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# One-Centimeter Orbits in Near-Real Time: The GPS experience on OSTM/Jason-2 4

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## Abstract

The advances in Precise Orbit Determination (POD) over the past three decades have been driven in large measure by the increasing demands of satellite altimetry missions. Since the launch of Seasat in 1978, both tracking-system technologies and orbit modeling capabilities have evolved considerably. The latest in a series of precise (TOPEX-class) altimeter missions is the Ocean Surface Topography Mission (OSTM, also Jason-2). GPS-based orbit solutions for this mission are accurate to 1-cm (radial RMS) within three to five hours of real time. These GPS-based orbit products provide the basis for a near-real time sea-surface height product that supports increasingly diverse applications of operational oceanography and climate forecasting.

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... There are 2 levels of GPS solutions for space based radio arrays. One is to have Real Time Gipsy (RTGx) running on all of the spacecraft, this gives an approximate location to within 22 ns that is good enough to synchronize the taking of data between the duty cycles of all the spacecraft [37]. ...
... These projected separations are important, and will be used later in the pipeline to simulate the synthetic beam of the array.Using models of the GNSS sidelobes and expected signal strengths at the SunRISE altitude, we generate a set of (3-D) position and timing uncertainties for each S/C. These values were given by the GNSS-Inferred Positioning System and Orbit Analysis Simulation Software package (GipsyX)[37]. This software has been successfully leveraged for many Low Earth Orbiting missions such as Jason-1, Jason-2/OSTM, and GRACE. ...
Thesis
Due to Earth’s ionosphere, it is not possible to image the sky below 10 MHz. Any waves below this cutoff frequency are absorbed by the plasma in Earth’s ionosphere, whose free electron density determines the cutoff. A constellation of small spacecraft above the ionosphere could enable radio imaging from space at frequencies below this cutoff, but the logistics and costs of doing this imaging using multiple satellites that are kilometers apart in a precise enough manner to form a radio array has until recently been unfeasible. With the lowering costs and increasing reliability of smallsats, the use of radio arrays in space is finally set to open up this new window through which we may observe the universe in a new light. For complex sources in the sky, analytical formulas are not enough to predict array performance; full simulations must be done to evaluate potential array configurations. Simulated outputs must be compared to a realistic input model to make sure that a given array configuration can meet its defined scientific requirements. Space-based arrays also introduce additional challenges in understanding novel data processing and errors from location retrieval of the receivers and budgeting for data transmission. In this thesis I demonstrate the feasibility for different space-based radio arrays by simulating their performance under realistic conditions. I outline the science goals involving radio imaging below 10 MHz for a range of solar, astrophysical, and magnetospheric targets. I then outline different strategies for creating synthetic apertures in space that are well suited for each of these targets. I describe the calculations needed for each style of correlation and create a data processing and science analysis pipeline for showcasing the imaging performance of each simulated array. I show that the SunRISE and RELIC array concepts are both able to meet their main scientific goals of localizing solar radio bursts and mapping radio galaxies respectively. I describe a novel way in which I use magnetohydrodynamic simulations of a solar eruption alongside real radio data to predict the sky brightness patterns of the radio bursts for input to the SunRISE pipeline across different theories of particle acceleration. This technique provides initial predictions of the location of solar type II burst generation in a coronal mass ejection that SunRISE can potentially confirm. I also demonstrate the feasibility of a lunar near side array powerful enough to image the Earth’s synchrotron emission, along with a zoo of brighter auroral emissions. Synchrotron measurements would provide a unique proxy measurement of the global energetic electron distribution in the Earth’s radiation belts. Such an array could also pinpoint the location of brighter transient events such as Auroral Kilometric Radiation with high precision, providing local, small scale electron data in addition to global data. The time finally seems ripe for low frequency radio astronomy to make its move to outer space. Increased feasibility of small satellites is a huge game changer for the entire space industry, incentivizing mission designs that can take advantage of the distributive nature of multiple small inexpensive spacecraft to do the jobs traditionally done, or unable to be done, by larger, more costly single spacecraft. In that same spirit, this work acts as a helpful starting point for the general mission design, data processing, and science analysis required for distributed space-based radio arrays.
... The used GPS receiver is a BlackJack receiver built by Spectrum Astro Inc. It features 16 dual frequency channels and is connected to a patch antenna with choke rings (Haines et al., 2011), shown inA secondary mission goal is to provide observations for a real time forecasting system of ionospheric irregularities. The system will be used to forecast possible navigation or communication outages for United States military forces (de La Beaujardière, 2004).To accomplish the scientific objectives the satellite is equipped with six different sensors to study the Earth's ionosphere. ...
