GPS and Beyond: The SatNav Transition

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Delays in implementing various aspects of the GPS system have left many in a quandary about why and perhaps when to move to the next generation of WAAS-capable avionics, and where LAAS fits in the big picture. This article offers some guidance for both VFR and IFR operators who may be considering the purchase of new SatNav avionics.

This is an update of an article originally published on AVweb in 1997, and includes information about new technologies and the status of WAAS and LAAS, as well as suggestions about avionics purchases.

Avionics

Most pilots think GPS is the best thing to happen to air navigation since the compass. And why not? Having accurate position anywhere in the world, being able to fly direct, being able to fly non-precision approaches that lead right to the threshold, and having more information about the flight than ever before, all make VOR and ADF navigation seem awkward, inaccurate and unreliable.

But GPS is just the starting point. GPS gives us a tremendous opportunity to start making a transition to SatNav for all phases of flight, to move to a more efficient area navigation (RNAV)-based airspace design and to reduce our dependence on traditional ground aids. It's an opportunity we can't afford to miss, but it means fielding wide and local area augmentation systems (WAAS and LAAS) in the near term. In the longer term, it means taking advantage of the next generation of GPS, the planned European Galileo system and the Russian GLONASS; these will all have more features than today's GPS.

A complete transition is not assured. The main barrier is the vulnerability of SatNav signals to intentional interference. This may require the retention of some traditional ground aids. Regardless, the operational benefits are clear, and achieving them requires focusing on the vision of a SatNav system providing seamless global guidance for all phases of flight.

This article aims to provide information on GPS, WAAS and LAAS issues and the plans for a SatNav transition.

Safety Standards

All electronic navigation aids have to meet three basic performance standards to ensure safety:

  • Accuracy -- as required for a given phase of flight;
  • Integrity -- the feature that protects against using hazardously misleading information by issuing timely warnings; and
  • Continuity - a very low probability of losing service during a given operation, such as during an approach.

These standards have to be met a high percentage of the time to support viable service, so availability, defined as the percentage of time that the system can support the desired phase of flight, is a fourth important element. Unless availability is very high, alternative systems or procedures must be used as a back-up.

Accuracy is clearly important, and fortunately accuracy is quite easy with SatNav. The real key is in fact integrity, because it is critical to safety. The greatest technical challenge is to deliver a high availability of integrity.

Integrity is straightforward with traditional aids. As an example, every ILS has monitors that evaluate the accuracy of the localizer and glideslope signals. If the localizer exceeds accuracy limits, the monitor shuts down the ILS within six seconds for a normal Category I approach (one second for Cat III). If only the glideslope exceeds accuracy limits, the glideslope shuts down and the localizer stays on the air.

Integrity is more complex with SatNav. GPS was not designed with civil aviation in mind, and there is no way to use accuracy monitors, as with the ILS. Instead, we rely on the avionics to use all the information available about satellites and from augmentation systems to compute a level of confidence in the service being provided. This level of confidence, computed continuously, is expressed in terms of protection levels - horizontal, and, in the case of WAAS and LAAS service, vertical too. These protection levels are compared with alert limits, which vary by phase of flight and are specified in international standards. The avionics will alert the pilot when a protection level exceeds an alert limit, hence the name. It is important to understand that the computed protection level has nothing to do with accuracy. In fact there could be an integrity alert when the aircraft is within a few meters of the desired track, because the statistical level of confidence in the received data is not high enough.

The integrity solution for today's GPS avionics is Receiver Autonomous Integrity Monitoring (RAIM). Horizontal alert limits are 2 NM for en route, 1 NM for terminal and 0.3 NM for non-precision approach; there is no vertical alert limit because raw GPS is not used for vertical approach guidance. The RAIM function depends on having extra satellites in view to calculate a protection level. The protection level is low when there are many satellites visible and they are well spread out across the sky - i.e., "satellite geometry" is good. When there are fewer satellites, or geometry is poor, the protection level is higher and may exceed the alert limit. When this occurs, the pilot receives an alert but the navigation function is not flagged. For obvious reasons, the availability of approach RAIM is lower than the availability of en route RAIM. The 0.3 NM (556 meters) non-precision approach alert limit is the lowest value that can be supported by RAIM with reasonable availability.

There is a second type of RAIM alert that occurs when there is a problem with a satellite. In this case the pilot will be alerted and the navigation function will be flagged.

