optimum altitudes for passive ranging satellite navigation systems,roger easton

8/3/2019 Optimum Altitudes for Passive Ranging Satellite Navigation Systems,Roger Easton

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Optimum Altitudes for PassiveRanging Satellite Navigation Systems

Roger L. EastonApplications Research Dil'isiollNm'lIl Research Labof(l/ory

Navigation is an old art, one of the oldest known to man. The worditself is derived from the Latin Ilal'igal\'s, a combination of the wordsfor ship (/lads) and to direct (ligereJ. Early navigation consisted ofmeasuring the altitude of known stars to obtain latitudes. One sailedto the desired latitude and then east or west to the proper longitude. Todetermine longitude, a timepiece was required. The earliest timepiecesused for this purpose were the moon and, for greater accuracy, the moonsof Jupiter. In due course, the chronometer came into general use, and somatters remained until the invention of radio. Various radio systemswere designed which improved the availability of fixes, but in most casesnot their accuracy. It took the advent of the near earth satellite to bringa significant worldwide improvement in fix accuracy. The first satellitesused for this purpose were low altitude (600 miles) polar objects thattransmitted continuous frequency (C\V) response.

This paper discusses a method for determining optimum altitudes for. navigation satellites having different types of transmission than the

present units.A navigator can find the position (or a component of position) of a

satellite by measuring the range to the satellite, by measuring a derivativeof position, or by an angle measurement. A direct measure of range hasthe advantage of providing great accuracy, relatively simple calculationsto find a line of position, and relative immunity to problems due to un-known navigator velocity.

Ranging can be accomplished by an active method in which the satelJiteis interrogated. The difficulty with active ranging methods is that thenavigator must transmit and possibly give away his position. A seconddifficulty is that the technique has a saturation level.In order to circumvent these difficulties, a passive ranging techniquewas conceived at ON R in 1964. Since that time it has inspired othernavigation proposals and has become a prime candidate for a DefenseNavigation Satellite System. Work has been done on this project bothat NRL and, under ONR contract, at the Astro Electronics Divisionof the Radio Corporation of America. Funding is and has been providedby the Naval Air Systems Command.

Passive ranging is accomplished by having electronic clocks locatedboth in the satellite and in the navigator's receiver. These clocks are

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optimum altitudes for passive ranging satellite navigation systems,roger easton

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