GPS navigation equipment

Global navigation satellite systems are transmitted so that the effects of refraction through the ionosphere can be compared between the two signals, and corrections applied. These carrier frequencies are modulated with complex digital codes that appear like random electrical noise; these are called pseudo random codes and they are a fundamental part of GPS. There are three sets of data to be modulated on the carrier waves:
• Course acquisition (CA) code
• Precise (or protected) P code
• Navigation system data.
The coarse acquisition gps ca code is a pseudo random string of digital data used primarily by commercial GPS receivers to determine the range of the transmitting satellite. The gps ca code modulates the carrier wave at 1.023 MHz and repeats every l ms. The gps P code (not available to civilian users) is modulated on both the carriers at a frequency of 10.23 MHz. The P code can be further encrypted as a Y code to provide a high level of security for military users.
Data is exchanged between each satellite and the monitoring stations via uplink and downlink frequencies in the S-band (2227.5 and 1783.74 MHz respectively).

Aircraft GPS signals

Each satellite transmits low power (20-50 watt) signals on two carrier frequencies: Ll (1575.42 MHz) and L2 (1227.60 MHz). GPS has various levels of operation depending on how many satellites are in view. Three satellites provide a two-dimensional position fix; four satellites or more is desirable for optimum navigation performance. The receiver seeks out at least four satellites by monitoring their signal transmissions; this acquisition process takes about 15-45 seconds. To speed up the navigation process, the receiver can obtain an initial position fix from the inertial reference system; this allows the receiver to search for satellites that should be in view. In the event of poor satellite coverage for defined periods (typically less than 30 seconds) the system uses other navigation sensor inputs to enter into a dead reckoning mode. For prolonged periods of poor satellite reception, the system reenters the acquisition mode.

GPS selective availability

Selective availability (SA) is a feature of GPS that intentionally introduces errors (typically 10 meters horizontally, and 30 meters vertically) into the publicly available Ll signals. This applied as a political strategy until 2000, then allowing all users access to the Ll signal without the intentional errors.

Navigation errors can arise from poor satellite visibility or less than optimum geometry from the satellites that are visible. Accuracy of ephemeris data (i.e. each satellite's positional information) is fundamental to the accuracy of the system.
There are external effects that will affect the GPS signal, introduce errors and subsequently affect accuracy. Multipath ranging errors can be caused by reflections of the GPS signals from mountains and tall buildings. Atmospheric conditions in the ionosphere and troposphere will affect GPS signals, these errors can be predicted to a certain extent and therefore correction factors can be built in. The ionosphere will refract the satellites' signals; however, since two frequencies are transmitted (L1 and L2), the time difference between when these signals are transmitted and received can be compared, and correction factors applied.
Calculating ranges from the intersection of two range measurements (whether satellite or ground navigation aids) requires optimum geometry. If the angle between the two satellites viewed by the receiver is acute, this does not provide an accurate position fix. In aviation gps navigation, this is referred to as geometric dilution of precision (GDOP). The closer two satellites are, when viewed from the aircraft, the greater is the GDOP.
Almanac data within the receiver, together with ephemeris data from the satellite, is used to assist the receiver in acquiring specific satellites for optimum geometry.
The aforementioned errors are all unintentional. There is, however, the ongoing concern of intentional interference known as spoofing, i.e. the deliberate attempt to disrupt GPS signals. The Federal Aviation Administration (FAA) and other authorities are constantly testing the quality of GPS signals and working on ways to mitigate such threats. There are several schemes in place or proposed to improve system accuracy, integrity, and availability including:
• Differential GPS (DGPS) for marine users of GPS, this is maintained by the US Coast Guard
• Wide area augmentation system (WAAS) for aviation users, this is maintained by the FAA
• Local area augmentation system (LAAS) for aviation users, this is maintained by the FAA
European geostationary navigation overlay service (EGNOS): this is a joint project of the European Space Agency (ESA). the European Commission (EC) and Eurocontrol.
All these augmentation systems operate on the principle of numerous ground stations in known geographical positions receiving GPS signals. Correction signals are then sent to users in a variety of ways. The wide area augmentation system (WAAS) was developed specifically for aviation users and is intended to enable GPS to be used in airspace that requires high integrity, availability and accuracy. WAAS improves a GPS signal accuracy of 20 metres to approximately 1.5 metres (typical) in both the horizontal and vertical dimensions. WAAS is based on a network of reference stations around the world that monitors GPS signals and compares them against the known position of the reference stations. These reference stations collect, process and transmit this data to a master station. Updated data is then sent from the master station via an uplink transmitter to one of two geostationary satellites; the aircraft receiver compares this with GPS data and messages are sent to the crew if the GPS signal is unreliable.

Improvements to aircraft gps navigation equipment

A further development of GPS augmentation for aircraft is the local area augmentation system (LAAS). This facility is located at specific airports and is intended to provide accuracy of less than one meter. Receiver stations are located in the local airport vicinity and these transmit integrity messages to the aircraft via VHF data links (VDL). The intention is for augmented GPS to gradually replace groundbased navigation aids, ultimately leading to global navigation satellite landing systems (GLS) to replace the instrument landing systems (ILS) for precision approaches and landings.
The GPS navigation receiver can also be installed with error detection software known as receiver autonomous integrity monitoring (RAIM). Monitoring is achieved by comparing the range estimates made from five satellites. In addition to this, failed satellite(s) can be excluded from the range estimates by comparing the data from six satellites. This technique is called fault detection and exclusion (FDE).

GPS airborne equipment

GPS can be used in isolation, or with other airborne systems to provide differing levels of operation. The level of integration determines if the GPS can be used for oceanic, en route, terminal area or non-precision approach. In addition to position calculations, GPS can provide derived navigation data:
• Track (from taking several position fixes)
• Ground speed (from calculating the distance between fixes over a period of time)
• Drift angle (from the difference between heading and track).
Global navigation systems for general aviation are often integrated with ILS-VOR and VHF communication systems. This is a self-contained panel mounted device. Text is displayed on the screen for selected frequencies, distances, bearings etc. Graphics are used to provide a multi-function type display, e.g. for navigation references, weather and traffic warnings.

Other global navigation satellite systems.

The Russian global navigation satellite system (GLONASS) features 24 satellites orbiting at a lower altitude of 19,100 km in three orbital planes; three satellites are in orbit as spares. The Russian defence organization owns the system and civilian usage is managed by the Russian Space Agency. There is limited take-up of GLONASS outside of Russia for civilian applications compared with the worldwide acceptance and usage of GPS. Several satellites have exceeded their design life thereby reducing system capability; these are being replaced on a progressive basis.
Galileo is a European system.