Some TSO C129a units go beyond RAIM to provide Fault Detection and Exclusion (FDE). With FDE, the avionics can identify a faulty satellite and exclude it from the navigation solution, allowing for continued GPS navigation. To do this, there typically has to be at least one more satellite in view compared with RAIM.

The main reason we need to retain ground aids at this stage is that the availability of RAIM is not high enough. The reason we need to go to WAAS and LAAS is to get a high availability with a much lower horizontal alert limit, and to get a similarly high availability with a vertical alert limit that will support approaches with vertical guidance.

The WAAS signal used by TSO C145a/C146a WAAS avionics contains integrity information, making it unnecessary for the receiver to use RAIM. WAAS avionics use this information, along with satellite information, to calculate horizontal and vertical protection levels, and alert the pilot when a protection level exceeds an alert limit. Unlike RAIM, there is no alert when there is a problem with a satellite the WAAS signal provides all the information needed for the avionics to ignore a faulty satellite and carry on navigating. The current WAAS design will support a high availability (greater than 99%) with a horizontal alert limit of 40 meters and a vertical alert limit of 50 meters. The former allows WAAS to use localizer lateral approach design criteria, and the latter allows WAAS to support vertical guidance. LAAS integrity works along the same lines as WAAS.

Navigation Satellites GPS, GLONASS and Galileo

GPS

Today's GPS is the basis for many timing, positioning and navigation applications; aviation users are actually in the minority. GPS was designed for military purposes, so aviation users have to rely on avionics and augmentation systems to meet civil aviation safety standards. The nominal GPS constellation contains 24 satellites, but for quite a while there have been 26 to 28 satellites in operation because many of them have exceeded their design life. This is a tribute to the people who designed and built the system, and has meant a high availability of RAIM. The downside is that the high reliability of today's satellites has slowed the development of the next-generation GPS III, with advanced features that will provide even better performance. It will likely be 2015 before there is a fully operational GPS III, based on current plans. In the meantime, the plan is to add a second coded signal to GPS satellites launched in the near future. Unfortunately, this signal is in a band that is not protected for aviation, so it will not enhance aviation operations.

Even with today's GPS, users saw an improvement in accuracy on May 1, 2000. Prior to that date, the GPS Standard Positioning Service (SPS) used by the civil community delivered 100 meter horizontal accuracy, 95% of the time (156 meter vertical) because the DOD, using Selective Availability (SA), introduced errors. After SA disappeared, the observed GPS SPS 95% accuracy has been about 7 meters in the horizontal and 12 meters in the vertical. The DOD's SPS performance standard uses less optimistic numbers (13 to 36 meters horizontal and 22 to 77 meters vertical, for "global average" or "worst site" conditions).

The next-generation GPS III satellites will provide service on two civil frequencies in bands protected for aviation. This provides an important advantage. The largest source of error in GPS is the variable delay as the signals come through the ionosphere. The delay is, however, proportional to a signal's frequency. It is therefore possible for a dual frequency receiver to calculate and correct for the delay, rather than using approximate models as in GPS or ionospheric correction and integrity data provided by WAAS. Another advantage of having two frequencies is that it makes it virtually impossible for unintentional interference to disrupt service entirely.

GLONASS

The Russian GLObal NAvigation Satellite System (GLONASS) could provide a service similar to GPS if there was a full set of satellites in operation. The problem has been funding. Russia is embarking on a GLONASS modernization program, and it is expected that the new satellites will also provide service on two frequencies. It remains to be seen whether GLONASS will be widely used for civil aviation in the future.

Galileo

Europe announced plans several years ago to develop Galileo, a system designed to serve many types of users and to be compatible and interoperable with GPS. The impetus for Galileo was concern about relying on another nation's system, industrial benefits for European industry and the requirement to tailor system performance to meet all users' requirements. The planned target date for Galileo operations is 2008; this is quite ambitious. The program has been delayed recently for an unusual reason too much funding! The European Space Agency (ESA) allocates contracts based on the fraction of funding provided by each country. In a rush to obtain contracts, European countries volunteered to contribute 133% of the total required. Sorting out shares that add up to 100% has caused a considerable delay.

On the technical side, the plan is for Galileo to provide service in the same two aviation frequency bands as GPS III. If aviation is to gain advantages from Galileo, ICAO and other bodies will have to develop standards for the signals and avionics. If this is done, Galileo holds considerable promise for aviation users.

