The 23rd International Flight Inspection Symposium (IFIS) will gather experts in San Salvador May 4-8. There, Rohde & Schwarz will demonstrate its test and measurement solutions for ground-based navigation aids. The exhibits address the rising traffic volumes and stricter safety requirements.
Rohde & Schwarz will take part in the conference’s technical sessions with a presentation on “Challenges for UAV Operations in RF Dense Aerodrome Environments.”
The aviation sector today faces increasing air traffic density, rapid technological advancements and heightened security concerns, the company explained. Operators need test equipment that delivers laboratory level precision while tolerating the harsh environment of an airport runway or a remote navigation site.
Among the exhibits at the Rohde & Schwarz booth is the R&S EVSD1000 VHF/UHF Nav/Drone Analyzer, designed to conduct GBAS, ILS and VOR measurements in line with ICAO Doc 8071 and ICAO Annex 10. The receiver delivers laboratory precision, supports an air to ground Wi‑Fi datalink and gapless measurements with improved location accuracy during flight inspections. Customers benefit from a device that can be mounted on a drone, reducing the need for manned flights and lowering operational expenses.
Rohde & Schwarz gives airlines, airport operators and navigation service providers a reliable way to certify and maintain ground‑based aids under today’s demanding conditions. By combining high measurement accuracy, easy operation and durability, Rohde & Schwarz aims to help the industry keep pace with growth.
The Airbus A350 can now be equipped with the Collins Aerospace GLU-2100 multi-mode receiver. (Photo: pablorebo1984/iStock/Getty Images Plus/Getty Images)
The Collins Aerospace GLU-2100 multi-mode receiver (MMR) has received approval by Airbus, making it available as line-fit and retrofit on Airbus A320, A330 and A350 aircraft. This a major step toward Collins offering next-generation GNSS to the commercial aviation marketplace.
An MMR assists pilots in positioning, navigating and landing an aircraft. Building on the GNSS capabilities of previous MMRs, the GLU-2100 provides a satellite-based augmentation system (SBAS) and ground-based augmentation system (GBAS). This supports the integrity of the aircraft position, as well as the accuracy and availability of demanding aircraft operations such as landing in low visibility conditions.
The GLU-2100 MMR ensures that commercial aircraft can meet flight zone global mandates, while also proofing the technology by providing a solid foundation for future growth. It includes the flexible hardware baseline necessary to implement future GNSS capabilities, such as multi-frequency and multi-constellation (MFMC), and GBAS Category II/III via software-only update.
Acquisition of FlightAware tracking platform
In August, Collins Aerospace signed a definitive agreement to acquire privately held FlightAware, a digital aviation company providing global flight-tracking solutions, predictive technology, analytics and decision-making tools.
Closure of the acquisition is subject to the completion of customary conditions and regulatory approvals. Following closing, FlightAware will join Collins’ Information Management Services portfolio within the company’s Avionics strategic business unit. Financial terms of the agreement were not disclosed.
Based in Houston, Texas, with approximately 130 employees, FlightAware was founded in 2005 and is a provider of real-time and historical flight information and insights to the global aviation community. FlightAware serves all segments of the aviation marketplace through applications and data services that provide comprehensive information about the current and predicted movement of aircraft.
Through the collection, interpretation and enrichment of hundreds of sources of data, FlightAware transforms millions of raw flight data elements and delivers them as coherent, easy-to-consume flight stories. The company has a proprietary terrestrial ADS-B network with tens of thousands of receivers spanning seven continents in 200 countries and territories.
On Feb. 12, the White House released an “Executive Order on Strengthening National Resilience through Responsible Use of Positioning, Navigation, and Timing Services.”
It is gratifying to see White House attention to this issue. The increase in public awareness it brings will benefit individual users and the nation as a whole.
The order also hints at market driven solutions that could quickly improve America’s PNT resilience.
Needless delays
Unfortunately, the order fails to direct immediate action on this critical national and economic security issue. Instead it needlessly pushes most action and responsibility off for a year or more to do “more study.”
This is hard to understand as most of the “more study” has already been completed. For example, the order tells the Department of Commerce to take up to a year to examine PNT use in various sectors, and identify vulnerabilities and user needs. The Department of Homeland Security has already completed a National Risk Assessment and, according to congressional staff, has recently completed a report on user requirements mandated in 2017’s National Defense Authorization Act.
The Office of Science and Technology Policy is given a year to develop a plan to test robust and resilient non-GNSS PNT services (but is not required to actually do any testing). Congress mandated such a test program in 2017 and funded it with $10 million in 2018. After much delay, the Department of Transportation will complete the testing in May of this year.
The order gives the Department of Commerce six months to make available a time source to support critical infrastructure. For more than 60 years, the nation’s master clock has been available to users at the department’s NIST Laboratory in Boulder, Colorado.
Note the challenge has not been the clock, but that the nation has no way — other than vulnerable GPS signals — to distribute time at the needed level of accuracy to millions of critical infrastructure nodes. Government studies in 2007 and 2014 determined that the best way to do this was with a ground-based system. The Department of Transportation’s ongoing testing program is examining this issue again.
Market-driven solutions
Aside from increasing public awareness, the best thing the Executive Order does is to point a way forward for market-driven resilient PNT solutions.
The order calls for federal contracts to (in 21 months, if everyone does their jobs on time) require that vendors use existing and new resilient PNT sources.
If this eventually happens, the government could leverage its enormous influence in the market and stimulate creation of one or more commercial distribution systems for resilient, non-GNSS PNT. This is a great concept, and very much in keeping with America’s tradition of letting market forces solve some of its biggest problems.
But this solution will not spring into life on its own.
No commercial entity will invest tens of millions of dollars, or more, in a PNT system without assurance in advance of an income stream. Especially since federal contracting officers can and will waive the requirement if offerors cannot reasonably meet it.
If stimulating a market solution is the administration’s intent, it must stay actively involved and encourage the process for some time to come.
This includes complying with the 2018 law that requires establishment of at least one wireless, terrestrial, difficult-to-disrupt source to back up the timing signals provided by GPS.
Fortunately, this can be done by leveraging the free market at minimal cost and with little administrative effort.
By contracting to subscribe to a commercial service that will provide resilient PNT signals, the government need only invest a relatively small yearly sum using a fairly simple contract vehicle. Such a contracting technique has been used before with great success.
In 2007 the Federal Aviation Administration (FAA) did this as a way to establish its ADS-B aviation tracking and safety network. Once the subscription contract was let, the commercial provider was able to get financing and quickly build out the system.
Today, the FAA gets the information it needs, doesn’t have the headache of owning and maintaining a large network, and even shares in the revenue the system owner earns from selling data to other companies.
Additional leadership needed
It is important to remember that, regardless of the issue, presidential pronouncements are not enough.
In 2004, President G.W. Bush directed a number of actions to protect the nation’s critical PNT, including establishment of a GPS backup capability. While 16 years later his directive is still official executive branch policy, that mandate and many others from his order are still unexecuted.
Real improvements to PNT resilience and our nation’s security depend not on one-time pronouncements, but continued leadership focus and engagement.
This is always a challenge for initiatives driven by the White House. It will be doubly so in this case as there is no clear department leader for civil PNT issues the administration can rely on while it attends to the next issue of the day.
An artist’s concept of a GPS IIR-M satellite in orbit (courtesy of Lockheed Martin).
For more than 41 years, many of us who were there in the beginning have been discussing the attributes, capabilities, enabling features and shortcomings of GPS and other space-based PNT (position, navigation and timing) systems. You have likely heard most of them; historically they go something like this:
The signal is weak.
The signal is easily jammed.
The signal can be spoofed.
The signal is subject to atmospheric perturbations.
The signal doesn’t penetrate buildings.
The signal doesn’t penetrate dense canopies (urban or natural).
I am sure you have heard most of these. Now, allow me to update the situation with some of the developments enabled by modern signals, new techniques, and multi-frequency, multi-GNSS (Global Navigation Satellite System) “all-in-view” receivers. All of the above bulleted statements are still true, but to a lesser extent, virtually each day. As some well-known pop musicians once sang, “It’s getting better all the time.”
