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.