1. International Civil Aviation Organization
Global Navigation Satellite System (GNSS) Overview and Spectrum Implementation
International Civil Aviation Organization
Spectrum Seminar
Nairobi, Kenya
February 17-19, 2004

2. Basic GNSS System Space Segment Ground Segment Orbiting Satellites
GPS
GLONASS
Future … Galileo?
Geostationary Satellites
SBAS
Ground Segment
Satellite Control Stations
Augmentation Systems
GBAS
Future … GRAS?
Global Navigation Satellite System (GNSS) is currently composed of two non-geostationary satellite constellations, and the augmentations for those two systems.
Constellations:
Global Positioning System (GPS); United States; 24 SV; 11,000 nm orbit
Global Navigation Satellite System (GLONASS); Russian Federation; 11 satellites (SVs), 8 w/o restrictions [per ANC11, expect to increase the number of SVs with the establishment of a special “Global Navigation System” program]
Future possibilities … Galileo, China, Japan
Augmentations improve signal availability, accuracy (since currently SVs are single frequency), integrity
Satellite-based providing wide area coverage, lesser accuracy and integrity. LPV now, possible CAT I.
Ground-based providing very good local corrections and short time-to-alert values. CAT I now, possible CAT II/III.
Future … regional augmentation. Essentially wide-area GBAS.

3. Satellite Navigation … Basically Multilateration
By knowing your distance from at least 3 points of known-position, you can determine your own position.
Satellite navigation is basically multilateration. The user receives the satellite position and time in the form of a broadcast almanac. All of the satellite clocks are closely synchronized, however the user may not be. As a result, with at least 4 satellites in view the user can solve for the four “unknowns” of latitude, longitude, altitude and time.
For Satellite Navigation: a, b & c are satellites, and a fourth is needed to solve for clock variations.

4. GNSS Ranging and Timing
Earth’s
Ionosphere
Actual Path
Assumed Path
Approach: tarrival – ttransmitted ~ distance from satellite
Assumes straight path of radio frequency signals
Earth’s ionosphere actually disrupts/bends that path
Augmentations correct for that bend using dual-frequency measurements
Currently not possible in aircraft; L2 signals not protected.
Errors accrue during single frequency operations due to the bending of the RF signals as they traverse the Earth’s ionosphere. This bending is frequency-dependent and therefore can be compensated for in the receiver if two signals of different frequency, from the same satellite, are received. Currently there is only a single protected civil signal available to aviation users ( MHz band). As a result those users rely on correction information from SBAS or GBAS. Those augmentations utilize an unprotected signal also broadcast by the satellites on a frequency in the MHz (L2) band to derive corrections. Though not suitable for aircraft use, those L2 signals can be sufficiently protected at the SBAS ground reference stations through careful station siting and antenna design.
GNSS modernization is expected to provide a second protected civil signal in the MHz band. At that time corrections could be made in the aircraft. Augmentations may still be required to meet signal integrity requirements.

5. Satellite Navigation’s Mission SBAS/GBAS Implementation
GNSS by itself provides a global navigation and timing capability. With the addition of SBAS and/or GBAS, the benefits are increased.
SBAS: Enroute oceanic and domestic, terminal operations including departure guidance, and approach/missed approach through perhaps eventually CAT I.
GBAS: Precision terminal, approach and surface operations. CAT II/III goal.
GBAS

6. Satellite Based Augmentation System (WAAS used as example)
Example shows the United States version of SBAS, termed the Wide Area Augmentation System (WAAS). This system was commissioned July 10, 2003, and is currently providing Lateral Navigation/Vertical Navigation (LNAV/VNAV) and LPV capability, using the accuracy of the WAAS signal to provide vertical (glide path) capability.
The system receives signals from GPS satellites at a number of widely spaced Wide Area Reference Stations. Unlike aircraft, those stations receive both the GPS L1 civil signal, and the encrypted GPS L2 signal. The latter is used in a semi-codeless fashion to derive correction factors to compensate for bending of the radio-frequency signal as it goes through the Earth’s atmosphere. These corrections are transmitted via land line to the WAAS Wide Area Master Stations. Derived corrections are sent via geostationary satellites and downlinked on the GPS L1 frequency, providing both the enhanced accuracy/integrity information, and additional ranging sources.
Accuracy improved from about 20m to meters in both horizontal and vertical dimensions.
Integrity data informs users of where GPS is unusable due to system or other errors. 6 second time-to-alarm supported.
Adding satellites in the future to ensure redundant coverage over all CONUS and most of Alaska. Working with Canada and Mexico to further expand coverage.
For more information on WAAS see
FAA

