History of Aircraft Navigation and Landing Systems
History of GPS in Aviation
GPS and Aviation Applications
GPS Landing and Navigation Systems (WAAS and LAAS)
• Creating a better navigation system and eliminating errors
Effect of GPS in Aviation
Various Augmentation systems used worldwide
1900’s -Pilots would land their planes in a field in the direction that gave
them the best angle relative to the wind
1920’s -Lights and approach lighting
Radio navigation (Morse Code )
1980’s-Microwave landing systems (MLS)
1991- GPS began to be used
GPS was originally intended for the United States Military and Air Force
In June of 1991, portable GPS receivers began to be used in correspondence
1994, the FAA halted further development of MLS and focused on making
GPS the standard for navigation and landing.
Brief History of Navigation and
Landing Aids in Aviation
Commercial GPS aviation applications appeared in 1996
In 1998, Vice President Al Gore revealed plans to upgrade GPS with two
new civilian signals for improved user accuracy and reliability, particularly
with respect to aviation safety
GPS and Aviation Applications
The satellites serve as a precise reference point
Aviation navigators, equipped with GPS receivers, use satellites as precise
reference points to locate the aircraft's position anywhere on or near the
The distance from the receiver to the GPS satellites can be determined by
estimating the amount of time it took the signal to reach the GPS receiver
GPS Landing and Navigation
There are two kinds of GPS landing and Navigation systems presently in
Wide Area Augmentation System (WAAS) &
Local Area Augmented System (LAAS)
Wide area augmentation system
Basic GPS fails to meet the accuracy, availability, and integrity needed in
navigation and landing of an aircraft
WAAS is based on a network of approximately 25 ground reference stations
throughout the United States
Signals from GPS satellites are collected and received by the reference
stations called wide area ground reference stations (WRSs)
• Precisely placed reference stations receive GPS satellite signals and
determine if any errors exist
WRSs linked to form the U.S. WAAS network
Accuracy from WAAs improves the positional accuracy 7 meters vertically
and horizontally (Less than 3 Meters)
Local Area Augmented System
A ground based amplification to GPS that focuses on a radius of
approximately 20-30 miles from an airport
LAAS broadcasts its correction through a very high frequency radio data
link from transmitter located on the ground
Accuracy of less than one meter in both horizontal and vertical axis
compared to 15 years ago (100 Meter accuracy)
• Problems with LAAS: System only offers a 20 to 30 mile radius of use
while the WAAS is across the entire country
Potential for failure, which means that the system could be effected under
weather conditions, solar activity, or jamming
Creating a Better Navigation
System and Eliminating Errors
Combine two common systems used in aircraft navigation, INS (Inertial
Navigation System) and GPS
Neither system alone provides sufficiently accurate and complete time series
of aircraft positions and velocities because of inherent technological
The INS allows accurate navigation with less than four satellites, helps
reject multipath and aids the GPS in re-acquiring signals
The intermittent loss of satellite signals caused by the aircraft moving into
or out of a satellite’s view, by an aircraft maneuver, or by the weather
Problems may occur during a turn.
Effects of GPS in Aviation
Enhanced safety of flight throughout the region
Seamless navigation service based on a standardized navigation service and
More efficient, optimized, flexible, and user-preferred route structures
Significant savings from shortened flight times and reduced fuel
• Reduced costs to each individual State while increasing overall benefits to
individual States and the entire region
Further economies from reduced maintenance and operation of unnecessary
Improved ground and cockpit situational awareness
Increased landing capacity for aircraft
Various SBAS(Satellite- based
Augmentation systems)used worldwide
European Geo-stationary Navigation Overlay Service(EGNOS)- European
GPS Aided Geo-Augmented Navigation System (GAGAN)- India
Multi-functional Transport Satellite (MTSAT) Satellite- based
Augmentation System(MSAS)- Japan
StarFire & OmniSTAR are the commercial SBAS presently available.