Thesis
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Observing temporal changes of gravity has become a vital source of information about changes in the system Earth. Currently these variations are observed by the satellite mission Gravity Recovery and Climate Experiment (GRACE). Besides this mission no other technique is capable of providing the same resolution, both in space and time. Although a follow-on mission is under preparation, it is likely that the highly valuable observational record might be interrupted. Hence, there is a great interest in developing an additional opportunity to observe variations in the Earth’s gravity field. One possible method to observe the gravity field is based on precise positions of low Earth orbiting satellites. The position of the satellite can be observed by an on-board Global Navigation Satellite System (GNSS) receiver. It provides measurements which can be used to compute a kinematic orbit without introducing a priori information. Subsequently the positions can be used to estimate the Earth’s gravity field. The approach, denoted as Satellite-to-Satellite Tracking in high-low mode (SST-hl), is well known and widely used. However, the accuracy of the derived gravity field solutions depends on the quality of the introduced orbit positions. Current state-of-the-art orbits are only capable of resolving the largest gravity variations. Available orbit estimates are degraded by systematic effects or the measurement noise exceeds the amplitude of the sought for signals. The goal of this work was to develop a new method for kinematic orbit determination based on raw GNSS measurements. The essence of the proposed raw observation approach is to leave the GNSS measurements unchanged and process all observables jointly in an iterative least-squares adjustment. Systematic effects are either corrected or set up as additional parameters. The combination of different observation types necessitates a realistic weighting scheme in combination with a flexible outlier detection. The validation of computed kinematic orbits revealed that the raw observation approach is capable of producing orbit positions with the same or better accuracy than existing methods. Estimated satellite positions were then used to generate monthly gravity field solutions. Investigations based on a 13 year time series, including data from 15 satellites, showed that it is possible to observe gravity changes. An analysis of major river basins revealed that mass variations can be detected for areas larger than 500 000 km2 , if the amplitude of the signal is sufficiently large. In view of the continuously increasing number of satellites equipped with GNSS receivers and the ongoing evolution of GNSS in general, the results obtained in this work suggest that SST-hl could be a true alternative or at least a supplement to other technologies.
Chapter
This chapter addresses the fundamentals of global navigation satellite system‐based orbit determination, highlighting the unique aspects of the technique relative to conventional terrestrial positioning. The unique challenge in satellite orbit determination is not the formulation or the solution of the estimation problem but, rather, the validation of the solution. Much of the special technology and expertise in precise orbit determination revolves around techniques and approaches for assessing solution accuracy. The inversion of the orbit determination problem to estimate the epoch state and model parameters can be accomplished with a variety of techniques, from classical least squares to various flavors of Kalman filtering or other statistical techniques. Regardless of the employed estimation technique, the first step is always the linearization of the problem around an initial approximation of the solution, consisting of a trajectory and a corresponding set of model parameters.
Article
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Satellite laser ranging (SLR) to low Earth orbiters (LEOs) provides optical distance measurements with mm-to-cm-level precision. SLR residuals, i.e., differences between measured and modeled ranges, serve as a common figure of merit for the quality assessment of orbits derived by radiometric tracking techniques. We discuss relevant processing standards for the modeling of SLR observations and highlight the importance of line-of-sight-dependent range corrections for the various types of laser retroreflector arrays. A 1–3 cm consistency of SLR observations and GPS-based precise orbits is demonstrated for a wide range of past and present LEO missions supported by the International Laser Ranging Service (ILRS). A parameter estimation approach is presented to investigate systematic orbit errors and it is shown that SLR validation of LEO satellites is not only able to detect radial but also along-track and cross-track offsets. SLR residual statistics clearly depend on the employed precise orbit determination technique (kinematic vs. reduced-dynamic, float vs. fixed ambiguities) but also reveal pronounced differences in the ILRS station performance. Using the residual-based parameter estimation approach, corrections to ILRS station coordinates, range biases, and timing offsets are derived. As a result, root-mean-square residuals of 5–10 mm have been achieved over a 1-year data arc in 2016 using observations from a subset of high-performance stations and ambiguity-fixed orbits of four LEO missions. As a final contribution, we demonstrate that SLR can not only validate single-satellite orbit solutions but also precise baseline solutions of formation flying missions such as GRACE, TanDEM-X, and Swarm.
Chapter
Signals transmitted by global navigation satellite system (GNSS ) satellites are not confined to the surface of the Earth but can likewise be used for navigation in space. Satellites in low Earth orbits, in particular, benefit from a similar signal strength and experience a full-sky visibility. On the other hand, the harsh space environment, long-term reliability requirements and the high dynamics of the host platform pose specific challenges to the design and operation of space-borne GNSS receivers. Despite these constraints, satellite manufacturers and scientists have early on started to exploit the benefits of GNSS technology. From the first flight of a Global Positioning System (GPS ) receiver on Landsat-4, GNSS receivers have evolved into indispensable and ubiquitous tools for navigation and control of space vehicles. Following a general introduction, the chapter first describes the specific aspects of GNSS signal tracking in space and highlights the technological challenges of space-borne receiver design. Subsequently, the use of GNSS for spacecraft navigation is discussed taking into account both real-time navigation and precise orbit determination. Relevant algorithms and software tools are discussed and the currently achieved performance is presented based on actual missions and flight results. A dedicated section is devoted to the use of space-borne GNSS for relative navigation of formation flying satellites. The chapter concludes with an outlook on special applications such as spacecraft attitude determination, GNSS tracking of ballistic vehicles as well as GNSS radio science.