7. Ground Based Augmentation System (GBAS) Architecture
Pseudolite
Focuses on service to a local are (20-30 mile radius).
Correction/integrity information broadcast over VHF frequency in the MHz ARNS band.
Foreseen uses include CAT I/II/III landings; curved approach paths to avoid obstacles, restricted airspace, noise sensitive areas and/or congested airspace. Reduces operating expenses and enables more efficient use of the airspace.
FAA awarded contract in April 2003 to purchase 10 systems beginning in Award went to Honeywell.
For more information on LAAS see
Pseudolite
GBAS
Reference Station
(Integrity
Accuracy
Availability)
Processor
GNSS
Receiver
VHF
Transmitter
Monitor
Status
Pseudolite

8. Planned GNSS Modernization
Addition of satellite constellations
Galileo, additional GLONASS satellites
Improves user availability
Addition of civil signals
MHz band
Facilitates user ionospheric corrections
Possible broadcast of integrity signal
May limit need for external augmentations
Increased power, improved coding
Better resistance to interference

9. GNSS Frequency Bands Frequency (MHz) Function 108-117.975
GBAS broadcast link
GPS L5, Galileo E5, future SBAS, GLONASS L3
GPS L2 (site-by-site ground use only)
SBAS, GPS L1, GLONASS, Galileo E1
Currently most of GNSS occurs in MHz with GBAS broadcast in the lower VHF band. As noted in previous slides, the MHz band is used at SBAS ground reference stations to enable ionospheric corrections. At this point the L2 signal cannot be reliably used in aircraft as the band is not controlled by aviation and is heavily used for radar. Ground use can be protected on a site-by-site basis. The signal corrections and integrity information for SBAS are transmitted in the MHz band.
Future GNSS modernization will result in the addition of new civil signals in the MHz band. These signals, available to civil users and occurring in frequency bands with allocations identical to those in MHz, will allow airborne users to perform their own ionospheric corrections.

10. Spectrum Issues GNSS signals are very weak
~ 50,000 times weaker than the minimum specified edge-of-coverage DME signal
Aviation spectrum managers must be constantly watching to ensure spectrum incursion from in-band/adjacent band systems does not cause interference.
One example: ITU Footnotes and 5.359
Allow fixed service in GNSS bands in some countries
Countries encouraged to remove their names from the footnotes.
5.355 Additional allocation:  in Bahrain, Bangladesh, Congo, Egypt, Eritrea, Iraq, Israel, Kuwait, Lebanon, Malta, Qatar, Syrian Arab Republic, Somalia, Sudan, Chad, Togo and Yemen, the bands 1 540-1 559 MHz, 1 610-1 645.5 MHz and 1   660 MHz are also allocated to the fixed service on a secondary basis.     (WRC‑03)
5.359 Additional allocation:  in Germany, Saudi Arabia, Armenia, Austria, Azerbaijan, Belarus, Benin, Bosnia and Herzegovina, Bulgaria, Cameroon, Spain, France, Gabon, Georgia, Greece, Guinea, Guinea-Bissau, Hungary, Jordan, Kazakhstan, Kuwait, Lebanon, Libyan Arab Jamahiriya, Lithuania, Mauritania, Moldova, Mongolia, Uganda, Uzbekistan, Pakistan, Poland, Syrian Arab Republic, Kyrgyzstan, the Dem. People’s Rep. of Korea, Romania, the Russian Federation, Swaziland, Tajikistan, Tanzania, Tunisia, Turkmenistan and Ukraine, the bands 1 550-1 559 MHz, 1 610-1 645.5 MHz and 1   660 MHz are also allocated to the fixed service on a primary basis. Administrations are urged to make all practicable efforts to avoid the implementation of new fixed-service stations in these bands.     (WRC‑03)