Wide Area Augmentation System (WAAS) – United States
– Operational since 2003
– Supports en route, terminal and approach operations
CAT I-like approach capability (LPV-200)
Multi-function Transport Satellite (MTSAT) Satellite-based
Augmentation System (MSAS) - Japan
– Operational since 2007
– Supports en route, terminal and non-precision approach operations
European Geostationary Navigation Overlay Service
(EGNOS) – European Union
– Open Service was declared in October 2009
– Safety-Of-Life Service has been operational since March 2011
– Supports En Route, Terminal and Approach operations
- APV-1 (LPV equivalent) operational capability
• Global Positioning System (GPS) Aided Geostationary Earth Orbit
Augmented Navigation (GAGAN) - India
– In development with plans for horizontal and vertical guidance
– Final Acceptance Testing planned in 2012
Fig.1. Various SBAS Systems Worldwide
Abousalem et al. (2000) presented a study on the DGPS performance
results using the newly modified receivers with the WAAS and EGNOS
Ochieng et al. (2003) assesses the capability of GPS to provide the level of
safety required for different aircraft flight navigation operations. It presents
an analysis of the protection offered against potential catastrophic GPS
failures at system and user levels. This is followed by an assessment of the
different approaches to augmenting GPS for civil air navigation. Results
show the inadequacy of GPS as a system for real-time safety critical use.
Witte et al. (2005) demonstrated accuracy of a WAAS-enabled GPS unit for
the determination of position and speed. Comparison with the new and
published data showed significant enhancements in both position and speed
accuracy over a non-WAAS system. Position data collected during straight
line cycling showed significantly lower sample-to-sample variation (mean
absolute deviation from straight line 0.11 vs. 0.78 m) and greater
repeatability from trial to trial (mean absolute deviation from actual path
0.37 vs. 4.8 m) for the WAAS-enabled unit compared to the non-WAAS
Rao et al. (2007) discussed the implementation of the GAGAN-TDS
(Technology Demonstration System). GAGAN TDS (Technology
Demonstration System) is a forerunner for the operational Satellite Based
Navigational System over the Indian region. The TDS phase of the project
implements minimum set of elements for demonstrating the SBAS proof of
concept over the Indian region.
Ettore et al. CIRA (2011) describes the algorithm implemented to process
the broadcasted EGNOS SIS in order to obtain a position solution and
integrity information compliant with RTCA DO229C. Moreover, they
present test procedures and experimental results that may be used as a
design guideline for monitoring manufacturing compliance and, in certain
cases, for obtaining formal DO229C certification of equipment design and
Jiwon Seo et al. (2011) studied aviation availability during a severe
scintillation period observed using data from the previous solar maximum is
analyzed. The effects from satellite loss due to deep fading and shortened
carrier smoothing time are considered. Availability results for both vertical
and horizontal navigation during the severe scintillation are illustrated.
Finally,a modification to the upper bound of the allowed reacquisition time
for the current Wide Area Augmentation System (WAAS) Minimum
Operational Performance Standards (MOPS) is recommended based on the
availability analysis results and observed performance of a certified WAAS
1. Korean Wide Area Differential Global Positioning
System Development Status and Preliminary Test
-Ho Yun* and Changdon Kee, Doyoon Kim
(Int’l J. of Aeronautical & Space Sci. 12(3), 274–282 (2011)
Since 1999, the Korea has installed the NDGPS reference stations and has
been providing local area DGPS service. Eleven coast reference stations and
six inland reference stations cover the whole area of South Korea.
From 2003 to 2005, WADGPS research groups in SNU have developed the
Korean WADGPS Test Bed (KWTB). The objectives of the KWTB are to
develop the related essential technology, to verify the feasibility of Korean
• The test bed consists of four WRSs (Wide-Area Reference Stations) and
one WMS(Wide-Area Main Station). WRSs have been installed in the
existing facilities of NDGPS reference stations.
WRS receives the measurements and navigation messages from the
GPS receiver, and validates these data by quality monitoring.
WRS also plays an added role as a permanent test user for monitoring
and analyzing performance of the demo system including accuracy,
integrity, availability, and continuity.
WMS receives the raw data and WRS data from the multiple WRSs. After
time synchronization of multiple WRS data, it checks the integrity flags and
determines the optimal issue of data ephemerides (IODE).
WMS estimate the ionospheric grid point vertical delays As in Eq. , the
ionospheric delay which is estimated from reference station contains
receiver interfrequency bias (Rx IFB) and transmitter IFB (Tx IFB).
Subscript i means the i-th reference station and superscript j means the j-th
Tx IFB can be easily eliminated using time of group delay value, which is
from GPS navigation data.
Eq. is an Tx IFB compensated ionospheric delay
Fitting the ionosphere as spherical harmonics model,WMS estimates the Rx
IFB of WRS. Eq. shows the second order spherical harmonics model.
