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The main objective of this work is to investigation the feasibility and performances of LEO communication satellite constellations as potential navigation augmentation platforms. The further examination of the existing and upcoming LEO communication satellite constellations has been conducted, such as Iridium, Globalstar, Teledesic, One Web, Boeing, SpaceX, Samsung, etc. The comprehensive performances of LEOs for navigation augmentation are evaluated and analyzed in terms of constellation characteristics, footprint, coverage, signal strength, dilution of precision (DOP, including GDOP, PDOP, VDOP, HDOP), and number of in-view satellites, with comparison of these to the current GPS, Galileo and BeiDou systems. The results showed that LEOs present superior performances compared with GNSS systems, and demonstrate promises as navigation augmentation platforms for challenging environments.
Moreover, the real-time signal-aided navigation method is explored, from user geometry and signal ranging errors to position errors. Then, we proposed a navigation system based on signals of opportunity from LEO platform. The proposed system relay on a terrestrial benchmark network consists of several monitoring stations with time synchronization. It would acquire the downlink communication signal from LEO platforms, and then estimate the time difference of arrival (TDOA) between stations with a correlation-based blind detection algorithm. The TDOA estimations and geographical position information are utilized to develop the time-delay-based spatio-temporal distribution model, which can determine the user’s position by matching the model with its estimated TDOA values. The proposed navigation system can operate stand-alone and facilitates the integration of communication and navigation system.
My last e-newsletter column discussed the basic foundation parameters of the North American-Pacific Geopotential Datum of 2022 (NAPGD2022); that is, a global geopotential model, the GRAV-D project, and the GEOID2022 geoid model. It emphasized that NAPGD2022 will provide a more efficient and cost-effective way to maintain consistent orthometric heights, but evaluating the relative accuracy of the geoid model is critical to a successful implementation of NAPGD2022. Performing GNSS/Leveling evaluation surveys will help in evaluating the relative accuracy of GEOID2022. NGS realizes that users will still have the need to perform leveling to obtain millimeter-level accuracy between closely spaced stations, and to evaluate the relative accuracy of a geoid model. NGS is developing geodetic routines and tools to assist users in transforming heights from NAVD 88 to NAPGD2022, and enabling the incorporation of geodetic leveling data into NAPGD2022 to establish NAPGD2022 orthometric heights. This newsletter will highlight NGS’ current plans for estimating NAPGD2022 GNSS-derived orthometric heights and incorporating geodetic leveling data into NAPGD2022 to establish orthometric heights consistent with GNSS-derived NAPGD2022 orthometric heights. Dan Gillins and Kandell Fancher did an excellent presentation titled “Leveling after 2022” at the 2017 Geospatial Summit. This e-newsletter will highlight some sections of the presentation.
As described in my last e-newsletter, NAPGD2022 will not be realized with leveling data. So, how will users access the National Spatial Reference System (NSRS) in 2022? NGS has prepared frequently asked questions about the new datums (https://www.ngs.noaa.gov/datums/newdatums/FAQNewDatums.shtml#CAN ). The following is the answer to the question How will accessing the National Spatial Reference System (NSRS) change with the release of the new datums?
How will accessing the National Spatial Reference System (NSRS) change with the release of the new datums?The NSRS will be accessed using Global Positioning System (GPS) technology that references Continuously Operating Reference Stations (CORS) and relies on a time-dependent gravimetric geoid model. This method of accessing the NSRS is a paradigm shift from accessing NAD 83 and NAVD 88 through the use of geodetic survey marks.
As described in previous newsletters, GNSS-derived Orthometric Heights are computed using the following formula: orthometric height (H) = ellipsoid height (h) minus geoid height (N). (See box titled “Slide 9 from Gillins and Fancher presentation titled ‘Leveling after 2022’ presented at the 2017 Geospatial Summit.”) It will not be necessary to connect to a geodetic monument, i.e., a bench mark, because the NATRF2022 ellipsoid height (hNATRF2022) is determined using the NGS CORS and the geoid model (NGEOID2022) is consistent with NATRF2022. In other words, GNSS observations combined with the geoid model will become the primary means for deriving orthometric heights on marks.
Slide 9 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit
Gillins and Fancher addressed the expected relative accuracy of a 2022 NAPGD2022 GNSS-derived orthometric height difference in slide 11 of their presentation. (See box titled “Slide 11 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit.”) Their estimation assumes a 1 cm sigma for each ellipsoid height value and 1 cm sigma for the relative geoid height value. This results in an estimated relative accuracy of a NAPGD2022 GNSS-derived height difference of +/- 1.7 cm. Gillins and Fancher also addressed the expected accuracy of leveling-derived heights in their slide 12. (See box titled “Slide 12 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit.”)
Slide 11 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit
This slide is just meant to give an idea of the error budget of GNSS leveling. Actually, if both stations are observed simultaneously, then there is a correlation term that must be tracked and added to the equation for sigma delta H. Further, the value for sigma delta N is poorly understood over very short distances (which are typical for leveling). However, it is reasonable to assume that differences in orthometric height of approx. 2 cm can be achieved with GNSS and a geoid model. The point is to say differences in height are to around 2 cm when only using GPS+geoid
Slide 12 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit
Comparing slides 11 and 12, it’s obvious that leveling-derived orthometric height differences are more accurate than GNSS-derived orthometric height differences between closely spaced stations. NGS recognizes that some users will require a high level of relative accuracy and will continue to perform leveling; and, therefore, they will want their leveling-derived orthometric heights consistent with NAPGD2022. Gillins and Fancher’s presentation stated that NGS has ongoing research to develop models to combine and adjust GNSS-derived heights and/or observations with leveling, and to develop software applications and tools for incorporating leveling-derived heights into NAPGD2022. NGS has performed some preliminary tests of adjusting GNSS derived heights with leveling data using weighted constraints. Slides 16-18 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit” depicts the basic concept.
