Fundamental in the determination of GNSS solutions is resolving the correct number of full cycles of the carrier signal (so-called fixing ambiguities) in order to resolve the ambiguity differences between the base and the rover. Distances measured from GNSS receivers contain errors caused by inaccuracies in the satellite and receiver clocks, the satellite orbits, and by the ionosphere and troposphere. When a base station is used, these errors are nearly identical to both the rover and base station receivers when the baseline distance is short. By removing these common errors through RTK processing, centimeter-level accurate vectors can be calculated between the base station and the rover.
Multipath, the reflection of GNSS signals from nearby objects and structures, creates its own indirect measurements from the satellites to the GNSS receiver and is the most critical source of inaccuracy in precision GNSS applications. The worst case is when the receiver doesn’t see the direct signal at all, such as when satellite is behind a building but is still receiving the signal reflected off of the nearby structure. Such indirect signals are usually strong, unhelpful and misleading.
A TRIUMPH-LS collecting a point under tree canopy.
The other aspect impacting the veracity of a fixed solution is when there are weak GNSS signals. Frequently, weak signals are due to their penetration directly through tree canopy. While the TRIUMPH-LS can’t move the obstacles that are creating multipath out of the way, its sophisticated engineering is designed to handle even the weakest signals like no other system with its RTK Verification System (patent pending).
When located in difficult environments and under tree canopy, all GNSS receivers are prone to give bad fixed solutions that may appear to be acceptable if they are not verified. Existing methods to verify GNSS solutions include “dumping” the receiver, turning it upside down to cause the RTK engines to reset, and re-observing the point at a later time.
The TRIUMPH-LS automates these processes with its built-in software features of Verify and Validate. Verify automatically resets the RTK engines after every fixed epoch is collected in the first step of its process. Epochs are sorted by distance and placed into groups during the first step. Once a group has built up a set level of confidence, the RTK engines are allowed to collect the remaining epochs without resetting. If epochs fall too far away from the best selected group from the first step, they are rejected and the RTK engines are reset.
Validation is the final step of the process. With this feature enabled, the RTK engines will reset one final time at the end of the observation and collect 10 additional epochs. Allowing sufficient time between the first step and the final validation step will guarantee a bad solution is not allowed to be accepted. From extensive testing of these features in the worst of multipath environments, a bad solution has yet to be accepted when the Verify and Validate features are used and 120 epochs are collected.
After using a TRIUMPH-LS system, many land surveyors who have used other GNSS receivers in the past without preforming any type of verification are starting to realize that they may have accepted many bad fixed solutions over the years. If you are not using a receiver like the TRIUMPH-LS that has the ability to automatically reset the RTK engines and verify the results, it is essential that you manually “dump” the receiver or re-observe the point at a later time so that you don’t make this same mistake.
Google is reorganizing under a new name, Alphabet, separating its moneymaking businesses from its cutting-edge ventures such as the self-driving car and drone delivery service. The move is being made because Google’s penchant for experimentation made traditional investors nervous, according to the New York Times.
Alphabet would be the parent entity, housing several companies, with Google the biggest among them. Alphabet Inc. will replace Google Inc. as the publicly traded entity and all shares of Google will automatically convert into the same number of shares of Alphabet, with all of the same rights. Google will become a wholly-owned subsidiary of Alphabet.
“For Sergey and me this is a very exciting new chapter in the life of Google — the birth of Alphabet,” Larry Page, the chief executive of Google, wrote in a blog post on Monday. “We liked the name Alphabet because it means a collection of letters that represent language, one of humanity’s most important innovations, and is the core of how we index with Google search. We also like that it means alpha‑bet (Alpha is investment return above benchmark), which we strive for!””
“Sergey and I are seriously in the business of starting new things.” Page writes in the blog. “Alphabet will also include our X lab, which incubates new efforts like Wing, our drone delivery effort. We are also stoked about growing our investment arms, Ventures and Capital, as part of this new structure.”
Rohde & Schwarz designed its GNSS simulator for the R&S SMBV100A with a focus on production testing of GNSS receivers.
Rohde & Schwarz now offers a new, speed-optimized production tester — the R&S SMBV100A vector signal generator equipped with the R&S SMBV-P101 package.
During production testing of modules and receivers for satellite-based communications, the basic GNSS signal reception and the connection between the antenna and GNSS chipset need to be checked. The GNSS production tester simulates separate satellites for the GPS, GLONASS, BeiDou and Galileo navigation standards in the L1/E1 band specifically for these production tests.
The four satellite constellations can be activated individually, each with a high dynamic range of 34 dB. Level changes can be made on the fly without interrupting the signal, enabling users to simultaneously perform independent sensitivity tests for each navigation system. The 1 pps or 10 pps GNSS marker allows exact time synchronization between the tester and the DUT. Pure, level-stable CW signals can also be generated to calibrate the test setup or to simulate interferers.
The R&S SMBV-P101 option additionally offers test functions for efficient characterization of GNSS chipsets, Rohde & Schwarz said. As a result, a receiver’s ability to handle high-movement dynamics can be verified quickly and cost-effectively. To do this, users can access both predefined and user-defined Doppler profiles, from which the R&S SMBV100A automatically generates the appropriate satellite signal.
The R&S SMBV-P101 GNSS production tester package for the R&S SMBV100A is now available from Rohde & Schwarz.
Trimble has launched its Trimble VRS Now correction service in New Mexico. The commercial subscription service provides surveyors, civil engineers and geospatial professionals in the region with instant access to real-time kinematic (RTK) GNSS corrections without the need for a base station.
Using both the GPS and GLONASS constellations, the Trimble service delivers centimeter-level RTK corrections customized for each GNSS receiver’s location anywhere in the network via cellular communications. The Trimble VRS Now service supplies accurate, reliable and easy-to-use GNSS positioning for a variety of applications including surveying, urban planning, urban and rural construction, environmental monitoring, resource and territory management, disaster prevention and relief and scientific research, Trimble said.
“As we continue to expand our VRS Now network infrastructure throughout the U.S., users in New Mexico now have increased reliability from both GPS and GLONASS corrections to enhance their work,” said Lisa Wetherbee, business area director of Trimble’s Positioning Services Division. “Our suite of correction services offers a variety of performance options, designed to meet the different requirements and budgets of our customers. VRS Now in New Mexico delivers centimeter-level accuracy to a wide range of industry professionals.”
Service in New Mexico is a continuation of Trimble’s focus on providing solutions that enable customers to increase productivity by simplifying access to high-precision positioning around the world. Similar VRS Now services are operating in Illinois, Indiana, Iowa, Nebraska, Colorado, Florida, Alabama, Mississippi, Texas, Oregon and parts of Europe and Australia.
Garmin’s eTrex Touch 25, 35 and 35t outdoor handhelds have an updated user interface and 2.6-inch capacitive touchscreen display. The eTrex Touch series also features activity profiles for navigation for multiple activities and an enhanced track manager to start and stop recording.
The eTrex Touch series has a high-sensitivity, WAAS-enabled GPS receiver with GLONASS support and HotFix satellite prediction to locate users’ position quickly and precisely, even in heavy cover and deep canyons. All units have a three-axis tilt-compensated electronic compass, which gives directional information even when standing still. The eTrex 35 and 35t also have a barometric altimeter to get more accurate altitude, elevation and climb information, as well as indications of weather changes.
