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Gagan LogoWith the Indian GAGAN (GPS-aided geo-augmented navigation) system expected to be fully operational by year end, The Flying Engineer visited a Diamond DA40NG today, at Bangalore, to check if the aircraft was SBAS (Satellite Based Augmentation System) enabled, and how GPS information is presented.

GPS is the acronym for the Navstar Global Positioning System, a space-based radio navigation system owned by the United States Government (USG) and operated by the United States Air Force (USAF). Due to its global availability, the Navstar GPS is a Global Navigation Satellite System (GNSS).

A SBAS system, in principle, detects errors responsible for low accuracy and integrity of GPS receiver positions, and broadcasts those errors via geostationary satellites. SBAS enabled GPS receivers apply these corrections, to compute a more accurate GPS position, with 99.99999% certainty. Sources of errors include the satellite (timing errors), and signal propagation delay (as it passes through the ionosphere). Satellite errors are applicable worldwide, but ionosphere errors are location specific.

The Garmin G1000 system relies on the GIA 63 IAU (Integrated Avionics Unit), which functions as the main communications hub, linking all other units (LRUs) with the PFD. Each IAU contains a GPS receiver, a very high frequency (VHF) G1000_SBAS01communication/navigation/glideslope (COM/NAV/GS) receiver, and system integration microprocessors. The GIA 63W (Note the extra “W”) contains a GPS WAAS receiver. WAAS is the United States’ SBAS.

Although labeled as a WAAS receiver, the unit can receive satellite corrections from other operational SBAS as well: Europe’s EGNOS, and Japan’s MSAS, as seen in the photo on the left.

When GAGAN becomes fully operational, supporting ILS CAT-I like GPS approaches, Garmin International is expected release a navigation database update cycle that will allow the Garmin G1000 display units to list the GAGAN system under “SBAS Selection”. It may then be prudent to de-select WAAS, EGNOS and MSAS, and select only GAGAN.

G1000_SBAS02The GPS Signal Strength box, as seen in the GPS Status page, in the photo on the right, shows the GPS satellites (these satellites have a code, called a PRN (Pseudo Random Noise), between 1 and 32), and the SBAS satellites (124, 126, 129). Satellite 124 is Artemis (EGNOS), 126 is INMAR3F5 (EGNOS), and 129 is MTSAT1R (MSAS).

GAGAN’s SBAS satellites, GSAT-8 and GSAT-10, will be seen as satellites with PRN 127 and 128, respectiely.

The green bars show satellites that are actually being used in the position calculation, the height of the bar proportional to the signal strength. The blue bar shows satellite 25 is locked on but not yet being used in the position calculation. The hollow signal strength bars for satellites 31, 126 and 129 show that the receiver has found the satellite and is collecting data, before the satellite may be used for navigation, and the bar becomes solid. No signal strength bar, as seen for satellite124, shows that the receiver is looking for the indicated satellite.

SBAS CrudeThe “D” indication on signal strength bar shows that the satellite is being used for differential computations. The differential computations, which is the consideration of the “error” to improve positional accuracy, is based on transmissions from EGNOS and MSAS. Since India is not in the intended geographical coverage area of EGNOS or MSAS (see image above, courtesy AAI), Ionosphere corrections are unavailable, but satellite error corrections, which are globally valid, are available, and being used.

With these corrections, the Estimated Position Uncertainty (EPU): the radius of a circle centered on the GPS estimated horizontal position in which actual position has 95% probability of lying, is 0.05NM, as seen in the Satellite Status Box.

The Horizontal and Vertical Figures of Merit (HFOM and VFOM), seen as 23ft and 33ft respectively, is the current 95% confidence horizontal and vertical accuracy values reported by the GPS receiver.

Based on GAGAN’s trials by the Airport Authority of India (AAI), the observed accuracies are 3ft horizontal and 5ft vertical: a dramatic increase in positional accuracy, which the same aircraft will observe when the GAGAN is switched on for civilian use: something that is hoped to happen by the end of Jan 2014, as per the AAI General Manager (CNS) heading the Ground Based Elements of the GAGAN Project at Bangalore, India.