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Plot Assignor

Page history last edited by Alan Hartley-Smith 11 years, 9 months ago

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 Introduction

(Input by Ian Gillis)

Plot Combiners are traditionally associated with the sensor; they take digital radar plots from primary and secondary radars, combine and transmit them in digital form to remote display centres. However this combination process can only be easily accomplished when the sensors are collocated; if they are separate then the earth's curvature means that the aircraft position is recorded in different coordinate systems and "combination" is a risky process.

 

In addition there existed a number of small airfields that were fitted only with primary approach radars - as such they lacked the data such as aircraft identity and height that secondary radars can provide.

 

The idea of providing these short-range approach radars with data from those long-range SSR stations sufficiently near to give cover in the airfield TMA and on approach was mooted - correcting the coordinate systems so they were either combined or at least closely visually associated.

 

What follows is a short description of one of the many success stories which remain largely unsung in Marconi History - because they were low in monetary value - and successful.

 

Plot Assignor Study

The study, sponsored by the CAA, was let in 1988.  A systems engineer (Ian Gillis) and a programmer (Peter Snell) were delegated to the task. Ian was fortunate in being the jealous guardian of an Apple Lisa desktop computer. The study compared the various systems and established the potential errors due to coordinate conversion and asynchronous rotation rates.

 

The Apple Lisa Office Suite had a well-integrated set of applications - a word processor for the study text (a 50-page report), LisaDraw for the diagrams and LisaCalc for the spherical geometry calculations and error graphs.

 

It was soon seen that the calculations could provide excellent results but the processing power required to solve equations of the form: sin(D/p) = sin(y/p) + 2 cos v sin L sin²(x/2p cos v), when extant processors required some 280 μS for the cosine function, was a serious limitation. Cue a visit to Baddow Research Laboratories where a mathematician (Bob Treciokas) advised that a "flat-earth approximation" was possible - this "first order" correction avoided unnecessary complex calculations to produce second-order adjustments. Essentially the coordinate positions and the angle between the primary and secondary radars were "tweaked" so that the flat-earth approximation gave perfectly acceptable results.

 

Successful Outcome

A Plot Assignor/Combiner unit implemented in hardware was subsequently constructed by Marconi Radar and tested in 1991 by the CAA using the Watchman approach radar at the Telecommunications Engineering Establishment (TEE) and the en-route SSR systems at Heathrow and Pease Pottage.

 

The National Air Traffic Services Air Traffic Evaluation Unit concluded in ATCEU Memorandum 162 "...that the equipment gave very good results, few anomalies were displayed and that the processor is suitable for use in an approach radar control role".

 

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Comments (1)

Ian Gillis said

at 12:50 pm on Feb 14, 2016

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