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The radar imaging satellite mission TerraSAR-X requires precisely determined satellite orbits for validating geodetic remote sensing techniques. Since the achieved quality of the operationally derived, reduced-dynamic (RD) orbit solutions limits the capabilities of the synthetic aperture radar (SAR) validation, an effort is made to improve the estimated orbit solutions. This paper discusses the benefits of refined dynamical models on orbit accuracy as well as estimated empirical accelerations and compares different dynamic models in a RD orbit determination. Modeling aspects discussed in the paper include the use of a macro-model for drag and radiation pressure computation, the use of high-quality atmospheric density and wind models as well as the benefit of high-fidelity gravity and ocean tide models. The Sun-synchronous dusk–dawn orbit geometry of TerraSAR-X results in a particular high correlation of solar radiation pressure modeling and estimated normal-direction positions. Furthermore, this mission offers a unique suite of independent sensors for orbit validation. Several parameters serve as quality indicators for the estimated satellite orbit solutions. These include the magnitude of the estimated empirical accelerations, satellite laser ranging (SLR) residuals, and SLR-based orbit corrections. Moreover, the radargrammetric distance measurements of the SAR instrument are selected for assessing the quality of the orbit solutions and compared to the SLR analysis. The use of high-fidelity satellite dynamics models in the RD approach is shown to clearly improve the orbit quality compared to simplified models and loosely constrained empirical accelerations. The estimated empirical accelerations are substantially reduced by 30% in tangential direction when working with the refined dynamical models. Likewise the SLR residuals are reduced from $$-3\,\pm \,17$$ to $$2\,\pm \,13$$ mm, and the SLR-derived normal-direction position corrections are reduced from 15 to 6 mm, obtained from the 2012–2014 period. The radar range bias is reduced from $$-10.3$$ to $$-6.1$$ mm with the updated orbit solutions, which coincides with the reduced standard deviation of the SLR residuals. The improvements are mainly driven by the satellite macro-model for the purpose of solar radiation pressure modeling, improved atmospheric density models, and the use of state-of-the-art gravity field models.
Conference Paper
This paper presents a novel concept and algorithms for the precise positioning of geostationary data relays using Low Earth Orbit (LEO) satellites. Simulations show positioning accuracies at centimetre level. A time synchronization between the LEO satellite and the ground station is decisive for achieving such high accuracies. The estimated positions enable orbit predictions for geostationary data relays with accuracies below one meter for more than six hours in advance.
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Observations of the Global Positioning System (GPS) will enable a reduced-dynamic technique for achieving subdecimeter orbit determination of earth-orbiting satellites. With this technique, information on the transition between satellite states at different observing times is furnished by both a formal dynamic model and observed satellite positional change (which is inferred kinematically from continuous GPS carrier-phase data). The relative weighting of dynamic and kinematic information can be freely varied. Covariance studies show that in situations where observing geometry is poor and the dynamic model is good, the model dominates determination of the state transition; where the dynamic model is poor and the geometry strong, carrier phase governs the determination of the transition. When neither kinematic nor dynamic information is clearly superior, the reduced-dynamic combination of the two can substantially improve the orbit-determination solution. Guidelines are given here for selecting a near-optimal weighting for the reduced-dynamic solution, and sensitivity of solution accuracy to this weighting is examined.
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TOPEX/Poseidon has been collecting altimeter data continuously since October 1992. Altimeter data have been used to produce maps of sea surface height, geostrophic velocity, significant wave height, and wind speed. This information is of proven use to mariners as well as to the scientific community. Uses of the data include commercial and recreational vessel routing, ocean acoustics, input to geographic information systems developed for the fishing industry, identification of marine mammal habitats, fisheries management, and monitoring ocean debris. As with sea surface temperature data from the Advanced Very High Resolution Radiometer (AVHRR) in the late 1980s and early 1990s, altimeter data from TOPEX/Poseidon and ERS-1 and -2 are in the process of being introduced to the marine world for operational maritime use. It is anticipated that over the next few years companies that specialize in producing custom products for shipping agencies, fisheries and yacht race competitors will be incorporating altimeter data into their products. The data are also being incorporated into weather and climate forecasts by operational agencies both in the US and Europe. This paper will discuss these products, their uses, operational demonstrations and means of accessing the data
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We describe a near-real-time (NRT) precise orbit determination (POD) system for the Ocean Surface Topography Mission/Jason-2 satellite altimeter mission that processes tracking data from the onboard "BlackJack" Global Positioning System (GPS) receiver. The NRT POD system is now operational, and the resulting orbit solutions are being used to generate a value-added NRT sea surface height product for operational altimetry applications. The NRT GPS-based orbit solution features radial orbit accuracies of 1 cm (RMS) with a latency of < 4 hours. These orbit accuracies are achieved through the use of Ultra-Rapid solutions for the orbits and clocks of the GPS constellation of satellites. We use satellite laser ranging tracking data and sea surface height crossover residuals as external metrics for evaluating orbit accuracy. We also provide comparisons to other orbit solutions of varying latencies to illustrate the trade between accuracy and timeliness.
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Radar altimeter missions require precise estimates of the satellite radial orbit position in order to support measurement of surface heights. Traditional applications of altimeter data have not placed serious demands on the timeliness of this precise orbit information. Many of these applications involve retrospective scientific analysis of phenomena such as ocean current patterns, permanent geoid features, and slow changes in the thickness of the polar ice sheets. For these studies, latencies of days to weeks in receiving the precise altimeter data sets provide little impediment to successfully completing the research. Owing to the continuing successes of the Topex/Poseidon (T/P) and ERS missions, however, we are presently witness to the beginnings of a new operational era in satellite altimetry. The most demanding of the operational applications, e.g., short-term climate forecasting, require that accurate orbits be made available in near-real time. Precise orbit determination techniques based on data from on-board global positioning system (GPS) receivers have contributed significantly to meeting these requirements and show great promise for keeping pace with the demands of future missions. In this paper, we describe recent advances in near-real-time orbit determination for the T/P mission. New results suggest that GPS-based orbits computed within one day of recording the last element of tracking data have radial accuracies of about 3 cm in a root-mean-square (RMS) sense. These orbits are used by a variety of specialized users in the oceanographic community in order to support climate forecasting and real-time monitoring of the global ocean.