Today, multi-GNSS signals in a fully modern multi-GNSS receiver can to some degree resist interference — intentional (jamming) or unintentional — and spoofing. It is extremely difficult for a jammer or spoofer to disrupt GPS, GLONASS, Galileo and BeiDou all at the same time. And more help is on the way.
Today, multi-GNSS signal corrections remove a large amount of error due to atmospheric perturbations and can sometimes deliver centimeter and millimeter accuracy in real time (in the case of short-baseline real-time kinematic (RTK) using only L1 carrier-phase as data, and/or in some other special situations.)
Today, multi-GNSS signals and augmentation signals show some improvement in penetrating dense canopies and canyons by virtue of their multiplied numbers and dispersed geometry.
Today, new ground-based technologies show promise at penetrating buildings to provide indoor location. When combined with GPS/GNSS, this is starting to get us closer to the Holy Grail, the ubiquitous PNT solution.
Debate
The future looks bright for PNT solutions, ground and space-based. I know it all sounds like a debating society, and you may have heard some of these arguments before. My point, my premise if you will, or bottom-line-upfront in military parlance, being: the GPS (space-based) limitations of the past are gradually giving way to the improved multi-GNSS capabilities of today and the combined ground-based and space-based PNT technologies of the present and rapidly arriving future.
Unfortunately, there are many uninformed so-called PNT pundits who love to posture for the press — and who are living in the past. The future is right in front of them, or in many cases in their hands, and they cannot or will not acknowledge its existence.
It’s all in the numbers
Current estimates are that more than 4 billion users depend on PNT daily for position, navigation and timing, or the multitude of services each of these resources enables. More than half of that number is attributable to smartphone users, which means, at a minimum, more than 2 million PNT users have a two-way communications device incorporated into their PNT receiver/sensor.
Let’s look at current high-end smartphones as examples of commercial multi-frequency, multi-GNSS “all signals available” devices. The user has a true multi-GNSS device incorporating:
GPS — Global Positioning System, United States government
GLONASS — Globalnaya Navigazionnaya Sputnikovaya Sistema, the Russian space-based PNT system
BeiDou — the Chinese BeiDou Navigation Satellite System, a regional system now, soon to be global (2020 the advertised date).
with augmentations such as
WAAS — U.S. Wide Area Augmentation System
EGNOS — European Geostationary Navigation Overlay Service
Other SBAS — additional Satellite-Based Augmentation System signals by region
Wi-Fi — Signals compatible with a set of broadband wireless networking standards.
The latest high-end smartphones incorporate an inertial system, a digital compass, a rate gyro, and a pressure sensor integrated with pedometer software that keep track of position, heading and velocity when external signals are lost. Add cellular tower and network-enabled positioning and timing technology, and you have a two-way communications and PNT-based multi-GNSS sensor that, as long as it has power, is never lost.
Atomic numbers
The rubidium-based (atomic-reference system) timing signals from GPS satellite vehicles (SV) are among the most stable timing frequencies ever broadcast from space. The true accuracy of the signal in space is classified, but approaches an accuracy 10 times better than what was once thought to be adequate for our warfighters.
The best clocks in any current GNSS system are the passive hydrogen masers of Galileo. Thus a PNT set-up that adds Galileo to GPS improves in more ways than one.
Ephemeris numbers
Twenty-five years ago, the U.S. military kept track of GPS satellite orbit locations (known as the ephemeris of the satellite) using actual GPS measurements at the control segment tracking stations. The GPS satellite ephemeris was known to a much lesser degree of accuracy than now. At the time, that accuracy was considered good enough.
Today, the ephemeris is known much more precisely, and this can be on the order of some centimeters. This has to do with not only the location of the satellite’s center of mass (c.o.m.), but the actual location from which the signal is broadcast. The position of the satellite’s broadcast antenna is known reasonably well most of the time, by very high-end users, after correcting for the arm lever between the c.o.m. and the antenna phase center. The c.o.m. itself can vary by some centimeters over time because of depletion of onboard expendables, but here we are getting into very high-order minutiae.
Suffice it to say that certain multi-GNSS scientific high-precision receivers today are used to measure tectonic movements on the order of centimeters over the course of a full year.
Number of signals
Just recently, with the addition of certain QZSS signals (the Japanese Quasi-Zenith Satellite System) along with the Indian (GAGAN) and Russian (SDCM) equivalents of WAAS and EGNOS, the number of multi-GNSS PNT signals available to a truly international multi-GNSS receiver exceeds 200. For example, one set of global commercial receivers routinely receive and process more than 190 PNT signals in a six-hour period. The receivers are both static and dynamic, and they are networked. The static receivers know their actual location to within millimeters, and use this location as a truth set from which all other signal data is compared.
Accuracy numbers
For our example (and all parameters are software-defined and user-programmable), the location parameter may be set at 10 centimeters, meaning that any position derived from PNT signals or augmentations that differ by more than 10 centimeters from the “truth set” are immediately rejected, and that data is broadcast on the systems network, which keeps the dynamic receivers in sync as well.
The individual receivers each contribute to their own and a networked website with metadata usable by Kalman filters to which other users may choose to subscribe. This makes the multi-GNSS receivers not only receivers, but system and PNT monitors and sensors that can detect jamming, interference and spoofing attempts, which are reported.
This monitoring and tracking system is constantly evolving and incorporating new technologies while becoming more secure everyday. This is not a totally new concept, as the core system is a mature enterprise system that has been in operation and commercially viable for more than seven years.
This should be comforting information for those of you who stay up at night worrying about the safety of autonomous vehicles on land, sea and in the air.
Don’t let me give you the impression that GPS is just waiting around for other GNSS to come to its aid. GPS is aggressively modernizing itself. In Air Force parlance, “GPS III space vehicles will introduce new capabilities to meet higher demands of both military and civilian users.” As stated by GPS III contractor Lockheed Martin, the modernized system will:
• Deliver signals three times more accurate than current GPS spacecraft.
• Provide military users up to eight times improved anti-jamming capabilities.
Augmentations and improvements
The bottom line is that a greatly increased number of space-based PNT platforms — along with quantum improvements in computing power, cheap non-volatile memory and software-defined capabilities — have produced a multi-GNSS PNT capability that increases availability via sheer numbers, with more security and reliability on the way.
A pair of LocataLite transmit antennas overlook a section of the White Sands Missile Range blanketed by the Locata high-precision ground-based positioning system.
We are rapidly developing a PNT system that goes far in countering the naysayers. It takes advantage of augmentations and complimentary systems such as newer versions of Loran, (Long-Range Navigation System) and local PNT implementations such as Locata, just to name a couple of examples.
These ground-based systems are critical to the future of PNT, and have very strong signals. For instance, eLoran is extremely difficult to jam, if not actually unjammable. If a monstrous sunspot were to temporarily knock out the majority of space-based systems, the ground-based systems would more than likely still be available, if — big if here — they are fully developed. At the moment, this is not a sure thing. It is a work in progress.
Ground-based augmentations and complimentary/backup systems can in the future add a level of security for GPS and other space-based PNT systems: Why bother trying to knock out these space-based systems when there is a suitable and readily available ground-based system as a backup?
The U.S. government maintains a number of monitor stations around the globe. However, it has not historically taken advantage of the incredible capabilities of multi-GNSS receivers and sensor technology. Although NASA and other U.S. non-military agencies have been involved with multi-GNSS — specifically the Russian GLONASS — for the past 20 years or so, the use has not been widespread. Fortunately, recent changes now permit multi-GNSS receivers for government users, including the military, in certain non-targeting activities, and the government would do well to take advantage of the changes. The good news is that the majority of the capability is in the receiver design, a capability on which the current director of the GPS Directorate at the Space and Missile Systems Center (SMC) “made his bones.”
To all those critics who take every opportunity to denigrate space-based PNT, both inside and outside the government, I say: Pay attention to multi-GNSS. Stop your diatribes, because the future is arriving. Secure space-based PNT systems are here to stay.
