If you wish to contribute or participate in the discussions about articles you are invited to join Navipedia as a registered user
|Year of Publication||2011|
The Attitude Determination is one of the many applications where GNSS can be effectively employed.
The attitude of an aircraft, i.e., the orientation in space, can be determined by measuring the relative positions of multiple GNSS antennas mounted on different positions of the aircraft. Usually, a set of 3 or more GNSS antennas placed on board of an aircraft can provide the complete information to compute the aircraft's attitude. 
The aircraft's attitude information is currently obtained by spinning rotor or ring laser gyroscopes. In General Aviation (GA) applications a vertical gyroscope is used for pitch and roll while a separate directional gyroscope is used for heading. The display of the information to the pilot is presented mechanically by the gyroscopes themselves. Commercial and military aircraft generally have computer-based CRTs or LCD displays that are driven by inertial measurement units (IMUs). These attitude systems are very precise but cost more than most small aircrafts.
GNSS based attitude determination is just an extension of differential carrier phase position determination. Very precise relative position (mm level) is determined between a pair of antennas. This relative position can then be translated into angular measurements. Two baselines [nb 1] composed of three antennas completely define the Euler angles associated with aircraft attitude and can be used to compute pitch, roll, and yaw angles. 
The advantages in using GNSS based attitude determination by means of two or more antennas on board an aircraft, as compared to the classical gyroscope/inertial navigation attitude function, are the following:
- Low cost,
- Low weight,
- Small volume and
- Low power consumption.
Therefore it is to be expected that, in the future, reliable and accurate GNSS based attitude measurement equipment will be developed for application in aviation. Aviation requires reliable attitude determination in terms of accuracy, availability, integrity and continuity. For this reason there is a requirement to be able to test newly developed attitude measurement equipment thoroughly.
GNSS based attitude determination is considered as a safety critical application.
GNSS based attitude determination can be applied in a number of different manners. Well known is the 3 antennas, 2 baseline (or more) configuration, which can perform full attitude determination onboard aircraft. With a 2 antennas, 1 baseline configuration, it is possible to use GNSS as a pointing device similar to a magnetic compass. With a 1 receiver, 0 baseline configuration, GNSS could be used as a backup attitude sensor for certain applications.
Via the baseline processing, the information on the relative positioning between the receivers can be translated into angular estimation of the aircraft's attitude. Algorithms which fully exploit the high precision of the GNSS carrier phase data, can achieve high accuracies on the attitude estimation within a short time.
To this purpose, new methods and numerically efficient routines are being developed, aiming to provide correct, fast and high-accurate full attitude estimations with stand-alone GNSS measurements.
There is a limit to the attitude accuracy that can be obtained by a GNSS alone attitude system. The accuracy of the system can be enhanced by combining GNSS with inexpensive inertial sensors. Other benefits that are also realized when GNSS is fused with inertial sensors is an increased bandwidth and robustness. That is, inertial sensors can provide attitude information at rates as high as several hundred Hz and can be used in high dynamic environments. They will also provide a degree of immunity against temporary GNSS outages.
A wide spectrum of applications can benefit from the research in this field: guidance of UAVs (Unmanned Aerial Vehicle), control of space platforms, precise docking of vessels, precision farming, among others. 
Avidyne and Garmin are the two leading manufacturers of glass cockpit products for small airplanes. Garmin owns some products with attitude determination feature included in the glass cockpit system, obtained by inertial measurement units, such as for example:
- G500H and
- ^ Baseline can be defined as a pair of antenna specifications. According to TUDelft - Attitude Determination and Formation Flying, the information on the relative positioning between the antennas can be translated into angular estimation of the platform's attitude, through baseline processing. The GNSS receiver computes the orientation of the aircraft from the differences in the carrier phase angles measured between the antennas.
- ^ a b c TUDelft - Attitude Determination and Formation Flying
- ^ a b Static and dynamic GNSS attitude function testing of airborne equipment, H. Kannemans, National Aerospace Laboratory NLR, April 2005
- ^ a b c GPS-Based Attitude for Aircraft, Roger C. Hayward, Demoz Gebre-Egziabher, J. David Powell, Department of Aeronautics & Astronautics, Stanford University, May 1998
- ^ a b The Baseline Constrained LAMBDA Method for Single Epoch, Single Frequency Attitude Determination Applications, Peter Buist, Delft Institute of Earth Observation and Space Systems (DEOS) Delft University of Technology, September 2007