Pseudolite Signals Research Articles

GPS 소프트웨어 수신기에 대한 GPS 의사위성의 영향 분석

이 논문은 의사위성 신호에 대한 GPS 소프트웨어 수신기의 성능을 분석하고자 한다. 의사위성이 계속 켜져있을 경우와 펄싱이 주어졌을 경우에 대해서, 의사위성의 신호세기와 의사위성과 수신기 사이의 거리에 따라 의사위성이 GPS 소프트웨어 수신기에 미치는 영향을 알아본다. This paper considers the performance of the non-participating GPS software receivers under the presence of GPS pseudolite(PL). For the continuous PL signal and pulsed PL signal, the impact of PL signals on the GPS software receivers is investigated according to the signal strength of the PL and the distance between the PL and the GPS receiver.

의사위성의 펄싱 방법에 대한 GPS L2C 신호획득 성 소능프분트석웨어 수신기의 L1, L2C 신호획득 성능분석

의사위성은 우주상공의 GPS 위성과는 달리 지상이나 비행체에 설치되어 GPS 위성과 같이 거리를 측정할 수 있는 ranging 신호를 전송하는 전송기이다. 의사위성은 정확성, 가용성, 무결성을 향상시킬 수 있으나 GPS 위성 신호에 대해 신호의 포화 또는 간섭을 일으킨다. 의사위성의 근원거리 문제를 해결하기 위해 의사위성의 신호를 펄싱이 있을 때만 내보내는 기법을 주로 사용한다. 본 논문에서는 의사위성의 정적 펄싱, 이동 펄싱, 랜덤 펄싱 기법과 의사위성의 개수가 GPS 소프트웨어 수신기의 L1 및 L2C 신호획득에 미치는 영향을 분석하였다. GPS L1신호의 경우 의사위성이 1개일때는 정적펄싱에 대한 GPS 소프트웨어 수신기의 신호 획득 및 추적 성능이 가장 좋았고 2개 이상일 때는 랜덤펄싱에 대한 성능이 가장 좋았다. L2C 신호는 세가지 펄싱에 대한 성능이 비슷하게 안정적으로 나왔는데 정적펄싱에 대한 성능이 약간 좋게 나왔다. L1, L2C 모두 위사위성 3개까지는 모든 펄싱에 대해서 측위가 가능한 것으로 나왔다. Pseudolites are ground-based transmitters that can be configured to emit GPS-like signals for enhancing the GPS by providing increased accuracy, integrity, and availability. However, a pseudolite (PL) can interfere with GPS satellite signals while it is transmitting or cause saturation to automatic gain control circuit. To solve these problems pulsing scheme is used, which transmits PL signal during a short period of time. In this paper the effect of the number of PL and pulsing scheme on the software GPS L1 and L2C signal acquisition performance is studied for the three pulsing schemes such as static pulsing, sweep pulsing, and pseudo random pulsing. For GPS L1 signal, static pulsing shows the best signal acquisition and tracking performance with one PL, and random pulsing shows the best performance with more than or equal to two PLs. For GPS L2C signal, all three pulsing schemes show the similar signal acquisition and tracking performance, but static pulsing shows a little better performance. For GPS L1 and L2C signals, software GPS receivers can do positioning with up to three PLs.

Reduced complexity crosscorrelation interference mitigation in GPS-enabled collaborative ad-hoc wireless networks – Theory

Localization based services rely on Global Positioning System (GPS) receivers embedded in the network nodes, but the satellite signal availability is often limited in indoors environment. Collaborative network-assisted GPS algorithms addressed this issue by communicating various assistance data between the nodes or transforming open-sky nodes into virtual GPS satellite transmitters (pseudolites). Even though such approach improves the coverage, the crosscorrelation problem surfaces due to masking of relatively weak available satellite signals by stronger pseudolites. This paper proposes reduced complexity algorithms to mitigate such a self-jamming (near/far) effect in ad hoc networks. The idea is based on adaptive modifications of dispreading GPS codes in receiver nodes to minimize interference caused by strong pseudolite signals. An optimization problem is formulated for the minimization of interference using mean squared error (MSE) as a cost function. Then computational optimization is achieved through adaptive implementation and parameterized dimension reduction of the optimization problem.

Design of the pulse duty cycle of the pseudolite signal in local area augmentation systems

The short-pulse TDMA signal is an excellent solution to the near-far problem in the pseudolite augmentation systems, and the pulse duty cycle is an important element in the solution. This paper aims at designing the duty cycle of the pseudolite pulse signals with a theoretical method. At first, the scope of the duty cycle is derived theoretically with the object of ensuring the reception of both the pseudolite signals and the satellite signals, and the expression for the theoretical upper limit is given. Then based on the proposed expression, the duty cycles of Galileo pseudolite pulse signals are designed. The theoretical upper limit can guide the design of the short-pulse TDMA pseudolite signals.