Garmin, www.garmin.com
Fleet Management
Supervisor App for Fleets
The Supervisor app for the FieldMaster suite of mobile applications allows managers to leave the office and still have visibility into their fleet and mobile workers from their smartphone or tablet, as well as manage day-to-day operations remotely.
FieldMaster Supervisor is available with Trimble Fleet Management and Work Management. Features include viewing the team’s locations on a map; seeing their job progress, including tasks at risk; finding the nearest worker to another team member or customer; turn-by-turn navigation; inspecting job performance and documenting status in the field; and receiving vehicle and driver performance alerts in real-time.
NovAtel’s SPAN GNSS/INS technology is now available on the company’s OEM625S dual-frequency SAASM GPS plus civil RTK receiver. SPAN offers system developers with SAASM requirements the benefit of continuously available 3D positioning, velocity and attitude (roll, pitch, yaw) for their defense applications. Authorized defense customers need access to the Precise Positioning Service (PPS) for DOD applications. When keyed, the existing OEM625S board-level receiver provides an RTK PPS solution by taking the raw measurements from an L-3 XFACTOR SAASM and applying them to NovAtel’s RTK algorithm. SPAN technology couples NovAtel’s precision GNSS receivers with robust IMUs to provide a more reliable, stable solution, even during short periods of time when satellite signals are blocked or unavailable.
The Averna RP-6100 series is an RF tool offering high-performance record-and-playback and real-time simulation in one platform for RF application validation.
The RP-6100 can capture all GNSS bands, as well as HD Radio, Wi-Fi, LTE, radar, and cognitive radio — plus impairments — to advance RF projects and harden product designs. It features up to four channels, 160 MHz of recording bandwidth, tight channel synchronization, an extended frequency range of 10 MHz to 6 GHz and 14-bit resolution. The RP-6100 can be equipped with Skydel Solutions’ software-defined, real-time GNSS simulator, which delivers easy setups, integrated maps, dynamic scenario creation, high precision and tight parameter controls to enable highly repeatable simulations of current and future GNSS conditions, as well as corner cases.
The AirPrime WP Series of smart wireless modules is designed for the development of connected products. The WP Series provides an integrated device-to-cloud architecture enabling developers to build a Linux-based product using a single module that sends user and product data to the cloud. The AirPrime WP series offers an application processor, GNSS receiver, and cellular modem with an optional ultra-low power mode that reduces power consumption by 200 times, opening up new use-case possibilities for cellular connectivity.
Researchers at Qihoo 360, a Chinese Internet security firm, say they have found a way to make a GPS emulator that can falsify the location of smartphones and in-car navigation systems, reports Forbes. The system is inexpensive compared to expensive, sophisticated GPS emulators that can cost thousands of dollars.
Qihoo lead researcher Lin Huang is the first Chinese woman to present at the yearly hacker conference Defcon, held in Las Vegas on Aug. 6-9. Huang said her team used common software-defined radio (SDR) tools to create their module and software. They also used open-source software found on Github that had come from researchers at a Chinese university, along with their own code.
The SDR tools used include HackRF, described by Forbes as the $300 wireless Swiss army knife for hackers. The small board can move between radio frequencies, and read and transmit to a broad range of radio frequencies. On smartphones, the attack targets navigation signals delivered at the chipset level, on both Apple or Android smartphones.
Huang suggests that chipset manufacturers consider introducing new software that can better detect GPS spoofing.
One potential target of such spoofing is a drone., which could be commandeered by the spoofer and taken into restricted airspace. Alternatively, it’s possible to make drones believe they’re in a no-fly area.
The Qihoo team demonstrated such attacks using the free and open source GNU Radio, among other tools, to alter the GPS coordinates on a DJI Phantom 3. In a video at Forbes, filmed from a drone-mounted camera, the hackers force a UAV to crash land.
The researchers said the weaknesses could be fixed by DJI and other drone makers, but they would have to do so at the GPS chip level, meaning any drones already out there are unlikely to receive an update.
The Inertial+ by OxTS improves measurements from a GPS receiver.
OxTS has successfully integrated a Locata receiver with its Inertial+ to create the first Locata+INS device, according to both companies. The device is capable of achieving centimeter-level accuracy where GPS systems fail.
The Inertial+ series, first developed in 2008, was designed for users who had an external GNSS receiver already, but still wanted to gain the benefits of an inertial system. The company has been able to combine OxTS’ Kalman filter and expertise in GNSS/IMU integration with its existing systems, meaning the user doesn’t have to pay for survey-grade integrated receivers.
Over the years, a number of popular GNSS receivers have been integrated with the Inertial+ to keep up to date with the market and make sure customers with the latest models can take advantage of the benefits the Inertial+ brings, OxTS said. Now, the Inertial+ has expanded from GNSS receivers and become the first inertial navigation system to integrate a Locata receiver, combining the many benefits of both systems, the companies said.
Locata is an innovative positioning system designed to complement rather than replace GPS, by addressing the issues and shortfalls of GNSS. As always, the Inertial+ allows Locata users to take advantage of their existing technology while enjoying the extra layer of measurements an aided-inertial navigation system brings.
Locata enables positioning in environments where GPS is either marginal or unavailable. Instead of using signals from satellites, a network of ground-based Locata transmitters (known as a LocataNet) can be set up around any specified local area. The LocataNet transmits GPS-like signals that allow any Locata receiver in the network to accurately calculate its position and time. Unlike GPS, where signals are too weak to penetrate into buildings, Locata’s signals are very powerful — more than one million times more powerful than GPS.
Additionally, with a locally based system (rather than a global satellite system), a user gains the benefit of having total control over both the reliability and quality of positioning solutions within the LocataNet coverage area. Locata systems are being sold today in many markets where GPS is unusable or unreliable, such as inside warehouses, on dockyards, in open-pit mines, for UAVs in urban areas, and for military uses where GPS is being actively denied by an adversary.
By combining the technologies of an inertial navigation system and a local positioning system, users have access to an extremely reliable and robust navigation solution, the companies said. Locata positioning data is fused with the IMU data in the Inertial+ with OxTS’ custom Kalman filter, creating a full 3D navigation solution with precise position, orientation, heading, velocity and acceleration measurements.
SVN49 in space (artist’s rendering). The signal anomaly from SVN 49 alerted researchers to new possibilities in analysis and monitoring.
Chip Transition-Edge Based Signal Tracking for Ultra-Precise GNSS Monitoring Applications
By Sanjeev Gunawardena, John Raquet and Frank van Graas
Tracking GNSS signals using their underlying spreading sequence chip transition edges reveals positive versus negative chip asymmetries that are characteristic to each satellite. This asymmetry is due to various types of natural signal deformation that is known to occur within the satellite’s signal generation and transmission hardware. This novel concept of monitoring chip asymmetry can extend the state of the art in the areas of GNSS signal-quality monitoring and authentication. A technique to directly monitor chip asymmetry within a specially designed ChipShape GNSS receiver architecture employs separate code discriminators that align themselves to the chip rising-edge and falling-edge zero crossings.
