Hang Gong

Comparison of Short-Term Stability Estimation Methods of GNSS On-Board Clock

The short-term frequency stability of satellite on-board clock is of great significance for GNSS applications, such as satellite clock modeling, simulation and clock offset prediction. In this paper, four short-term frequency stability estimation methods are analyzed, which need observation data from only one station in contrast with ODTS needing continuous observation of large ground monitoring networks. Performance and accuracy of all these four methods is analyzed and compared using GPS and COMPASS observation data, and their advantage and disadvantage are discussed correspondingly. For COMPASS and Galileo systems which are currently in the early stage of construction, this paper proposes some meaningful suggestions of on-board clock short-term frequency stability estimation.

Detection of weak frequency anomalies for atomic clocks with a Kalman filter

An algorithm presented in a previous paper by the authors detects frequency anomalies. The algorithm was shown to be effective for early detection of weak frequency jumps, and also valid for frequency drift jumps that belong to the class of errors that are most difficult to detect. A modification to the original frequency jump detector algorithm is suggested by the addition of a noise-removal filter. The performance is then compared with the original algorithm and improvement is shown. An analysis of real data from a real satellite is also adopted.

Estimation of COMPASS on-board clock short-term stability

The short-term frequency stability estimation of GNSS on-board clocks is very important for clock modeling and prediction. Orbit Determination and Time Synchronization (ODTS) method is commonly used to estimate the clock offset, which needs continuous observation by large monitoring ground network and complicate algorithms. COMPASS is on its early stage of construction and has not globally distributed station network for satellites tracking. In this paper, we have analyzed an estimation method based on a single monitoring station observation. Based on this method, a simple method for COMPASS MEO/IGSO on-board clocks estimation using smoothed broadcast ephemeris is presented, and a method for COMPASS GEO on-board clocks estimation using satellite radio ranging (SRR) is also discussed. At the end of this paper, the preliminary estimation results for COMPASS IGSO and GEO on-board clocks by these two methods are presented.

An Integrity Monitoring Algorithm for Satellite Clock Based on Test Statistics

As a core part of satellite payload, satellite clock has a direct influence on GNSS service performance. Therefore, it is necessary to monitor its integrity. In this paper, real-time test statistics for satellite clock phase and frequency anomaly are built, and an integrity monitoring algorithm for satellite clock based on these statistics is proposed. It is composed of two parts, including phase anomaly monitoring and frequency anomaly monitoring, which mainly aims at three anomalies comprising occasional bad or outlier points, anomaly jumps that later return to stable values and phase or frequency steps. The new algorithm can not only detect phase and frequency anomalies, but also identify and deal with clock anomaly instantly, including eliminating and replacing the anomaly data. Satellite clock anomalies are simulated using the IGS observation clock data and performance of the new algorithm is verified. The results show that the new algorithm has a perfect performance of detecting the satellite clock anomaly including both phase anomaly and frequency anomaly. At the same time, it can also eliminate and replace the anomaly data, and the replaced error is negligible. The integrity monitoring algorithm for satellite clock proposed in this paper is helpful to GNSS satellite clock autonomous integrity monitoring.

Non-Reciprocity Correction Using Broadcast Ephemeris in Two-Way Satellite Time and Frequency Transfer (TWSTFT)

Two-way satellite time and frequency transfer (TWSTFT) is applied in BeiDou Navigation Satellite System for time synchronization of the ground stations. In fact, the geostationary (GEO) satellites have periodic motions around their planned orbits instead of being stationary to the ground stations, which results in periodic fluctuations of Sagnac correction and signal path geometry correction in TWSTFT. Precise TWSTFT is relaying on the non-reciprocity corrections. An analysis method with broadcast ephemeris of GEO satellite for Sagnac correction and signal path geometry correction is introduced in this paper. With this method, TWSTFT experiment result using BeiDou GEO satellite at 140°E of three links between Beijing and three different sites is analyzed and the periodic fluctuation of Sagnac correction and signal path geometry correction are presented in detail. The fluctuation range of Sagnac correction is from 0.04 to 0.16 ns and that of signal path geometry correction is from 0.01 to 0.53 ns for the three links, which shows that Sagnac correction and signal path geometry correction can not be ignored in time transfer requiring sub-nanosecond precision generally for the three links.