Substituting , ionospheric delay and Rx IFB can be modeled as a function of local
time and geomagnetic latitude. For estimating the Rx IFB in real time, Kalman
filter has been implemented.The below Eqn has been used as an observation
Satellite orbit and clock errors are estimated using inverted GPS methods with
Kalman filter After estimating the correction it generates the integrity information
and SBAS messages.
Korean WADGPS development plan
The main goal of this phase is to show the capability of Korean WADGPS
using pseudolite and existing NDGPS infrastructures in real-time.This
project is scheduled for 2010 to 2014.
According to this plan, after this project, Korea will launch a geostationary
multifunctional satellite with a navigation payload which will be
broadcasting augmenting signals.
This project is under active development to satisfy the following objectives:
- Increased overall navigation performance (land/ air/marine, civil/military)
- Independent & interoperable with other SBASs
- Certified quality of service
- Qualified for safety critical applications.
Fig.2. Milestones of Korean wide area differential global positioning system development.
To show the initial capability of KWTB, a preliminary
test was conducted via simulation. Satellite orbit and clock errors were made
by RINEX navigation files and precise orbit data from IGS SP3 files.
• Satellite orbit and clocks calculated from SP3 are assumed as true.
Ionospheric delay was generated from IONEX files.
• The other error sources such as tropospheric delay or receiver noise were
generated by accurate modeling.
Fig.4.Time history of satellite range
Fig.5.Histogram of satellite range errors
for each PRN.
The history and the current status of the Korean WADGPS development
plan is presented.
The Korean WADGPS development phase 1 has been successfully
Phase 2 has just started, with the participation of one government office and
seven research institutes and universities.
In this phase the technologies of the WADGPS ground system and
pseudolite broadcasting system are secured.
In phase 3, Korea will launch multi-functional GEO satellites and initial
operation of Korean SBAS will be started.
After this project, Korea will join the ranks of advanced countries in GNSS.
2. Modified Ionospheric Tomography
algorithm using GAGAN data
- D.Venkata Ratnam1, A.D. Sarma1, V. P.V.D. Somasekhar Rao and
B.M.Reddy (The Journal of navigation )funded by ISRO ,Bangalore Vide
Order No: CAWSES:05
Modeling of ionospheric delay is one of major challenges for GPS Aided
Geo Augmented Navigation (GAGAN) system.
An attempt is made to characterize the Indian ionosphere using tomography
technique. One of the prominent ionospheric tomography model is spherical
harmonics model with Empirical Orthogonal Functions (EOF).
But it requires more number of coefficients. Therefore, the model is
modified to reduce the coefficients.
The paper describes a 3-D ionospheric model, which is developed on the
basis of tomographic techniques with GPS data.
Tomography refers to the cross-sectional imaging of an object from either
transmission or reflection data acquired by illuminating the object from
many different directions.
The modified tomograhic algorithm is tested using real time data over the
The goal of ionospheric tomography is to find the electron distribution as a
function of latitude, longitude and height in ionosphere.
The function based models have been chosen for development of
ionospheric model. The reason for this is that they require less processing
time and in turn would be useful in real time applications.
The TEC data is applied to tomographic algorithm to determine electron
density with respect to latitude, longitude and altitude. TEC can be
expressed in terms of electron density as,
TEC =∫N( r ) dl ( r )
R(r) represents GPS receiver station.
S(r) represents satellite
N(r) represents Electron density
dl(r) is unit length of altitudes
Eq can be writeen in terms of electron density N(r) as
S( r )
R ( r )
∫N(r)dl(r) ∫ Σ aΓ (h) Σb Y ( , )dl(r)
S( r )
R( r )
S( r )
R( r )
kk 1 1
Σa Γ(h) denotes EOFs
k represents number of EOFs.
q and f represent IPP longitude and latitude respectively.
Σ b Y (q,f) , represents spherical harmonics function.
bl is number of spherical harmonics coefficients.
Eq is rewritten in terms of two variables as
TECi=[Hi11 Hi12 …Hi1m Hi21……Hikl]x
where, i is the number of TEC measurements
H =∫ Γ ( h ) .Y ( θ, φ) d l( r ) and
x=[a1b1 a1b2.........a kbl a2b1....................akbl]T
The design matrix [H] is formed using spherical harmonics and EOF
Eq can be further approximated as
S( r )
R( r )
or simply, TECi=Hx
We can estimate x (electron density) by least-square solution and
reconstruct the ionosphere from basis functions with in terms of
Modified tomographic algorithm
In the modified tomographic algorithm the numbers of coefficients for
obtaining electron densities are reduced.