They continue to improve and become more secure as they incorporate space- and ground-based augmentations, new PNT technologies, software-defined capabilities, multi-GNSS signals, and enhanced computing. “It’s getting better all the time.”
Allow me to repeat myself all over again. Space-based PNT is here to stay.
Until next time, happy navigating, and remember: GPS is brought to you free of charge by the United States Air Force.
Analysis of new Galileo signals at an experimental ground-based augmentation system (GBAS) compares noise and multipath in their performance to GPS L1 and L5. Raw noise and multipath level of the Galileo signals is shown to be smaller than those of GPS. Even after smoothing, Galileo signals perform somewhat better than GPS and are less sensitive to the smoothing time constant.
By Mihaela-Simona Circiu, Michael Felux, German Aerospace Center (DLR), and Sam Pullen, Stanford University
Several ground-based augmentation system (GBAS) stations have become operational in recent years and are used on a regular basis for approach guidance. These include airports at Sydney, Malaga, Frankfurt and Zurich. These stations are so-called GBAS Approach Service Type C (GAST C) stations and support approaches only under CAT-I weather conditions; that is, with a certain minimum visibility. Standards for stations supporting CAT-II/III operations (low visibility or automatic landing, called GAST D), are expected to be agreed upon by the International Civil Aviation Organization (ICAO) later this year. Stations could be commercially available as soon as 2018.
However, for both GAST C and D, the availability of the GBAS approach service can be significantly reduced under active ionospheric conditions. One potential solution is the use of two frequencies and multiple constellations in order to be able to correct for ionospheric impacts, detect and remove any compromised satellites, and improve the overall satellite geometry (and thus the availability) of the system.
A new multi-frequency and multi-constellation (MFMC) GBAS will have different potential error sources and failure modes that have to be considered and bounded. Thus, all performance and integrity assumptions of the existing single-frequency GBAS must be carefully reviewed before they can be applied to an MFMC system. A central element for ensuring the integrity of the estimated position solution is the calculation of protection levels. This is done by modeling all disturbances to the navigation signals in a conservative way and then estimating a bound on the resulting positioning errors that is valid at an allocated integrity risk probability.
One of the parameters that is different for the new signals and must be recharacterized is the residual uncertainty attributed to the corrections from the ground system (σpr_gnd). A method to assess the contribution of residual noise and multipath is by evaluating the B-values in GBAS, which give an estimate of the error contribution from a single reference receiver to a broadcast correction. Independent data samples over at least one day (for GPS) are collected and sorted by elevation angle. Then the mean and standard deviations for each elevation bin are determined.
Here, we evaluate the E1 and E5a signals broadcast by the operational Galileo satellites now in orbit. In the same manner as we did for GPS L5 in earlier research, we determine the σpr_gnd values for these Galileo signals. As for GPS L5, results show a lower level of noise and multipath in unsmoothed pseudorange measurements compared to GPS L1 C/A code.
DLR GBAS Facility
DLR has set up a GBAS prototype at the research airport in Braunschweig (ICAO identifier EDVE) near the DLR research facility there. This ground station has recently been updated and now consists of four GNSS receivers connected to choke ring antennas, which are mounted at heights between 2.5 meters and 7.5 meters above equipment shelters. All four receivers are capable of tracking GPS L5 (in addition to GPS L1 and L2 semi-codeless) and Galileo E1 and E5a signals. Figure 1 gives an overview of the current ground station layout, and Table 1 gives the coordinates of the antennas.
Figure 1. DLR ground facility near Braunschweig Airport, also shown in opening photo at left.
Table 1. Ground receiver antenna coordinates.
Smoothing Techniques
The GBAS system corrects for the combined effects of multiple sources of measurement errors that are highly correlated between reference receivers and users, such as satellite clock, ephemeris error, ionospheric delay error, and tropospheric delay error, through the differential corrections broadcast by the GBAS ground subsystem. However, uncorrelated errors such as multipath and receiver noise can make a significant contribution to the remaining differential error. Multipath errors are introduced by the satellite signal reaching the antenna via both the direct path from the satellites and from other paths due to reflection. These errors affect both the ground and the airborne receivers, but are different at each and do not cancel out when differential corrections are applied.
To reduce these errors, GBAS performs carrier smoothing. Smoothing makes use of the less noisy but ambiguous carrier-phase measurements to suppress the noise and multipath from the noisy but unambiguous code measurements.
The current GBAS architecture is based on single-frequency GPS L1 C/A code measurements only. Single-frequency carrier smoothing reduces noise and multipath, but ionospheric disturbances can cause significant differential errors when the ground station and the airborne user are affected by different conditions. With the new available satellites (GPS Block IIF and Galileo) broadcasting in an additional aeronautical band (L5 / E5), this second frequency could be used in GBAS to overcome many current limitations of the single-frequency system.
Dual-frequency techniques have been investigated in previous work. Two dual-frequency smoothing algorithms, Divergence Free (Dfree) and Ionosphere Free (Ifree), have been proposed to mitigate the effect of ionosphere gradients.
The Dfree output removes the temporal ionospheric gradient that affects the single-frequency filter but is still affected by the absolute difference in delay created by spatial gradients. The main advantage of Dfree is that the output noise is similar to that of single-frequency smoothing, since only one single-frequency code measurement is used as the code input (recall that carrier phase noise on both frequencies is small and can be neglected).
Ifree smoothing completely removes the (first-order) effects of ionospheric delay by using ionosphere-free combinations of code and phase measurements from two frequencies as inputs to the smoothing filter. Unlike the Dfree, the Ifree outputs contain the combination of errors from two code measurements. This increases the standard deviation of the differential pseudorange error and thus also of the position solution.
Noise and Multipath in New GNSS Signals
GBAS users compute nominal protection levels (H0) under a fault-free assumption. These protection levels are conservative overbounds of the maximum position error after application of the differential corrections broadcast by the ground system, assuming that no faults or anomalies affect the position solution. In order to compute these error bounds, the total standard deviation of each differentially corrected pseudorange measurements has to be modeled. The standard deviation of the residual uncertainty (σn, for the nth satellite) consists of the root-sum-square of uncertainties introduced by atmospheric effects (ionosphere, troposphere) as well as of the contribution of the ground multipath and noise. In other words, these error components are combined to estimate σn2 as described in the following equation:
(1)
The ground broadcasts a value for σpr_gnd (described later in the section) associated with the pseudorange correction for each satellite. These broadcast values are based on combinations of theoretical models and actual measurements collected from the ground receivers that represent actual system characteristics. Unlike the ground, σpr_air is computed based entirely on a standardized error model. This is mainly to avoid the evaluation of multipath for each receiver and each aircraft during equipment approval.
In addition to the characteristics of nearby signal reflectors, multipath errors are mainly dependent on signal modulation and other signal characteristics (for example, power, chip rate). In earlier research, we showed that the newly available L5 signals broadcast by the GPS Block IIF satellites show better performance in terms of lower noise and multipath. This mainly results from an increased transmitted power and a 10 times higher chip rate on L5 compared to the L1 C/A code signal.
In this work, we extend this evaluation to the new Galileo signals and investigate their impact on a future multi-frequency, multi-constellation GBAS. Characterization of these new signals is based on ground subsystem measurements, since no flight data with GPS L5 or Galileo measurements are available at the moment. We assume that the improvements observed by ground receivers are also applicable to airborne measurements. This assumption will be validated as soon as flight data are available.
The measurements used were collected from the DLR GBAS test bed over 10 days (note that Galileo satellite ground track repeatability is 10 sidereal days) between the December 14 and 23, 2013. In that period, four Galileo and four Block IIF GPS satellites were operational and broadcast signals on both aeronautical bands E1 / L1 and E5a / L5.
