Degradation of AHRS Heading Accuracy During Typical FI Maneuvers
Stefan Jagieniak and Claus-Sebastian Wilkens, Aerodata AG, Germany
Location: Regency Ballroom
In a modern flight inspection system (FIS), heading information is used to feed azimuth-dependent antenna gain models, to perform lever arm transformations and to compute e.g. the ADF bearing error.
A wide spread technical solution to obtain heading data in the FI aircraft with minimum aircraft modification effort is to interface the primary attitude and heading reference system (AHRS) or gyro system. These are coupled to a magnetic north sensor for long term stability. Following the documentation, this combination should provide sufficient accuracy for flight inspection.
However, these sensors are designed for commuter aircraft, not for special FI maneuvers and may suffer from effects that degrade the performance significantly beyond specification under certain circumstances. Especially during narrow NDB or VOR orbits, the heading may be wrong by several degrees! This leads to an inaccurate determination of the antenna gain and consequently the field strength measurements are degraded.
Further options are to use heading references based on inertial navigation systems (INS), inertial reference systems (IRS) or GNSS antenna arrays. These do not suffer from the described effects, but come with higher cost and aircraft modifications.
This paper discusses the fundamental physical effects leading to the mentioned errors. It depicts actual data obtained during routine flight calibration as well as comparisons against independent heading systems like INS. Potential countermeasures are pointed out in terms of operational procedures and scientific methods in order to predict and mathematically reduce such effects.