361 research outputs found
PPP-based Swarm kinematic orbit determination
The Swarm mission of the European Space Agency (ESA) offers excellent opportunities to study the ionosphere
and to provide temporal gravity field information for the gap between the Gravity Recovery and Climate Experiment
(GRACE)
and its follow-on mission (GRACE-FO). In order to contribute to these studies, at the Institut für Erdmessung (IfE) Hannover, a
software based on precise point positioning (PPP) batch least-squares
adjustment is developed for kinematic orbit determination. In this paper, the
main achievements are presented.The approach for the detection and repair of cycle slips caused by
ionospheric scintillation is introduced, which is based on the
Melbourne–Wübbena and ionosphere-free linear combination. The results show
that around 95 % of cycle slips can be repaired and the majority of the
cycle slips occur on L2. After the analysis and careful preprocessing of
the observations, 1-year kinematic orbits of Swarm satellites from
September 2015 to August 2016 are computed with the PPP approach. The
kinematic orbits are validated with the reduced-dynamic orbits published by
the
ESA in the Swarm Level 2 products and SLR measurements. The differences between
IfE kinematic orbits and ESA reduced-dynamic orbits are at the 1.5, 1.5 and
2.5 cm level in the along-track, cross-track and radial directions,
respectively. Remaining systematics are characterized by spectral analyses,
showing once-per-revolution period. The external validation with SLR
measurements shows RMSEs at the 4 cm level. Finally, fully populated
covariance matrices of the kinematic orbits obtained from the least-squares
adjustment with 30, 10 and 1 s data rate are discussed. It is shown that for
data rates larger than 10 s, the correlation between satellite positions
should be taken into account, for example, for the recovery of gravity field
from kinematic orbits.</p
Multi-frequency and multi-GNSS PPP phase bias estimation and ambiguity resolution
Multi-frequency and multi-GNSS measurements from modernized satellites are properly integrated for PPP with ambiguity resolution to achieve the state-of-the-art fast and accurate positioning, which provides an important contribution to GNSS precise positioning and applications. The multi-frequency and multi-GNSS PPP phase bias estimation and ambiguity resolution, which is accomplished by a unified model based on the uncombined PPP, are thoroughly evaluated with special focus on Galileo and BDS
Multi-frequency and multi-GNSS PPP phase bias estimation and ambiguity resolution
Multi-frequency and multi-GNSS measurements from modernized satellites are properly integrated for PPP with ambiguity resolution to achieve the state-of-the-art fast and accurate positioning, which provides an important contribution to GNSS precise positioning and applications. The multi-frequency and multi-GNSS PPP phase bias estimation and ambiguity resolution, which is accomplished by a unified model based on the uncombined PPP, are thoroughly evaluated with special focus on Galileo and BDS
Analysis and Quality Assessment of LEO GPS Data for Geophysical and Ionospheric Applications
During the last few years, an ever-increasing fleet of Low Earth Orbiting (LEO) satellites for scientific purposes became operative. Most of these satellites carry dual-frequency Global Positioning System (GPS) receivers. The highly accurate dual-frequency observations allow mitigating the ionospheric signal contribution to estimate precise orbits and eventually the earth's gravity field. However, when comparing the obtained GPS only gravity fields derived from Swarm to gravity field solutions obtained by the dedicated gravity field mission GRACE, systematic band-shaped differences are visible in the vicinity of the geomagnetic equator.
In this work, an empirical approach for the appropriate weighting of GPS observations is derived to mitigate these ionospheric artifacts. The cause of the
artifacts is further analyzed by investigating the loop filter implementation. A tracking loop-specific transfer function is derived and used to invert the loop filter response to derive corrections for the GPS phase observations. Both methods are evaluated to achieve the best possible Swarm GPS only gravity field. Vice versa, the collected GPS observations from the LEO precise orbit determination antenna can also be used to gain insight into the topside ionosphere and plasmasphere. A three-dimensional model approach is developed using a fleet of LEO satellites to estimate a model of the electron density distribution between LEO and GPS satellites.