The basic concept is that the user will first establish NAPGD2022 orthometric heights at two stations using GNSS observations and a geoid model. Then, the user will observe leveling height differences between the two stations (see box titled “Slide 16 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit”), and finally the user will perform a least squares adjustment to estimate NAPGD2022 orthometric heights using appropriated weighted constraints of the NAPGD 2022 GNSS-derived orthometric heights and appropriated weighted leveling observations (See box titled “Slide 18 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit.”).
Slide 16 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit (Before Adjustment)Slide 18 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit (After Adjustment)
We will address this topic in more detail in another newsletter but the major takeaways are given in slide 22 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit. Basically, the GNSS and a high-accuracy geoid model connects the user to NAPGD2022 and provides the overall network accuracy, and the leveling data improves the accuracy of height differences between marks and provides the local accuracy. The addition of leveling with GNSS increases the overall redundancy in a survey network which increases the ability to detect outliers and improves the relative accuracy of the final adjusted height differences.
To assist users in obtaining accurate relative NAPGD2022 height differences, NGS has plans to develop software applications and tools for incorporating leveling-derived heights into NAPGD2022. They have a project called “OPUS-Projects for GNSS & Leveling.” The box titled “Slide 25 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit” is a mockup of the proposed tool. This tool will apply the appropriate corrections to the leveling data and perform a least-squares adjustment to estimate NAPGD2022 heights based on weighted constraints.
Slide 25 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit
This newsletter focused on NGS’ current plans for estimating NAPGD2022 GNSS-derived orthometric heights and incorporating geodetic leveling data into NAPGD2022 to establish orthometric heights consistent with GNSS-derived NAPGD2022 orthometric heights. It emphasized that after NAPGD2022 is established, the primary means for deriving orthometric heights on monuments will be using GNSS observations combined with the geoid model. Future newsletters will discuss in more detail some of NGS’ ongoing research to develop models and tools to combine and adjust GNSS-derived heights and/or observations with leveling.
Doug Flewelling, Ph.D., gives an overview of the University of Redland‘s master’s degree program for GIS at the 2017 Esri User Conference, which took place July 10-14 in San Diego, California. The program is specifically designed for is professionals in the field looking to further their careers, as well as international students.
TomTom‘s Kenneth Clay highlights traffic features the company offers with its mapping services. TomTom exhibited at the 2017 Esri User Conference, which took place July 10-14 in San Diego, California.
Pennsylvania State University assistant professor of geography Anthony Robinson discusses the university’s online geospatial program at the 2017 Esri User Conference, which took place July 10-14 in San Diego, California. The university added a graduate certificate in remote sensing and earth observation to its offerings.
Teledyne Optech Business Unit Manager Albert Iavarone talks about the features of the Polaris Terrestrial Laser Scanner Series. The unit, which has a touchscreen display, was showcased at the 2017 Esri User Conference, which took place July 10-14 in San Diego, California. Iavarone also touches on the Maverick mobile scanner.
Leica Geosystems showed off its Zeno GG04 smart antenna and DS2000 Utility Detection Radar at the 2017 Esri User Conference, which took place July 10-14 in San Diego, California. The Zeno GG04 improve mobile devices’ GNSS accuracy with Real-Time Kinematic (RTK) and precise point positioning (PPP), while the Leica DS2000 Utility Detection Radar detects and positions shallow and deep targets simultaneously.
The Trimble Catalyst software-defined GNSS receiver for Android devices is now available through Trimble’s global distribution network. Trimble’s Gareth Gibson gives an overview of its features at the 2017 Esri User Conference, which took place July 10-14 in San Diego, California.
Space Debris: Artist’s impression based on density data, shown at an exaggerated size to make objects visible. Image: ESA
In April, the European Space Agency (ESA) hosted the 7th European Conference on Space Debris at ESA’s Satellite Control Centre in Darmstadt, Germany. There, international experts discussed ways to head off the threat of space junk.
ESA estimates there are roughly 5,000 objects larger than 1 meter, 20,000 objects over 10 centimeters and 750,000 “flying bullets” of around one centimeter.
Risks of a collision are statistically remote, but “The growth in the number of fragments has deviated from the linear trend in the past and has entered into the more feared exponential trend,” warns Holger Krag, in charge of ESA’s space debris office.
Many of the objects are traveling at enormous speed, up to 56,000 kilometers per hour, giving them the potential explosive force of a hand grenade on impact, said ESA experts.
In the U.S., more than 16,000 objects are tracked and cataloged daily by crews in the Joint Space Operations Center at Vandenberg Air Force Base. Only 1,100 of the tracked items are functional spacecraft, including GPS satellites.
Dealing with existing debris will call for innovative solutions — the purpose of the four-day summit, held every four years since 1993.
“It’s clear to us that the issue of space debris is serious,” Jan Woerner, ESA chief, told the conference. “No country can stand or act alone.”
Spirent Federal Systems‘ Kalani Needham discusses the company’s GSS6450 RF record and playback system and GSS7000 signal generator at Xponential 2017 in Dallas, Texas.