The research progress of two way time synchronization with fiber based on spread spectrum signal

High-precision time synchronization is a basic element in some areas of aeronautical engineering, such as satellite navigation and deep space exploration. It is more accurate and stable to use the optical fibers while performing time frequency transfer than using other media such as GNSS common view and two-way satellite time frequency transfer. The frequency transfer is the main focus of the current research in optical fiber time frequency transfer. However, there is little study on time transfer, and the accuracy of time transfer is commonly in nanosecond level. In this paper, a two way time transfer method based on spread spectrum ranging is studied. For the proposed method, the accuracy is improved and extra links for data check are not needed any more. The designing schemes and the implementation progress of the engineering prototype are presented. The experimental results indicate that the time synchronization uncertainty is less than 30ps by use of the proposed method when the optical fiber transmission distance is within 2km.

GCE-BOC Modulation: A Generalized Multiplexing Technology for Modern GNSS Dual-Frequency Signals

For the purpose of structural enhancement and backwards compatibility, more than two binary signals on one carrier frequency are required to emit. In order to maximize the power efficiency, constant-envelope modulation is adopted. Alt-BOC and ACE-BOC modulations, the constant-envelope modulation that multiplexes four binary spreading codes as two QPSK signals at two different carrier frequencies, are proposed by Galileo and BeiDou respectively. A constraint of these new modulations is that the number of signals to be multiplexed must be no larger than four signal channels. In this paper, a generalized dual-frequency constant-envelope multiplexing technology, named generalized constant envelope BOC (GCE-BOC) modulation, is presented. The modulation can be regarded as the extended form of Alt-BOC and ACE-BOC modulations and provides a solution to dual-frequency constant envelope multiplexing problem for worldwide GNSS.

Frequency stability estimation of Compass On-board clock based on smoothed broadcast ephemeris

For Compass and Galileo systems which are currently in their stage of construction, it is difficult to obtain precise on-board clock short-term stability for common users. In this paper, a simple estimation method based on smoothed broadcast ephemerides is proposed, which only needs observation from one single receiver. The principle of this method is discussed, followed by performance evaluation using GPS data. We conclude that the relative error of short-term stability estimated with this method is less than 10% for average time of 1~800 seconds. Short-term stability estimation of all the current 14 Compass on-board clocks are presented, with the result of approximately 6×10-12 at average time of 1s and 2×10-13 at 1000s.

Research on Robust Timing Receiver with Joint Verification of Location and Clock Offset

GNSS timing receiver is widely used in various fields. Unfortunately, it has weak capability of anti-jamming and anti-spoofing, so an integrated processing technique based on joint verification of location and clock offset is proposed. The algorithm is verified by simulation with timing receiver data. Simulation results show that 95% probability of the spherical error of location result can be used as the threshold of the location to detect strong intentional or unintentional jamming signal efficiently. When the location check is passed, the clock offset checking is used to monitor the receiver clock offset anomaly, with processing error less than 0.2 ns. The joint verification of location and clock offset method can greatly improve the robust performance of timing receivers.

Research and Application on Enhanced Reception Techniques Based on Distributed Antennas in Ground Station

In order to enhance navigation warfare capabilities of ground station, miniaturization and motorization is a trend. However, the demand of antenna aperture for downlink signal processing is diversity. Then contenting the diversity of antennas is a major factor that restricts the development of ground control systems. For the demand of multi-target tracking and single target enhancing, based on the characteristics of precise-time synchronization between ground stations of satellite navigation systems, an enhanced reception techniques based on distributed antenna system (DAS) is proposed with a plurality of distributed small aperture as the basic element. With the knowledge of wave direction, phase difference between the received signals is computed and compensated. Then downlink signal synthesis is realized which means larger aperture synthesizing antenna is achieved and the ability of receiving signal is enhanced. Numerical analysis shows that when the phase compensation accuracy is up to 0.2, synthesizing efficiency can reach to 75 % within the scanning range [π/18, π/2]; while a digital multi-beam antenna can only reach about 70 % at elevation angle of 30°. Distributed enhanced reception techniques, by arraying small aperture to achieve flexible combinations and configurations of beam and gain, can be applied to ground station for miniaturization and self-propelled construction and improve the multi-target tracking and controlling capability of satellite navigation and aerospace based on flexibility and low cost of small-diameter antenna.