E a r t h M e a s u r e m e n t s V i a S a t e l l i t e s (*) A . L . C O T C H E R R i c e v u t o il 19 F e b b r a i o 1903 S U M M A R Y . — Geodetic p a r a m e t e r s a n d i n t e r c o n t i n e n t a l ties c a n be d e t e r m i i i e d using a n e w t e c h n i q u e whicli combine» eleotronic d i s t a n c e - m e a s u r i n g e q u i p m e n t (DME) a n d artiflcial satellites. [IL initial field t e s t s , a v e r a g e r a n g e r e s o l u t i o n s of 30 cm b a v e been o b t a i n e d a t d i s t a n c e s g r e a t c r t b a n 3000 k m . R I A S S U N T O . — Nella n o t a l ' A . m e t t e in e v i d e n z a l ' u s o di n u o v e t e c n i c h e per d e t e r m i n a r e i p a r a m e t r i geodetici e i c o l l e g a m e n t i intercon- t i n e n t a l i , a b b i n a n d o l ' e q u i p a g g i a m e n t o elettronico per m i s u r a - d i s t a n z e (DME) ai satelliti artificiali. The purpose of this paper is to introduce t h e concept of using ar- tiflcial earth satellites for measuring the size and shape of the earth, and the relative position of places on its surface. An advanced electronic system which has been built to implement this eoncept will be described in general terms, with a re vi e w of the experimental data obtained to date. This system, called Secor, provides a new and powerful tool for extending man's knowledge of t h e planet on which he lives. Before discussing Secor itself, I should like to talk about the basic building blocks of which it is eomposed . . . electronic DME (Distance- Measuring Equipment). DME operates on a very simple principle, which says tliat t h e pliase shift experienced by any propagated electro- magnetic wave is directly proportional to the propagation p a t h length. Therefore, if this phase shift can be measured by comparing t h e wave at the beginning of the p a t h with the wave at the end, the p a t h length (*) N o t a p r e s e n t a t a al 2° Congresso I n t e r n a z i o n a l e Tecnico Scientifico dello Spazio. R o m a , 19-23 Giugno 1962. 3 H 2 A . L . C O T C H E R can be determined. Making such a measurement if flit' beginning a n d end of t h e patii are f a r a p a r t poses serious problems, which m a y be easily avoided, however: the beginning and end are a t t h e sanie place, b u t in traveling f r o m one to the other, t h e wave is propagatori via t h e point whose distance is of interest. Then this point's distance is, of course, exactly half t h e patii length indicated by t h e phase dift'erence measurement a t the common beginning-end. For example, a c-\v signal is t r a n s m i t t e d f r o m a ground station to some airborne veliicle carrying a transponder; the transponder re- ceives t h e signal and retransmits it, back to the ground station, where it is compared in phase with t h e originai transmission to determine t h e distance from the ground station to t h e airborne veliicle. Techniques have been developed in recent years which m a k e this kind of distance measurement, using phase comparison of continuous-wave signals, ex- tremely accurate. Or, more precisely speaking, extremely accurate whenever t h e phase dift'erence measured is less t h a n one wavelength (360°); phase differences greater t h a n t h a t raise measurement problems t h a t are not readily solved. In general, t h e accuracy of m e a s u r e m e n t , expressed as a certain small fraction of a wavelength, depends only on t h e equipment used, but does not depend 011 the wavelength; in other words, equipment t h a t can measure a phase dift'erence as small as one thousandth of a wavelength can m a k e p a t h length measurements with a resolution of 1 k m if t h e wavelength is 1000 k m , b u t with a resolution of 1 cm if the wavelength is 10 meters. In s u m m a r y , then, t h e two f u n d a m e n t a l f a c t s abont phase-com- parison DME are (1) e s t r e m e accuracy inversely proportional to wave- length and (2) b u t only a t distances less t h a n one wavelength. Thus, we m u s t choosé a short wavelength (high frequency) to ensure high accuracy, and a t t h e same time choose a long wavelength (lovv frequen- cy) if we w a n t to measure long distances. Here is t h e basic dilemma. I t is solved, in Cubie DME, by temporarily ignoring t h e a c t u a l trans- mitted frequency (carrier) and considering modulation signals impressed upon