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We present strategies and results for near real-time (NRT) precise orbit determination (POD) of the Global Positioning System (GPS) constellation. The POD for the GPS constellation is performed using a global network of 40 ground stations. The resulting products are available with a latency of about one hour, and include orbit and clock estimates for the GPS satellites, as well as widelane phase bias information from the global solution. The widelane information, when used with the orbit and clock estimates, enables singlereceiver, ambiguity resolved GPS-based positioning. Comparisons to definitive final products from the Jet Propulsion Laboratory and International GNSS Service show that NRT orbit accuracies of 5 cm RMS (3D) and clock accuracies of 5 cm RMS are achieved. Daily point positioning of a variety of static ground station receivers using these products yields repeatabilities of 1 cm. An additional NRT process, in turn, utilizes the GPS orbit, clock, and widelane products to perform POD for the Ocean Surface Topography Mission (OSTM)/Jason-2 satellite, which carries an advanced dual-frequency "Blackjack" GPS receiver. The radial accuracy of the resulting OSTM/Jason-2 orbits is typically 1 cm (RMS) with a latency of 2 hours. These new orbit solutions provide the basis for computing accurate sea-surface height information for operational oceanographic and low-latency scientific applications of satellite altimeter data.
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Modern applications with satellite-borne altimeters, which include operational altimetry, place requirements on Precision Orbit Determination (POD) that include automation and near real-time or low latency (e.g., < 12 hours) in the generation of the POD products. The Ice, Cloud and land Elevation Satellite (ICESat) has achieved some POD automation in support of laser altimetry but the latency requirements are not near real-time. Nevertheless, ICESat data provides an opportunity to explore approaches for near real-time POD by adapting the ICESat methodologies to experiment with various options to reduce the POD product latency from a few days to less than 12 hours. For these experiments, it was assumed that the POD would be generated in a stand-alone ground environment using an ICESat-like on-board GPS receiver. Factors that influence the overall latency include the latency associated with the flight GPS receiver and the latencies associated with other data required for the generation of the POD product. Focus is given to a scenario which produces an ephemeris for the GPS receiver host satellite with < 5 cm radial accuracy and with a net latency of less than 7 hours, where the latency is primarily determined by delays in receipt of data from the flight GPS receiver.
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Integer ambiguity fixing is routinely applied to double-differenced GPS phase measurements to achieve precise positioning. Double-differencing is interesting because it removes most of the common errors between the different signal paths. However, if common errors can be estimated it becomes attractive to fix integer ambiguities on undifferenced measurements. Phase measurements then become pseudorange-like measurements with a noise level of a few millimeters. This paper introduces a new method for fixing dual-frequency GPS ambiguities on undifferenced phase measurements either locally or globally. The clocks for the GPS constellation obtained during this process can be used for precise point positioning of ground based receivers and for precise orbit determination of low Earth orbiting satellites. The resulting positioning precision is comparable to that of standard differential positioning without the need for a reference station. Ambiguity-fixed satellite orbits for the GRACE and Jason satellites are more precise than the most precise solution available today.
Article
The Ocean Surface Topography Mission/Jason-2 (OSTM/Jason-2) satellite altimetry mission was successfully launched on June 20, 2008, as a cooperative mission between CNES, EUMETSAT, NASA, and NOAA. OSTM/Jason-2 will continue to precisely measure the surface topography of the oceans and continental surface waters, following on the same orbit as its predecessors, TOPEX/Poseidon and Jason-1. To maintain the high-accuracy measurements, the mission carries a dual-frequency altimeter, a three-frequency microwave radiometer, and three precise positioning systems. The objectives of the mission are both operational and scientific. The mission will provide near-real time high-precision altimetric measurements for integration into ocean forecasting models and other products. The mission will also extend the precise surface topography time series started by TOPEX/Poseidon in 1992 over two decades in order to study long-term ocean variations such as mean sea level variations and interannual and decadal oscillations. The measurement system has been adapted to provide quality data nearer to the coasts, and over lakes and rivers. This paper provides an overview of the OSTM/Jason-2 mission in terms of the system design and a brief introduction to the science objectives.
Article
recision orbit determination (OD) methodologies have evolved over the past 50 years through research by astrodynamics specialists from industry, university, and government organizations. Refinements have included improvements in mod- eling techniques from analysis of satellite tracking data over a wide range of orbits. Methods have been developed to evaluate force models and the enhancement of model fidelity using a variety of geodetic-quality satellites placed into orbit since the early days of the space program and continuing today. This article provides an over- view of OD methodologies and their evolution as well as a brief description of modern OD and estimation methods that are being used routinely in the 21st century by the astrodynamics community. The subject matter should also be useful reading for the nonspecialist.
Article
The U.S./French Jason-1 oceanographic mission is carrying state-of-the-art radiometric tracking systems (GPS and DORIS) to support precise orbit determination (POD) requirements. The performance of the systems is strongly reflected in the early POD results. Results of both internal and external (e.g., satellite laser ranging) comparisons indicate that the root-mean-square (RMS) radial accuracy is in the range of 1–2 cm. This paper reviews the POD strategy underlying these orbits, as well as the challenging issues that bear on the understanding and characterization of an orbit solution at the 1 cm level. It also describes a GPS-based system for producing science-quality orbits in near real time to support emerging applications in operational oceanography.