The detailed study of naturally-present deformations in GNSS signals is a relatively new activity that was sparked by the GPS SVN49 anomaly and the associated research activities that followed. This research area has numerous applications that include:
Informing the design of sudden signal deformation detection and alerting algorithms for safety-of-life differential GNSS applications (such as aviation).
GNSS signal “fingerprinting” and authentication.
The detailed study of long-term degradation effects of GNSS satellite signal generation and transmission hardware.
Analysis of the impact to the first item in this list of swapping a satellite’s signal generation modules by its control segment.
Multipath detection, characterization, and mitigation are also closely tied to all research relating to GNSS signal deformation monitoring (SDM).
High-fidelity SDM can be performed using two methods:
observation of actual GNSS signals above the thermal noise floor using a high-gain dish antenna;
the combination of long coherent integration and multi-correlator processing.
Our previous research has revealed that these two methods are highly complementary for gaining full insight into the effects and causes of observed natural signal deformations.
Among the handful of multi-correlator processing techniques that can be applied for SDM, ChipShape processing allows the correlation function resolution to be finely adjustable while providing good numerical processing efficiency. This processing technique also allows chip-transition eye diagrams to be constructed in order to provide additional insight such as positive and negative chip width asymmetries.
One goal of our SDM research involves developing capabilities to observe GNSS signals with the highest levels of fidelity practically achievable in order to further the application areas described above. Key to this is developing techniques to track GNSS signals using a reference point that is both consistent and invariant (to the greatest extent possible) to nominal signal deformations and environmental effects such as multipath. Traditional multipath mitigating techniques such as narrow correlator and double-delta correlator are sub-optimal in this regard. This is because a significant portion of the signal around the chip transition point (that is, 10 percent and 20 percent for 0.1 chip correlator spacing, respectively) must be integrated to realize these discriminators and maintain robust tracking in moderate dynamics conditions. This integration tends to low-pass filter the desired observables.
Chip Transition Edge-Based Code Tracking
Figure 1 illustrates normalized C/A code chip rising edges for the GPS constellation of June 2014. These chip shapes were processed using a front-end with 24 MHz bandwidth. For visual comparison purposes, this and other related plots were obtained using 600 seconds of coherent integration.
Figure 1. Normalized ChipShape rising edges for the GPS SPS constellation of June 2014; each color represents a different GPS satellite.
The code tracking loop used to obtain this result employed an empirical normalized coherent rising-edge discriminator given by:
(1)
Where τ is relative code phase in chips, d is Early-Late correlator spacing,R’XYZ(i) is the differential correlation output for integer bin i obtained using ChipShape processing with masking sequence XYZ. bin(x) is a function that selects the closest ChipShape vector index that corresponds to relative code phase x. Each ChipShape processing bank is configured to span one chip early and one chip late with a resolution of N bins per chip, thus producing a ChipShape vector of 3N bins. α is a scale factor obtained through trial and error to yield robust tracking performance as observed by the code-minus-phase measurement. For the result shown in Figure 1, N=240 and d ≈ 0.017 chips.
The figure clearly shows that the rising-edge zero crossings vary by SV. This variation is due to nominal signal deformation present in each GPS-SPS signal.
Figure 2 illustrates the rising-edge zero crossings aligned to zero relative code phase. This alignment was performed by interpolating each R’NPN vector, precisely estimating code phase at the zero-crossing point, and shifting the curve appropriately.
Figure 2. Normalized ChipShape rising edges for the GPS SPS constellation of June 2014: Zero crossing compensated.
Figure 3 shows zero crossings for the falling edges after all rising edges were aligned to zero. The figure clearly illustrates subtle asymmetries between positive and negative chips which span a range of approximately ±1.5 meters. These asymmetries are not directly observable using typical GNSS receiver processing. However, they can lead to pseudorange biases through the resulting distortion that occurs to the traditional correlation function.
Figure 3. Normalized ChipShape falling edges for the GPS SPS Constellation of June 2014 when rising edges are aligned to zero.
In general, a family of code discriminators that precisely track chip rising-edge zero crossings can be defined by:
(2)
Where R’NPX is a linear combination of orthogonal ChipShape components that preserve the rising-edge transition, e.g.: R’NPX =R’NPN +R’NPP. R’FFX is a linear combination of orthogonal ChipShape components that preserve the non-transitioning (that is, flat) sections of chips, for example: R’FFX =R’PPP + R’PPN −R’NNP − R’NNN. a and b define an integration interval within the range −1 to +2 chips with respect to the chip transition edge. β is a bias compensation term. represents the real or imaginary component function for the coherent discriminator (depending on the modulation phase of the signal being tracked), or the magnitude function for a non-coherent discriminator implementation.
Similarly, a family of code discriminators that precisely track chip falling-edge zero crossings that occur one chip after the rising edges tracked by the discriminator of Equation 2 can be defined by:
(3)
Then, a two-step technique to precisely monitor chip asymmetry can be described as follows:
Setup two identical ChipShape processing channels to track a given PRN. Progressively tighten the code tracking loops to track the rising-edge zero crossings of the underlying signal using the discriminator of Equation 2.
After steady-state zero-crossing rising-edge tracking is achieved, switch the second channel’s code discriminator to that of Equation 3. This will cause the second channel to track the zero crossings of the falling edges that occur one chip later in the underlying signal’s spreading sequence. The discriminator’s linear range must be wide enough to pull-in the chip asymmetry shown in Figure 3.
When the second channel re-converges as a result of Step 2, the relative pseudorange displacement that occurs is equal to the chip asymmetry in meters. Hence, chip asymmetry can be monitored for the entire visible pass of a satellite. It is expected that positive and negative chip transitions are equally affected by channel distortions (that is, code and carrier multipath, ionosphere, troposphere, and the receiver antenna and front-end transfer function). Hence, the rising-edge-code-minus-falling-edge-code measure of chip asymmetry is expected to be invariant to most if not all channel distortions.
Estimating Compensation Parameters
As shown in Equations 2 and 3, due to natural signal deformation of many types, the rising and falling-edge zero-crossing discriminators are expected to be SV number, PRN code and elevation angle dependent. Hence, α and β must be estimated for a given correlator spacing d separately for all SV signals of the constellation. These values will also be specific to a given antenna and receiver front-end.
Figure 4 illustrates the procedure used to estimate the scale factor and bias terms starting with the empirical rising-edge tracking process described above.
Figure 4. Procedure for estimating scale factors and biases for rising-edge tracking early-late and double-delta code discriminators.
The following figures illustrate the edge tracking discriminator calibration process using R’NPN for a single SV.
Figure 5 illustrates the early-plus-late functions computed for various correlator spacings. As described previously, these functions typically do not cross through zero codephase due to natural signal deformation.
Figure 5. Uncorrected rising-edge early-late discriminator functions for various correlator spacings.
Figure 6 illustrates the rising-edge discriminator functions after bias compensation.
Figure 6. Rising-edge early-late discriminator functions for various correlator spacings after bias compensation.
Figure 7. Calibrated rising-edge early-late discriminator functions for various correlator spacings.
Figure 8 illustrates the multipath error envelopes for the rising edge-based coherent code discriminators. The performance of these discriminators is similar to the traditional Early-Late discriminators for the same correlator spacings. This result is consistent with the theoretical bounds for code multipath.
Figure 8. Multipath error envelopes for various rising edge-based coherent early-late code discriminator functions.
