Preparing for GAGAN: SBAS vs Non-SBAS Receiver

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GAGAN's GSAT 8 (closer to Africa) and GSAT 10 provide the SBAS correction & integrity signals.

GAGAN’s GSAT 8 (closer to Africa) and GSAT 10 provide the SBAS correction & integrity signals.

With the GPS Aided GEO Augmented Navigation (GAGAN; Indian term for the country’s SBAS system) availability just a few days away, excitement is in the air, especially those who realize the benefits of the Satellite Based Augmentation System (SBAS) and the benefits it brings to aviation applications.

Today, we get to see the Wide Area Augmentation System (WAAS; US term for their SBAS system) as an option on a high sensitivity WAAS enabled Garmin receiver, and how it compares with a non-specialized commercial grade GPS receiver (A Nokia E-72 was used for this).

The Garmin unit picked up 11 Satellites, while the Nokia E72 picked up only 8 (blue bars).

The Garmin unit picked up 11 Satellites, while the Nokia E72 picked up only 8 (blue bars). Note that the Nokia GPS cannot receive signals from satellites beyond #32.

The Garmin handheld unit (eTrex-H, now a discontinued model from Garmin, but used by many for aviation applications, though not certified for such use) features a high sensitivity receiver. With higher sensitivity, it can pick up weak GPS signals, which are too weak for standard sensitivity GPS receivers to pick up. As a result, it receives signals from more satellites, making the reported position very accurate and stable. (with a 3 meter accuracy, you can be assured of landing within 10ft on either side of a runway centreline)

The Garmin Unit's accuracy was rock solid stable at 3 meters, while the Nokia's accuracy fluctuated, and came nowhere close.

The Garmin Unit’s accuracy was rock solid stable at 3 meters, while the Nokia’s accuracy fluctuated, and came nowhere close.

In addition, the Garmin eTrex-H also has a the ability to receive signals from ANY SBAS satellite, and apply the necessary corrections to make the signals more accurate. Considering that the GPS unit already has an accuracy of 3m, it may be unlikely that a greater accuracy may be noticed with the WAAS system, although the corrections will be applied. This is because, closer to the equator, the ionosphere introduces a lot many errors, which disturb the GPS signals. An SBAS attempts to provide a 7 meter accuracy; anything better than that must be treated purely as a bonus!

WAAS ellitenabled, and the Garmin unit looking for Satellite 39 from EGNOS

WAAS ellitenabled, and the Garmin unit looking for Satellite 39 from EGNOS

In the settings, WAAS was enabled, and as a result, the Garmin GPS unit received satellite number 37 (Jan 10) and 39 (Jan 11). A standard non-WAAS / SBAS receiver will not see more than 32 satellites. GPS satellites have a PRN (Pseudo Random Noise code that allows the receiver to decode that specific satellite’s information) between 1 and 32, both inclusive. Any satellite beyond 32 is a SBAS Satellite, part of WAAS, EGNOS (the European Geostationary Navigation Overlay Service), MSAS (Multi-functional Satellite Augmentation System (Japanese)), or, as will be seen in a few days, the GAGAN system’s. Satellite numbers 37 and 39 are from the European EGNOS, but the corrections received will not be applied by the receiver as the satellite signals specify the area of applicability.

The GAGAN system’s satellites, with a PRN of 127 (GSAT-8) and 128 (GSAT-10), will appear as satellites 40 and 41, respectively, on a GPS receiver. Both satellites transmit the same information. That satellite from which the GPS receiver receives stronger signals will be selected. For Bangalore, this is GSAT-10 (Seen on the GPS receiver as 41).

The excitement is building!

4 thoughts on “Preparing for GAGAN: SBAS vs Non-SBAS Receiver”

  1. GAGAN Purely is a show of technology and power.They were launching a Geo Stationary sattelite and decided lets put a NAV transponder on it. The system still rely’s on the GPS .With an expected service life of approx 10 to 15 years are replacements planned ?? .Who is it set to benefit.How far is the airspace capacity going to increase.India does not certify the GPS as a navigation equipment be it An aircraft or a Ship or a Car.dGps has been in use in the country for about 10 years now by DGS. Regional connectivity seems to be the only reason why AAi has spend their head on GAGAN and RNV5 routes in the recent past.User preferred routes were a requirement of the all major Intl operators in the EU and SEA/PAC hence AAI had to implement it . Air Nav Charges being a major chunk of AAI revenue from BOBASMA & EMARSSH .The state of the Art CNS facility and the upper area control at Chennai was a necessity of AAI to ease their own head ache.Archaic separation techniques inadequately trained controllers who are engineers by profession and do not care how an aircraft flies or its capabilities.So whom does and how does GAGAN benfit me as a domestic operator?? I still only fly the published air routes.As far as RNP approach go.. Cochin has always had one.

    • You’ll now see ILS CAT-I like approaches a fields that have no ILS. You’ll see approaches into fields have have no VOR. You’ll see Q Routes, and T routes. You’ll see the “GPS not to be used for primary navigation” go away. You can now use the GPS, because AAI keeps an eye on how good or bad the data is. Finally, the country has control, in part. The other part relies on the pormise by the US to not shut the US down. GAGAN sets the stage for India’s own GNSS constellation.

      • I still do not see the reason.Cochin has had an RNAV app without GAGAN. Q12345678…. exist without GAGAN.Any and all airfields which are operated to hav VORS baring Finger few.Why hasnt a Q route come up between Delhi and Chennai , Delhi and Kolkata.The first RNAV routes in India were not for the benefit of any Local operators but were for EU/SEA/PAC regional op’s serving BOBASMA & EMARSSH.Time will tell how DOMESTIC aviation will benefit from GAGAN. Some of the ISRO transponders are primarily for AWS data collection.Majority of AWS them remain in-operational.There are remote airfields to which major domestic carriers operate which have AWS however the system has not been able to provide updated weather even till date.

      • You have RNAV because of the RNAV capability on board. Smaller GA aircraft will benefit from the T routes.

        Cochin’s RNAV Approach has a DH of 410 feet. APV-1 will give you 200 / 250ft.

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