IFR Briefing: An Introduction to GNSS Approaches in Europe

Aim

Provide een introduction to concepts and procedures for conducting precision and non-precision RNAV approaches in Europe

Theoretical Background

The History of RNAV Navigation

Area Navigation (or RNAV for short) is a method of navigation that was developed in the 1960s in the USA in response to a rapidly developing aviation market.  It allowed aircraft to navigate any course within a network of beacons, conserving flight distance, reducing congestion, and allowing flights into airports without beacons.  It essentially replaced the method of navigating from beacon to beacon.

The navaids that RNAV traditionally relied on for land-based operations are VOR and DME, and a combination thereof (with the exception of NDBs).  By simultaneously querying one or more of the navaids it is possible to calculate a lateral position in space, thus enabling navigation between points that are situated anywhere where such navaids are available.  For ocean based operation an IRS or INS is typically used.  For aircraft that are equipt with FMS (Flight Management Systems), this is a automated process, whereby the FMS automatically chooses a combination of the most reliable navaids such as to be able to calculate a position in space.

Most general aviation aircraft, however, do not have the luxury of a FMS that has become standard equipment in most jets.  Although this is rapidly changing with more advanced avionics found in GA aircraft these days. However, for IFR flights within Europe, extensive usage is made of RNAV navigation by the use of inbuilt GPS, or GNSS systems for the enroute segment of an IFR flight.  The usage of GPS for the enroute segment is commonly referred to as B-RNAV, or Basic-RNAV, or RNAV-5 (these terms essentially all mean the same), and covers the necessary precision and reliability for that purpose.   Two systems commonly found in General Aviation (GA) aircraft are the G1000 avionics suite, and the G430 / G530 Com/Nav/GPS systems.  These systems rely on GPS to calculate a position in space.

G100 avionics system
Figure 1: G1000 avionics system
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Figure 2: G430 GPS

RNAV-5 navigation for the enroute segment is not new for GA operations and has been used for a number of years.  However, what is new is the usage of GPS to achieve the necessary precision in order  to safely fly STARs, SIDs and approaches into airports.  More concretely the usage of GPS as such is referred to as PBN, or Performance-based Navigation.  Through this method, external information is integrated into the position calculation process, such as to achieve a higher accuracy than would otherwise possible when relying on standalone GPS.  More specifically augmentation systems are used in combination with GPS such as to provide a greater accuracy of position information.

GPS and Augmentation Systems

Standalone GPS suffers from inaccuracies caused by ionosferic delays on the microwave signals that are received by GPS antennas.  Humidity levels and the composition of gasses also have een effect on the accuracy.  These delays are known as dispertion, and several methods are available to handle the inaccuracies, thereby increasing position accuracy.

The GPS system was originally developed for military applications.  Two signals are transmitted by a GPS satellite, a L1 signal and L2 signal.  For military applications, higher accuracy was possible by decrypting the encrypted L2 signal.  By comparing the L1 and L2 signals, which are transmitted on different frequencies, it is possible to calculate, and subsequently apply a correction for the error introduced by dispertion.  Some more expensive civil GPS receivers, relied upon a method of tracking the carrier wave on the L2 signal, as opposed to the modulation code to calculate the correction, thereby avoiding the need to decrypt the P(Y) which is carried on the L2 signal.  To facilitate this on lower cost receivers, a new civilian code signal on L2, called L2C, was added, allowing direct comparison of the L1 and L2 signals using the coded signal instead of the carrier wave.

The effects of the ionospheric delay generally change slowly, and can be averaged over time. Those for any particular geographical area can be easily calculated by comparing the GPS-measured position to a known surveyed location. This correction is also valid for other receivers in the same general location. Several systems send this information over radio or other links to allow L1-only receivers to make ionospheric corrections. The ionospheric data is transmitted via satellite in Satellite Based Augmentation Systems (SBAS) such as Wide Area Augmentation System (WAAS) (available in North America and Hawaii), EGNOS (Europe and Asia) or Multi-functional Satellite Augmentation System (MSAS) (Japan), which transmits it on the GPS frequency using a special pseudo-random noise sequence (PRN), so only one receiver and antenna are required to pick up the L1 signal.

The SBAS system relies upon several ground locations with survey points, that upload corrections to geostationary satellites.  These corrections are  subsequently downloaded by GPS receivers that are SBAS enabled.

As for the GA GPS receivers, the G1000 is able to use SBAS out of the box.  The G430 and G530 also have this capability when updated with the latest software.  In the latter case, a ‘W’ is added to the denomination, such that G430 become G430W.  G430W is SBAS enabled and certified.  As many receivers are built and sold in the USA, WAAS is often used as a denomination for SBAS capable GPS receivers.  WAAS is the name of the SBAS system in the USA, for Europe it is EGNOS, and this explains the ‘W’ that is added to the GPS name.  Both systems are fully compatible, such that a WASS receiver made in the USA, can also be used on the EGNOS system in Europe.