As shown in Figure 4, the edge-tracking discriminators described in Equations 2 and 3 that are based on Early-Late bin spacings can be combined to obtain edge-tracking double-delta discriminators. Double-delta discriminators provide significantly improved multipath performance.
In general, the edge-tracking double-delta discriminator for inner correlator spacing d is formed by the linear combination of two early-late edge-tracking discriminators, as follows:
(4)
Scale factor γ is estimated such that overall multipath error is minimized according to a given design criteria.
Figure 9 illustrates the double-delta rising-edge discriminator with inner spacing of 0.017 chips. This discriminator has a pull-in range of approximately ±0.01 C/A chips.
Figure 11 illustrates the multipath error envelope for the coherent rising-edge double-delta discriminator. Performance is consistent with a traditional second-derivative discriminator.
Figure 11. Multipath error envelope for coherent rising-edge double-delta code discriminator with inner spacing of ~0.017 C/A chips.
Figure 12 illustrates the performance of the various rising-edge tracking discriminators for a live-sky GPS-SPS signal (de-trended code-minus-carrier measurement). This figure clearly demonstrates robust code tracking and the multipath and noise mitigating benefit of ultra-narrow rising-edge discriminators.
Figure 12. Code tracking performance for live sky data of various rising edge-based coherent early-late code discriminator functions.
Conclusions
An empirical chip rising edge-based tracking technique was used to observe the underlying chip shapes of live sky GPS-SPS signals at high fidelity. These results reveal positive versus negative chip asymmetries that are characteristic to each satellite. The novel concept and technique of directly monitoring chip asymmetry has potential to extend the state of the art in the areas of GNSS signal quality monitoring and authentication.
Disclaimers. The views expressed in this paper are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense, or the United States Government.
Acknowledgments. This research was supported by the Air Force Research Laboratory Sensors Directorate.
The authors thank Ohio University Avionics Engineering Center for making available a cluster of high-performance computers to process the 20 TB dataset for this research, and Kadi Merbouh of Ohio University for maintaining and overseeing operation of this equipment.
The ChipShape processing is an extension of the signal compression technique first published by Larry Weill and licensed by NovAtel for use in its Vision Correlator technology.
This article is based on a paper presented at ION Pacific PNT 2015 in Honolulu.
SANJEEV GUNAWARDENA is a research assistant professor with the Autonomy & Navigation Technology (ANT) Center at the Air Force Institute of Technology (AFIT). He earned a Ph.D. in electrical engineering from Ohio University.
JOHN RAQUET is a professor of electrical engineering and the Director of the ANT Center at AFIT. He has been involved in navigation-related research for more than 25 years.
FRANK VAN GRAAS is the Fritz J. and Dolores H. Russ professor of electrical engineering and principal investigator with the Avionics Engineering Center at Ohio University. He received the ION Johannes Kepler, Thurlow and Burka awards, and is a Fellow and past president of the ION.
NASA’s Ikhana is being used to test a system that will allow uncrewed aircraft to fly routine operations within the National Airspace System. (Credit: NASA)
NASA plans to install a Locata network (LocataNet) as the core positioning technology for safety-critical unmanned aerial systems (UAS) research at its Langley Research Center in Hampton, Va., according to an announcement by Locata.
NASA Langley is tasked with performing rigorous and repeatable scientific evaluation of new UAS safety and technology concepts under development. The LocataNet will provide high-precision non-GPS-based positioning, navigation and timing (PNT) that is essential for this work. Known for its long history of aeronautics research, NASA Langley is a key center for UAS research and development. In June, one of Langley’s unmanned hexacopters (a drone with six rotors) delivered medical supplies to a clinic, the first such delivery by an unmanned drone.
Locata’s centimeter-accurate positioning will now assist NASA to develop and improve flight-critical technology systems that support air transportation safety, efficiency and performance. Langley’s extensive state-of-the-art facilities will be further enhanced with the installation of the LocataNet.
The NASA LocataNet is scheduled to be installed and commissioned before the end of 2015. Locata will supply the LocataLite Transmitters and Locata receivers required by NASA for the installation. Aviation-quality Locata antennas, developed by Cooper Antennas (UK) and previously used by the U.S. Air Force in its own military LocataNets, will also be installed. Locata engineers will support the physical installation, ongoing training and the future technical support required by NASA Langley for this world-first UAS deployment.
Locata Corporation has invented new terrestrial positioning networks which function as local, ground-based replicas of GPS. These networks can be thought of as “GPS hotspots,” according to the company. Locata has amassed 146 granted patents to date protecting these innovations, with many more patents in the works.
Locata is currently shipping commercial systems to demanding and professional end users such as the USAF, NASA, Leica Geosystems, and many others. Locata enables their integration partners to extend GPS-like positioning coverage to modern industrial, commercial, consumer and government applications in areas where GPS is erratic, jammed or unavailable.
“Locata is proud and delighted to have received an order for NASA’s first LocataNet. Globally significant installations like this prove Locata’s new technology is delivering unprecedented levels of performance to many important new applications,” said Nunzio Gambale, Locata CEO. “As our technology roll-out begins to gain pace, the exceptional value Locata brings to next-gen mobile apps has attracted interest from players all over the world. In fact, our list of relationships is now looking like a roster of the world’s crème-de-la-crème. I honestly can’t think of a better or more prestigious name than NASA to add to our growing partner list.”
“Our team is savoring the opportunity to work alongside NASA engineers and we’re excited that Locata will help advance the safety-critical performance of Unmanned Aerial Systems,” he continued. “Almost all future mobile devices or machines, be they on the road, in the air, on a mine site, in a port, in a warehouse, in your mobile phone, or part of the inevitable Internet of Things — all of them are critically dependent on pervasive, reliable, high-accuracy positioning. Locata is being leveraged into these next-gen systems because it’s clear that satellite-based solutions alone can no longer deliver what’s required. Soon, as we bring miniaturized Locata transmitters and receivers to market, our innovations will enable even greater advances in cutting-edge consumer, commercial, and government applications.”
NASA Testing Program. As part of its UAS research, NASA is testing a system that would make it possible for unmanned aircraft to fly routine operations in United States airspace. Through the agency’s Unmanned Aircraft Systems Integration in the National Airspace System (UAS-NAS) project, NASA, General Atomics Aeronautical Systems, Inc. (GA-ASI) and Honeywell International, Inc., are flying a series of tests which began on June 17 and will run through July at NASA’s Armstrong Flight Research Center in California.
“We are excited to continue our partnership with GA-ASI and Honeywell to collect flight test data that will aid in the development of standards necessary to safely integrate these aircraft into the National Airspace System,” said Laurie Grindle, UAS-NAS project manager at Armstrong.
This is the third series of tests that builds upon the success of similar experiments conducted late last year that demonstrated a proof-of-concept sense-and-avoid system. The tests engage the core air traffic infrastructure and supporting software components through a live and virtual environment to demonstrate how a remotely piloted aircraft interacts with air traffic controllers and other air traffic.
“This is the first time that we are flight testing all of the technology developments from the project at the same time,” Grindle said.
This series of tests is made up of two phases. The first is focused on validation of sensor, trajectory and other simulation models using live data. Some of the tests will be flown with an Ikhana aircraft, based at Armstrong, that has been equipped with an updated sense-and-avoid system, as well as other advanced software from Honeywell.