Consequently, the amount of correction data to be transferred is reduced. In
this, the design matrix (H) is modified tonon linear form. Eq can be
expressed in non linear form as ,
Finally the unknown coefficients can be estimated using either kinematic or
Kalman filter. However, in this paper, linear least square estimator is used.
• A 3-D modified ionospheric tomographic method has been described in
this paper. The advantage of the function based model is that it requires
less computational time for estimating ionospheric delays.
• Accordingly, ionospheric corrections can be transferred within time to the
• This technique indicates the occurrence of maximum electron density at
• The accuracy of model is to be further validated using more data
corresponding to several seasons.
The Role of GPS in Aviation is a budding one and will only continue to grow as
air travel becomes the main mode of transport in later years.
There are 3 operational SBAS (satellite based augmentation systems) in
WAAS,EGNOS & MSAS and there are 2 which are nearing completion
(GAGAN,Korean WADGPS) and many more in the pipeline in various contries.
GPS has become an integral part of the aviation industry and it’s importance
will only continue to grow in coming years.
But is is not all rosy as GPS has some disadvatages too like it’s availability
during strong ionospheric scintillation .
There is also the question of GPS Integrity and it’s impact on Aviaton safety but
at the end of the day, the advantages it offers far outweigh it’s short-comings.
Many studies are being conducted and many new algorithms are being tested to
overcome the short-comings and make GPS all the more better for aviation.
Ettore De Lellis, CIRA (Centro Italiano Ricerca Aerospaziale)- An
EGNOS Based Navigation System for Highly Reliable Aircraft Automatic
Ho Yun* and Changdon Kee, Doyoon Kim (Int’l J. of Aeronautical &
Space Sci. 12(3), 274–282 (2011) DOI:10.5139/IJASS.2011.12.3.274)-
Korean Wide Area Differential Global Positioning System Development
Status and Preliminary Test Results
Jitu Sanwale, Dhan Jeet Singh, U G Salawade(International Journal of
Scientific & Engineering Research, Volume 4, Issue 12, December-2013 ) -
The Global Navigation Satellite System (GNSS) and Indian Satellite Based
Augmentation System (GAGAN)
Jiwon Seo,Todd Walter,Per Enge (IEEE VOL. 47, NO. 3 JULY 2011)-
Availability Impact on GPS Aviation due to Strong Ionospheric
• K.N.Suryanarayana Rao,ISRO (IJRSP Vol.36,August 2007)-GAGAN-
The Indian Satellite based Augmentation System
• Mohamed Abousalem, Dr. Sergei Lusin, Mr. Oleg Tubalin, Mr. Javier
de Salas(GNSS 2000 Conference, Edinburgh, Scotland, UK, May 1-4,
2000)- Performance Analysis of GPS Positioning Using WAAS and
Rajat Acharya, Neha Nagori, Nishkam Jain, Surendra Sunda,
Sawarmal Regar, M R.Sivaraman & Kalyan Bandopadhyay(Indian
Journal of Radio Space Physics, Vol.36, Oct 2007, 394-404 pgs)-
Ionospheric studies for the implementation of GAGAN
pal(ISRO) (58th International Astronautical Congress 2007)- GAGAN (
GPS AIDED GEO AUGMENTED NAVIGATION) - INDIAN SBAS
• T.H. Witte, A.M. Wilson,(Journal of Biomechanics 38 (2005) 1717–
1722)- Accuracy of WAAS-enabled GPS for the determination of position
and speed over ground
Venkata Ratnam, A.D. Sarma1, V. P.V.D. Somasekhar Rao and
B.M.Reddy (The Journal of navigation )funded by ISRO ,Bangalore Vide
Order No: CAWSES:05)-Modified Ionospheric Tomography algorithm
using GAGAN data
Washington Y. Ochieng and Knut Sauer, David Walsh and Gary
Brodin, Steve Griffin and Mark Denney (THE JOURNAL OF
NAVIGATION (2003), 56, 51–65.)- GPS Integrity and Potential Impact on