Performance Based Navigation

Not surprisingly, space-based navigation, or in other words, the usage of the GPS system to achieve the necessary precision and reliability as a replacement of ground-based navaids, is a primary focus of Eurocontrol and EASA, for the simple reason of it being more cost-efficient.  It is easy to see that maintaining thousands of ground-based navaids is much more expensive than maintaining a limited number of satellites in orbit.  EASA has currently opted for using RNP (Required Navigation Performance) approaches next to ILS approaches, with ILS approaches being the preferred and primary method, and RNP approaches being used as a backup when the ILS is unavailable.  However, for airports that do not have ILS approaches, or only have them on one runway, RNP approaches may become the primary and only available procedure, thus giving thousands of airports IFR approach and departure capabilities at a fraction of the cost of maintaining an ILS, or other non-precision approach system.  The system is in effect already there, and only the procedures need to be drawn up.

Performance-based navigation relies on the concept of achieving and maintaining, during the time that the performance-based navigation is required, a required navigation performance (or RNP).  The system we use for the performance-based navigation must therefore be capable of continuously calculating the actual navigation performance (or ANP), and comparing this with the RNP (required navigation performance) for the chosen approach.  In other words, our GNSS system must be able to continuously calculate that it is providing the necessary accuracy for the approach.  The required accuracy must be attained at a 95% confidence level.

In order to be able to calculate whether the GNSS receiver is operating within limits, it makes use of RAIM (Receiver Autonomous Integrity Monitoring).  RAIM relies on the usage of more than 4 satellites to calculate the integrity of the position.  When the receiver is able to receive and interpret the signal of 5 satellites, it is possible to recognise a fault in one of the satellites. It is however not always possible to detect which satellite is causing the fault.  The system is therefore said to work in a fault detection mode.  By including an additional satellite, such that 6 are simultaneously received, the receiver is also able to exclude the faulty satellite, and is said to operate in a fault detection and exclusion (FDE) mode.

The GPS satellite constellation provides 6 or more satellites in almost all circumstances, however, this is not guaranteed. On a particular route at a particular time, it may be that the geometry of the satellite constellation, or a failure of one or more of the satellites, prevents RAIM availability.  As such, although the position may be accurate and within limits, it is not possible to assure the integrity of the position. In Europe, AUGUR is the approved method of querying whether RAIM will be available at a specific time, and is available through http://augur.ecacnav.com.  All IFR certified GNSS receivers are also capable of calculating RAIM based on the Almanac data in the Navigation Message, and in Garmin receivers, is available in the auxiliary pages.   However, the GNSS receiver may not be able to query the most up to date information for predicting RAIM availability, for instance, because on of the satellites is or is expected to be out of service.  Therefore the AUGUR system should be used, in particular when the GNSS system is to be used for the terminal approach segment (P-RNAV / RNP-1).

If RAIM integrity is lost during any phase of flight, the GNSS receiver shall display a Loss of Integrity warning message.  This message is triggered for one or more of the following reasons:

  • A loss of RAIM availability
  • The detection of a fault in satellite signals which compromises position accuracy
  • An unfavourable satellite geometry and dilution of position, such that accuracy does not meet the protection limit

RAIM integrity is coupled to the required RNP level.  For instance, RNP is superceerded by a number. RNP-10 means that the system must be able to calculate our position within a circle with a radius of 10 NM (Nautical Miles) at a 95% confidence interval.  Similarly, RNP-0.3 must be able to calculate our position within a circle with a radius of 3 tenths a NM.  In Europe there are 2 specifications, Basic-RNAV equivalent to RNP-5, and Precision-RNAV, equivalent to RNP-1.  RNP-0.3 is used for the final approach segment in a RNP APCH mode.  RAIM integrity is therefore be stricter in the final approach segment versus the enroute segment.

The different Types of RNP Approaches

The ICAO Resolution at the 36th Assembly in 2007, and the publication of ICAO’s PBN Concept define the various types of PBN methods that are available for the approach segment:

  • RNP APCH LNAV (Lateral Navigation only and relies on GPS)
  • RNP APCH LNAV/VNAV (with Vertical Navigation added and relies on GPS and Barometric VNAV). This is also referred to as an APV Baro.
  • RNP APCH LP (Localizer Performance only and relies on GPS and EGNOS, the European satellite-based augmentation system (SBAS)).
  • RNP APCH LPV (with Vertical Navigation added and relies on GPS and EGNOS). This is also referred to as an APV SBAS.