Using Combinations of Signals

More satellite signals means better service, so using signals from a combination of satellite constellations should mean benefits for users and less reliance on augmentation systems to meet performance standards. ICAO is starting to address the subject of combinations of systems. One goal is to identify the most likely combinations with the most potential benefits.

Wide Area Augmentation System (WAAS)

WAAS is the American version of a system that the international community calls a Satellite-Based Augmentation System (SBAS), because the system provides augmentation signals from satellites. Work is underway in the USA, Europe, Japan and India on SBASs that will broadcast ICAO-standard signals, thus supporting seamless global coverage. The FAA faced a number of technical and program challenges during the development of WAAS, but the system was finally commissioned July 10, 2003. Japan is working on the "MT-SAT Based Satellite Augmentation System" (MSAS). Europe aims to commission the "European Geostationary Navigation Overlay System" (EGNOS) in 2004. India has started work on the GPS and GEO Augmented Navigation (GAGAN) system. ICAO standards ensure global signal compatibility, and representatives from each project office are working together to find the best ways to ensure system interoperability and to avoid duplication of effort.

All four SBASs are being designed to support en route, terminal, non-precision approach and a new type of "approach with vertical guidance" - APV in international terms, LPV in U.S. terms. The FAA uses the term LPV because the lateral guidance is as accurate as a precision approach localizer, and because it provides vertical guidance (Lateral Precision, Vertical guidance). These new approaches will be flown like an ILS to a decision altitude as low as 250 feet. LPV approaches will mean lower minima, hence higher airport usability at many sites. This is a very significant level of service, in that we can now move toward the elimination of the "dive and drive" non-precision approach and gain the safety benefits of vertical guidance, without the requirement for a ground system at the airport.

How WAAS Works

A network of WAAS reference stations monitors GPS signals and relays data to a master station, which assesses signal validity, computes corrections and creates the WAAS message. It sends this to a ground uplink station for relay to geostationary satellites orbiting over the equator. These satellites rebroadcast the message on the GPS L1 frequency; their signals cover a hemisphere, except for polar regions.

Wide-Area Augmentation System

The WAAS signal contains correction and integrity data as well as ranging signals. The correction data allows the receiver to compensate for satellite clock and orbital position errors, as well as for the signal delays caused by the ionosphere. The resulting horizontal and vertical accuracy is usually better than 2 meters. The integrity data is used as described above, and solves the availability problem with RAIM. The ranging signals make the geostationary satellites look like GPS satellites and this also helps boost availability of service.

The technical challenge with WAAS was achieving a high availability of integrity. In designing anything in aviation, the strategy used to meet safety standards determines performance. For example, designers have to guard against structural failure by using sufficiently strong components. This determines the weight of the aircraft and its maximum payload and range. With SatNav, integrity equals safety, and the formulas used in the integrity calculation determine the best level of service and the availability of that service. When the WAAS project started, it was expected that it would deliver Cat I precision approaches. Further analysis showed, however, that integrity could be assured only by taking a conservative approach, particularly with respect to characterizing ionospheric delays. The result is that at this point, WAAS cannot deliver a high enough availability of integrity at the Cat I level, despite the fact that WAAS is more accurate than ILS. It is possible, however, that experience with WAAS will provide the information needed to support a less conservative approach.

Local Area Augmentation System (LAAS)

LAAS is the American name for a system that the international community calls a Ground-Based Augmentation System (GBAS) because the system provides augmentation signals from ground stations. The ultimate goal with LAAS is to support all categories of precision approach, and eventually surface movement guidance. With LAAS, a ground reference station at the airport broadcasts satellite ranging corrections via VHF to aircraft within 25-30 nm, so a single system serves all runways. The cost of a single LAAS should be less than the cost of an ILS, which serves only one runway.

Local Area Augmentation System

Trials in the U.S. and elsewhere going back over a decade showed that LAAS can deliver Category III (autoland) accuracy. Like WAAS, however, accuracy is easy and integrity is the challenge.

