Why GPS approaches and landings might very well be the future of aviation.

The Global Positioning System (GPS) has been an integral part of our lives for a long time. It is present in our cars, computers, and even in mobile phones. So, it should not be a surprise to learn that it is also used by aircraft to find their location in the sky. GPS can also be used to guide an aircraft to a specific runway in an airport and allow it to make a safe landing.

The Instrument Landing System (ILS) forms a part of the aerodrome infrastructure and can both laterally and vertically lead an aircraft to its landing. The ILS is still the most precise form of an approach. However, GPS comes very close to it, or some would argue that it is more accurate than an ILS when implemented correctly. In this article, we will look at the theory of the GPS, the GPS-guided approaches and landings, and why GPS approaches and landings might very well be the future of aviation.

In strict terms, the GPS is one of the many Global Navigation Satellite Systems (GNSS) that are currently in use. When we say GPS, we are referring to the GNSS deployed by the United States. It is the world’s first space-based global navigation system and is also the most widely used GNSS. The other GNSS are the Russian GLONASS (Global'naya Navigatsionnaya Sputnikovaya Sistema), the European Galileo, and the Chinese-owned BeiDou. The working principle of all these GNSSs is similar. To reduce confusion, this article will continue to use the term GPS in the place of the GNSS as most refer to the GNSS as GPS.

The GPS has two segments. They are:

The space segment consists of several space vehicles (SVs), or satellites. The GPS has 31 SVs, GLONASS 24 SVs, Galileo 24 SVs, and BeiDou consists of 35 SVs. The SVs have an orbital period of 12 hours and the height of the orbit is about 20,000 km above the surface. The SVs sent out signals at the speed of light and broadcast pseudo-random noise (PRN) codes of one-millisecond duration. They are carried by two frequencies in the UHF (Ultra High Frequency) band.

These frequencies are known as L1 and L2 frequencies. The L1 frequency is 1575.42 MHz and transmits a coarse acquisition (C/A) code and the Precision (P) code. The C/A code is the only code available for civilian users, while the P-code is used by the military.

The control segment has on-ground monitoring stations that keep an eye on the SVs. The SV's orbits are subject to errors due to the gravitational influence of the sun, and are also affected by solar radiation. This results in errors in SVs' calculated position. The positional errors are detected by the ground stations and this data is then fed to the SVs so that they can correct the position.

So, how do the satellites or the SVs pinpoint the location of an aircraft? For this, at least three SVs are required. We can imagine each SV constructing a sphere. With one SV active, the aircraft can be in any place of its respective sphere. When two SV spheres come together, the aircraft position can be narrowed down to a circle because the intersection of two spheres creates a circle. When a third SV comes into the picture, the position of the aircraft is further narrowed down to two positions - one on the surface of the earth and the other out in space. The SVs simply reject the location in outer space and the position of the aircraft is known.

Even though the location of the aircraft can be calculated by three SVs, a fourth SV is also required. This is because the SVs use a very accurate atomic clock to measure time. However, the GPS receiver clocks are not that accurate. As GPS signals travel at the speed of light (300,000 km/sec), a time difference of even a millisecond results in a position error of 300 km.

To solve this issue the receiver clock is kept at a known error. For example, if the receiver clock is set up so that it is 2 milliseconds faster than the actual time, it can correct that error during the signal processing. The fourth SV thus gives the time component of the equation that ultimately calculates the position of the aircraft.

Before we look at the GPS approaches, it is important to consider the efforts that have been made to ensure that accurate GPS signals are always available to the aircraft. Pilots do not want an aircraft to miss the runway in bad weather due to erroneous GPS signals.

The GPS signals have some inherent errors. These include errors in SV clocks and errors caused by gravitational effects on the SV called ephemeris error. There is also an error caused by the ionosphere. When the signals hit the ionosphere, the signal energy gets attenuated slowing it down resulting in erroneous data being received by the receivers. One way to correct these errors is to use a GPS augmentation system. There are two main types of augmentation systems. One is ground-based and the other is space-based.

The GLS uses a set of antennae that are located at a surveyed position of the airport. These antennae receive signals from the GPS satellites and send them to a control room which is also located at the airport. Here, the signals are corrected or augmented for SV clock errors, ephemeris errors, ionospheric delay, etc. The corrected data is then sent as a VHF signal to a dedicated VHF antenna.

This antenna then gives out a vertical and lateral trajectory beam to any aircraft with a GLS receiver. The pilots simply must tune to the correct GLS channel which is available on the approach chart and the channel is unique for each runway. The GLS is flown like an ILS approach, and it is considered a precision approach by ICAO.

The only difference is that a GLS can be used up to Category I (CAT I) minima whereas an ILS can be utilized down to CAT II and III minima which allows for low visibility landing operations. As it stands, there is work on going to develop GLS that can be used to fly down to CAT II and CAT III minima.