In order to able to conduct RNP APCH, the operator is required to take additional training in addition to the equipment being certified to conduct LPV approaches to minima.

To conclude, figure 3 provides a visual representation of the different PBN methods and where they can be found on the approach.

RNPpng
Figure 3: Navigation methods – Eurocontrol (2013)

 

Practice

Common GNSS (RNAV) Approaches

As in traditional approaches, GNSS (RNAV) approaches can be precision or non-precision, or with or without vertical guidance.  In this section both will be discussed.  Before discussing the specifics of precision and non-precision approaches, we will first discuss the commonalities.

As discussed in the previous section, a pilot will be notified with a Loss of Integrity alert, should the position no longer be reliable within required limits, or/and should RAIM be unavailable.  When GNSS is used for approaches, more specifically in P-RNAV and RNP APCH modes, a Loss of Integrity alert requires specific action:

  • During a P-RNAV procedure, the pilot must advise ATC of the RAIM failure and request radar vectors or a conventional alternative approach.
  • During a RNP APCH, the pilot must initiate the missed approach procedure, and advise ATC

On the Garmin G430W/G530W receivers, loss off integrity is displayed as follows:

Loss of Integrity

Loss of Integrity
Figure 4: Loss of Integrity

The Loss of Integrity warning may only be applicable of a specific segment of the approach, for instance, between the FAF (Final Approach Fix) and the MAP (Missed Approach Point) where higher precision is required (RNP-0.3).  The receiver will generally provide a message giving additional details of the integrity loss.

Some receivers may also reduce the approach to a lower grade, such that other minima will be applicable.

It is important to note, that a exception to the RAIM requirement does not apply to an LPV approach. The reason is that with an LPV approach, integrity is managed directly by the SBAS geostationary satellites instead of the receiver.

Non-precision GNSS (RNAV Approaches)

Non-precision GNSS approaches are those that rely upon satellite navigation to provide the required accuracy for approaches without vertical guidance.   In effect there are 2 types of approaches defined by ICAO’s PBN Concept that conform to this.

  • RNP APCH LNAV (Lateral Navigation only and relies on GPS)
  • RNP APCH LP (Localizer Performance only and relies on GPS and EGNOS, the European satellite-based augmentation system (SBAS)).

Operating Minima

The MDH/A selected by the operator must not be less than the greater of the following values

  • MDH/A corresponding to the aircraft category* if published by the authority in charge of the aerodrome;
  • OCH/A (obstacle clearance height) corresponding to the aircraft category*;
  • 300ft.

Precision GNSS (RNAV Approaches)

  • RNP APCH LNAV/VNAV (with Vertical Navigation added and relies on GPS and Barometric VNAV). This is also referred to as an APV Baro.
  • RNP APCH LPV (with Vertical Navigation added and relies on GPS and EGNOS). This is also referred to as an APV SBAS.

RNP APCH LNAV/VNAV relies on using barometric input in order to determine a vertical and horizontal position in space. The barometric input in effect works as a substitute of the fourth satellite required for  RAIM predictability.

Operating Minima

The DH/A selected by the operator should not be less than the greater of the following values:

The DH/A selected by the operator should not be less than the greater of the following values:

  • DH/A corresponding to the aircraft category* if published by the authority in charge of the aerodrome;
  • OCH/A (obstacle clearance height) corresponding to the aircraft category *;
  • 250ft

Missed Approaches

A missed approach must be initiated in the following cases:

For LNAV, LNAV/VNAV and LPV approaches:

  • Loss of the function checking the position integrity or position error alarm (e.g.: GPS Primary loss, Unable RNP, RAIM loss/not available, RAIM position error/alert, etc.)
  • Suspected database error.
  • Loss of RNAV/GNSS guidance (case of architectures without lateral deviation indicator in the PFD).
  • Discrepancy between the two RNAV/GNSS devices for an installation certified with two systems.
  • Excessive technical error (excessive deviation noted on the lateral deviation indicator)

LNAV/VNAV and LPV approaches:

  • In the event of loss of vertical guidance (even if lateral guidance is still displayed)
  • Excessive flight technical error (excessive deviation8 observed on the vertical deviation indicator)

Example APV Precision Approach into Le Touqet

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Figure 5: Example RNAV precision approach into Le Touquet

Example RNAV Non-precision Approach into Hamburg

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Figure 6: Example RNP (Non-precision) approach into Hamburg

Airmanship

  • Training: It is easy to see that RNAV approaches introduces many new concepts and procedures.  In order to be able to safely conduct the approaches in an IFR setting, additional training is legally required.
  • Equipment: Equipment, and more specifically the GPS receiver needs to be certified.  Furthermore, an up to date navigation database is also required.
  • Availability: Depending on the approach, it may be necessary to consider RAIM availability prior to conducting the flight.