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

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.

Experimental Analysis of GPS/Pseudolite/INS Integration for Aircraft Precision Approach and Landing

This paper analyses flight trial results to study the overall performance and limitations of a GPS/Pseudolite/INS integration approach for aircraft precision approach and landing applications. For this purpose, the series of approaches were flown at Wedderburn Airfield, Australia. The analysed results show that pseudolite signals strengthen the ranging signal availability and the satellite geometry. Most of the geometry enhancement is found in the vertical position component, improving the accuracy of the aircraft's altitude. Furthermore, the results reveal that the inclusion of a pseudolite enhances both internal and external reliabilities. A dramatic improvement of the external reliability in the vertical component is observed.

Optimal Pulsing Schemes for Galileo Pseudolite Signals

Galileo, the European Satellite Navigation System, is currently under development. Even before first satellites of the constellation are launched, Galileo signals will be provided through ground based Navigation Signal Generators for the investigation of signal performance and characteristics. Currently various projects are ongoing to develop these Galileo pseudolites (pseudo satellites). Since pseudolites are part of the Galileo system architecture namely as "Local Elements" it is expected that they will be used together with GNSS for position determination. The main characteristic of pseudolite navigation is the relatively small distance between the signal transmitter and the receiver, compared to the distances from the GNSS satellites to the receiver. This short distance causes the so-called "Near-Far- Problem". Different attempts have been made in the past to overcome the near-far problem. A possible solution is to pulse the pseudolite transmitter signals, which has been proposed by many researchers and the success of pulsing has also been demonstrated. Basically these studies have been focused on the GPS pseudolites and the proposed pulsing schemes are optimised for the GPS signals (RTCM, RTCA). Due to major differences between the GPS and Galileo signal structures, these pulsing schemes cannot directly be adopted for use in the Galileo pseudolites. Thus new pulsing schemes and patterns have to be found and investigated. This paper summarises and assesses the existing GPS pulse patterns and pulsing techniques. The parameters which characterise a pulsing scheme are discussed and implemented. Simulations based on the Galileo signal structure (codes, chipping rates, cross correlation properties) have been performed and the results will be presented. These simulations form the basis for the proposal of a new optimised pulsing scheme for Galileo pseudolites w.r.t. pulse length, duty cycles and pulse patterns.

Autonomous Navigation Environment with Self-Calibrating Transceivers

An operable navigation system which demonstrates successful self-calibration and precise local navigation has been developed by EADS Astrium. This paper presents the architecture of the autonomous navigation environment with the ability to calibrate itself as well as the results of field tests. The Self-calibrating autonomous Navigation Environment (SekaN) can be used as stand-alone navigation system for applications where satellite signals are not available or where autonomy and high precision is required. Cargo drop, navigation in canyons and open pit mines, indoor navigation and extraterrestrial navigation are just examples of possible applications. The self-calibrating feature of SekaN is of special interest in conflict areas where a temporary autonomous navigation environment has to be installed quickly and where it is not possible to calibrate the locations of the pseudolites a priori. Furthermore, the system can be operated as augmentation system to classical satellite navigation systems. Therefore a mixed mode has been introduced which allows for simultaneous tracking of both satellite signals and pseudolite signals. Referencing of the local coordinate system to e.g. WGS84 becomes possible. The SekaN system comprises the following HW units developed by EADS Astrium: at least 4 Transceivers (TCs), a Rover receiver (ROV) and a Master Control Station (MCS). A WLAN data link is used between the units. Each TC comprises a GNSS signal generator NSG 5100 which supports both GPS and Galileo signals and an Astriumspecific GPS/PSL receiver. The number of TCs in the network is scalable and dependent on the specific application of the SekaN. Various TC-array sizes are supported as the output power of the pseudolites can be varied in a wide range. The rover receiver positioning takes place at the MCS. However, several receivers may be registered at the MCS. The TCs are operated unsynchronized and differential concepts are applied to eliminate the clock errors. Presently the pulsed signals with pseudolite spreading codes at the GPS L1 and dummy navigation messages are used as navigation signals. As soon as low-cost Galileo receivers are available the system can be switched to any Galileo frequency band. In a batch process the exact locations of the TC TX-antennas are determined without any a priori knowledge of the geometric array configuration. The general idea behind the self-calibration algorithms is based on the solution algorithm for self-calibrating pseudolite arrays presented in (LeMaster and Rock, 2002). However, several modifications were necessary to adapt the algorithms to the SekaN system requirements. The rover which is used for data collection during the self-calibration process is designed as a Receiver-only module instead of a TC module. This makes the rover hardware less complex, smaller and lighter, but also complicates the self-calibration process. Self-differencing between the stationary TCs and the rover TC can no longer be applied. The ranges between the rover RX and the TCs are therefore not directly observable. The selfcalibration and navigation algorithms developed for the SekaN work in both 2-D and 3-D scenarios. Although multipath effects, non-linearities and the near-far-effect are inherent in these kinds of ground-based navigation systems, precise user positioning at the sub-meter level becomes possible even with low-cost receivers within the self-calibrated navigation environment.

