Rendezvous Technology of SNU CubeSat (SNUGLITE-III) Using Only GPS Receivers

Hanjoon Shim,O-jong Kim,Sunkyung Yu,Yonghwan Bae,Changdon Kee1 한국항공우주학회 2022 한국항공우주학회 학술발표회 논문집 Vol.2022 No.11

Extending Operational Area of Pseudolite Using Long Integration Time and Data-less Pseudolites

KEE, Changdon,LEE, Taikjin,SO, Hyungmin THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIEN 2009 Transactions of the Japan Society for Aeronautical Vol.52 No.176

<P>A pseudolite can be a solution to strengthen GNSS or create an independent navigation system. However a pseudolite has very serious near-far problem where the operational area becomes smaller and the usage limited. The aim of this work is to extend the pseudolite operational area. To minimize the near-far problem, we split the pseudolite pulse into several pulses. To maintain tracking of weak pseudolite signals, we increased receiver integration time. To validate these solutions, we simulated distributed pulsing and tested the increased integration time. To increase the integration time of conventional and low-cost receivers, we used a data-less pseudolite with no navigation data. Using these solutions, we extended the mid zone of the operational area and reduced the near and far zone.</P>

Tomographic Estimation of the Ionosphere

Changdon Kee,Youngho Sohn 한국기상학회 2005 한국기상학회 학술대회 논문집 Vol.2005 No.-

Ionospheric time delay is the biggest error source for single-frequency DGPS applications, including time transfer and Wide Area Differential GPS (WADGPS). Currently, there are many attempts to develop real-time ionospheric time delay estimation techniques to reduce positioning error due to the ionospheric time delay. Klobuchar model is now widely used for ionospheric time delay calculation for single-frequency users. It uses flat surface at night time and cosine surface at daytime[1]. However, the model was developed for worldwide ionosphere fit, it is not adequate for local area single-frequency users who want to estimate ionospheric time delay accurately[2]. Therefore, 3-D ionosphere model using tomographic estimation has been developed. 3-D tomographic inversion model shows better accuracy compared with prior algorithms[3]. In this paper, we explained the basic algorithm of tomographic estimation of ionosphere and showed comparisons with other methods.

Robust and Practical Indoor Navigation System and Its Application to Automatic Control of Miniature Vehicle

Kee, Changdon,Jun, Haeyoung,Yun, Doohee 서울대학교 항공우주신기술연구소 2001 항공우주신기술연구소 연구보고 Vol.2 No.2

By using pseudolites, we developed an indoor navigation system and obtained centimeter-level positioning accuracy. To make this system a more robust and practical one, we implemented automatic cycle ambiguity resolution and cycle-slip recovery functions. In addition, we also made a two-dimensional attitude determination system using a two-antenna array. Our final research goal was to control a miniature vehicle by use of only the indoor navigation system and to show that this system is a sufficient position and attitude sensor for practical applications.

GPS를 이용한 항공기 이착륙시스템

기창돈(Changdon Kee) 한국항공우주학회 1999 韓國航空宇宙學會誌 Vol.27 No.5

Frequency Tracking Error Analysis of LQG Based Vector Tracking Loop for Robust Signal Tracking

Park, Minhuck,Kee, Changdon The Institute of Positioning 2020 Journal of Positioning, Navigation, and Timing Vol.9 No.3

In this paper, we implement linear-quadratic-Gaussian based vector tracking loop (LQG-VTL) instead of conventional extended Kalman filter based vector tracking loop (EKF-VTL). The LQG-VTL can improve the performance compared to the EKF-VTL by generating optimal control input at a specific performance index. Performance analysis is conducted through two factors, frequency thermal noise and frequency dynamic stress error, which determine total frequency tracking error. We derive the thermal noise and the dynamic stress error formula in the LQG-VTL. From frequency tracking error analysis, we can determine control gain matrix in the LQG controller and show that the frequency tracking error of the LQG-VTL is lower than that of the EKF-VTL in all C/N0 ranges. The simulation results show that the LQG-VTL improves performance by 30% in Doppler tracking, so the LQG-VTL can extend pre-integration time longer and track weaker signals than the EKF-VTL. Therefore, the LQG-VTL algorithm is more robust than the EKF-VTL in weak signal environments.

Korean Wide Area Differential Global Positioning System Development Status and Preliminary Test Results

Ho Yun,Changdon Kee,Doyoon Kim 한국항공우주학회 2011 International Journal of Aeronautical and Space Sc Vol.12 No.3

This paper is focused on dynamic modeling and control system design as well as vision based collision avoidance for multi-rotor unmanned aerial vehicles (UAVs). Multi-rotor UAVs are defined as rotary-winged UAVs with multiple rotors. These multi-rotor UAVs can be utilized in various military situations such as surveillance and reconnaissance. They can also be used for obtaining visual information from steep terrains or disaster sites. In this paper, a quad-rotor model is introduced as well as its control system, which is designed based on a proportional-integral-derivative controller and vision-based collision avoidance control system. Additionally, in order for a UAV to navigate safely in areas such as buildings and offices with a number of obstacles, there must be a collision avoidance algorithm installed in the UAV’s hardware, which should include the detection of obstacles, avoidance maneuvering, etc. In this paper, the optical flow method, one of the vision-based collision avoidance techniques, is introduced, and multi-rotor UAV’s collision avoidance simulations are described in various virtual environments in order to demonstrate its avoidance performance.