Article
The U.S./French Jason-1 satellite is carrying a state-of-the-art GPS receiver to support precise orbit determination (POD) requirements. The performance of the Jason-1 “BlackJack” GPS receiver was strongly reflected in early POD results from the mission, enabling radial accuracies of 1–2 cm soon after the satellite's 2001 launch. We have made further advances in the GPS-based POD for Jason-1, most notably in describing the phase center variations of the on-board GPS antenna. We have also adopted new geopotential models from the Gravity Recovery and Climate Experiment (GRACE). The new strategies have enabled us to better exploit the unique contributions of the BlackJack GPS tracking data in the POD process. Results of both internal and external (e.g., laser ranging) comparisons indicate that orbit accuracies of 1 cm (radial RMS) are being achieved for Jason-1 using GPS data alone.
Article
To achieve maximum benefit from the altimetric data collected by the French-aAmerican TOPEX/POSEIDON spacecraft, radial orbit accuracy of 10 cm or better is required. This unprecedented requirement led the French Space Agency Centre National d'Etudes Spatiales (CNES) to develop a new high-accuracy tracking system, Doppler orbitography and radiopositioning integrated by satellite (DORIS), and a new precision orbit production facility, the Service d'Orbitographie DORIS. A global effort produced new models and a new orbit determination strategies. The result of these efforts has been assessed after 1 year of operation. The original goal has clearly been met, and the TOPEX/POSEIDON orbits produced by NASA and CNES agree beter than the 5 cm root mean square (RMS) level in the radial direction. At this level of accuracy, traditional techniques cannot correctly describe the actual orbit error, and some new procedures are proposed.
Article
The TOPEX/POSEIDON (T/P) altimeter satellite has recently become an integral part of NOAA's operational satellite system for monitori ng the oceans. The transformation was achieved through the joint efforts of NOAA, JPL, and NAVOCEANO. Since late 1996, this team has produced accurate T/P sea level observations with a delay of only two daysófast enough to be included in NOAA's weekly ocean model run. Operational assimilation began in March 1997. The T/P data improve both the ocean initial conditions and the sea surface temperature forecast s with lead times of up to 6 months.
Article
Until now, TOPEX/Poseidon precise orbits were needed only for the production of Geophysical Data Record files, and thus were not required until about 5 weeks after data acquisition. Recent developments in operational oceanography now require the rapid delivery of precise altimeter data within days, and possibly hours, of data acquisition. The processing of the altimeter measurements can be accomplished according to this schedule, and the only difficulty rests with the production of the precise orbit ephemerides. The long delay involved in the current production scheme results from the necessity to collect laser tracking data from ground stations and also from the need to wait for the final and most accurate values of the solar activity and Earth orientation parameters. A reduction in the orbit production delay forces the processing to deal with DORIS data only and with predicted values for the parameters. In addition, this reduces the amount of validation that can be performed before delivery. Fortunately, the spatial and temporal coverage of the DORIS tracking system is such that the DORIS data by itself is sufficient to produce a precise orbit. Also, predictions of solar activity and Earth orientation parameters have improved considerably over the last few years, so that using them instead of actual data does not significantly degrade the orbit accuracy. Using this strategy, DORIS orbits have been computed on a daily basis within 24 to 48 hours of data acquisition. And since the beginning of October 1997, these orbits have been included on the Posëdon Interim Geophysical Data Record files for all cycles when this altimeter is on. Evaluations of these daily orbits reveal that their radial accuracy is very close to that of the standard precise orbits ephemerides.
Article
The Jason-1 altimeter satellite and its follow-on mission Jason-2/OSTM were launched in December 2001 and June 2008, respectively, to provide the scientific community with a high-accuracy continuous record of observations of the ocean surface topography. Both missions carry on board three state-of-the-art tracking systems (DORIS, GPS, SLR) to meet the requirement of better-than-1.5 cm radial accuracy for the operational orbit included in the geophysical data record (GDR) product.This article outlines the common set of models and processing techniques applied to both Jason reprocessed and operational orbits included in version C of the GDR, referred to as GDR-C standards for precision orbit determination (POD), and describes the systematic components of the radial error budget that are of most interest for the altimeter data analysts. The nonsystematic component of the error budget, quantified by intercomparison of orbits using similar models or with reduced dependency on the dynamic models, is generally at or below 7 mm RMS (root-mean-square). In particular, the average daily RMS of the radial difference between the JPL and CNES reduced-dynamic orbits on Jason-2 is below 6 mm. Concerning the dynamic models employed, the principal contributors to residual systematic differences appear to be the time varying gravity and solar radiation pressure, resulting in geographically correlated periodic signals that have amplitudes at the few-mm level. Concerning the drifts of the orbits along the North/South direction, all solutions agree to better than the 1 mm/year level.
Article
We assess the accuracy of JPL's estimated OSTM/Jason-2 Global Positioning System (GPS)-determined orbits based on residuals to independent satellite laser ranging (SLR) data, compared with orbits produced by different software from different data (SLR/DORIS), Geophysical Data Record version C (GDR-C) orbits, and altimeter crossover tests. All of these tests are consistent with sub-cm radial accuracy: high elevation SLR residual standard deviation lies at 6.8 mm, RMS differences from GDR-C in the radial component typically fall below a cm, and altimeter crossovers from JPL orbits have a variance 89 mm smaller than altimeter crossovers from GDR-C orbits. Although RMS differences between radial components of different orbit solutions typically lie below a cm, we observe systematic dependences on both time and geography.The improved precision and accuracy of JPL's OSTM/Jason-2 orbit solutions rely on a new algorithm for applying constraints to integer carrier phase ambiguities. This algorithm is sufficiently robust to improve solutions despite half-cycle carrier phase identification issues in OSTM/Jason-2's BlackJack receiver. Although Jason-1 receiver performance differs, our algorithm should extend to Jason-1 processing (during the time span of nominal GPS receiver operations).