Attitude Determination Methods Using Pseudolite Signal Phase Measurements

ABSTRACT: In this paper, new attitude determination algorithms using pseudolite signal phase measurements are developed and presented with realistic simulations. Pseudolite signals are used to replace GPS signals which are often not available due to blocking by nearby large structures, for example, a crew return vehicle under the International Space Station. A new observation model needs to be applied because spherical wavefronts are present for pseudolite signals, which are much closer to the antennas than GPS signals. Both static and filtering methods are developed. An iterative algorithm using the Levenberg-Marquardt method and a nonlinear Predictive filter provide a deterministic solution; however, they are not suitable for moving cases. Instead, the nonlinear Predictive filter, the extended Kalman filter and the Unscented filter using angular velocity measurements as well as pseudolite signal phase measurements are developed. In the convergence comparison, the Unscented filter and the Predictive filter show more robust behavior than that of the extended Kalman filter.

Tropospheric Delay Estimation for Pseudolite Positioning

Pseudolites, ground-based GPS signal transmitters, can significantly enhance the GPS satellite geometry or can even be an independent positioning system. However, as pseudolites are very close to the receivers, error effects are different from the traditional GPS and should be considered and modeled in a different way. Tropospheric delay is one of the largest error sources in pseudolite positioning, as pseudolite signal propagates through the lower troposphere which is very difficult to be modeled due to spatial variations in atmosphere. The objective of this research is to analyse pseudolite tropospheric delay modelling methods and to select the optimal tropospheric delay models for different applications. Several methods to estimate the tropospheric delay for pseudolite positioning are introduced and compared. One approach is to utilize single-differenced GPS tropospheric models. Another one is to compute the tropospheric delay as a function of the local refractivity along the pseudolite signal path. The ratio method used for Electronic Distance Measurement (EDM) can also be applied to estimate tropospheric delay. Experiments with simulation and real flight test data are conducted in this study to investigate the proposed methods. The advantages and limitations of each method are analysed. The mode defined by RTCA and its modification are suitable for a low elevation and short range application, such as LAAS and local ground based applications. Models derived from single-differenced NMF and Saastamoinen models perform well in long range and high elevation but have a big bias in low elevation. And the model derived from the Hopfield model performs relatively well in all the range and elevation.

A Novel Antenna Array for GPS/INS/PL Integration

In order to improve signal reception performance in GPS/INS/PL (Pseudolite) integration applications, a semi-sphere antenna array is proposed in this paper. It inherits the wide coverage characteristic of conventional spherical arrays and utilizes only about half the number of elements compared to a planar array to cover the upper-semi-sphere space above earth plane. It can process signals from both overhead and horizontal directions at the same time. Thus, unlike common planar arrays, this novel antenna array with a special geometry can receive satellite and Pseudolite (PL) signals from all directions, even from the horizon. Both Capon's and constraint methods have been used in the simulations of Direction of Arrival (DOA) estimation and beam- forming. These simulation results have demonstrated the advantages of the new array.

Preliminary Tests of GPS/HAPS Positioning System Using a Pseudolite on Helicopter

Japan Aerospace Exploration Agency (JAXA) has proposed a new positioning/navigation service using pseudolites installed on the High altitude Platforms Systems (HAPS). If pseudolites were mounted on the HAPS, their GPS-like signals would be stable augmentations that would improve the performance of GPS. In order to establish the fundamental technologies, the augmentation tests using a GPS pseudolite installed on a helicopter were conducted. The position of the pseudolite antenna underneath the helicopter, which is analogous to the GPS ‘precise ephemeris’, was precisely determined by the inverted GPS method. Also, the results demonstrate the efficacy of integrating the pseudolite signal with GPS.

Precise calibration method of pseudolite positions in indoor navigation systems

A pseudolite is a signal generator that transmits a CPS-like signal. Using pseudolites, we developed an indoor navigation system. An indoor navigation system has several positioning error sources, such as time-tag error, multipath error, near/far problem, and pseudolite position error. Among them, we can remove the pseudolite position error by precise calibration because pseudolites are fixed. Otherwise, there would be bias-type positioning errors in indoor navigation solutions. Therefore, how exactly the pseudolite positions are calibrated determines the limit of the indoor navigation system's performance. With this motivation, we made efforts to develop a precise calibration method of pseudolite positions in an indoor navigation system. This paper proposes a practical and accurate calibration method, which uses only pseudolite signals. Simulation results showed that we can calibrate all the pseudolite positions within two or three centimetres by this method.

