Geographic Orientations

Introduction

The alignment of an inertial navigation system in a ground based device is one of the attractive problems. Clearly, the scope for applying motion to aid the process of alignment is very limited in such applications. Attention is focused now on the requirement of determining the orientation of a set of sensor axes with respect to the local geographic frame. For convenience, the local geographic axis set is often chosen to be the reference frame.

On the other hand, the site survey could be carried out to establish the geographic north line. Heading information would then be transferred to the aligning navigation system using theodolites and a prism attached to the aligning system. Although high accuracy can be obtained using this approach, it is both time consuming and labour intensive. The methods discussed in the following sections are usually more convenient to implement and avoid such problems.

One of the experimental works relevant to the problem of determining geographic directions is that of Bain (1961) which is based on the previously published work of the same author Bain (1956). In this work, an instrument capable of improved performance in conditions of multimode propagation Bain (1961) is used. The measurements were provided during the winter of 1954- 55, where a number of observations were carried out at the Radio Research Station in which phase differences were measured between the signals picked up at aerials spaced by several wave-lengths. The four aerials used in this experiment were short unipolar; they were placed on the circumference of a circle of diameter 180 m so that one pair of aerials was aligned in the north-south direction and another in the east-west direction. Inside a hut at the center of the circle each of these pairs of aerials was connected to phase-measuring equipment of the type described by Ross, Bramley and Ashwell (1951). When the receivers were tuned to the desired station, the overall phase balance of the system was checked, and the phase differences between the signals picked up on the north-south and east-west pairs were recorded photographically at three-second intervals for a period not exceeding five minutes, thus giving about 70 values for each. The conditions of ionosphere were characteristic of the time in question and were never greatly disturbed. Observations were confined to the hours of daylight.

On the other hand, the National Oceanic and Atmospheric Administration (NOAA) maintains a Web site that gives up-to-date magnetic declination values for any site on the Earth’s surface basing on two models: World Magnetic Model (WMM) and International Geomagnetic Reference Field (IGRF). For example, in Damascus, Syria, the angle of magnetic declination calculated using WMM model is 4° 46' E ± 0° 18' with a yearly change by 0° 5' E. The angle of magnetic declination calculated using IGRF model is 4° 45' E with a yearly change by 0° 5' E per year.

Accounting for declination is fairly straightforward using a compass. If the magnetic declination is towards east, then the declination should be subtracted from the magnetic north (0°; 360°) and magnetic south (180°) readings to get the true directional readings. Using the angle of Damascus magnetic declination of 4° 45' E, true north and south align with 355.25° (360° – 4.75°) and 175.25° (180° – 4.75°). To correct for a west declination, the declination should be added to (0°; 360°) and 180° to get the true directions.

However, to maximize the collection of the daily and seasonal solar energy possible, solar collectors (PV modules, solar absorbers, etc...) should be oriented geographically. In the Northern Hemisphere the optimum orientation for a solar receiver is the true south while the true north is the optimum orientation for a solar receiver in the Southern Hemisphere. True south or true north are where the sun will be at its highest during the day. Unfortunately simply using a compass needle aligned north-south isn't good enough. Therefore, before using compass to site an array of solar receivers, the declination error must be corrected for the considered site. In the Northern Hemisphere, a compass needle aligns itself along the magnetic north-south line. The solar receivers should be oriented to geographic south, so an account for magnetic declination—the angular difference between true and magnetic north— should be considered. The main cause for this discrepancy is the Earth’s non-uniform, conductive, fluid outer core that consists mainly of iron and nickel. This layer pushes the compass needle off of true north. Depending upon the site’s location on the planet, the “push” varies in strength and direction (Chisholm, 2007).