Article
Plans are presented for flying a Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) system on SPOT platforms. DORIS is designed for precise orbit positioning for use as part of the Topex/Poseidon payload. The measurement precision of DORIS depends upon the characteristics of Ultra Stable Oscillators (USOs). Laboratory experiments testing the stability of the USOs for long, medium, and short term behavior are presented.
Article
The Jason-1 Operational Sensor Data Record (OSDR) is intended as a wind and wave product that is aimed towards near-real-time (NRT) meteorological applications. However, the OSDR provides most of the information that is required to determine altimetric sea surface heights in NRT. The exceptions include a sufficiently accurate orbit altitude, and pressure fields to determine the dry troposphere path delay correction. An orbit altitude field is provided on the OSDR but has accuracies that range between 8-25 cm (RMS). However, tracking data from the on-board BlackJack GPS receiver are available with sufficiently short latency for use in the computation of NRT GPS-based orbit solutions. The orbit altitudes from these NRT orbit solutions have typical accuracies of < 3.0 cm (RMS) with a latency of 1-3 h, and < 2.5 cm (RMS) with a latency of 3-5 h. Meanwhile, forecast global pressure fields from the National Center for Environmental Prediction (NCEP) are available for the NRT computation of the dry troposphere correction. In combination, the Jason-1 OSDR, the NRT GPS-based orbit solutions, and the NCEP pressure fields can be used to compute sea surface height observations from the Jason-1 mission with typical latencies of 3-5 h, and have differences with those from the 2-3 day latency Interim Geophysical Data Records of < 5 cm (RMS). The NRT altimetric sea surface height observations are potentially of benefit to forecasting, tactical oceanography, and natural hazard monitoring.
Article
We have used GPS carrier phase integer ambiguity resolution to investigate improvements in the orbit determination for the Jason-1 satellite altimeter mission. The technique has been implemented in the GIPSY orbit determination software developed by JPL. The radial accuracy of the Jason-1 orbits is already near 1 cm, and thus it is difficult to detect the improvements gained when the carrier phase ambiguities are resolved. Nevertheless, each of the metrics we use to evaluate the orbit accuracy (orbit overlaps, orbit comparisons, satellite laser ranging residuals, altimeter crossover residuals, orbit centering) show modest improvement when the ambiguities are resolved. We conservatively estimate the improvement in the radial orbit accuracy is at the 10–20% level.
Article
Precise measurements of sea surface height from the Jason-1 and Jason-2/Ocean Surface Topography Mission satellite altimeter missions are being generated within seven and four hours, respectively, of real time by applying high accuracy orbit altitudes to their near-real-time altimetry products. The near-real-time orbit altitudes have respective accuracies of <2 and <1 cm (RMS), and the corresponding sea surface height measurements have accuracies of <4.0 and <3.5 cm, respectively. These near-real-time orbit solutions are computed using GPS-based precise orbit determination for Jason-2, and Jason-1/Jason-2 sea surface height crossover differences for Jason-1.
Chapter
The basic concept of satellite altimetry is to measure the range from the satellite to the sea surface. The altimeter transmits a short pulse of microwave radiation with known power toward the sea surface. The pulse interacts with the rough sea surface and a part of the incident radiation reflects back to the altimeter. The chapter emphasizes on the correction algorithms applied to the dual-frequency altimeter onboard the TOPEX/POSEIDON (T/P) satellite. This state-of-the-art altimeter sets the standard for future altimeter missions as it is significantly more accurate than any of the other altimeters that have been launched to date. To provide assurance that the performance requirements for altimeter measurement accuracy are met or exceeded, extensive calibration and validation (cal/val) are important elements of altimeter missions. Cal/val embraces a wide variety of activities, ranging from the interpretation of information from internal-calibration modes of the sensors to the validation of the fully corrected sea-level estimates using in situ data. The chapter concludes with a summary of the T/P mission design and an assessment of the performance of the T/P dual-frequency altimeter in addition, as well as an overview of future altimeter missions.
Article
JPL's BlackJack receiver currently represents the most widely used geodetic grade GPS receiver for space applications. Using data from the CHAMP science mission, the in-flight performance of the BlackJack receiver has been assessed and the impact of various software updates performed during the 2.5 years since launch is described. Key aspects of the study comprise the channel allocation, anomalous data points, and the noise level of the code and carrier data. In addition, it has been demonstrated that the code measurements collected onboard the CHAMP satellite are notably affected by multipath errors in the aft-looking hemisphere, which can be attributed to cross-talk between the occultation antenna string and the primary precise orbit determination antenna. For carrier smoothed 10 s normal points, the code noise itself varies between a minimum of 5 cm at high elevations and 0.5 m (C/A) to 1.0 m (P1, P2) at 10° elevation. Carrier-phase data exhibit representative errors of 0.2 to 2.5 mm. The results of the CHAMP GPS data analysis contribute to a better understanding and possible improvement of the BlackJack receiver and support the design of optimal data editing and weighting strategies in precise orbit determination applications.
Article
DIODE (DORIS Immediate Orbit on-board Determination) is a real-time on-board orbit determination software, embedded in the DORIS receiver. The purpose of this paper is to focus on DIODE performances. After a description of the recent DORIS evolutions, we detail how compliance with specifications are verified during extensive ground tests before the launch, then during the in-flight commissioning phase just after the launch, and how well they are met in the routine phase and today. Future improvements are also discussed for Jason-2 as well as for the next missions.