Other tests will involve an S-3B plane from NASA’s Glenn Research Center in Cleveland, serving as a high-speed piloted surrogate aircraft. Both tests will use other aircraft following scripted flight paths to intrude on the flight path the remotely-piloted craft is flying, prompting it to either issue an alert or maneuver out of the other aircraft’s path. These flights will also conduct the first full test of the traffic alert and collision avoidance system (TCAS II) on a remotely piloted aircraft.
During the June 17 test, which lasted a little more than five hours, the team accomplished 14 encounters using the Ikhana aircraft and a Honeywell-owned Beech C90 King Air acting as the intruder. A second test was flown the following day, with a total of 23 encounters. The project team plans to fly more than 200 encounters throughout the first phase of the test series.
“Our researchers and project engineers will be gathering a substantial amount of data to validate their pilot maneuver guidance and alerting logic that has previously been evaluated in simulations,” said Heather Maliska, Armstrong’s UAS-NAS deputy project manager.
The second phase of the third test series will begin in August and will include a T-34 plane equipped with a proof-of-concept control and non-payload communications system. It will evaluate how well the systems work together so that the aircraft pilots itself, interacts with air traffic controllers and remains well clear of other aircraft while executing its operational mission. The aircraft, which will have an onboard safety pilot, will fly an operationally representative mission in a virtual airspace sector complete with air traffic control and live and virtual traffic.
The four MMS spacecraft host the highest ever operational GPS receivers in space. (artist’s rendition, credit: NASA)
Editor’s Note: See additional coverage of the MMS mission here.
September Marks Start of Magnetosphere Mission, but Navigators Already Perform
A NASA mission to explore magnetic reconnection also made GPS history this spring. The Magnetospheric Multiscale (MMS) mission, led by NASA’s Goddard Space Flight Center in Greenbelt, Md., is flying four identically equipped spacecraft in a tight formation to take measurements 100 times faster than any previous space mission.
Each of the four spinning MMS spacecraft — roughly the size of a ballpark once eight booms deploy — is equipped with 25 sensors and other components provided by more than 40 partner institutions in the U.S., Europe and Japan. One key component is a GPS receiver dubbed Navigator.
Magnetic reconnection is a fundamental, yet poorly understood process.While reconnection occurs throughout the universe when magnetic field lines within plasma connect and disconnect, it can impact our technological society, since it drives virtually all space weather events that can disrupt low-Earth-orbiting spacecraft and lead to GPS, communications and power blackouts on Earth.
During the mission’s first phase, which begins in September, the spacecraft will travel through reconnection sites on the sun-side of Earth, where the orbit extends out toward the sun to around 47,500 miles. One year later, ground controllers will move the spacecraft to Earth’s night-side or magnetotail where the magnetic fields also reconnect — an orbit that extends away from Earth to almost 99,000 miles, nearly halfway to the moon.
However, science operations can’t begin before the four move into a highly elliptical orbit and assume their pyramid-shape formation that places the spinning spacecraft just 6.2 miles apart. It required a breakthrough to accomplish such an exacting formation, and the Goddard-developed Navigator GPS provided the solution.
Begun in the early 2000s as an enabling technology for MMS-type missions, the Navigator receiver and associated algorithms can quickly acquire and track GPS radiowaves even in weak-signal areas well above GPS’s 30-plus-satellite constellation positioned about 12,550 miles above Earth. In addition to continuously tracking weak signals, the Navigator also must operate as the spacecraft spin at three revolutions per minute. As a result, each MMS satellite is equipped with two Navigator receivers (primary and redundant), with four antennas placed around the perimeter of each, assuring continuous contact with the tracked GPS satellites
“Spinning adds a whole new dimension to trying to figure out where you are,” said Ken McCaughey, MMS GPS Navigator Product Development Lead at Goddard. “As the spacecraft rotates we have an algorithm running that allows us to hand off from one antenna to the next without losing the signal.”
Robust Receivers. To the satisfaction of the technology’s architect, Goddard technologist Luke Winternitz, the receivers have proven very robust. Shortly after the GPS receivers were powered on after the launch, Navigator became, at more than 43,500 miles above Earth’s surface, the highest ever operational GPS receiver in space. “We’re tracking up to 12 GPS satellites at maximum altitude and track on average about nine,” Winternitz said. “We’re really excited about their performance so far.”
Even if the receiver were to lose all GPS signals for part of the orbit, Navigator is specifically designed to handle such dropouts. By gathering as many observations as possible, integrated software called GEONS — Goddard Enhanced Onboard Navigation System — can still compute the orbit by incorporating additional information including drag force, gravity, and solar radiation pressure.
The red ellipses show the MMS orbit paths during the first and second phases of the mission. Each spacecraft uses GPS signals — which come from satellites situated along the green circle shown surrounding Earth — from the far side of Earth to track its position. (Credit: NASA/MMS)
This system will be even more important during the second phase of the MMS mission when the orbit will double in size and travel all the way out to 95,000 miles from Earth.
“It’s going to be very interesting to see how far out MMS can still receive signals,” said Mission Deputy Project Manager Brent Robertson. “But Navigator has already far exceeded expectations.”
Almost all activities associated with operating the mission depend on where the satellites will be positioned a few days hence. That includes everything from determining the best time to downlink telemetry and scientific data to calculating when ground controllers would command the firing of the satellites’ onboard thrusters, which move and help maintain their orbital formation — an exercise that will happen at least once every couple weeks.
“I think there’s a good chance we’ll end up being able to use GPS and save us some of the expense of using ground observations,” Robertson said.
While Navigator technology and GPS receivers were previously flown for testing and to help navigate a low-earth-orbit mission, this is the first time that the complete Navigator package has been used to actively navigate a high-altitude mission. Now that the team knows it works so well, Navigator can be used for other missions that travel in similar high orbits.
The four MMS observatories are processed for launch in a clean room at the Astrotech Space Operations facility in Titusville, Fla. The MMS mission launched March 12, 2015. (Credit: Ben Smegelsky/NASA)
Navigator Highlights
At the highest point of the MMS orbit, at more than 43,500 mile above the surface of the earth, Navigator set a record for the highest ever reception of signals and onboard navigation solutions by an operational GPS receiver in space.
At the lowest point of the MMS orbit, Navigator set a record as the fastest operational GPS receiver in space, at velocities over 22,000 miles per hour.
At the farthest point in its orbit, some 43,500 miles away from Earth, Navigator can determine the position of each spacecraft with an uncertainty of better than 50 feet.
Eos Positioning’s Arrow 200 Bluetooth receiver now supports Hemisphere’s Atlas correction service,
The Arrow 200 Bluetooth GNSS receiver by Eos Positioning Systems now supports the new Atlas H10 GNSS correction service. Using the H10 service, the Arrow 200 GNSS receiver is able to achieve 8-cm accuracy, in real-time, virtually anywhere in the world, the company said. The H10 corrections are delivered by geostationary satellite or via Internet connection.
The Hemisphere GNSS Atlas correction service, announced in June, is a real-time correction service that meets or exceeds existing correction services. It has three service levels, with H10 having the highest accuracy.
