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Copyright © 2000, MCB UP Limited
Helicopter recovery at sea
Helicopter recovery at sea
Keywords: Helicopters, Landing
Over the last decade or so, there has been a general trend for Armed Forces world-wide, and certainly in the UK, to adopt a more utilitarian approach towards major platform developments. Clearly such developments are able to satisfy many operational needs in one go, but it also presents one or two new challenges for the Operational Requirements staff to meet. A good example of this may be found in the question...
How do you safely and accurately bring a large helicopter (a new development), down on the landing deck of a small ship (a older existing platform), in all weather conditions?
In the UK the MoD has turned to industry in partnership with the government research organisations to find a solution - in this instance based on satellite radio navigation and the latest technological developments in GPS receiver design.
In 1995 Raytheon Systems Limited was funded by the UK Defence Evaluation and Research Agency (DERA) to develop a High Integrity GPS Guidance Enhanced Receiver (HIGGER) to support a variety of future aircraft navigation, guidance and control applications. At the same time the All Weather Operations group at DERA Bedford began researching the total system solutions to address:
The launch and recovery of large helicopters operating from small ships.
The launch and recovery of fixed wing aircraft to restricted sites ashore and ships at sea.
Airborne rendezvous for air-to-air refuelling.
Recovery to restricted and austere sites.
The launch and recovery of Uninhabited Air Vehicles.
The primary research work has focused on the recovery of the EH101 Merlin Helicopter to the existing Type 23 Frigate (Plates 1 and 2). A successful recovery consists of a covert approach from the task area to an alongside hover at the ship and will be in all weathers, day and night, and in adverse sea conditions up to sea state 6.
Simulation and flight trials identified the minimum horizontal guidance system accuracy requirement. From the alongside hover the helicopter transitions across the deck to hover above the landing grid. When ship motion is within acceptable limits the helicopter descends and secures to the deck with a harpoon. This guidance accuracy requirement was identified as 0.3 metres R95 and allows for helicopter flight technical error, ship motion and the capture area for the securing system.
The HIGGER unit, developed by Raytheon in Harlow, UK, provides a GPS based guidance solution configured around the existing production STR2515 receiver modified for all-in-view operation. The HIGGER units, located onboard ship and helicopter, communicate via a covert data link. Using a conventional standalone GPS a user would be subject to a number of external errors (variation of the GPS signal in space, satellite model errors) and local errors (atmospheric errors, receiver noise and multipath), that would make it impossible to achieve the required three-dimensional accuracy. The HIGGER unit comprises a 10-Hz measurement process coupled to one or more Kalman Filters producing the appropriate navigation solutions.
To meet the accuracy requirements differential or relative GPS operation is required to cancel common-mode receiver error terms. That is, the errors observed at each receiver have a large degree of commonality and when the measurements from each unit are subtracted the error terms cancel. The differential GPS solution was provided for use where one receiver may be static and can benefit from the reduced platform dynamics. This allows one receiver to be configured as a reference station optimising the bandwidths used for signal tracking, and optimising the navigation filter performance for a stationary solution. The relative GPS solution was provided to allow for both receiver platforms to be moving. The unit was also augmented with Receiver Autonomous Integrity Monitoring (RAIM) to improve the integrity of the GPS by use of an over-determined consistency check solution. A tightly coupled GPS/INS solution was also added to accurately model INS errors and help maintain a solution where GPS is not available.
Plate 1 Type 23 Helo Deck
Plate 2Bond Dauphin at test landing
The receiver development was split into two phases. The first phase provided the basic operations of an all-in-view receiver with relative and differential based solutions. The second phase focussed on the development of enhanced relative modes of navigation. The relative navigation mode allows precise navigation by eliminating common mode errors in the navigation solution between the two receivers. To achieve this, a Kalman Filter was implemented forming relative clock, frequency bias, position, and velocity terms. This was provided with the 10 Hz GPS pseudorange and range rate measurements allowing rapid determination of the relative solution. This solution, however, was dominated by the noise in the pseudo-range measurements and gave a solution of 3 metres R95, consistent with conventional relative solutions.
Having identified the viability of the system solution in flight, the second phase of development extended the relative solution to improve the underlying accuracy. Raytheon implemented a technique called Kinematic Carrier Phase Tracking which makes use of the extremely accurate measurements of the GPS satellite carrier phase measurements. A traditional wide-lane double difference equation is formed utilising L1 and L2 signal measurements, eliminating the common mode space and control segment errors, and removing all local receiver common mode effects (i.e. receiver clock variation). The resultant solution shows centimetric accuracy sufficient for any recovery requirement. Plots of the initial relative solution accuracy and improved KCPT accuracy under dynamics are shown in Figure 1. The Float and Fix solutions show the integer ambiguity being resolved.
However, helicopter trials at sea are an expensive place to evaluate performance. More recent work has now extended these solutions under a simulation environment. The Virtual HIGGER (VHIGGER) system provides a PC based simulation of the whole GPS/INS system. Utilising many models (GPS satellites, user vehicle, antenna etc.) the VHIGGER outputs navigation solutions on RS422, Mil-Std-1553 and Ethernet. Because this runs the same HIGGER software core the solutions produced closely mimic the expected real life output, and similarly produces comparable SNU-84 quality inertial data. This has allowed the assessment of various effects such as introduction of more GPS tracking channels, enhanced KCPT with inertial integration, and loss of signals due to vehicle masking. The use of such simulation tools will substantially improve subsequent integration phases, and substantially reduce the cost of equipment trials. DERA Bedford have now incorporated this integrated GPS/INS simulation with a sophisticated Real Time All Vehicle Simulator allowing "pilot in the loop" equipment trials.
Figure 1HIGGER relative accuracy (metres) vs. time (secs)
Having demonstrated the required three-dimensional accuracy, the objectives of the project for the year 2000 are to assess the issues of solution continuity, integrity and availability. Continuity is the ability of the equipment to provide a continuous solution and comprises many factors such as equipment MTBF. Integrity is the confidence in the given navigation solution and an assessment of the underlying architecture. Availability is the definition of how often the navigation solution is available during a particular approach. The continued operational success of the equipment will be defined by these parameters and are critical to any future GPS based guidance system.
Graeme MyersRaytheon Systems Limited