2009: Precise Orbit Determination of Giove-A using DDIC and Initial Results

Published during the ION ITM from 26-28 January 2009 in Anaheim, California, US.

Abstract:

GIOVE-A is a first Galileo test satellite launched on December 28, 2005. The satellite payload transmits two frequency signals at L1+E5 or L1+E6. There are 13 world wide located Galileo Experimental Sensor Stations (GESS) to track and monitor the GIOVE-A signals. High accuracy of GIOVE-A ephemeris or orbit, however, is essential for navigation applications. The paper presents a new method to deal with the precise orbit determination of GIOVE-A satellite using pure carrier phase measurements and the initial results.

It is well known that the highest accuracy of orbit determination can be achieved using double differences of carrier-phase measurements. But the problem for such a solution is that in order to use double differences of carrier-phase measurements, at least two Galileo satellites are needed. For GIOVE-A satellite, it is difficult to form such double differences as not enough Galileo satellites are currently in space. Therefore the GIOVE-A carrier-phase measurements combined with GPS measurements would be a reasonable solution. This method is called Double Differences of Carrier Phase Measurements Inter Constellations of GPS/Galileo (DDIC).

In order to use DDIC, some problems shall be solved in advance, first of all the synchronization of GIOVE-A satellite measurements with GPS time reference, secondly the different time reference bias between GIOVE-A and GPS satellites, thirdly the different system bias between GPS and GIOVE-A satellites due to different coordinate systems and different system configuration and finally the weight normalization of GIOVE-A and GPS measurements for unified processing.

The time reference bias can be solved using navigation solution (or called PVT) with time synchronization parameters from GPS. All the GIOVE-A measurements are synchronized with GPS time after the navigation solution. This step implied the synchronization of Galileo time with GPS time. The system bias is also solved at the navigation step, but later after the first solution of the orbit determination. After the time synchronization, the weight normalization will be taken place. The following factors are considered for the normalization, satellite orbit characteristics (altitude and movement, etc.) and measurement accuracy. Then GIOVE-A orbit determination together with GPS satellite orbit is performed using dynamic method. After the GIOVE-A orbit is determined, the GIOVE-A satellite clock corrections are solved separately using pseudorange measurements.

The initial results using one day of DD (for GPS) and DDIC (for GIOVE-A) measurements show that the GIOVE-A orbit accuracy was achieved to about 20 – 80 cm level deduced implicitly from GPS orbit results compared with IGS final orbit. The implication means in this case, GIOVE-A is considered as another GPS satellite in the orbit determination. The accuracy will be further improved using multi-day measurements. The accuracy of the GIOVE-A satellite clock solution was in ns-level, but compared with GPS satellite clock solution, the GIOVE-A satellite clock bias increased quickly with time. For example, the GIOVE-A satellite clock bias was 800.522156 (ms)  at 00:00:00 on Jan. 9 2008. The bias became 1258.956921 (ms) at 00:00:00 on Jan. 21, 2008. At the same time period, the determined GPS satellite clocks, for example,  PRN02 Rb Clock corrections (processed with GIOVE-A satellite together) changed from 164.783651 (ms) to 167.890185 (ms).

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2008: GalTeC Flyer

Published on the Munich Satellite Navigation Summit from 19-21 February 2008 in Munich, D.

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2008: GNSS Performance Monitoring Services with GalTeC

Published during the ION GNSS from 16-19 September 2008 in Savannah, Georiga, US.

Abstract:

Currently two Global GNSS are in operation, the GPS and in the near future a fully deployed GLONASS. Galileo operates from April on with two experimental satellites Giove-A and -B. Additionally several Satellite Based Augmentation Systems (EGNOS, WAAS, MSAS) are in more or less advanced operational modes. They all provide different levels of services to the users. These users need different levels of information about the systems recent or past operational condition and performance. Also each user community describes performance in its own words. On the other hand, we have an increasing variety of satellite navigation based application services. The providers of such services need to assess the possible performances of the different satellite navigation systems before designing and implementing their services. The way to have a comparative view on GNSS performance will become more and more complex with the gradual introduction of Galileo and the stepwise replenishing of GPS and GLONASS with the next generation of space vehicles.

Therefore it seemed appropriate to develop an independent center to evaluate actual and future services and service levels in terms of performance parameters such as Accuracy, Integrity, Availability and Continuity. At a system level also the SIS performance is monitored using different statistical figures of merit. For this purpose, a project was started to develop a Galileo Service Center - called GalTeC (Galileo Technology Center), which considers not only Galileo but all relevant GNSS information. The project partners are Thales and NavPos Systems. GalTeC is a development project under contract of the German Aerospace Center (DLR) funded by the German Ministry of Economics and Technology. GalTeC will provide its services to the public via an Internet application. On-line information as well as on-request services will be available.
 