Both aspects represent possibilities of using GPS/GNSS on-board of LEO satellites for geophysical applications
Integer ambiguity Resolution in Multi-constellation GNSS for LEO Satellites POD
Precise Orbit Determination (POD) of Low Earth Orbit (LEO) satellites is essential for future LEO-augmented Positioning, Navigation and Timing (PNT) service based on the use of Global Navigation Satellite Systems (GNSS) measurements. Compared with the ambiguity-float LEO satellite POD, Integer Ambiguity Resolution (IAR) reduces number of parameters, eliminates the high correlations between the ambiguities and other estimable parameters, and strengthens model strength. In this study, using real data from Sentinel-6A tracking dual-frequency GPS and Galileo observations, the wide-lane (WL) and narrow-lane (NL) ambiguity fixing rates and the effects of the IAR on orbital accuracy are assessed in the single- and dual-constellation scenarios. Post-processed high-accuracy GNSS satellite clocks, orbits and Observable-specific Signal Biases (OSBs) from the final products of the Center for Orbit Determination in Europe (CODE) and the rapid products of the GeoForschungsZentrum (GFZ) are used for the analysis. Results showed that both the WL and NL fixing rates in the Galileo-only scenario are higher than those in the GPS-only scenario, reaching more than 98%. This implies a better signal quality of the Galileo observations. Applying IAR has improved the orbital accuracy for all single- and dual-constellation scenarios, and was shown to be especially helpful in reducing the once-per-revolution systematic effects in the along-track orbital errors, with over 50% improvement when using the COM products. With the IAR enabled, when using the COM final products, the 3D RMS of the orbital errors amounts to 1.2, 1.2 and 1.1 cm in the GPS-only, Galileo-only and GPS+Galileo combined scenarios, and the RMS of the Orbital User Range Errors (OUREs) amounts to 0.7, 0.7 and 0.6 cm, respectively. When using the GFZ rapid products, the IAR-enabled 3D RMS were 1.8, 2.1 and 1.4 cm in the GPS-only, Galileo-only and GPS+Galileo combined scenarios, with OURE RMS of about 1 cm
BDS GNSS for Earth Observation
For millennia, human communities have wondered about the possibility of observing
phenomena in their surroundings, and in particular those affecting the Earth on which they live.
More generally, it can be conceptually defined as Earth observation (EO) and is the collection of
information about the biological, chemical and physical systems of planet Earth. It can be undertaken
through sensors in direct contact with the ground or airborne platforms (such as weather balloons and
stations) or remote-sensing technologies. However, the definition of EO has only become significant
in the last 50 years, since it has been possible to send artificial satellites out of Earth’s orbit.
Referring strictly to civil applications, satellites of this type were initially designed to provide
satellite images; later, their purpose expanded to include the study of information on land
characteristics, growing vegetation, crops, and environmental pollution. The data collected are used
for several purposes, including the identification of natural resources and the production of accurate
cartography. Satellite observations can cover the land, the atmosphere, and the oceans.
Remote-sensing satellites may be equipped with passive instrumentation such as infrared or
cameras for imaging the visible or active instrumentation such as radar. Generally, such satellites are
non-geostationary satellites, i.e., they move at a certain speed along orbits inclined with respect to the
Earth’s equatorial plane, often in polar orbit, at low or medium altitude, Low Earth Orbit (LEO) and
Medium Earth Orbit (MEO), thus covering the entire Earth’s surface in a certain scan time (properly
called ’temporal resolution’), i.e., in a certain number of orbits around the Earth.
The first remote-sensing satellites were the American NASA/USGS Landsat Program;
subsequently, the European: ENVISAT (ENVironmental SATellite), ERS (European Remote-Sensing
satellite), RapidEye, the French SPOT (Satellite Pour l’Observation de laTerre), and the Canadian
RADARSAT satellites were launched. The IKONOS, QuickBird, and GeoEye-1 satellites were
dedicated to cartography. The WorldView-1 and WorldView-2 satellites and the COSMO-SkyMed
system are more recent. The latest generation are the low payloads called Small Satellites, e.g., the
Chinese BuFeng-1 and Fengyun-3 series.
Also, Global Navigation Satellite Systems (GNSSs) have captured the attention of researchers
worldwide for a multitude of Earth monitoring and exploration applications. On the other hand,
over the past 40 years, GNSSs have become an essential part of many human activities. As is widely
noted, there are currently four fully operational GNSSs; two of these were developed for military
purposes (American NAVstar GPS and Russian GLONASS), whilst two others were developed for
civil purposes such as the Chinese BeiDou satellite navigation system (BDS) and the European
Galileo. In addition, many other regional GNSSs, such as the South Korean Regional Positioning
System (KPS), the Japanese quasi-zenital satellite system (QZSS), and the Indian Regional Navigation
Satellite System (IRNSS/NavIC), will become available in the next few years, which will have
enormous potential for scientific applications and geomatics professionals.
In addition to their traditional role of providing global positioning, navigation, and timing (PNT)
information, GNSS navigation signals are now being used in new and innovative ways. Across the
globe, new fields of scientific study are opening up to examine how signals can provide information
about the characteristics of the atmosphere and even the surfaces from which they are reflected before
being collected by a receiver.
EO researchers monitor global environmental systems using in situ and remote monitoring tools.
Their findings provide tools to support decision makers in various areas of interest, from security
to the natural environment. GNSS signals are considered an important new source of information
because they are a free, real-time, and globally available resource for the EO community
Prospects for commercialization of SELV-based in-space operations
A workshop was hosted by the Langley Research Center as a part of an activity to assess the commercialization potential of Small Expendible Launch Vehicle-based in-space operations. Representatives of the space launch insurance industry, industrial consultants, producers of spacecraft, launch vehicle manufacturers, and government researchers constituted the participants. The workshop was broken into four sessions: Customers Small Expendible Launch Systems, Representative Missions, and Synthesis-Government role. This publication contains the presentation material, written synopses of the sessions, and conclusions developed at the workshop
- …