Bussola Solare

La sinergia di competenze in vari campi del Laboratorio Sorgenti di Radiazione
dell’ENEA di Frascati (ottica, matematica, progettazione di meridiane, ecc.) unita alla presenza in ENEA di gruppi di ricerca operanti nel settore dell’energia solare sia di tipo termodinamico (Unità Tecnica Fonti Rinnovabili, UTRINN) che di tipo fotovoltaico a concentrazione (Unità Tecnica Tecnologie Portici, UTTP), ha portato allo sviluppo di una bussola solare elettronica di elevata accuratezza (fino ad un minuto d’arco), in grado di funzionare ovunque nel mondo e senza la presenza di un operatore 1. I due prototipi finora realizzati ne hanno confermato sia la prevista accuratezza che la versatilità d’uso.

La bussola solare elettronica brevettata da ENEA è uno strumento compatto, automatico, accurato (risoluzione migliore di un primo d’arco, pari a 0,016 gradi) e veloce (fornisce l’orientamento in pochi secondi). La bussola ENEA è stata già usata con successo per rilevamenti di orientamento in Antartide (in collaborazione con INGV), per il controllo della movimentazione e dell’orientamento dell’asse di rotazione degli specchi delle centrali solari a concentrazione (in collaborazione con D.D. s.r.l.), e per la progettazione di meridiane.
In questo rapporto presentiamo l’uso della bussola ENEA nelle misure di orientamento di alcune strutture della Grotta di Tiberio adiacente alla Villa appartenuta nel I secolo d.C. all'imperatore romano Tiberio, presso l’area annessa al Museo Archeologico Nazionale di Sperlonga, in provincia di Latina. Le misure di orientamento sono finalizzate a verificare se tali strutture presentino una direzione privilegiata rispetto a particolari eventi astronomici, collegati al moto apparente del Sole.

The Electronic solar compass, patented by ENEA, is compact, automatic, accurate (the orientation/azimuth is determined with a resolution better than 0.016°), and fast (it provides the orientation in few seconds). The ENEA compass has been successfully used for orientation measurements in Antarctica (in collaboration with INGV), for handling the movement and checking the rotation axis orientation of the mirrors in the concentrated solar power plants (in collaboration with D.D. srl,), and for the design of sundials.
In this report, we exploit the ENEA solar compass in archeoastronomy, specifically in orientation measurements of some structures at the Grotta di Tiberio, a cave adjacent to the Villa of the emperor Tiberius (1st century AD) located in the archaeological site nearby the National Archaeological Museum of Sperlonga, in the province of Latina. The measures are aimed at verifying whether these structures provide references to particular astronomical events, connected to the apparent motion of the Sun.

An electro-optical solar compass, designed and developed at the Frascati Research Centre of the Italian National Agency for New Technologies, Energy and Sustainable Economic Development
(ENEA), has been tested for geomagnetic measurement at the Concordia Station polar observatory. This device, compact, not expensive, fast and very accurate, is able to measure the Sun direction and, consequently, the North Pole direction, within 1/100 of degree.
It is extremely more accurate and reliable than a magnetic compass and its performance is comparable with that of the expensive modern GPS and gyroscopic systems, without the drawback of long measuring time and/or latitude-dependency.
The ENEA compass provides the direction of the true (geographic) North and can be used to determine the exact orientation of solar concentrating power plants as well as to calibrate other compasses or for topographic surveying and, finally, for geomagnetic measurements.
The ENEA compass has been used during the last Antarctic campaign, in December 2017, carried out by the italian ‘Istituto Nazionale di Geofisica e Vulcanologia’ (INGV)and aimed at magnetic declination measurements. The results show that at a very high latitude the measurements done with the solar compass are much more reliable than those obtained with GPS sensors.

ENEA ha progettato, realizzato e brevettato una bussola solare, basata su un dispositivo elettro-ottico (composto da una fenditura e da un sensore amatrice di pixel) e su un algoritmo per il calcolo delle effemeridi appositamente sviluppato, in grado di determinare la direzione del Nord geograficocon un’accuratezza migliore di 1/100 di grado.
Tra le applicazioni di tale bussola vi sono l’ingegneria civile, la topografia, l’installazione di sistemi dienergia solare a concentrazione, l’installazione di radar aeroportuali, la costruzione di tunnel, la misura della declinazione magnetica, la taratura dialtri dispositivi per la misura di azimut.

We present a novel electronic solar compass which is able to determine the true North direction with an accuracy better than 1/100 of degree, superior to that of any other magnetic or electronic compass which do not resort to differential GPS. The compass has an electronic sensor to determine the line of sight of the Sun and a simple but effective algorithm to calculate the position of the Sun. The excellent results obtained during the experimental tests demonstrate the advantages of this compass, which is also compact and not expensive.

Solar compasses are designed to accurately find the true North in sunny days. However, no data on their performance are available when sunlight is partially blocked, e.g., by a cloud. We have measured for the first time the performance of one of the most accurate electro-optic solar compasses (accuracy better than 0.01 degrees) as a function of the solar disk obscuration during the Sun eclipse occurred on March 20, 2015.  The measurements show that the accuracy level is mainly dependent on the asymmetry of the obscuration with respect to the main axis of the optical detection system, and to a lesser extent by the percentage of solar disk covered. In particular, the azimuth measurement suffered of a maximum deviation of 0.08 degrees when 35% of the solar disk was asymmetrically obscured. The deviation was smaller when 46% of the solar disk was more symmetrically obscured. This experiment demonstrates that, even in case of partially obscured Sun, the electro-optical solar compass maintains an accuracy better than magnetic and electronic compasses.