With 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) communication/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.
The 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.
The “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.
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). 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.
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
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).
GPS Satellites from which signals could be received at 2145IST (1615UTC) on 9th January 2014.
The Flying Engineer visited the Master Control Centre of the GAGAN system, the equivalent to the United States’ WAAS. This piece talks of the GPS system, as available today, and the changes expected, in a few days, to aviation navigation in India.
Navigation information may be from a self contained source (such as an inertial navigation system), or from land external radio aids, such as VOR, DME, ILS, NDB (almost on its way out), or from space based radio aids: Satellites. The most commonly used satellite navigation system is the NAVSTAR Global Positioning System, popularly known as the GPS.
The GPS signals as received by the on-board GPS receiver of a Nokia E-72. The screenshots are for different orientations of the phone: North-East-South-West. As seen at 21:37 IST (16:07UTC) on 9th December 2014.
A simple GPS receiver in a mobile phone (I didn’t pull out my Garmin as the battery is dead) can show you the satellites in the vicinity, and the positional accuracy. If you’ll notice, the mobile phone receiver shows 32 slots for 32 possible active GPS satellites (identified by their PRN number: see the table below), not all of which are in the line of sight of the receiver at any given point of time, as the satellites orbit the earth. GPS signals are weak, and hence by making the mobile phone face North, East, South and West, different satellites could be picked up, all those which were “visible” (line of sight) from the ground (see the table of satellites).
GPS Satellites “visible” over Bangalore as of 2146IST (1616UTC). This table matches with the GPS satellites visible on the phone.
The advantage with a satellite based navigation system, such as the GPS, which offers navigation signal coverage globally, and hence called GNSS or Global Navigation Satellite System, is that it overcomes line of sight and range issues associated with all land based radio aids, and doesn’t drift like the INS. Today, most aircraft have a GNSS receiver on board, and is used to supplement navigational information obtained from the VOR, ILS, and the INS, if present on board.
The “supplement” in the statement above must be paid attention to. Because a GNSS’s control is exclusively in the hands of just one country / union, other countries do not have a way of controlling or monitoring the signal. Further, errors that creep into the signal as it passes through the ionosphere degrade the positional accuracy. Hence, on all airplanes in India, “GPS Not to be used for Primary Navigation” is often seen in the flightdeck, especially in general aviation (GA) aircraft, even though the accuracy of GPS receiver is greater than that of a VOR, and the INS, but worse than that of an ILS.
Note the horizontal and vertical accuracies, which are sufficient for enroute, but poor for a precision approach.
The GPS system (which includes the receiver) guarantees an accuracy within 100m (0.05NM), but practically observed GPS accuracies at the receiver level are encouraging: usually, the accuracies go up to 3 meters for good receivers with higher sensitivity (like a simple handheld Garmin eTrex H), and is around 10-40 meters for GPS receivers like those found in mobile phones. With 0.05NM accuracy, it may immediately seem evident that with a GPS receiver, an airplane can comfortably fly a RNP 0.1 route / arrival.
It can, but it may not. The problem is that, if all the satellites behave equally bad, (or ionospheric disturbances introduce too much error), fooling the GPS receiver into believing that it is computing a valid, accurate GPS position, the outcome may be as bad as a controlled flight into terrain (CFIT). There must be a means to inform pilots if the GPS signals are not reliable. That requires a second system based on the GPS, that monitors the GPS signal’s integrity, and lets users know if the signals are reliable or not. Once information about integrity is made available to pilots, GPS may be used to navigate, for as soon as the signals go bad, pilots will receive a notification which will allow them to discard GPS data, and switch to land based radio navigation aids to continue navigating safely, and sufficiently accurate.
In India, this role of monitoring the signals is the responsibility of the GPS aided geo augmented navigation (GAGAN) system. The GAGAN system has 15 ground stations scientifically scattered across the India, to monitor GPS signals. The system offers integrity monitoring only within India’s flight information regions (FIRs), besides providing information that allows GPS receivers to compensate for errors induced due to either the satellites or the propagation through the ionosphere. This make the GPS receivers determine position with far greater accuracy: as much as 7.6 meters, with a guarantee.
In 3-5 days from today, the GAGAN system will be switched on, available to everybody, not just to airborne receivers. However, the information crucial to aviation, which is reliability & accuracy, needs something more than a normal GPS receiver. The GPS receiver needs to have the ability to receive the additional information: about signal integrity, and error information (that may be applied to increase accuracy). This information is made available through additional satellites: in the case of the GAGAN system, these are satellites with codes 127 and 128, transmitted by the Indian GSAT-8 and GSAT-10, respectively. GPS receivers which sell with a “WAAS-enabled” tag (like my Garmin eTrex H) will be able to offer the accuracies promised.
WAAS enabled Airborne GPS receivers, such as the Garmin GNS530W (Note the “W” for WAAS) will be required to fly in Indian airspace, if the aircraft is to fly a GPS arrival, approach, or route. These receivers are readily available, and when installed, the “GPS not to be used for primary navigation” will be a sticker of the past.