Like most others, I was so surprised by this blue sky research that I was moved to ask the question why?
It transpires that this is the last of four studies that were conducted as a result of the flight trials of DGPS guidance for helicopter approaches to offshore platforms - reported in CAA Paper 2000/5 - which, in turn, was instigated in response to the findings of the Helicopter Human Factors Working Group, reported in CAA paper 87007 (formed as a result of the HARP report - CAP 491).
The General Foreword of CAA Paper 2003/2 (which contains the first three studies) contained the statement “Perhaps the most significant finding of all of the work reported in this paper is the poor satellite tracking performance observed during the flight trials, and identified in the third study. As a consequence, further experimental work addressing the effects of helicopter rotors on GPS reception has been commissioned by CAA and has recently been completed. This work will be published in a separate CAA paper”.
It transpires that the foreword (reproduced below) had been omitted from the referenced report: it will have been added by the time you read this post.
When reading the latest report it is important to note that it is not accuracy but availability of satellites that is the main issue: at present the constellation contains 27 satellites when only 24 are required; it would also appear that most, if not all, of these are operating at above specification. A reduction to the minimum number of satellites, a reduction of power to specification and a loss of signal due to tail rotor interference, could result in an unacceptable reduction of performance if GPS is the sole method of navigation or the only available approach aid.
Effect of Helicopter Rotors on GPS Reception
Foreword
The research reported in this paper was funded by the Safety Regulation Group of the UK Civil Aviation Authority, and was performed by Cranfield Aerospace Ltd and the CAA Institute of Satellite Navigation at the University of Leeds. The work comprises a study of the effect of helicopter rotors on GPS reception which was highlighted as an issue for investigation during the flight trials of DGPS guidance for helicopter approaches to offshore platforms reported in CAA Paper 2000/5. Further impetus was added by the results of the third of the three follow-on trial studies reported in CAA Paper 2003/2 which identified a significant difference between the number of satellites tracked and those which were expected to have been tracked; the evidence available indicated the most likely cause to be the tail rotor (the GPS antenna had been mounted on the tail fin on the trials aircraft).
The CAA accepts the results of the study which will be taken into account in overseeing the use of GPS in helicopter operations, for which the most significant use is currently offshore enroute navigation. The GPS antennas on all helicopters currently operating in the North Sea region are understood to be located on the tail boom under the main rotor. The results of this study suggest that such installations may be expected to be less susceptible to the rotor effects identified and, based on the favourable in-service experience to date, this appears to be the case. However, little or no objective data for in-service operations exists, and it is noted that most of this experience is based on a GPS space segment with more satellites and a higher signal strength than that guaranteed by the system provider (US DoD).
In view of the foregoing, and in line with the recommendations contained within this paper, CAA proposes to pursue two courses of action:
(i) Establish a requirement and a means of compliance for confirming the acceptability of GPS antenna installations on helicopters. It is anticipated that this will take the form of a ground test procedure for use by helicopter operators and/or helicopter manufacturers in validating both existing and new antenna installations.
(ii) Establish a practical means of monitoring GPS performance in order to detect any significant degradation. This may involve monitoring of the GPS space segment at a fixed location and/or monitoring of overall performance on board helicopters during routine operations.