The SBAS, unlike the GBAS, uses a satellite to correct for errors in GPS signals. There is also a ground component of it. Like the GBAS system, the signals for GPS satellites are received by ground reference stations that are placed in a wide area. These stations then correct for the signal errors and uplink them back to a geostationary satellite which then sends the data to aircraft with SBAS receivers.

In the industry, we call SBAS approaches, LPV (Localizer Performance with Vertical guidance) approaches. The LPV approach is not considered a precision approach. However, it can be used to fly down to ILS CAT I minima. Again, this approach is flown like an ILS approach where both the vertical and lateral guidance is provided to the pilot.

There are several SBAS systems across the globe. The Wide Area Augmentation System (WAAS) of the United States, the European Geostationary Navigation Overlay System (EGNOS) of Europe, the System for Differential Corrections and Monitoring (SDCM) of Russia, the Multifunctional Transport Satellite Augmentation System (MSAS) of Japan and the recently tested Geo and GPS Augmented Navigation (GAGAN) of India.

The RNP approaches are a part of the ICAO’s Performance Based Navigation (PBN) initiative. This type of navigation is purely concerned with the performance of the navigation system. It does not matter, how or from where the navigation gets the data as long as the navigation performance criteria are always met. The navigation performance must be able to meet three navigation specifications. They are:

The navigation accuracy requirement states that the aircraft's computed position must remain within a determined area, 95% of the total flight time. For this, a number is denoted for each PBN-related operation. For example, for an RNP approach, the value is set at 0.3. And for an aircraft to meet the accuracy requirement it must remain within time of the RNP value. In this case, it must be within 0.3 nm of the approach path 95% of the approach time.

The navigation integrity ensures a high level of trust in aircraft navigation computation. It states that the aircraft's computed position must remain within the determined area 99.999% of the total flight time. The integrity is given an area of two times the RNP value. If in the approach this means that the aircraft must remain 0.6 nm of the approach path 99.999% of the time. As of now, this level of integrity can only be provided by the GPS. Other navigation systems such as IRS, VOR, DME, etc cannot give such a level of integrity. So, without a GPS signal, RNP approaches and other RNP terminal procedures are not possible.

The continuity aspect makes sure that computation of aircraft position is always available throughout the flight.

The RNP also requires an Onboard Performance Monitoring and Alerting system (ObMA). This system alerts the pilots if the aircraft navigation system fails to meet the requirements so that they can decide to either continue or discontinue the approach based on the failure. For instance, if the aircraft loses the GPS signal, that information needs to be shown to the pilot so that he can immediately go around during a GPS-aided approach.

The RNP approaches do not require special aircraft or ground or satellite augmentation systems. You can shoot an RNP approach if you have a working GPS receiver. Depending on how capable the aircraft is, RNP approaches can be used with both lateral and vertical guidance. When vertical guidance is used in such approaches, it is generated by barometric data from the aircraft altimeter and not the GPS. Only the lateral path is defined by GPS signals. The lateral guidance is known as LNAV (Lateral Navigation) and the vertical guidance is known as Baro-VNAV (Vertical Navigation). The advantage of having the capability to fly LNAV and VNAV is that it allows for lower minima when compared to LNAV-only minima.

The ILS approaches were first introduced in the 1940s. It is indeed a very old technology, and it is very expensive to fix and maintain an ILS system. Moreover, a single ILS facility can only serve a single runway. So, for each runway, a separate ILS system must be maintained. If we compare it to a GLS approach, the latter is way cheaper as one GLS system can accommodate several runways in an aerodrome.

Then if we look at the LPV approach, it does not require a separate airport infrastructure and can provide very accurate signals for an approach if the aircraft is within the range of an SBAS. The RNP approaches are also accurate and can be used to replace the current non-precision approaches such as VOR-guided approaches. As GPS approaches require minimum ground facilities or in some cases no equipment at all, it helps the airport operators to cut costs.

The GPS signals are also not affected as much as ILS signals. ILS beams (vertical and lateral) are very sensitive to interference. Close vehicles and aircraft can cause signal scalloping. For this reason, during low visibility landing operations airports keep aircraft holding near the runways in special holding locations so that the signals' interference is reduced for incoming aircraft.

It may still take a lot of time for the changeover to happen. We have been using ILS for a very long time and to fully replace the system, it would take a lot of paperwork and a lot of convincing from a regulatory point of view. Keep in mind that the airports already have invested a lot in their ILS facilities and removing them suddenly does not make financial sense. And, with the GPS we are still relying heavily on one single source for navigation. This, for now, is not acceptable.

Journalist - An Airbus A320 pilot, Anas has over 4,000 hours of flying experience. He is excited to bring his operational and safety experience to Simple Flying as a member of the writing team. Based in The Maldives.