In Figure 2, the suppression of multipath and noise on the Galileo signals can be observed, where the code multipath and noise versus elevation for GPS L1 C/A BSPK(1), Galileo E1 (BOC (1,1)) and Galileo E5a (BPSK(10)) signals are shown. The code multipath and noise was estimated using the linear dual-frequency combination described in equation (2), where MPi represents the code multipath and noise on frequency i, ρi the code measurement, and ϕi,and ϕj represent the carrier-phase measurements on frequencies i and j, respectively. Carrier phase noises are small and can be neglected.
(2)
Figure 2. Raw multipath function of elevation for GPS L1, Galileo E1 (BOC (1,1)) and Galileo E5a (BPSK(10)) signals.
The multipath on the Galileo E1 (BOC(1,1)) signal (the magenta curve) is lower than the GPS L1 C/A (BPSK(1))(black curve), especially for low elevation, where the advantage of the E1 BOC(1,1) is more pronounced. The lower values can be explained by the wider transmission bandwidth on E1 and the structure of the BOC signal. Galileo E5a (green data in Figure 2) again shows a better performance than Galileo E1. This was expected due to the higher chip rate and higher signal power. A comparison of the raw multipath and noise standard deviations for GPS L1, L5 and Galileo E1, E5a signals is presented in Figure 3.
Figure 3. Ratios of the multipath and noise standard deviation function of elevation.
The curves there show the ratios of the standard deviations for each elevation bin. The values for GPS L1 are almost 1.5 times larger than those for Galileo E1 BOC(1,1) (green curve) for elevations below 20°. For high elevations, the ratio approaches 1.0. This corresponds to the observations in the raw multipath plot ( Figure 2). With the same signal modulation and the same chip rate, E5a and L5 have very similar results (red curve), and the ratio stays close to 1.0 for all elevations.
The blue and the purple curves in Figure 3 show the ratio of GPS L1 C/A (BPSK(1)) and GPS L5 (BPSK(10)), and Galileo E1 (BOC(1,1)) and Galileo E5a (BPSK(10)), respectively. The ratio of GPS L1 to GPS L5 (blue curve) increases with elevation from values around 2.5 for low elevations, reaching values above 3.5 for elevations higher than 60°. As Galileo E1 performs better, the ratio between Galileo E1 and Galileo E5a (purple curve) is smaller, from a value of 1.5 for elevations below 10 degrees to a value of 3.0 for high elevations.
Until now, we have presented the evaluation of raw code noise and multipath. However, in GBAS, carrier smoothing is performed to minimize the effect of code noise and multipath. The value that describes the noise introduced by the ground station is represented by a standard deviation called σpr_gnd and is computed based on the smoothed pseudoranges from the reference receivers. In the following section, we focus on the evaluation of σpr_gnd using different signals and different smoothing time constants. Note that, in this study, σpr_gnd contains only smoothed multipath and noise; no other contributions (for example, inflation due to signal deformation or geometry screening) are considered.
B-values and σpr_gnd
B-values represent estimates of the associated noise and multipath with the pseudorange corrections provided from each receiver for each satellite, as described in Eurocae ED-114A and RTCA DO-253C. They are used to detect faulty measurements in the ground system. For each satellite-receiver pair B(i,j), they are computed as:
(3)
where PRCTX represents the candidate transmitted pseudorange correction for satellite i (computed as an average over all M(i) receivers), and PRCSCA(i,k) represents the correction for satellite i from receiver k after smoothed clock adjustment, which is the process of removing the individual receiver clock bias from each reference receiver and all other common errors from the corrections. The summation computes the average correction over all M(k) receivers except receiver j. This allows detection and exclusion of receiver j if it is faulty. If all B-values are below their thresholds, the candidate pseudorange correction PRCTX is approved and transmitted. If not, a series of measurement exclusions and PRC and B-value recalculations takes place until all revised B-values are below threshold. Note that, under nominal conditions using only single-frequency measurements, the B-values are mainly affected by code multipath and noise.
Under the assumption that multipath errors are uncorrelated across reference receivers, nominal B-values can be used to assess the accuracy of the ground system. The standard deviation of the uncertainty associated with the contribution of the corrections (σpr_gnd) for each receiver m is related to the standard deviation of the B-values by:
(4)
where M represents the number of the receivers and N represents the number of satellites used. The final sigma takes into account the contribution from all receivers and is computed as the root mean square of the standard deviation of the uncertainties associated with each receiver (Equation 4).
Figure 4 shows the evaluation of (σpr_gnd) for the Galileo E1, BOC(1,1) signal and the GPS L1 C/A signal for increasing smoothing time constants (10, 30, 60, and 100 seconds). Starting with a 10-second smoothing constant, Galileo E1 shows much better performance than GPS L1. The difference shrinks as the smoothing constant increases due to the effectiveness of smoothing in reducing noise and short-delay multipath. However, even with 100-second smoothing (the purple curves), Galileo E1 BOC(1,1) shows lower values of (σpr_gnd).
Figure 4. σ(pr_gnd) versus elevation for Galileo E1 (dotted lines) and GPS L1 (solid lines for different smoothing constants: red (10s), green (30s), cyan (60s), purple (100s).
A similar comparison is presented in Figure 5, of the performance of GPS L1 and Galileo E5a. The Galileo E5a signal is significantly less affected by multipath, and the difference stays more pronounced than in the Galileo E1 – GPS L1, even with 100-second smoothing. It can be also observed that the Galileo signals have a lower sensitivity to the smoothing constant. The Galileo E1 signal shows an increase of sensitivity for low elevations (below 40°), while on E5a, a smoothing constant larger than 10 seconds has almost no impact on the residual error. Thus, a shorter smoothing constant on Galileo E5a generates approximately the same residual noise and multipath a 100-second smoothing constant on GPS L1.
Figure 5. σ(pr_gnd) versus elevation for Galileo E5a (dotted lines) and GPS L1 (solid lines) for different smoothing constants: red (10s), green (30s), cyan (60s), purple (100s).
The values for (σpr_gnd) are, however, impacted by the number of satellites which are used to determine a correction. Since only a very limited number of satellites broadcasting L5 and Galileo signals are currently available, these results should be considered preliminary. The first evaluations strongly indicate that with the new signals, we get better ranging performance. Based on the performance advantage of the new signals, a decrease of the smoothing constant is one option for future application. This would reduce the time required (for smoothing to converge) before including a new satellite or re-including a satellite after it was lost.
In the current GAST-D implementation, based on GPS L1 only, guidance is developed based on a 30-second smoothing time constant. A second solution, one with 100 seconds of smoothing, is used for deriving the Dv and Dl parameters from the DSIGMA monitor and thus for protection level bounding (it is also used for guidance in GAST-C). During the flight, different flight maneuvers or the blockage by the airframe can lead to the loss of the satellite signal.
Figure 6 shows the ground track of a recent flight trial conducted by DLR in November 2014. The colors represent the difference between the number of satellites used by the ground subsystem (with available corrections) and the number of satellites used by the airborne subsystem in the GAST-D position solution. One of the purposes of the flight was to characterize the loss of satellite signals in turns. In turns with a steeper bank angle, up to 3 satellites are lost (Turns 1, 3, and 4), while on a wide turn with a small bank angle (Turn 2), no loss of satellite lock occurred. It is also possible for airframe to block satellite signals, leading to a different number of satellites between ground and airborne even without turns.
Figure 6. Ground track of a flight trial conducted by DLR. The colors represent difference between number of SVs used by the ground system and number of SVs used by the airborne.
With this in mind, a shorter smoothing constant would allow the satellites lost to turns or to airframe blockage to be re-included more rapidly in the position solution. However, a new smoothing constant would have to be validated with a larger amount of data. Data from flights trials has to be evaluated as well to confirm that similar levels of performance are reresentative of the air multipath and noise.
In a future dual-frequency GBAS implementation, an important advantage of lower multipath and noise is to improve the Ifree position solution. In earlier research, we demonstrated that the error level of the Dfree solution is almost the same as for single-frequency, but an increase in error by a factor of 2.33 was computed for the Ifree standard deviation based on L1 C/A code and L2 semi-codeless measurements.