“Eos is proud to introduce the first GNSS receiver that supports the H10 service,” said Chief Technology Officer Jean-Yves Lauture. “It will allow our customers in every country in the world to have access to sub-decimeter real-time accuracy on all mobile platforms, including iOS, Android and Windows devices.”
The H10 correction service and the Arrow 200 support all active constellations including GPS, GLONASS, Galileo, BeiDou and QZSS, giving the user ultra-fast convergence time to real-time decimeter accuracy, Eos Positioning said.
The Arrow 200 employs long-range (1 km) universal Bluetooth connectivity so the user can interface to any brand of smartphone or tablet, whether it’s iOS, Android or Windows-based. The Arrow 200 has been optimized to run all day on battery power. The battery pack is field-replaceable and rechargeable separately. All Arrow receivers have been designed to meet IP-67 specifications for immersion in water and are completely dust-proof so they will survive in the harshest environments.
The Arrow 200 GNSS receiver with Atlas H-10 service is targeted at high-accuracy applications like GIS, environmental, agriculture, electric/gas/water utilities, surveying, machine control, and federal, state and local government.
Part 1 of this column appeared in the June Survey Scene newsletter.
Basic Procedures for Establishing Accurate GNSS-Derived Ellipsoid Heights
David B. Zilkoski
In my first newsletter column of this series, Part 1, I discussed the basic concepts of GNSS-derived heights. My article discussed the three types of heights involved in determining GNSS-derived orthometric heights: ellipsoid, geoid, and orthometric. I also mentioned that each of these heights has its own error sources that need to be detected, reduced or eliminated by following specific procedures or applying special models.
GNSS-derived ellipsoid heights are the basis for GNSS-derived orthometric heights, so it makes sense to make these ellipsoid heights as close to error free as possible. This article will discuss guidelines for detecting, reducing and eliminating error sources in ellipsoid heights. It will focus on guidelines for establishing accurate ellipsoid heights in a local geodetic network.
Based on the Federal Geographic Data Committee publication “Geospatial Positioning Accuracy Standards, Part 2: Standards for Geodetic Networks,” guidelines were developed by the National Geodetic Survey (NGS) for performing GNSS surveys that are intended to achieve ellipsoid height network accuracies of 5 cm at the 95 percent confidence level, as well asellipsoid height local accuracies of 2 cm and 5 cm, also at the 95 percent confidence level. These guidelines were developed in partnership with federal, state and local government agencies, academia and private surveyors, and are the result of processing various test data sets and having extensive discussions with various GNSS users groups. These guidelines, known as NGS 58, have been documented in a publication titled “Guidelines for Establishing GPS-derived Ellipsoid Heights (Standards: 2 cm 9and 5 cm), Version 4.3″ and can be downloaded from the NGS website. NGS is reevaluating the guidelines and, based on its research results, will update the document appropriately (NGS, Personnel Communication).
Guidelines have also been written to establish GNSS-derived orthometric heights that approach these same accuracies, 2 cm and 5 cm. The slight differences between the accuracies of GNSS-derived ellipsoid heights and GNSS-derived orthometric heights will be generally due to the accuracy of the geoid model and published orthometric heights used to evaluate the differences between the three height systems: ellipsoid, geoid and orthometric heights. The topic “procedures for estimating accurate GNSS-derived orthometric heights” will be addressed in a future newsletter in this series.
If users follow the NGS guidelines, they will reduce or eliminate errors in ellipsoid height or, at a minimum, they will detect problems or errors in data. If these problems or errors are detected and corrected before the project is completed, then they will not be problems to the end users.
Basic Procedures for Detecting, Reducing, and Eliminating Errors in GNSS Ellipsoid Heights
The basic concepts listed below are very simple, but they all need to be followed as prescribed.
First and probably one of the most important procedure is to repeat baselines on different days and at different times of the day. This helps to detect and reduce the effects of: multipath, differences in height values due to different satellite geometry, and the amount of time a user must occupy a station for a short baseline, for instance, 30 minutes of good, valid data over baselines less than 10 km. (Although, it should be noted that to obtain 30 minutes of good, valid data, the user may have to obtain 45 to 60 minutes of data.)
The observing scheme for all stations requires that all adjacent stations (base lines) be observed at least twice on two different days and at two different times of the day. The purpose is to ensure different atmospheric conditions (different days) and significantly different satellite geometry (different times) for the two baseline measurements.
Keep baseline lengths under 10 km. The closer the two stations are, the better chance that common errors will cancel or nearly cancel, such as unmodeled atmospheric errors. It helps to reduce the amount of time the user must occupy a station in order to collect enough good, valid data to correctly fix all the integers.
Use fixed height poles. This helps eliminate errors due to incorrectly measuring the height of the antenna above the mark. Of course, when listening to GNSS users, nobody has ever measured the height of the tripod wrong. But, it’s strange how that turns out to be the most common error when fixed-height poles are not used.
Antenna set-up is critical. Plumbing bubbles on the antenna pole of the fixed-height tripod must be shaded when plumbing is performed. Plumbing bubbles must be shaded for at least 3 minutes before checking and/or re-plumbing. The perpendicularity of the poles must be checked at the beginning of the project and any other time there is suspicion of a problem. The user should also ensure the antenna is properly seated in the mount.
Use a geodetic antenna with ground plane and/or choke ring. This helps reduce effects of local multipath.
Final processing shall consist of fixing all integers for each vector for all sessions except to some control sites. Users should be able to fix the integers over baselines that are less than 10 kilometers. If the integers cannot be fixed, there is probably something wrong with the data, such as bad multipath effects, missing data due to blockage, or interference. Baseline solutions with fixed integers prove to be more reliable, consistent and accurate.
Simultaneously observe baselines between neighboring stations. This helps to ensure that closely spaced stations (neighboring stations) will have the desired local accuracy and are the stations that most users will want to use to validate their classical leveling results.
Establish a high-accuracy 3-D fiducial network that encompasses the entire project. This network helps to detect and reduce the effects of remaining systematic errors in the local network observations. This also ensures that when two local networks are eventually connected, they will be consistent with each other. This is a very important aspect of establishing accurate GNSS-derived ellipsoid heights using the guidelines documented in NGS 58. The survey should be referenced to at least three existing Continuous Operating Reference Stations (CORS) [NOAA CORS or equivalent] near the project area. The survey should also consist of at least three control stations that are referenced to the three CORS and interspersed throughout the project. For these control stations, receivers should collect data continuously and simultaneously for at least three, 5-hour sessions on three different days at different times of the day during the project. As previously stated, NGS is reevaluating the guidelines and will update them based on the results of their research. Until NGS updates the guidelines, the user should continue to collect long data sets at these control stations, because they are extremely important to detecting potential errors in the stations established using short data observing sessions.
Evaluating the Quality of Published NAD 83 (2011) Ellipsoid Heights
A description of the National Adjustment of 2011 Project (Alignment of passive control with the latest realization of the North American Datum of 1983: NAD 83(2011/PA11/MA11) epoch 2010.00) is available online.
I’ve listed a few paragraphs (and highlighted a few statements) from the write-up that I believe are important to anyone using published NAD 83 (2011) ellipsoid heights as control stations.