it. These modulation signals undergo exactly the same sort of pathlength-dependent phase shift I previously described, and in Cubie DME t h e r e are usually four or live of t h e m , covering the range be- tween t h e longest distance ever to be measured (say, 20,000 km) and t h e d e s i m i accuracy (say, 1 meter). The lower-frequency modulating signals provide t h e unambiguous d a t a revealing less and less roughly how f a r away t h e transponder is, wliile t h e highest-frequency modulating signal pinpoints the distance, much as the numerals in a I-digit n u m b e r teli E A R T H M E A S U R E M E N T S V I A S A T B L I . I T E S 3 8 3 you, one by one from t h e left, t h e thousands, hundreds, decades, and fìnally t h e units involved. So much for a simple DME, measuring even very long ranges with very liigh accuracies. B y measuring the distarne to some airborne object, it tells us in effect t h a t the object is somewhere on a hemisphere whose center is at the ground station and whose radius is the measured distance. If the distance to t h e sanie airborne object is simultaneously measured f r o m a second ground station somewhere else on t h e e a r t h ' s surface, a second hemisphere is established, and the object m u s t lie on this second hemisphere as well. Therefore, it must lie on t h e semicircle which is t h e intersection of t h e two hemispheres. A distance measure- m e n t from a t h i r d ground station provides a hemisphere which inter- sects the semicircle a t only one point, thereby uniquely determining the location of t h e object in space. Secor measures spatial positions in just this way, nsing t h r e e or more ground stations, except t h a t t h e measurements are not iliade quite simultaneously. I n s t e a d , to p e r m i t using only a single transponder in t h e airborne vehicle, where space and weight are nsually a t a pre- mium, this transponder is rapidly time-shared among t h e ground sta- tions; system accuracy is not appreciably affected. Secor is being used today on m a j o r test ranges throughout t h e world for tracking and guiding both long-range and short-range missiles. Now let's consider an a c t u a l e a r t h measurement problem, like determining t h e distance between N o r t h America and Hawaii. Be- cause of certain difflculties associated with celestial observations, such distances are now known to within perhaps the nearest kilometer. W i t h Secor, it is anticipated t h a t this u n c e r t a i n t y can be reduced to as little as 30 meters. The f u n d a m e n t a l technique is as follows: (1) place three DME ground stations a t accurately surveyed points, a few lmndred miles a p a r t , in N o r t h America; (2) place a f o u r t h DME ground station in Hawaii; (3) orbit a n artificial satellite carrying a Secor transponder; (4) determine the satellite's orbit (with respect to N o r t h America) pre- cisely from t h e North American ground stations; (5) determine t h e Hawaiian station's position (with respect to t h e orbit) f r o m t h e satellite. Perhaps t h e last step requires a brief explanation. As it moves along its orbit, t h e satellite can measure t h e distance to the Hawaiian station whenever t h e lattei- is " in sight " , which might be for several thousand orbitai miles. Measurements at merely t h r e e points, perhaps 500 miles a p a r t , however, are enough to pinpoint t h e Hawaiian site; we bave three intersecting hemispheres, just as before, except t h a t now 3 8 4 A . L . C O T C H E R their centers lie along t h e orbit while their only point of intersection is on t h e e a r t h ' s surface. At present, the Secor earth-measurement program is preparing for large-scale evaluation. A number of helicopter-transportable ground stations bave been built and tested against Secor transponders carried aboard three e a r t h satellites; in tliese tests, average range resolutions of 30 cm were obtained a t distances up to 2000 miles. More ground stations, of advanced a n d ruggedized design, as well as additional trans- ponders and satellites, are built and building. I t is anticipated t h a t t h e first intercontinental d a t a will be available witliin less t h a n a year, thus m a r k i n g the beginning of a long and useful life for Secor as a unique scientific i n s t r u m e n t for exploring the earth from t h e newest of its moons.