If the errors on L1 (E1) and L5 (E5a) code and carrier phase measurements are statistically independent the standard deviation of the σIfree can be written as,
(5)
where α=1−f 21 ∕ f 25, and σL1,σL5 represent the standard deviations of the smoothed noise and multipath for L1 (E1) and L5 (E5a), respectively. Considering σpr_gnd,L1(E1)) = σpr_gnd,L5(E5a)) in equation (5), the noise and multipath error on Ifree (σIfree) increases by a factor of 2.59.
Figure 7 shows the ratio σIfree/σL1 using measured data. We observe that the measured ratio (the black curve) is below the theoretical ratio computed based on the assumption of statistically independent samples (the constant value of 2.59). This is explained by the fact that the multipath errors in the measurements are not independent but have some degree of statistical correlation. The standard deviations are computed based on the same data set used in the raw multipath and noise assessment using 100-second smoothed measurements sorted into elevation bins of 10° spacing.
Figure 7. Measured ratio σIfree/σL1 function of elevation.
Conclusion
We have shown how GBAS can benefit from the new signals provided by the latest generation of GPS and Galileo satellites. We have demonstrated improved performance in terms of lower noise and multipath in data collected in our GBAS test bed. When GBAS is extended to a multi-frequency and multi-constellation system, these improvements can be leveraged for improved availability and better robustness of GBAS against ionospheric and other disturbances.
Acknowledgment
Large portions of this work were conducted in the framework of the DLR internal project, GRETA.
Manufacturers
The ground facility consists of four JAVAD GNSS Delta receivers, all connected to Leica AR 25choke ring antennas.
Mihaela-Simona Circiu is is a research associate at the German Aerospace Center (DLR). Her research focuses on multi-frequency multi-constellation Ground Based Augmentation System. She obtained a 2nd level Specialized Master in Navigation and Related Applications from Politecnico di Torino.
MIchael Felux is is a research associate at the German Aerospace Center (DLR). He is coordinating research in the field of ground-based augmentation systems and pursuing a Ph.D. in Aerospace Engineering at the Technische Universität München.
Sam Pullen is a senior research engineer at Stanford University, where he is the director of the Local Area Augmentation System (LAAS) research effort. He has supported the FAA and others in developing GNSS system concepts, requirements, integrity algorithms, and performance models since obtaining his Ph.D. from Stanford in Aeronautics and Astronautics.
By Mirko Stanisak, Mark Bitter, and Thomas Feuerle
INNOVATION INSIGHTS by Richard Langley
GPS = SAFER FLIGHT. While reviewing material for an article celebrating the 25th anniversary of the launch in February 1989 of the first Block II or operational GPS satellite, I was yet again annoyed by many articles on the Web stating that GPS only became available for civil use after the launch of this satellite. Some sources get closer to the truth when they say that GPS was opened for civil use in 1983, following the shoot-down of the Korean Airlines Flight 007. In fact, GPS was designed to serve the needs of both the military and civil communities from the outset. A government memo from April 1973 clearly states: “Civil user needs should be considered in the design of the spaceborne equipment.”
One of the first demonstrations of the use of GPS for aircraft navigation occurred in July 1983, when a Sabreliner business jet was flown in stages from Cedar Rapids, Iowa, to the Paris Air Show, flying only when a sufficient number of the experimental or Block I satellites were in view. The first standalone GPS receivers certified for aviation use (with Receiver Autonomous Integrity Monitoring or RAIM) became available by the mid-1990s. But already the Federal Aviation Administration had been looking into the development of a system to provide higher accuracies and better integrity than that afforded by standalone receivers. In 1994, the FAA announced the development of the Wide Area Augmentation System, its brand of a system generically known as satellite-based augmentation. Geostationary satellites transmit corrections and integrity information to GPS receivers, permitting GPS use for en route navigation all the way down to traditional Category I approach and landing. CAT I approaches can be flown down to a decision height of 61 meters (200 feet). WAAS was declared operational on July 10, 2003, but enhancements to the system continue. Japan, Europe, and India also have operational SBAS based on GPS.
Ground-based GPS augmentation was first developed for maritime applications with the U.S. Coast Guard’s low-frequency system coming on line in the mid-1990s. Also in the mid-1990s, the FAA began the development of the Local Area Augmentation System, generically known as a ground-based augmentation system (GBAS), to provide aircraft with approach and landing capabilities from CAT I down through CAT II (30-meter or 100-foot decision height) and CAT III (no decision height but certain visual range minima) using a VHF datalink. Initial CAT I systems are being operated at Bremen, Germany, and at Newark Liberty International Airport and Houston George Bush Intercontinental Airport.
While a GPS-based GBAS will definitely offer improved navigation services for aircraft, might these services be even better if the systems were to use satellites from other constellations besides GPS? In this month’s column, we look at a straw-man concept for modifying the GBAS protocols to accommodate multiple constellations and the results of preliminary tests using GPS, GLONASS, and Galileo simultaneously.
“Innovation” is a regular feature that discusses advances in GPS technology and its applications as well as the fundamentals of GPS positioning. The column is coordinated by Richard Langley of the Department of Geodesy and Geomatics Engineering, University of New Brunswick. He welcomes comments and topic ideas. Write to him at lang @ unb.ca.
Ever since the declaration of Full Operational Capability (FOC) of the U.S. Global Positioning System in April 1995, GPS has dominated satellite navigation, especially in aviation applications. By contrast, the Russian GLONASS system cannot be used in western aviation because no approval guidelines exist for GLONASS equipment. Thus GPS has been the de-facto standard in aviation for years.
However, within the last few years, major changes have evolved in the field of GNSS, providing a wide variety of useable satellite navigation systems. The European Union launched its Galileo project, which will provide global multi-frequency services in the near future. China is upgrading its BeiDou system (formerly called Compass) to provide global coverage with more medium-Earth-orbit (MEO) satellites. The operators of GPS and GLONASS have started modernization programs that will enable multi-frequency operations in the future, too. Therefore, a large number of usable satellites and signals from multiple systems will soon be available.
In aviation, almost all phases of flight can be assisted by satellite navigation systems nowadays. The most challenging phase of flight with respect to accuracy, continuity, availability, and integrity is the approach and landing phase. The Ground Based Augmentation System (see FIGURE 1; courtesy of the European Organization for Civil Aviation Equipment) allows precision approaches to be performed using satellite navigation. It uses a VHF data link to broadcast differential GNSS corrections, integrity information, and approach definitions to approaching aircraft. These aircraft combine the differential corrections with their own GNSS measurements, calculate a GBAS-corrected position solution, and determine path deviations based on the selected approach.
FIGURE 1. GBAS principle. (Source: EUROCAE WG 28, ED-114)
From a technical perspective, GBAS can use either GPS or GLONASS for differential corrections. For this, the International Civil Aviation Organization (ICAO) Standards and Recommended Practices (SARPs) include GPS and GLONASS side by side. On the other hand, some standardization documents (for example, those from RTCA) are limited to GPS only, effectively excluding GLONASS from being used in the western world. Nevertheless, Russian GBAS systems provide differential corrections for GPS and GLONASS, and are expected to be certified in Russia in the near future. Additional GNSS such as Galileo or BeiDou are not yet included within these documents, as these systems are not approved for aviation use themselves. This article will focus on how a multi-constellation GBAS with GPS, GLONASS, and Galileo could work.
GBAS installations can provide multiple services for different kinds of operation, based on GNSS L1 corrections only. On the one hand, the differentially corrected positioning service (DCPS) is intended to be a generic service for high accuracy positioning. On the other hand, two different GBAS approach services have been defined. GBAS Approach Service Type C (GAST-C) allows Category I (CAT I) procedures and is already in operation. GAST-D is still under development and will enable precision approaches and landings down to CAT II/III minima once certified. To mitigate all possible hazards, GAST-D will require some additional broadcast messages.