As part of continuing efforts to improve the NSRS, on June 30, 2012, NGS completed the National Adjustment of 2011 Project. This project was a nationwide adjustment of NGS “passive” control (physical marks that can be occupied with survey equipment, such as brass disk bench marks) positioned using GNSS technology. The adjustment was constrained to current North American Datum of 1983 (NAD 83) latitude, longitude and ellipsoid heights of NGS Continuously Operating Reference Stations (CORS). The CORS network is an “active” control system consisting of permanently mounted GNSS antennas, and it is the geometric foundation of the NSRS. Constraining the adjustment to the CORS optimally aligned the GNSS passive control with the active control, providing a unified reference frame to serve the nation’s geometric positioning needs.
For the final constrained adjustments, the median network accuracy for all stations was 0.9 cm horizontal and 1.5 cm vertical (i.e., ellipsoid height) at the 95% confidence level. The median change in coordinates from the previous published values was about 2 cm horizontally and vertically. However, some station coordinates changed by more than 1 meter horizontally and 60 cm vertically. Although some of the large coordinate changes resulted from new data and adjustment strategies, most horizontal changes greater than about 6 cm occurred in geologically active areas and were likely due to tectonic motion.
Results of the 2011 national adjustment for 79,677 passive control marks are available on NGS Datasheets, including their network and local accuracies.Of these passive marks, 79,161 are referenced to the North America tectonic plate as the 2011 realization (including CONUS, Alaska and the Caribbean); 345 are referenced to the Pacific plate as the PA11 realization (the central Pacific, including Hawaii, American Samoa and the Marshall Islands); and 171 are referenced to the Mariana plate as the MA11 realization (the western Pacific, including Guam, Palau and the Commonwealth of the Northern Mariana Islands). Although the passive marks are referenced to three different tectonic plates, all refer to a common 2010.0 epoch date. With the completion of the national adjustment, all passive marks on NGS Datasheets with NAD 83(2011/PA11/MA11) epoch 2010.00 coordinates will be consistent with results obtained using CORS and the NGS Online Positioning User Service (OPUS). Note that 183 stations were excluded from the final national adjustments due to lack of enabled vector connections; where possible, these stations will be reconnected to the network in subsequent individual adjustments.
Other technical issues addressed in the project include:
1. appropriate down-weighting of the up component of GNSS vectors to account for subsidence in the northern Gulf Coast region of CONUS;
2. use of variable weighted (stochastic) constraints for CORS based on formal accuracy estimates derived from the NGS MYCS1;
3. scaling of GNSS vector error estimates for all projects to ensure consistent weighting of observations;
4. use of down-weighting (rather than removal) for vector rejections;
5. splitting the conterminous U.S. into a Primary and Secondary network, as mentioned above, such that vectors observed prior to about 1994 were assigned to the Secondary network. This allowed the Primary network to be adjusted separately without the problems associated with older observations (e.g., single frequency receivers, no antenna phase center models, poor orbit accuracy, incomplete satellite constellation, lack of CORS, etc.).
Each of these technical challenges (and others) was satisfactorily resolved, and completion of the National Adjustment of 2011 Project represents a significant step toward a more integrated, consistent, and accurate NSRS.
First, I’d like to commend NGS for performing the NAD 83 (2011) national adjustment; it was a great accomplishment by NGS. It provides users with a consistent, accurate set of geodetic coordinates (latitude, longitude and ellipsoid height) that should serve the nation’s positioning requirements for many years. Saying that, there are some issues that the user needs to consider when using published NAD 83 (2011) ellipsoid heights as constraints in GNSS network adjustments:
Generally, the NAD 83 (2011) network design was sufficient for determining accurate horizontal coordinates (latitude and longitude) but may not have been sufficient for establishing the vertical component (ellipsoid height) accurate enough for use as control stations in NGS Height Modernization Projects (see this webpage for more information on NGS’ Height Modernization Program) . Many of the earlier GNSS projects, prior to the publication of NGS 58, did not repeat baselines; stations were, however, usually occupied at least twice and observing sessions lasted for two hours or more. They were generally evaluated using loop closures and adjustment statistics, but loop analysis and adjustments do not always detect, reduce and/or eliminate all problems.
In addition, prior to NGS 58, not all closely spaced stations (neighboring stations) were simultaneously observed during the same session. In my opinion, the published formal errors may be too optimistic for some of these stations. These stations may be very precise but based on the survey field procedures performed prior to the publication of NGS 58, it is my opinion that the relative ellipsoid height accuracy for closely-spaced stations that were not simultaneously observed during the same session may not be as accurate as their listed median accuracy value.
Stations that were observed following the NGS 58 document are labeled as Height Modernization stations on the NGS datasheet and their ellipsoid height values should be good to the 2-cm level if they were involved in the same project.
It is important to understand the quality of published NAD 83 (2011) ellipsoid heights because your project’s GNSS-derived ellipsoid height values will be evaluated by them. The project’s control stations help to detect and reduce the effects of remaining systematic errors in the local network so they need to be very accurately determined.
Identifying good, valid published NAD 83 (2011) ellipsoid heights accurate enough to evaluate the results of a GNSS project isn’t an exact science, but there are ways to identify good candidates. I’ve listed three ways of using NGS published datasheets to help the user evaluate the quality of NAD 83 (2011) ellipsoid heights.
Identify stations that were established in Height Modernization Projects (that is, the stations were established following NGS 58 guidelines).
Analyze the network and local accuracy values to identify stations with accuracy values less than 2 cm.
Use local accuracy tables of stations to determine if closely spaced monuments (neighboring stations) were occupied during the same session.
The user can retrieve NGS datasheets in text form or as a shape file using NGS’ Datasheet retrieval program. Identifying stations involved in a NGS Height Modernization Project is simple because the datasheet adds a note stating that a particular station is a Height Modernization Survey Station. The user can assume these stations were determined following NGS 58 guidelines. An example of a station involved in a height modernization project is station CARGO, DJ5933 (see the datasheet below). The NGS datasheet also lists the station’s network and local accuracies. On the datasheet, the network accuracy value is listed below the coordinates (for instance, 1.39 cm for station CARGO). Below the network accuracy value, the user can obtain the local accuracy values by clicking on the following link in the datasheet: “Click here for local accuracies and other accuracy information.” You can obtain the full NGS datasheet for CARGO.
The NGS Data Sheet for Height Modernization Station CARGO (DJ5933) PROGRAM = datasheet95, VERSION = 8.71 National Geodetic Survey, Retrieval Date = JULY 12, 2015 DJ5933*********************************************************************** DJ5933 HT_MOD – This is a Height Modernization Survey Station. DJ5933 DESIGNATION – CARGO DJ5933 PID – DJ5933DJ5933 STATE/COUNTY- NC/NEW HANOVERDJ5933 COUNTRY – US DJ5933 USGS QUAD – WILMINGTON (1979)DJ5933DJ5933 *CURRENT SURVEY CONTROL DJ5933 ______________________________________________________________________ DJ5933* NAD 83(2011) POSITION- 34 12 27.89075(N) 077 57 16.40009(W) ADJUSTED DJ5933* NAD 83(2011) ELLIP HT- -34.732 (meters) (06/27/12) ADJUSTED DJ5933* NAD 83(2011) EPOCH – 2010.00 DJ5933* NAVD 88 ORTHO HEIGHT – 2.05 (meters) 6.7 (feet) GPS OBS DJ5933 ______________________________________________________________________ DJ5933 NAVD 88 orthometric height was determined with geoid model GEOID03 DJ5933 GEOID HEIGHT – -36.78 (meters) GEOID03DJ5933 GEOID HEIGHT – -36.80 (meters) GEOID12BDJ5933 NAD 83(2011) X – 1,101,934.174 (meters) COMPDJ5933 NAD 83(2011) Y – -5,164,049.037 (meters) COMPDJ5933 NAD 83(2011) Z – 3,565,508.167 (meters) COMPDJ5933 LAPLACE CORR – -5.30 (seconds) DEFLEC12B
DJ5933
DJ5933 Network accuracy estimates per FGDC Geospatial Positioning Accuracy
DJ5933 Standards:
DJ5933 FGDC (95% conf, cm) Standard deviation (cm) CorrNE
DJ5933 Click here for local accuracies and other accuracy information.