Review

This briefing has introduced the most important concepts for RNAV precision and non-precision approaches. Furthermore it has provided an introduction to the procedures.

RNAV approaches provide another dimension to IFR flying, and expand the possibilities of flying in low visibility.

For more information take a look at the documents provided in the references.  In particular the training manual provided by Vasa Babic provides a wealth of information.

References

Eurocontrol (2013), Introducing Performance Based Navigation (PBN) and Advanced RNP (A-RNP).

Vasa Babic (2008), PPL / IR Europe: RAV Training Manual, http://www.pplir.org .

DSAC (2011), Operational Guidelines to Conduct RNAV (GNSS) approaches LNAV, LNAV/VNAV and LPN, second edition.

EASA (2009), AMC – Airworthiness Approval and Operational Criteria for RNP APPROACH (RNP APCH) Operations Including APV BARO-VNAV Operations.

4 comments

  1. RAIM is a method by which a GNSS receiver can determine the integrity of a fix by using more satellites than necessary for computing a position. To compute a position, GNSS requires a minimum of 4 satellites. By using a 5th satellite, the receiver can determine the integrity of the position on the assumption that all computed positions must agree with each other. If all satellites do not agree, then the position can not be guaranteed (i.e., we are getting inconsistent positions from the satellites). By using a 6th satellite, it is also possible to identify and exclude the faulty satellite (with only 5, you can determine that there is an inconsistency, but not which one is inconsistent). If the receiver is unable to determine the integrity of the position, then the RAIM alert will be given. This simply means that the receiver is unable to guarantee the integrity of the position. It however does not mean that the position is inaccurate, or say anything about the accuracy of the position.

    The accuracy of a computed position is an issue separate to RAIM, which only handles integrity. Depending on the receiver and installation, it will be certified for a particular accuracy (or RNP specification). Generally speaking, the accuracy of the position is determined by the number of satellites the receiver is able to receive, and the geometry of the satellites in space. The higher the number of satellites received, and also the further spaced out the satellites are, the more accurate the computed position will be. One of the largest errors in calculating a very accurate position, is the error introduced due to ionospheric delay. GNSS receivers rely on determining the time it takes for a signal to be sent from the satellite to the receiver. As the speed at which radio waves propagate is known to equal the speed of sound, it is possible to determine a distance from the satellite using simple algebra. However, due to the ionospheric delay, radio waves do not always propagate at exactly the speed of sound. This is where SBAS (satellite based augmentation system) or GBAS (ground based augmentation system) send corrections for this error, ionospheric delay, based on known surveyed positions, greatly enhancing the accuracy of a computed position. WAAS is the name of the US system of SBAS, EGNOS is the name for the European one. As such, it is not RAIM that is responsible for a level of accuracy, but this is determined by several factors such as number of satellites, geometry, and availability of SBAS or GBAS.

    What a receiver will do, is continuously monitor that the computed position is accurate enough to a required specification. For instance, RNP APCH requires a standard navigation accuracy of 1 NM for the initial, intermediate and missed sections of the approach, and a 0.3 NM accuracy for the final approach segment. If the receiver er is unable to compute a position to the required several of accuracy at 95% of the time, it will alert the pilot will then in most cases be required to go missed.

    As a final note, RAIM is not used, nor is it required to determine if RAIM will be available, for approaches that rely on SBAS (such as WAAS or EGNOS) or GBAS. This is because the information for determining the integrity is already contained in the information from the SBAS or GBAS system.

  2. Shouldn’t this part “RNP APCH LNAV/VNAV (with Vertical Navigation added and relies on GPS and Barometric VNAV). This is also referred to as an APV Baro.” Be under Non-precision approaches rather than precision approaches ?

    In the later section, the Jeppesen chart shows LNAV/VNAV as NPA

    1. Yes, you are correct. Thanks for the correction.

      A lot has happened since the article has with written. The definitions have changed and in EASA instrument rated pilots are now required to have PBN privileges added to their license. The article has therefore somewhat become outdated.

      ICAO has now defined approaches under type A and type B. Type B are approaches with a minima >= 250 ft, and type B with a minima < 250 ft. Type B approaches are all precision by definition. This category now includes the SBAS approaches which were previously considered non-precision (only conventional approaches were considered precision by the old definition). Type A approaches are categorized in two subcategories, NPA (non-precision approaches) and APV (approaches with vertical guidance). These can include all kinds of RNP approaches, and the baro-VNAV approach would fit into the latter subcategory. I think the main point to be taken is that ICAO is increasingly recognizing non-conventional approaches relying on SBAS or GBAS (SBAS CAT 1) as worthy for precision approaches.

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