VHF Data Broadcast
The VHF Data Broadcast (VDB) is used to communicate binary GBAS messages to approaching aircraft. It operates in the VHF band (108.025 – 117.975 MHz) and uses time-division multiple access (TDMA) to allow the operation of multiple GBAS ground stations on a single frequency. As shown in FIGURE 2, VDB uses UTC time to have a common time frame. Two frames are transmitted each second, lasting 0.5 seconds each. Within each frame, eight slots with durations of 62.5 milliseconds can be used for transmission. Binary application data is encoded using a differentially encoded eight-phase-shift-keying modulation (D8PSK) and a symbol rate of 10,500 symbols per second. With three bits transmitted per symbol, up to 31,500 bits per second can be transmitted. Each slot can contain up to 222 bytes of binary application data. Usually, only a subset of slots is allocated to a particular ground facility. This way, multiple GBAS ground facilities can share a common VDB frequency.
Within each slot, multiple VDB messages can be transmitted as application data. The coding of information in VDB messages is defined in the RTCA’s GNSS-Based Precision Approach Local Area Augmentation System (LAAS) Signal-in-Space Interface Control Document (ICD) and depends on the VDB message type. (LAAS is the U.S. GBAS.) Currently, message types (MT) 1, 2, 3, 4 and 11 are defined. Figure 2 is derived from this document.
Message Type 1 – MT1. Within VDB Message Type 1, differential corrections based on 100-second smoothing are transmitted. These corrections are required by all GBAS approach services (GAST-C and GAST-D). Aside from the differential corrections, additional information for the first broadcast satellite is transmitted. This includes an ephemeris cyclic redundancy check (CRC), mitigating the effects of wrongly received GNSS navigation data, and the Issue of Data (IOD) flag, indicating the time of applicability for the ephemeris data to be used. To transmit this information for all satellites, the satellite for which differential corrections are transmitted first has to be alternated continuously.
Each MT1 message can contain up to 18 pseudorange- and range-rate corrections for individual satellites. Nevertheless, it is possible to link two consecutive MT1 messages using the Additional Message Flag (AMF). The value of this parameter indicates whether this is a single message (0), or the first (1) or second (3) part of a linked MT1 message. Up to 36 differential corrections can be transmitted using two consecutive VDB time slots with 18 corrections each.
All MT1 measurement blocks must be transmitted at least once per frame. The maximum transmission rate is once per slot for all measurement blocks.
Message Type 2 – MT2. VDB Message Type 2 contains station and integrity parameters such as the coordinates of the reference point to which all differential corrections refer. MT2 messages can include (next to a “core” MT2 message) multiple Additional Data Blocks (ADBs) to transmit information required for different GBAS services. At the moment, the Additional Data Blocks 1, 3, and 4 are defined.
ADB1 contains the maximum distance to the reference point at which the corrections may be used (Dmax) as well as parameters to calculate the remaining risk of incorrect GNSS ephemeris data (Kmd,e). Within ADB3, additional information required for GAST-D is transmitted. ADB4 implements the VDB authentication feature. If this ADB is broadcast by a ground facility, MT2 messages must be transmitted first and contain additional indications about which VDB slots are allocated to the ground facility.
MT2 messages must be transmitted at least each 20th frame, but may be repeated up to once per frame.
Message Type 3 – MT3. The VDB Message Type 3 is a fill message, which is only used in conjunction with the GBAS authentication feature (MT2, ADB4). Among other things, this feature requires a minimum slot occupancy of at least 95 percent. Thus, MT3 messages are broadcast only by ground facilities that support the authentication feature and are completely ignored by airborne GBAS receivers.
Message Type 4 – MT4. With VDB Message Type 4, approach information can be broadcast to approaching aircraft. A pilot can select a specific approach by simply tuning to a given channel number.
Currently, GBAS only uses Instrument Landing System look-alike straight-in approaches called Final Approach Segments (FAS). Each FAS represents one approach. This way, a single GBAS ground facility can provide multiple approaches for all runways of an airport. All approaches must be broadcast at least once per 20 consecutive frames.
Message Type 11 – MT11. The VDB Message Type 11 provides differential corrections in a way very similar to MT1 messages. The main difference is that MT11 corrections are based on 30-second smoothing, which is required for GAST-D service. As for MT1, all MT11 measurement blocks must be transmitted at least once per frame.
Enhancements for GBAS with Galileo
At the moment, the GBAS standardization documents include information on GPS, GLONASS, and SBAS ranging sources. No information on Galileo or other constellations has been added yet. Thus, to include Galileo for GBAS, some Galileo-specific experimental additions to the standards are necessary. These proposed modifications have been made in such a way as to keep as close to the other system standards as possible to preserve consistency. This way, hardly any new functionality is added, but additional satellites can be used. The additional Galileo signals (E5a, E5b, E6) are not used at the moment; however, they might be highly beneficial for multi-frequency applications in the future.
All modifications presented here are purely experimental and will most probably not be exactly the same as those in future standards documents. Nevertheless, they provide a way to test Galileo together with GPS and GLONASS for GBAS on an experimental basis.
Ranging Source ID. The Ranging Source ID uniquely addresses a single satellite. It is used in MT1 and MT11 to transmit the differential corrections and other information for each ranging source. In ICAO Annex 10, Standards and Recommended Practices, the Ranging Source ID is defined for GPS, GLONASS, and SBAS only. To provide Galileo corrections as well, an experimental mapping for Galileo satellites was added; see TABLE 1.
TABLE 1. GBAS Ranging Source IDs.
In this way, up to 36 Galileo satellites can be addressed.
Navigation Data. Galileo provides two different sets of navigation data. The I/NAV data corresponds to the Safety-of-Life (SoL) service and is broadcast on E1 and E5b. The F/NAV data corresponds to the Open Service (OS) and is broadcast on E5a. In order to remain as close as possible to the legacy navigation systems, we selected the I/NAV navigation data for use, as it is broadcast on the E1 frequency and can thus be received with an L1-only GNSS receiver.
The navigation data is primarily used in VDB MT1. For the first transmitted correction in this message, the ephemeris set that shall be used in the aircraft is identified via the Issue of Data (IOD) field. To be consistent with the GPS ephemeris, we used Galileo’s IODnav parameter.
Together with the identification of the navigation data, a CRC parameter is transmitted in MT1 for the first satellite within the differential corrections. This parameter ensures that the receiver as well as the ground facility use identical navigation data for all calculations. The CRC algorithm uses the raw navigation data to generate a distinct CRC value.
For GPS and GLONASS, two ephemeris masks are defined. These masks ensure that only information relevant for GBAS processing are covered by the CRC. For Galileo, a similar mask had to be designed.
Additional Data Blocks in MT2. Within VDB MT2, station parameters and integrity information are transmitted. Some parameters for the over-bounding of possible ephemeris errors are specific to each satellite navigation system.
To extend MT2 to Galileo, parameters for the DCPS, GAST-C, and GAST-D must be added for Galileo. For downward compatibility, these parameters cannot be included in the existing Additional Data Blocks beside the existing parameters. Thus, a new Additional Data Block (ADB5) was defined on an experimental basis. This Additional Data Block is dedicated to Galileo and is structured as shown in TABLE 2. The coding of all values corresponds to the coding of the parameters for the existing systems.
TABLE 2. Additional Data Block 5 in Message Type 2 for Galileo parameters.
Optimized VDB Transmission Scheme
Having available a large number of ranging sources for differential corrections, the VHF VDB is a bottleneck for the transmission of this data. To demonstrate this, we first consider the number of visible satellites that there will be in the future. This leads to construction rules for an optimal VDB transmission scheme, which allows transmitting the maximum number of differential corrections.
Number of Satellites Available. To demonstrate the number of differential corrections enabled by the different systems in the future, we computed the number of visible satellites over a day for a stationary GNSS receiver in Braunschweig, Germany. Even though only four Galileo satellites were in orbit at that time, up to 26 different satellites (GPS, GLONASS, and Galileo) were in view simultaneously. Keeping in mind the preliminary Galileo constellation, it is obvious that more than 30 satellites will be available simultaneously in the future — considering only GPS, GLONASS, and Galileo. Adding BeiDou satellites for GBAS would further boost these numbers.