Article
The TOPEX/Poseidon, Jason-1 and Jason-2 set of altimeter data now provide a time series of synoptic observations of the ocean that span nearly 17 years from the launch of TOPEX in 1992. The analysis of the altimeter data including the use of altimetry to monitor the global change in mean sea level requires a stable, accurate, and consistent orbit reference over the entire time span. In this paper, we describe the recomputation of a time series of orbits that rely on a consistent set of reference frames and geophysical models. The recomputed orbits adhere to the IERS 2003 standards for ocean and earth tides, use updates to the ITRF2005 reference frame for both the SLR and DORIS stations, apply GRACE-derived models for modeling of the static and time-variable gravity, implement the University College London (UCL) radiation pressure model for Jason-1, use improved troposphere modeling for the DORIS data, and apply the GOT4.7 ocean tide model for both dynamical ocean tide modeling and for ocean loading. The new TOPEX orbits have a mean SLR fit of 1.79 cm compared to 2.21 cm for the MGDR-B orbits. These new TOPEX orbits agree radially with independent SLR/crossover orbits at 0.70 cm RMS, and the orbit accuracy is estimated at 1.5–2.0 cm RMS over the entire TOPEX time series. The recomputed Jason-1 orbits agree radially with the Jason-1 GDR-C orbits at 1.08 cm RMS. The GSFC SLR/DORIS dynamic and reduced-dynamic orbits for Jason-2 agree radially with independent orbits from the CNES and JPL at 0.70–1.06 cm RMS. Applying these new orbits, and using the latest altimeter corrections for TOPEX, Jason-1, and Jason-2 from September 1992 to May 2009, we find a global rate in mean sea level of 3.0 ± 0.4 mm/yr.
Article
The DORIS system has been developed to provide high accuracy orbit determination and beacon positioning. It is fully operational and has been working for 10 years with Spot-2, Spot-3, Spot-4 and Topex-Poseidon. This uplink radio-electrical system is based upon precise doppler measurements made with the dual-frequency signals emitted by the 50 autonomous beacons distributed homogeneously over the Earth surface. The DORIS Control Center performs system monitoring (on board receivers and ground beacons), receivers programming, data processing and archive.On board gathering of the measurements allows a precise on board autonomous real-time computation of the orbit. Experienced on board of Spot-4 since 1998, DIODE (the Navigation software) has demonstrated that on-board real-time orbits are now feasible with a few meters accuracy and an availability better than 99% over 3 years, even through manæuvers.Today, Earth observation missions, altimetry, oceanography, imaging, etc. and satellite constellations navigation often ask for a real time on board orbit restitution with a accuracy, or even better.Including an extended force model (40×40 Earth gravitational fields, moon and sun attractions, etc.), the last issue of DIODE is able to compute the orbit in real-time on board of the satellite, with an expected accuracy of RMS on the radial component.To be launched in 2001, Envisat-1 and Jason-1 are both equipped with a DORIS 2nd generation receiver: unfortunately, flight results will not be available in time to be presented in the paper. Future flights will occur in the next years (Spot-5, Cryosat, Pléiades, etc.).
Article
The ability of radar altimeters to measure the distance from a satellite to the ocean surface with a precision of the order of 2 cm imposes unique requirements for the orbit determination accuracy. The orbit accuracy requirements will be especially demanding for the joint NASA/CNES Ocean Topography Experiment (Topex/Poseidon). For this mission, a radial orbit accuracy of 13 centimeters will be required for a mission period of three to five years. This is an order of magnitude improvement in the accuracy achieved during any previous satellite mission. This investigation considers the factors which limit the orbit accuracy for the Topex mission. Particular error sources which are considered include the geopotential, the radiation pressure and the atmospheric drag model.
Article
TOPEX/POSEIDON is the first space mission specifically designed and conducted for studying the circulation of the world's oceans. The mission is jointly conducted by the United States and France. A state-of-the-art radar altimetry system is used to measure the precise height of sea level, from which information on the ocean circulation is obtained. To meet the stringent measurement accuracy required for ocean circulation studies, a number of innovative improvements have been made to the mission design, including the first dual-frequency space-borne radar altimeter capable of retrieving the ionospheric delay of the radar signal, a three-frequency microwave radiometer for retrieving the signal delay caused by the water vapor in the troposphere, an optimal model of the Earth's gravity field and multiple satellite tracking systems for precision orbit determination. Additionally, the satellite also carried two experimental instruments to demonstrate new technologies: a single-frequency solid-state altimeter for the technology of low-power, low-weight altimeter and a Global Positioning System receiver for continuous, precise satellite tracking. -from Authors
Article
The objectives and conclusions reached during the Seasat Precision Orbit Determination Experiment are discussed. It is noted that the activities of the experiment team included extensive software calibration and validation and an intense effort to validate and improve the dynamic models which describe the satellite's motion. Significant improvement in the gravitational model was obtained during the experiment, and it is pointed out that the current accuracy of the Seasat altitude ephemeris is 1.5 m rms. An altitude ephemeris for the Seasat spacecraft with an accuracy of 0.5 m rms is seen as possible with further improvements in the geopotential, atmospheric drag, and solar radiation pressure models. It is concluded that since altimetry missions with a 2-cm precision altimeter are contemplated, the precision orbit determination effort initiated under the Seasat Project must be continued and expanded.