The FAA's LAAS project has moved ahead unevenly, and the current plan is to commission Cat I LAAS approaches in late 2006, quite a bit later than originally thought. The aim is then to move on to Cat II/III, but that will not be easy. ICAO has not developed performance requirements or standards for Cat II/III, and the international community is divided on the prospects for these very demanding levels of service. Some experts feel that we will have to wait for the next-generation dual frequency satellites. The decision may, in fact, hinge on the business case. Development costs will be high, a small percentage of airports have the weather and traffic to justify Cat II/III costs, and the cost of related lighting and other airport systems is high. Moreover, ILS and MLS already meet all precision approach requirements, so LAAS does not support a new service level.

Taking a Total System Approach

SatNav has the operational and technical components shown in the diagram below. For a smooth transition, they all have to be considered carefully.

System Approach to SatNav

Performance depends on a very complex integration of GPS (and in future Galileo and GLONASS), augmentation systems and avionics to meet required navigation performance standards.

Safety depends on the validity of the way point coordinates stored in avionics. Databases have been used for years, but SatNav approaches bring a new urgency to database integrity. ICAO has developed surveying standards that will be used to collect the raw data used in designing approach procedures. The approach design process will keep the data intact, so that there's no chance of introducing errors. New standards for data handling processes will apply to WAAS LPV approaches, so there will be data integrity from survey through to avionics. For all other operations, pilots should not take database integrity for granted.

Procedure design is also very important. A properly designed approach will result in maximum efficiency benefits while ensuring flyability and minimum cockpit workload. Traditional procedures depend on the location of ground aids. SatNav brings total flexibility, so procedures and routes can be designed to make the best use of airspace and reduce flying time to minimize fuel burn. Pilot and air traffic experts are developing new standards to take maximum advantage of SatNav performance.

Ultimately, all components will come together in an operational approval, including training requirements. The terms of this approval have to reflect an understanding of the total system, or there may be unnecessary restrictions or inadequate safeguards.

The key to success is cooperative efforts of experts in many fields. The FAA and NAV CANADA SatNav offices have from the start worked closely with regulatory experts, academic institutions, aircraft operators, aircraft and avionics manufacturers, other GPS users and international organizations to avoid duplication of effort and take advantage of a broad pool of talent.

Avionics -- The Pilot Interface & Safety

A SatNav transition requires manufacturers to produce capable, affordable products. Unfortunately, some early GPS receivers worked well enough technically but had "user hostile" pilot interfaces. It's great that the avionics always know the aircraft's position; it's even better if the pilot knows. If avionics design increases pilot workload or makes it difficult to maintain situational awareness, accidents will happen.

The good news is that manufacturers listened to the marketplace, and today's new GPS and WAAS avionics are much more capable than their predecessors. They generally have moving map displays and they are designed to reduce workload. WAAS avionics must also meet a basic pilot interface standard, unlike first-generation GPS receivers.

The key to operating safely is for pilots to be completely familiar with their GPS avionics. This can be a challenge because most units have a myriad of features and functions. Some commercial operators have simplified matters by developing standard operating procedures for basic functions that must be used on every flight. Once pilots are completely comfortable with these functions, there is nothing wrong with taking advantage of long en route legs to explore other features that may be useful but are not essential. As always, safety depends on remembering that the number-one job is flying the aircraft.

Avionics -- When and What to Buy?

Many aircraft operators are trying to decide when to buy SatNav avionics, whether to buy a GPS or WAAS-capable unit and which model to choose. At this point WAAS panel-mount avionics are appearing on the market at a cost that compares with current GPS avionics market leaders. More WAAS models should appear soon. The decision should therefore be quite simple as long as the cost is the same or any extra cost is more than balanced by benefits - buy a WAAS panel mount, not a plain GPS unit. In some cases manufacturers are planning upgrades to GPS units, so buyers should ask questions to verify costs, timing and performance. Owners who already have GPS units should check with their manufacturer about WAAS upgrades.

IFR Operations -- Costs

There are four costs to be considered: the cost of the avionics; the cost of installation (including aircraft downtime); the cost of initial and recurrent training; and the ongoing cost of database updates.

GPS avionics used for IFR must meet TSO C129a. WAAS avionics must meet TSO C145a (WAAS sensor for installation in an FMS) or TSO C146a (stand-alone WAAS navigator i.e. a panel-mount WAAS unit). Comparing models means weighing features and performance against cost. Recent GPS and WAAS models have features that make first-generation GPS avionics seem obsolete. Potential buyers should be aware that there are three performance levels for WAAS avionics, and not all support LPV approaches. Level 1 avionics will support en route through non-precision approach; Level 2 will support en route through LNAV/VNAV; and Level 3 will support en route through LPV. Some manufacturers are offering an upgrade path to Level 3, based essentially on new software; in this case, confirm timing and costs.

