It is investigated, through theoretical analysis, the positional accuracies of DME-based navigation systems. It is supposed that the short range navaids, i. e. DME and VOR, are available. Four different types of navigation systems are examined, which utilize respectively; (1) single VOR/DME, (2) separate DME's, (3) one VOR/DME and another DME, (4) 3 DME's. In all the cases, the barometric altitude information is also assumed to be involved, in determination of position. The LMS error criterion is applied to calculate the estimation of position. The horizontal dilution of precision (HDOP) is so chosen as the evaluation parameter in comparison among the above-mentioned nav-systems. Obtained results are shown in the form of equi-HDOP contours around the typical configuration of navaids. Favourite discussions are given to the 3 DME's method from the theoretical viewpoint. But in actual operation, it is recommended that selection should be made between the four nav-systems, owing to such conditions as radio propagation, flight attitude, configuration of navaids etc.
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