The broadcast of such a large number of differential corrections is limited by the capacity of the VDB and thus by the number of slots assigned to a GBAS ground facility. The number of assigned slots for a facility should be limited as far as possible to be able to use the same frequency for other GBAS ground facilities. Thus, the available capacity must be used as effectively as possible.
Number of Bytes Required. Each VDB message is framed by a message block header (6 bytes) and the message block CRC (4 bytes).
The length of each message depends on the message type and the amount of information to be transmitted. The resulting length for a message of each type is given in TABLE 3.
TABLE 3. Size of different VDB message types (including message block header and CRC). Variable length message types are dependent on the number of corrections, N.
VDB Constraints. A GBAS ground facility must transmit the VDB data following some constraints. These are:
MT2 messages (including all Additional Data Blocks required) must be transmitted at least each 20th frame (that is, every 10 seconds).
If authentication is required, each MT2 message must be transmitted in the first slot assigned to the GBAS ground facility.
All differential corrections (both MT1 and MT11) must be transmitted at least once in each frame. However, it is possible to split the differential corrections into two adjacent slots using the Additional Message Flags in MT1 and MT11 messages.
Within each MT1 message, the ephemeris decorrelation parameter (Peph), the Issue of Data (IOD), and the ephemeris CRC is transmitted for the first satellite in the message. Thus, the first satellite must be alternated in order to broadcast the ephemeris information for all satellites.
Approach definitions are transmitted in MT4 messages. All MT4 messages must be transmitted within at least each 20th slot.
Based on these constraints, a VDB encoding scheme has been developed, which allows us to fulfill all the requirements listed above while optimizing the number of differential corrections that can be transmitted. Even though it is optimized for GAST-D-like services (including authentication parameters, MT11 messages, and experimental Galileo extensions), it can be used for legacy GAST-C systems, too.
Rules for Optimal VDB Transmission. To fulfill the requirement for the MT2 message to be transmitted first, a complete MT2 message must be transmitted each 20th frame at the beginning of the first slot assigned. If no MT2 message has to be transmitted, an MT4 message is transmitted instead. Thus, all messages are arranged in proper order by three simple rules:
MT2 (each 20th frame) or MT4 (otherwise)
MT11 (all corrections; can be split into two messages)
MT1 (all corrections; can be split into two messages).
Additionally, two more rules must be fulfilled. On the one hand, if supporting the authentication feature, each slot in which the ground facility may transmit VDB data must be filled to at least 95 percent. For this, MT3 null messages may be used to ensure that each slot is filled sufficiently. On the other hand, an additional rule for MT1 messages is necessary if more than three slots are assigned to the GBAS ground facility. In this case, to maximize the number of differential corrections the MT1 messages may be transmitted in the last two assigned slots only. This rule is necessary because the Additional Message Flag is limited to two slots for differential corrections.
Using this transmission scheme, the number of differential corrections is maximized while fulfilling the minimum requirements on the VDB data. Even in case of the maximum number of differential corrections, MT4 approach definitions can still be broadcast. However, in this case, the number of transmittable FAS segments is limited to 19. If more approaches (or different approach types such as Terminal Area Paths (TAPs)) have to be transmitted, the VDB generation scheme must be adapted.
Number of Transmittable Corrections. Using the optimized transmission scheme explained earlier, the number of transmittable corrections can be calculated easily for different numbers of assigned slots for GAST-C as well as for GAST-D services (see TABLE 4).
TABLE 4. Number of differential corrections that can be broadcast.
The exact distribution of VDB messages for the maximum number of differential corrections (18) is shown in FIGURE 3 for an MT1/MT11 configuration and two assigned slots.
FIGURE 3. VDB messages for two slots and 18 satellites (MT1 and MT11).
Experimental Realization of Multi-Constellation GBAS
The experimental GBAS multi-constellation extensions described earlier have been implemented in software for further testing. As these enhancements are purely experimental and might change in the future, we have ensured that these definitions can be changed easily.
Navigation Software. The Institute of Flight Guidance at Technische Universität Braunschweig has been developing an experimental navigation framework for many years. This software, called TriPos, can handle and combine different navigation technologies. TriPos can be used for simulations, post-processing of recorded data, and even for live (online) processing. It is written in C++ and supports various platforms.
The navigation framework can be extended easily. Originally, only GPS was supported within the software, but support for GLONASS and Galileo as well as augmentation systems like SBAS and GBAS were added over the past few years. Additionally, the software handles GNSS data of multiple frequencies internally and can thus be used for multi-constellation and multi-frequency applications. TriPos includes decoders for the binary protocols of most GNSS receivers currently available.
For GBAS research, two components can be simulated using the software. On the one hand, the Ground Facility simulation calculates the differential corrections and provides simulated VDB data. On the other hand, the GBAS receiver simulation emulates the behavior of an airborne GBAS receiver and uses VDB data and GNSS measurements to calculate a GBAS solution. Both simulations can use either recorded data in post-processing or live data for online-processing. This allows complete simulation of GBAS.
Multi-Constellation GBAS Ground Facility Simulation. The GBAS ground facility simulation uses raw binary data from multiple stationary GNSS receivers to calculate binary VDB data. The simulation can be freely configured to process either live or pre-recorded GNSS data. Even though it features all algorithms required by the standards, it does not contain additional monitor algorithms at the moment.
Nevertheless, it can provide a valid VDB signal-in-space (SIS), which can be used by GBAS receivers and simulation tools (such as Eurocontrol’s PEGASUS tool). The ground facility simulation supports legacy GBAS CAT-I (GAST-C) as well as GAST-D (including all additional VDB information required) using GPS and GLONASS. Support for Galileo has been added according to the experimental definitions described earlier. In addition to FAS data blocks, the ground facility simulation is also capable of providing curved approaches using TAP data blocks.
Multi-Constellation Airborne GBAS Receiver Simulation. The GBAS receiver simulation has been used for various GBAS-related projects. It supports GAST-C as well as GAST-D and can be configured flexibly to use GPS, GLONASS, and/or Galileo (using the experimental enhancements as described earlier). For GAST-D, all airborne monitoring algorithms required are present. Thus, the aircraft-specific parameters (for example for the airborne geometry screening) can be configured together with the other parameters.
Flight Trials
The practicability of the multi-constellation GBAS approach has been tested in flight trials. To ensure that all four Galileo satellites were in view and capable of providing valid data during our trials, an orbit prediction tool and the Notice Advisory to Galileo Users (NAGU) service of the European GNSS Service Center (GSC) were used prior to the flight.
The data processing configuration is shown in FIGURE 4 and includes the GBAS simulation components explained earlier. All processing is done in real time while recording all data for later post processing.
FIGURE 4. Schematic data processing for the flight experiments (ground components in orange, airborne components in blue).
Ground Processing. On the ground, two Septentrio AsteRx3 GNSS receivers connected to two roof-top antennas were used. The GNSS receivers were connected to the GBAS ground facility simulation via a network and provided binary GPS, GLONASS, and Galileo raw measurements with an update rate of 2 Hz as well as navigation data. Using this data, the ground facility simulation generated binary VDB data. The GBAS ground facility simulation was configured to generate multi-constellation GAST-D VDB data for a three-slot configuration. All required messages (MT1, MT2 including all required ADBs, MT3, MT4 and MT11) were generated and sent to the telemetry facility via the network.
Telemetry. Official VHF data broadcasts operate in a frequency band between 108 and 118 MHz, which is reserved for authorized aviation applications. However, for our experimental system, an alternative data link was used. The Institute of Flight Guidance operates a full-duplex telemetry system to share data between ground and aircraft. Even though the operating frequencies are different, the telemetry system allows the generated binary VDB data to be transmitted to research aircraft. The airborne telemetry receiver outputs data as if it were a VDB receiver to allow us to switch between a real VDB receiver and the telemetry receiver easily.
Research Aircraft. The Institute of Flight Guidance operates the research aircraft of the Technische Universität Braunschweig. The Dornier Do 128-6 with the call sign D-IBUF (see FIGURE 5) is a twin-engine turboprop aircraft without a pressurized cabin and has been used multiple times for GBAS-related research over the years.