Modified Tomographic Estimation of the lonosphere using Fewer Coefficients

Youngho Sohn,Changdon Kee 한국항공우주학회 2004 International Journal of Aeronautical and Space Sc Vol.5 No.1

Ionospheric time delay is the biggest error source for single-frequency DGPS applications, including time transfer and Wide Area Differential GPS (WADGPS). Currently, there are many attempts to develop real-time ionospheric time delay estimation techniques to reduce positioning error due to the ionospheric time delay. Klobuchar model is now widely used for ionospheric time delay calculation for single-frequency . users. It uses flat surface at night time and cosine surface at day time[1]. However, the model was developed for worldwide ionosphere fit, it is not adequate for local area single-frequency users who want to estimate ionospheric time delay accurately [2] Therefore, 3-D ionosphere model using tomographic estimation has been developed. 3-D tomographic inversion model shows better accuracy compared with prior algorithms[3]. But that existing 3-D model still has problem that it requires many coefficients and measurements for good accuracy. So, that algorithm has limitation with many coefficients in continuous estimation at the small region which is obliged to have fewer measurements. In this paper, we developed an modified 3-D ionospheric time delay model using tomography, which requires only fewer coefficients. Because the combinations of our base coefficients correspond to the full coefficients of the existing model, our model has equivalent accuracy to the existing. We confirmed our algorithm by simulations. The results proved that our modified algorithm can perform continuous estimation with fewer coefficients.

Verification of a Real-Time Attitude Determination Algorithm through Development of 48-Channel GPS Attitude Receiver Hardware

Jang, Jaegyu,Kee, Changdon Cambridge University Press 2009 The Journal of navigation Vol.62 No.3

<P>This paper describes the verification of a real-time attitude determination algorithm during GPS attitude receiver hardware development. The GPS attitude receiver of 24 channels had been already developed in Surrey University. However 24 channels were not enough for practical usage. For this reason, a 48-channel attitude receiver with 12 channels for each antenna has been developed. To estimate attitude in real time, precise relative positions of the GPS antenna array have to be determined as rapidly as possible. However, the calculation load based on the conventional algorithm is too burdensome to perform using the RISC microprocessor. Therefore, in this paper, the cycle ambiguities of each base vector are resolved using SNUGLAD (Seoul Nat Univ GNSS Lab Attitude Determination), the design focus of which is to allow the receiver to estimate the 10 Hz onboard solutions. To keep precise solutions continuously, after ambiguity removal, cycle slip must be detected or isolated. Otherwise, the receiver would output erroneous solutions after a short signal blockage or fading of the GPS signal. To prevent this, we defined the cycle slip detection and repair scheme using a standard extended Kalman filter, which can detect and repair cycle slip within one cycle. As a result, this paper shows that time synchronized measurement with good quality and a reliable solution can be provided by the hardware developed with inexpensive chipsets and that this may be a possible cost efficient sensor for UAV or microsatellites.</P>

Multiple-hypothesis RAIM algorithm with an RRAIM concept

Yun, Ho,Kee, Changdon Emerald Group Publishing Limited 2013 Aircraft engineering and aerospace technology Vol.86 No.1

<B>Purpose</B> - This paper aims to develop and analyse a new multiple-hypothesis receiver autonomous integrity monitoring (RAIM) algorithm. The proposed algorithm can handle simultaneous multiple failures as well as a single failure. <B>Design/methodology/approach</B> - The proposed algorithm uses measurement residuals and satellite observation matrices of several consecutive epochs for failure detection and exclusion. It detects failures by monitoring the error vector itself instead of monitoring the projection of the error vector. The algorithm reduces the minimum detectable bias via the relative receiver autonomous integrity monitoring (RRAIM) scheme. <B>Findings</B> - The algorithm is able to detect any instance of multiple failures, including failures that are not detected by the conventional RAIM algorithm. It is able to detect multiple failures with magnitudes of several tens of meters, although the algorithm has to solve an ill-conditioning problem. The detection capability of the proposed algorithm is not dependent on satellite geometry. <B>Research limitations/implications</B> - The algorithm assumes that the error vectors in three consecutive epochs have biases of similar magnitude. As a result, although the algorithm detects occurrences of drifting error, it cannot identify which measurement(s) has the critical error. <B>Practical implications</B> - The paper includes implications for the development of the RAIM algorithm for aviation users. Especially, it can be a candidate for future standard architecture in multiple constellations, multiple frequency satellite-based augmentation system (SBAS) users. <B>Originality/value</B> - The paper proposes a new multiple-hypothesis RAIM algorithm with an RRAIM concept. A detailed explanation of the algorithms, including rigorous mathematical expressions, is presented. The paper also includes an analysis of differences in detection capability between conventional algorithm and the proposed algorithm depending on satellite geometry.