The GalTeC is composed of five major modules. The Measurements and Data Collection module collects on a daily basis data from own receivers and from internet sources for further processing. The Reference Determination module calculates Precise Satellite Orbit and Clock data and derived Signal-In-Space Errors. It also includes capability for recalculation of Position Integrity using raw measurements and recorded ephemeris. The Prediction and Simulation module analyses the performance of past, current or future GNSS constellations, SBAS and GBAS using service volume simulation. Furthermore, it also will support service providers in dimensioning and performance prediction of their services (like e.g. Galileo Commercial Services). The Analysis module produces several graphical and statistical representations on the basis of the Receiver data or Reference Module data. Finally the Service module composes out of the various single products an Internet representation and a set of summary reports. The Service module will handle also user access management to the GalTeC Internet site.

The project nears its completion in December 2008 and is currently under testing. Once testing is accomplished GalTeC will be set online for a demonstration phase with public access.

This paper will present as focal point the capabilities of GalTeC in terms of offered products and organization of the services with its options for use by the online community. Examples  of GPS, GLONASS and Giove-A real data performances will be shown. For the upcoming constellations Galileo and GPS-III, performance predictions will be presented as example for the powerful simulation capability.

Finally the last progress of GalTeC will be presented and an outlook into the future operation and development of the system will be provided.

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2008: PORIMA (Integrity Concept)

Published during the ION GNSS from 16-19 September 2008 in Savannah, Georiga, US.

Abstract:

With the development of the satellite orbit prediction algorithms and international satellite monitoring network such as IGS, the precise GPS satellite orbit and clock prediction can achieve an accuracy of about 10 cm for orbit and 5 ns for clock (timing). It’s well known that the accuracy of precise GPS prediction orbit and clock are much better than that of the GPS broadcast ephemeris. Many GNSS data processing facilities and services can produce the precise predicted GPS orbit and clock based on international GNSS monitoring networks (IGS) and user regional monitoring networks. The IGS ultra rapid orbit and clock are one of the IGS precise predicted GPS satellite orbit products, which are updated four times a day, and any user can get them from IGS sites at any time, therefore the precise predicted GPS orbit and clock can be used for real-time applications. This paper presents a newly developed user integrity monitoring methodology based on the precise predicted GNSS orbit and clock, called PORIMA (Precise Predicted Orbit based Receiver Integrity Monitoring Algorithms) for aviation and various real-time related applications, which includes a related protection level computation algorithm.

Currently there are a number of types of GNSS integrity monitoring concepts and methods, i.e. user receiver self-monitoring - RAIM, systems supported monitoring - SBAS/GBAS and Galileo. RAIM integrity monitoring is developed based on the redundant measurements. In order to detect satellite failures, the satellite measurement error threshold is determined based on statistics tested on the measurement residuals. Without any reference information about satellites SIS (Signal-in-Space) performance, these thresholds are usually the same for all visible satellites and reflect the worst case satellite performance. For SBAS-like systems, the thresholds are determined based on the regional monitoring network and related to so-called the worst user location in the region. These systems check the measurement errors from the regional monitoring network against the thresholds and send “don’t use” flags to users if some measurement errors are exceeded. The Galileo integrity concept is currently under development.

Using PORIMA, initially, the GPS broadcast ephemeris (satellite orbits and clocks) received by users are compared with the precise predicted orbit and clock obtained previously in order to check the GPS broadcast ephemeris consistency. When the checking is successful, the satellite range error thresholds related to an actual user location can be determined based on the differences between the broadcast ephemeris and the precise predicted orbits dependent on a risk requirement. These thresholds are unique for each satellite and reflect the actual satellite SIS performance status. In order to detect potential satellite failures, the satellite range measurement errors (residuals) at each epoch are checked against these thresholds. If some of the measurement errors exceed the related thresholds, the corresponding satellites are labeled as “bad”. The PVT solution related to that epoch is performed again and the protection levels are computed accordingly when the measurements from “bad” satellites are ignored. With this reiteration the satellite failures such as satellite maintenance anomaly, maneuver and clock jumps etc. can be detected with the related risk requirement as defined.

This paper also discusses the relations between the broadcast ephemeris and the precise predicted orbit and their impact on the PORIMA integrity monitoring method. In this discussion, three scenarios are considered: (1) the broadcast ephemeris are in line with the predicted precise orbit. SISRE (Signal-in-Space Range Errors) is small, and the thresholds are also small, but the true satellite track deviates largely from the predicted and the broadcast tracks; (2) true satellite track is in line with the predicted orbit, but the broadcast track deviates largely from the true track, the threshold is large; (3) true satellite track is in line with the broadcast ephemeris, but the predicted orbit deviates largely from the true satellite track, the thresholds are increased due to the large deviation. In conclusion, for scenario 1 und 2, the satellite failures can be detected using PORIMA; for scenario 3, according to IGS analysis reports, the performance of the IGS Ultra-rapid orbit are in centimetre levels. In this instance, large deviations, e.g. greater than 1 m, should be quite improbable. However, with the application of PORIMA, a plausible checking of GPS ephemeris and predicted orbit has been proved to detect such problems.