FIGURE 5. Research aircraft D-IBUF (Dornier Do 128-6).
The research aircraft allows us to flexibly integrate experimental equipment for specific flight trials. For the multi-constellation GBAS flights, a JAVAD Delta GNSS receiver (capable of multiple constellations and frequencies), a telemetry receiver, and an experimental cockpit display were installed temporarily.
Airborne Processing. The online GBAS receiver simulator uses GNSS data from the JAVAD Delta GNSS receiver together with the VDB data received via telemetry. The receiver was configured to output raw GPS, GLONASS, and Galileo measurements with an update rate of 10 Hz. The simulator was configured to use this data to calculate a multi-constellation GAST-D solution. Based on the selected approach definition, the resulting information (deviations, distance to threshold, and so on) was displayed in the cockpit using an experimental cockpit display.
Results. The flight test was conducted in the evening of November 6, 2013 (16:52 – 17:58 UTC), at Research Airport Braunschweig (EDVE). We performed five approaches with a 10 nautical mile final segment. The flight path as calculated by the GBAS receiver subsystem is shown in FIGURE 6.
FIGURE 7 shows the number of satellites used for the GBAS receiver simulation, and distinguishes between the different satellite navigation systems used. Up to 22 satellites have been used simultaneously for GBAS processing, including up to 10 GPS satellites, eight GLONASS satellites, and four Galileo satellites.
FIGURE 7. Number of satellites used by the multi-constellation GBAS receiver simulation.
Even though no certified GBAS equipment was used for the flight trials, FIGURE 8 shows the resulting vertical and lateral protection levels (VPL and LPL) of the online multi-constellation GBAS receiver simulation. Both values fluctuate due to the differences between 100- and 30-second smoothing position solutions, which have to be added to the protection levels for GAST-D. Nevertheless, both sets of values remain clearly below the corresponding Alert Limits (FAS Lateral Alarm Limit (FASLAL): 40 meters, FAS Vertical Alarm Limit (FASVAL): 10 meters). A valid GAST-D service was achieved continuously.
FIGURE 8. Vertical and lateral protection levels (VPL and LPL).
FIGURE 9 shows a vertical integrity diagram, commonly known as a Stanford plot, for the integrity of the multi-constellation GBAS simulation. This plot shows the Vertical Protection Level (VPL) as determined by the GBAS receiver simulation against the actual Vertical Position Error (VPE). The Vertical Position Error is a direct measure for the Vertical Navigation System Error (V-NSE). This has been determined using a precise point positioning reference trajectory. Both values are normalized by the current VAL as these values change during the approaches. During the flight, the GBAS online processing ran at a rate of 10 Hz, resulting in 43,670 GAST-D epochs and an availability of 100 percent.
FIGURE 9. Normalized vertical Stanford plot of flight trials (GAST-D using GPS, GLONASS, and Galileo). Color scale indicates number of occurrences.
Of course, these results must not be misinterpreted as a multi-constellation GBAS performance assessment. The ground facility simulation was highly experimental and lacked any kind of long-term analysis. Even the GNSS antennas used do not meet formal requirements. However, aside from a quantitative judgment, these results show the practicability of this multi-constellation GBAS approach on an experimental basis.
Conclusion and Outlook
In this article, experimental extensions to GBAS have been developed to support GPS, GLONASS, and Galileo simultaneously. Based on these extensions, an optimized VDB transmission scheme has been created. In this way, the number of transmittable differential corrections could be maximized. Using flight trials, the multi-constellation GBAS concept has successfully been verified. The experimental airborne GBAS subsystem was able to calculate a valid GBAS solution including GPS, GLONASS, and Galileo satellites continuously.
It has been shown that multi-constellation GBAS is possible from a purely technical perspective. On the other hand, neither operational nor approval aspects for satellite navigation systems other than GPS have been addressed yet. Additionally, further testing would be necessary to ensure the compatibility with legacy GPS-only GBAS equipment. However, in theory, all modifications for Galileo are backward compatible. Nevertheless, it has to be assured that certified GBAS multi-mode receivers only use the GPS part of the VDB data and are not disturbed by additional VDB messages or additional ranging sources, for example. The required tests are planned for the future.
The operational benefit of multi-constellation GBAS systems cannot be foreseen yet. A certification for this will take several years and could only be addressed by the GBAS community after the completion of the GAST-D certification. Most probably, the use of GNSS signals on multiple frequencies could provide a highly improved GBAS service and will allow much more operational benefit. Many of the satellite navigation systems have already introduced additional frequencies, including signals in the protected L5 aviation band. The use of multiple frequencies for satellite navigation in aviation can remove most ionospheric errors effectively and mitigate a major source of uncertainty. Thus, multi-constellation GBAS can just be seen as a preliminary step on the way towards multi-frequency GBAS. The concepts and infrastructure described in this article will serve as a basis for more research in this area.
Acknowledgments
Most of our work on multi-constellation GBAS was done within the research project “Bürgernahes Flugzeug,” which was established in 2009 and is partly funded by the German federal state of Lower Saxony. This is gratefully acknowledged by the authors. Additionally, the authors would like to thank all colleagues involved for constructive discussions and their support. This article is based on the paper “Mulitple Satellite Navigation for the Ground Based Augmentation System” presented at ITM 2014, The Institute of Navigation 2014 International Technical Meeting, held in San Diego, California, January 27-29, 2014.
MIRKO STANISAK is a research assistant at the Institute of Flight Guidance (IFF) at the Technische Universität (TU) Braunschweig in Germany. He received his diploma in mechanical engineering (Dipl.-Ing.) in 2009 from TU Braunschweig.
MARK BITTER holds a Dipl.-Ing. in mechanical engineering from TU Braunschweig and has been employed as a research engineer at TU Braunschweig IFF since 2003.
THOMAS FEUERLE received his Dipl.-Ing. in mechanical engineering in 1997 from TU Braunschweig. He joined the TU Braunschweig IFF in May 1997. Since 2005, he has been the leader of the Air Traffic Management Team at the IFF. In April 2010, he completed his Ph.D. dissertation at TU Braunschweig.
FURTHER READING
• Authors’ Conference Paper
“Multiple Satellite Navigation Systems for the Ground Based Augmentation System,” by M. Stanisak, M. Bitter, and T. Feuerle in Proceedings of ITM 2014, the 2014 International Technical Meeting of The Institute of Navigation, San Diego, California, January 27–29, 2014, pp. 254–264.
• Standards Documents
Aeronautical Communications, Vol. 1, Radio Navigation Aids, Annex 10 to the Convention on International Civil Aviation, International Standards and Recommended Practices, International Civil Aviation Organization, Montreal, Draft Version, May 2010.
GNSS-Based Precision Approach Local Area Augmentation System (LAAS) Signal-In Space Interface Control Document (ICD), DO-246D, RTCA Special Committee 159, Global Positioning Systems, RTCA Inc. Washington, D.C., December 2008.
Minimum Operational Performance Standards for GPS Local Area Augmentation System Airborne Equipment, DO-253C, RTCA Special Committee 159, Global Positioning Systems, RTCA Inc. Washington, D.C., December 2008.
Minimum Operational Performance Specification for Global Navigation Satellite Ground Based Augmentation System Ground Equipment to Support Category I Operations, ED-114, EUROCAE Working Group 28 on Global Navigation Satellite System, European Organisation for Civil Aviation Equipment, Malakoff, France, September 2003.
• GBAS Research and Development
“Conception, Implementation and Validation of a GAST-D Capable Airborne Receiver Simulation” by M. Stanisak, R. Schork, M. Kujawska, T. Feuerle, and P. Hecker in Proceedings of ION GNSS 2012, the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation, Nashville, Tennessee, September 17–21, 2012, pp. 250–257.
“Initial GBAS Experiences in Europe” by A. Lipp, A. Quiles, M. Reche, W. Dunkel, and S. Grand-Perret in Proceedings of ION GNSS 2005, the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation, Long Beach, California, September 13–16, 2005, pp. 2911–2922.
