**Representative pseudorange and carrier phase measurement residuals of the Luojia-1A with error sources corrected by empirical models.

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Wang, L.; Chen, R.; Li, D.; Zhang, G.; Shen, X.; Yu, B.; Wu, C.; Xie, S.; Zhang, P.; Li, M.; Pan, Y. Initial Assessment of the LEO Based Navigation Signal Augmentation System from Luojia-1A Satellite. *Sensors* **2018**, *18*, 3919.
https://doi.org/10.3390/s18113919

**AMA Style**

Wang L, Chen R, Li D, Zhang G, Shen X, Yu B, Wu C, Xie S, Zhang P, Li M, Pan Y. Initial Assessment of the LEO Based Navigation Signal Augmentation System from Luojia-1A Satellite. *Sensors*. 2018; 18(11):3919.
https://doi.org/10.3390/s18113919

**Chicago/Turabian Style**

Wang, Lei, Ruizhi Chen, Deren Li, Guo Zhang, Xin Shen, Baoguo Yu, Cailun Wu, Song Xie, Peng Zhang, Ming Li, and Yuanjin Pan. 2018. "Initial Assessment of the LEO Based Navigation Signal Augmentation System from Luojia-1A Satellite" *Sensors* 18, no. 11: 3919.
https://doi.org/10.3390/s18113919