Assessment of pseudolite layout under urban environments using a simulation system for seamless positioning

Since the launch of the first Global Positioning System satellite, the demand for higher accuracy and wider applicability has been continuously increasing. In particular, personal navigation stresses requirements on “seamless” or continuous positioning with higher reliability are needed in urban areas. Unfortunately, urban canyons causing shadowing and multipath represent an adverse condition for Global Navigation Satellite System (GNSS) signal reception, thus leading to poor satellite visibility and low positioning accuracy. In order to overcome such problems, pseudolites can be set up to provide additional ranging signals and aid the carrier phase ambiguity resolution which is crucial for utilizing the full accuracy and wide range capability of GNSS. Pseudolites, which are ground-based instruments that transmit GPS-like signals, can improve the satellite-receiver layout and be used as additional range observations to improve the performance of a GPS-based deformation monitoring system. However, due to economic and environment constraints, the number of pseudolites that can be installed will be limited. More importantly, in order to reduce the areas affected by multi-path of pseudolite signals, locations and antenna pattern of pseudolites have to be carefully examined. This paper shows the development of a simulation system for evaluating the appropriateness and economics of installing pseudolites using precise orbital information of the satellite and a three-dimensional digital map.

Application of successive interference cancellation to the GPS pseudolite near-far problem

Ground-based transmitters called pseudolites have been proposed to augment the basic Global Positioning System (GPS) in environments where satellite visibility is limited. One difficulty in their use is the so-called near-far problem, where in close proximity to the ground transmitter, the pseudolite signal can be orders of magnitude stronger than the satellite signals. This large range of signal levels prevents a conventional receiver from simultaneously detecting both types of signals. This paper describes the application of a signal processing technique, known as successive interference cancellation (SIC), to the acquisition and tracking of weak satellite signals in the presence of a nearby pseudolite and possible multipath reflections of this pseudolite signal. The SIC architecture is implemented on simulated and experimental near-far data sets. The results are compared with a conventional detector and improvements in acquisition and tracking performance are illustrated.

Pseudolite Augmentation for OTF Ambiguity Resolution in Shipborne Mode

The use of shore-based pseudolite to augment differential global positioning systems (DGPS) for on-the-fly (OTF) carrier phase ambiguity resolution in the marine environment is investigated. OTF ambiguity resolution results in cm-level tridimensional accuracies. This high level of accuracy is required for applications such as depth determination in shallow navigation channels, dredging, and harbor maneuvers. Pseudolite augmentation results in improved satellite geometry and more effective and reliable ambiguity resolution. The use of a pseudolite can be crucial to resolving the ambiguities in areas where signal masking occurs because of topographic or manmade features. A shipborne test was conducted on Lake Okanagan, British Columbia, Canada, in November 1996, to validate these claims and assess pseudolite performance under field test conditions. A Stanford Telecom L1 pseudolite (Model 7201) was used, together with two NovAtel OEM-2 (RT-20) L1 12-channel Narrow Correlator receivers, which can receive the STel pseudolite signals without any modifications. The distance from ship to shore, where the reference receiver and pseudolite were located, reached 3,250 m. Between five and eight satellites were available during two tests. When using the full GPS constellation and with a masking angle of 10 deg (0.175 rad), it was found that pseudolite augmentation reduced the average time to ambiguity resolution by 35-40%, while improving the percentage of time when the ambiguities were solved correctly in the initial attempt from 95% to practically 100%. Weak geometry was simulated by removing a satellite from the data processing. Under such a condition, pseudolite augmentation reduced the average time to ambiguity resolution by more than 50% and resulted in a significant improvement in ambiguity resolution reliability.

RTCM SC-104 Recommended Pseudolite Signal Specification

Anyone involved in the design of GPS user equipment should be concerned with the potential impact of pseudolite signals. It is quite possible that pseudolite transmitters will be located near the majority of all airports and heliports, both rural and urban, in the United States and around the world. Therefore, even if a receiver is not intended for aircraft navigation or will not use the pseudolite signals, it may be affected by these signals. Differential GPS can improve the reliability, integrity, and accuracy of GPS navigation, but it cannot overcome the inherent geometric limitations of an 18 satellite constellation, especially when one of the satellites is out of service. Pseudolites transmit the differential correction message at the GPS L1 frequency, thus also providing an additional pseudorange measurement which overcomes poor satellite geometry. The key problem was to develop a GPS-like pseudolite signal structure which does not interfere with reception of GPS satellite signals even when a receiver is very close to a pseudolite transmitter. A simple and effective signal has been defined, but receiver manufacturers must assure that their designs are immune to pseudolite interference. This paper defines the proposed signal and describes the appropriate receiver design considerations.