Article
NASA is pursuing two key applications of differential positioning with the Global Positioning System (GPS): sub-decimeter tracking of earth satellites and few-centimeter determination of ground-fixed baselines. Key requirements of the two applications include the use of dual-frequency carrier phase data, multiple ground receivers to serve as reference points, simultaneous solution for use position and GPS orbits, and calibration of atmospheric delays using water vapor radiometers. Sub-decimeter tracking will be first demonstrated on the TOPEX oceanographic satellite to be launched in 1991. A GPS flight receiver together with at least six ground receivers will acquire delta range data from the GPS carriers for non-real-time analysis. Altitude accuracies of 5 to 10 cm are expected. For baseline measurements, efforts will be made to obtain precise differential pseudorange by resolving the cycle ambiguity in differential carrier phase. This could lead to accuracies of 2 or 3 cm over a few thousand kilometers. To achieve this, a high-performance receiver is being developed, along with improved calibration and data processing techniques. Demonstrations may begin in 1986.
Article
A reduced-dynamic technique for precise orbit determination of low earth satellites is described. This technique optimally combines the conventional dynamic technique with the nondynamic technique which uses differential GPS continuous carrier phase to define the state transition. A Kalman filter formulation for this reduced-dynamic technique is given. A covariance analysis shows that when neither the dynamic nor the nondynamic technique is clearly superior, the reduced-dynamic technique appreciably improves the orbit accuracy. Guidelines for selecting a near-optimum weighting for the combination are given. Sensitivity to suboptimal weighting is assessed.
Article
The largest nongravitational forces that will be acting on the TOPEX satellite will be those due to incident and emitted radiation on the spacecraft surfaces. In order to minimize the effects of these forces on orbit determination, a detailed model is being developed so that they may be predicted accurately. This model requires a precise description of the spacecraft shape and orientation, an evaluation of the solar and Earth radiation impinging on the surfaces, and a determination of the radiation being emitted from the surfaces as they heat and cool throughout the orbit. The TRASYS software system is used to evaluate the solar and Earth radiation (albedo and infrared) striking each surface of the spacecraft. This software has been modified to include an Earth radiation model that follows the seasonal variations in albedo and infrared radiation. The SINDA software system is then used to determine the transient temperatures of the spacecraft surfaces. Orbital thermal histories of significant features are given. These temperatures are used to determine the force exerted on each surface due to thermal emission. The emission forces are combined with the incident radiation forces to determine the total force acting on the satellite.
Article
(Previously announced in STAR as N80-20749)
Article
In this paper a computational scheme is presented for accurately predicting the farfield amplitude and phase characteristics of Global Positioning System (GPS) antennas flush-mounted to a corrugated groundplane. The algorithm developed is particularly well-suited in beamshaping of (GPS) antennas in order to provide a high level of multipath rejection. The usefulness of the analytical model has been verified by the excellent agreement achieved between experimental data and predicted amplitude and phase patterns
Gravity Model Improvement: A Decade Preparing for TOPEX/Poseidon: George Born, Byron Tapley and Jim Marsh
• F G Lemoine
• S Luthcke
• D Rowlands
• S Klosko
Gravity Model Improvement: A Decade Preparing for TOPEXlPoseidon
• F G Lemoine
• S Luthcke
• D Rowlands
• S Klosko
• N Zelensky
LEMOINE, F.G., LUTHCKE, S., ROWLANDS, D., KLOSKO, S., and ZELENSKY, N. "Gravity Model Improvement: A Decade Preparing for TOPEXlPoseidon: George Born, Byron Tapley and Jim Marsh," George H Born Symposium, American Astronautical Society, Boulder CO USA, May 13-14,2010.
An Introduction to Satellite Altimetry Satellite Altimetry and Earth Science: A Handbook of Techniques and Applications
• D Ribs
• J Callahan
• P Haines
• B Fu
CHELTON, D., RIBS, J., CALLAHAN, P., HAINES, B. and FU, L.L. "An Introduction to Satellite Altimetry," Satellite Altimetry and Earth Science: A Handbook of Techniques and Applications, International Geophysics Vol. 69, Academic Press, San Diego, 2001, pp. 463, doi: 1O.1016/Soo74-6142(01 )80146-7.
The I-Centimeter Orbit: Jason-l Orbit Determination Using
• S Luthcke
• N Zelensky
• D Rowlands
• F Lemoine
• T Williams
• Gps
• Slr
LUTHCKE, S., ZELENSKY, N., ROWLANDS, D., LEMOINE, F. and WILLIAMS, T. "The I-Centimeter Orbit: Jason-l Orbit Determination Using GPS, SLR, DORIS and Altimeter Data," Marine Geodesy, Vol. 26, No. 3-4, 2003, pp. 399-421, doi: 10.10801 714044529.
Re-sults of an Automated GPS Tracking System in Support of
• R I Muellershoen
• S Lichten
• U Lindqwister
• W Bertiger
• Topexiposeidon And Haines
MUELLERSHOEN, R.I., LICHTEN, S., LINDQWISTER, U., and BERTIGER, W. "Re-sults of an Automated GPS Tracking System in Support of TOPEXIPOSEIDON and Haines et al. GPSMet," Proceedings of the 1995 international Technical Meeting of the institute of Navigation (JON GPS 1995), Palm Springs, 1995, pp. 183-193.
An Introduction to Satellite Altimetry,” Satellite Altimetry and Earth Science: A Handbook of Techniques and Applications, International Geophysics
• J Callahan
• P Haines