Model: MC-1-M (2U)
Doc. Ref. no. : m05/om/101
Issue no. 14
User’s Manual
Page 10 of 103
3. GPS Fundamentals
GPS Master Clock Model MC-1-M device is a GPS/GNSS based receiver clock device which provides
accurate time output with 1PPS signal. Satellite Navigation system is a system of satellites that provide
autonomous geo-spatial positioning with global coverage. It allows small electronic receivers to determine
their location (longitude, latitude, and altitude) to high precision (within a few meters) using time
signals transmitted along a line of sight by radio from satellites
.
GNSS is a satellite navigation system that
is used multiple navigation systems mainly GPS and GLONASS. GNSS also include satellite navigation
systems of SBAS, QZSS, Galileo systems etc.
GPS satellite navigation system is maintained by United States of America since 1994 which consists of
at-least 24 operational satellites out of 32 satellites in six orbital planes orbiting at an altitude of
approximately 20,200 km. In typical GPS operation, four or more satellites must be visible to obtain an
accurate result. Satellite-based navigation systems use a version of triangulation to locate the user,
through calculations involving information from a number of satellites.
Figure 3.1: The Basis of GPS
If one considers Figure 1 which shows a flat plane. X and Y are two known fixed points on the plane. P is
an unknown point. If the distances PX and PY can be measured, then the position of point P can be
calculated. Actually there is an ambiguity in that point P’ would also fit the measurements. This can be
resolved if the position of a third fixed point Z is known since PZ is different to P’Z. This can be summed
up by saying that the unknown point P lies at the intersection of three circles based on the known points
X, Y and Z.
When the plane becomes three dimensional spaces, the circles become spheres. The intersection of two
sphere is a circle, and the intersection of three spheres is a pair of points analogous to the points P and
P’ of the flat plane case. As for the flat plane case a measurement from an extra fixed point is required to
absolutely resolve the ambiguity, although in many cases the ambiguous point would be below the
surface of the world. Thus to achieve the objective, GPS must provide accurate measurement of distance
from the unknown location of the receiver to 4 known points.