As an example of successful application, the paper presents analysis results using PORIMA to detect a GPS PRN 18 satellite maintenance anomaly that occurred on April 10, 2007 at about 15:53 (UTC), according to FAA GPS SPS Performance Analysis Report 58. In order to demonstrate our method, the measurements from two IGS sites, Auckland, New Zealand and Victoria, Canada were selected for the test. The satellite range measurement error thresholds at these two sites were computed based on the orbit and clock differences between the broadcast ephemeris and IGS ultra-rapid orbit updated at 12:00 (UTC) on that day. The measurements were processed epoch by epoch. The data analysis shows that the PRN 18 satellite failure was successfully detected at both sites, but there were some delays. The delay was related to a geographical site location and the satellite visibility.

According to the analysis and test results presented in this paper, it can be concluded that PORIMA can be used for user integrity monitoring for GNSS constellations to improve the user integrity performance. The test results show that GPS PRN 18 anomaly failure was successfully detected using the PORIMA. For a single frequency receiver, the performance of this approach is better than RAIM. For dual frequency receivers, the performance can be significantly improved. As a supplementary tool to RAIM, PORIMA can be used as a real-time monitor of the GPS SIS integrity, and is especially useful for the areas outside currently existing SBAS service areas.

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2007: GNSS Performance Monitoring Services with GalTeC

Published during the ION GNSS from 25-28 September 2007 in Forth Worth, Texas, US.

Abstract:

This paper presents a concept of integrating different GNSS information sources in one single GNSS Service Centre which provides comparable and understandable information for different user groups and their needs.

The first part of the paper will introduce the architecture and concept of GalTeC which is to be understood as a GNSS Service Centre for the Satellite Navigation Systems Galileo, GPS, and EGNOS as a first implementation.

GalTeC is based on four corner stones. The first of them is the Satellites Reference Orbit & Clock reconstitution, which serves as the basis of analysis in Signal In Space(SIS) domain. First results of this capability is presented upon the SIS related behaviour of each GPS satellite for a representative time period. The second cornerstone of GalTeC will be the analysis capability in the user domain, accounting raw measurements with all errors along the path. They serve for determination of positions, protection levels (SBAS, AIM), integrity risks (Galileo). The third cornerstone of GalTeC is its powerful servicevolume simulation capability for different kinds of constellations and different integrity methods (RAIM, SBAS, GBAS, Galileo Integrity). Within this paper, results will be presented using the new AVIGA Service Availability Module. The module results are the Availabilities of Accuracy, Integrity Risk, Continuity Risk and Availability of Service. The fourth cornerstone of GalTeC is the collection and structured handling and archiving of GNSS monitor station observations and computed data. For the results obtained in this paper the IGS provided observations are used on the one hand and one EGNOS/GPS receiver on the other hand. Finally an outlook on the Services Provision methodology and architecture is given and the offered products are presented showing an example of the internet browser based functionality and styles for condensing the GPS and EGNOS/GPS analysis at staged levels of information depth.

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2006: Independent validation of Galileo global and regional integrity performance using GalTeC

Published on the European Navigation Conference (ENC) from 7-10 May 2006 in Manchester, UK.

Abstract:

With the Galileo development programme now well on its way, the technical performances and merits driving the downstream business opportunities have been given considerable attention by the Galileo Concessionaire. The establishment of  facilities to analyse, validate and survey operational availability of services and their underlying technical performances are essential to guarantee Galileo objectives. One of such facilities is the [Gal]ileo [Te]chnology [C]enter GalTeC, representing one of the first Galileo Service Center prototypes, currently under development in joint cooperation by Thales ATM GmbH and NavPos Systems GmbH, supported by the DLR. This paper describes the GalTeC project.

Currently the first of two experimental Galileo satellites - Giove A has been placed in orbit and transmits successfully Galileo like signals for institutional and experimental analyses. Later in 2006 the second experimental Galileo satellite - Giove B will be launched. Finally in 2008 with the In-Orbit-Validation phase, the Signal and Service evaluation programmes will start using signals of four Galileo satellites. The evaluation will take place at systems level as well as externally by independent assessment. GalTeC will be such an independent GNSS data processing and system performance validation facility. As such it features four main branches, the precise Reference Orbit Determination Service, the Service Level Prediction and the Data Analysis Service complemented later on by a Network of Regional/User Ground Stations.

The Galileo services validation will be performed in the following two domains. First - in the Signal in Space (SIS) domain, i.e. precise Galileo satellite orbits and related reference parameters such as reference signal in space error SISRE and reference signal in space accuracy SISRA will be determined.  These reference parameters will be compared with the original broadcast SISA and SISMA from Galileo SIS. Performance Indicators will be generated in order to show the historical system performance.

Second - in the user domain, the global and regional Horizontal and Vertical Protection Levels will be computed based on the reference SISRE and SISRA, the broadcast SISA/SISMA and the global and regional ground station network. Statistical analysis will be performed on these results to show the Galileo integrity performance in the user domain.

The paper first describes in detail the objectives of the GalTeC project. Then an overview of the development status is given together with an outlook for possible types of services which could be provided to users and service providers. Additionally, some results of validation of Galileo integrity performance will be presented, using service prediction data including some scenarios as well as using real GPS measurements. As an outlook, GalTeC not only focuses on suitable validations of Galileo system performances but examines options to enhance the Galileo integrity performance in  regional and local areas.

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