Installation can be tricky and expensive, depending on how easy it is to link the GPS or WAAS unit to CDIs, HSIs, the autopilot and flight director. So don't just look at the sticker price -- find out what's involved in the installation, and how much that will cost.

Look for a unit that's user-friendly, because that will translate into lower training costs. These costs even apply to the owner of a light GA aircraft, who will have to spend some air time and perhaps hire an instructor to learn how to use the box properly. Don't even think about flying an approach in instrument weather until you are completely comfortable -- there's much more to it than tuning an ILS and setting the course.

Database update costs always take owners off guard. But once we all need databases, there's no excuse for today's prices; it might be worthwhile to point this out to database suppliers.

IFR units come in three flavors -- panel mounts, FMS look-alikes and FMS sensors. A panel mount is the only solution for a small aircraft. For high-end aircraft with FMSs, moving to GPS or WAAS might mean a new sensor and software, or it could require moving to a newer FMS. Larger GA aircraft and older air carrier aircraft with conventional avionics can be fitted with either panel mounts or FMS look-alike units. Many airlines who have looked at GPS for their older aircraft have decided on FMS look-alikes because they are more suitable for an airline environment.

IFR Operations -- Benefits

The most obvious benefits are fuel and time savings that come from flying direct or from a greater airport accessibility due to lower limits.

Another benefit could be avoiding the need to replace an aging ADF or DME, both fairly costly items.

The final benefit is somewhat intangible for users in areas with radar coverage and lots of traditional aids, but very real for users in sparsely settled areas like Alaska or northern Canada. That is the comfort that comes from always knowing position, groundspeed and time to the next way point or to destination.

Even in a busy terminal area, having a GPS or WAAS receiver with a moving map display helps with situational awareness. This feature is also very useful in an emergency, when you might want to go direct to the nearest airport and have valid information about distance and time to go.

And keep in mind that TSO C129 avionics will still be useful after WAAS is operational. The plan for approach charts is to show minima for LPV, LNAV/VNAV and LNAV. The latter requires only a TSO C129a receiver. It might make sense, therefore, to install a WAAS unit and retain the TSO C129a unit for back-up.

VFR Operations

Map reading and good weather are the keys to VFR flying, but a handheld or panel mount GPS or WAAS receiver can certainly help. But if the receiver doesn't have RAIM or WAAS integrity, then there's no warning if a satellite goes bad, so don't depend solely on GPS. Another safety tip -- avoid overconfidence or complacency -- we have seen accidents where the pilot set out in poor weather, counting on GPS to avoid getting lost, but either lost control in cloud or ran into an unseen hill.

There are good handheld receivers on the market, complete with moving map and a database, and most new ones use the WAAS signal to improve accuracy (but there is no integrity function). Some owners have installed external antennas to feed these units, and that is a good idea. In some cases the databases can't be updated easily, so beware of old data. Another database trap relates to the display of airspace boundaries. There is no standard for the data that drives these displays, so the boundaries may not always be accurate. We have had reports from pilots who inadvertently strayed into airspace where they shouldn't have been by relying too much on these units. There are some good panel mount GPS/NAV/COM units on the market, complete with moving map displays, and this might be a good replacement for an aging NAV/COM.

Summary

So where are we? Back in 1993 when the U.S. and Canada first approved GPS for IFR operations, we thought that ground aids would soon go the way of the dinosaur. We expected WAAS and LAAS to arrive before the end of the 90s and we minimized the impact of interference. Since then, technical issues, some politics and reality have slowed progress and made us think twice about ground aids. But we have learned a lot and have made real progress. Thousands of pilots are taking advantage of GPS as an aid to VFR navigation, for IFR en route and to fly approaches that have improved airport accessibility around the world. Avionics have become more useful and pilot-friendly. WAAS, representing the second generation, is here and it promises major benefits. New satellite constellations on the horizon will make SatNav more robust. So we're still on track to realize the vision of a seamless, global SatNav solution for all phases of flight. There will be more technical and operational challenges, but there is no reason to think that we can't find solutions. The final result will be improved service at lower cost, and that, by any measure, is a good deal for the aviation community.