Local accuracies provided on the NGS datasheet can be used to determine if closely spaced stations were simultaneously observed during the same session. If two stations were simultaneously observed during the same session, they will have a local accuracy value listed in their table. Station TOWN CREEK (EA0883) is an example of a station that was simultaneously observed by BR 7 (EA0873) in one GNSS project and by LILIPUT (EA0875) in a different project. (Figure 1 depicts these stations and their NAD 83 (2011) network accuracy values.) Looking at the highlighted section of the tables below, station EA0883 is listed in the local accuracy tables for EA0873 and EA0875, so it was simultaneously observed during sessions with EA0873 and EA0875.
Saying that, we can also use the tables to show that EA0873 and EA0875 were not simultaneously observed during the same session. That is, EA0873 is not listed on EA0875 local accuracy table and EA0875 is not listed on EA0873 local accuracy table so they were not processed simultaneous in a session. Figure 2 depicts the two GNSS projects that include observations involving stations EA0873 and EA0875. The user can perform the same procedure to determine that stations EB0217 and EA0873, 8.3 km apart, were not simultaneously observed during the same session, and similarly EA0873 and EA0665, 7.5 km apart, were not simultaneously observed during the same project. Please note I am not suggesting that anything is wrong with these surveys; there may be good reasons why these stations were not simultaneously observed during the same project. I am only using it as an example in this column. Network and local accuracy values are good indicators of potentially “how good” a station is relative to its neighbor, but they should always be evaluated and investigated. My intent is to provide the user with tools for evaluating the quality of published NAD 83 (2011) ellipsoid heights. This is important because published coordinates are used to evaluate the adjustment results of new projects.
Local and Network Accuracy Data for NGS Datasheet – EA0873 Program lna_ret Version 2.7 Date April 6, 2015 National Geodetic Survey, Retrieval Date = JUNE 30, 2015 EA0873 ************************************************************ EA0873 ACCURACIES – Complete network and local accuracy information. EA0873 DESIGNATION – BR 7 EA0873 PID – EA0873 EA0873 EA0873 Horiz and Ellip are the horizontal and ellipsoid height accuracies EA0873 at the 95% confidence level per Federal Geographic Data Committee EA0873 Geospatial Positioning Accuracy Standards. SD_N, SD_E and SD_h are EA0873 the standard deviations (one sigma) of the coordinates (NETWORK) or EA0873 of the difference in the coordinates (LOCAL) in latitude, longitude EA0873 and ellipsoid height. CorrNE is the (unitless) correlation EA0873 coefficient between the latitude and longitude components of either EA0873 the coordinate (NETWORK) or coordinate difference (LOCAL). Dist is EA0873 the three-dimensional straight-line slope distance, in km, between EA0873 station EA0873 and the corresponding local station. Local stations EA0873 are stations processed simultaneously in a session regardless of EA0873 distance. EA0873EA0873 Accuracy and standard deviation values are given in cm.EA0873EA0873 Type/PID Horiz Ellip Dist(km) SD_N SD_E SD_h CorrNEEA0873 ——————————————————————-
Local and Network Accuracy Data for NGS Datasheets – EA0875 Program lna_ret Version 2.7 Date April 6, 2015National Geodetic Survey, Retrieval Date = JUNE 30, 2015 EA0875 ********************************************************** EA0875 ACCURACIES – Complete network and local accuracy information. EA0875 DESIGNATION – LILIPUT EA0875 PID – EA0875 EA0875 EA0875 Horiz and Ellip are the horizontal and ellipsoid height accuracies EA0875 at the 95% confidence level per Federal Geographic Data Committee EA0875 Geospatial Positioning Accuracy Standards. SD_N, SD_E and SD_h are EA0875 the standard deviations (one sigma) of the coordinates (NETWORK) or EA0875 of the difference in the coordinates (LOCAL) in latitude, longitude EA0875 and ellipsoid height. CorrNE is the (unitless) correlation EA0875 coefficient between the latitude and longitude components of either EA0875 the coordinate (NETWORK) or coordinate difference (LOCAL). Dist is EA0875 the three-dimensional straight-line slope distance, in km, between EA0875 station EA0875 and the corresponding local station. Local stations EA0875 are stations processed simultaneously in a session regardless ofEA0875 distance.EA0875EA0875 Accuracy and standard deviation values are given in cm.EA0875EA0875 Type/PID Horiz Ellip Dist(km) SD_N SD_E SD_h CorrNE
I haven’t discussed all procedures documented in NGS 58 here. There are other minor, but very important, procedures that the user must follow, such as use of precise ephemerides, taking a rubbing of the mark; the reader is referred to NOAA Technical Memorandum NOS NGS-58, “Guidelines for Establishing GPS-derived Ellipsoid Heights (Standards: 2 cm and 5 cm), Version 4.3,” for more details.
This column discussed procedures that need to be followed to detect, reduce and eliminate error sources to estimate accurate GNSS-derived ellipsoid heights. Analysis of the quality of project data should be based on repeatability of measurements, adjustment residuals and analysis of loop closures. Please be aware that repeatability and loop closures do not always disclose all problems, and that is why it is important to adhere to the procedures outlined in NGS’ publications.
It is important to understand geoid models when estimating GNSS-derived orthometric heights. The user should understand the differences between NGS’ scientific gravimetric geoid model and hybrid geoid models, and why it is important to use both types of geoid models in an analysis. As I mentioned in Part 1, the latest NGS hybrid geoid model, Geoid12B, is made consistent with the published NAVD 88 heights. This means you will be consistent with NAVD 88 when using GEOID12B to estimate GNSS-derived orthometric heights. However, this doesn’t guarantee that your GNSS-derived orthometric heights are accurate. NGS’ new Beta experimental geoid height model xGEOID14B is not distorted to fit the published NAVD 88 heights so it is useful for identifying valid NAVD 88 benchmarks. In my next column, I’ll address how to use these geoid models and published NAD 83 (2011) ellipsoid heights to evaluate potential issues with published NAVD 88 heights.
Figure 1. NAD 83 (2011) Ellipsoid Network Accuracies – units cm (Network accuracies were obtained from NGS datasheets).Figure 2. NAD 83 (2011) Network Design for Stations EA0873 and EA0875. [Note: GNSS Vectors for GNSS projects GPS 1588 and GPS 2057 were provided by NGS].
