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JNSGyro-2T Operator’s Manual
17
I N T R O D U C T I O N
Principles of Operation
Calculating Positions
Once the receiver locks on to a satellite, it starts recording measurements
and receiving various digital information (ephemeris, almanac, and so on)
the satellites broadcast. To calculate a position, receivers use the following
formula:
Velocity x Time = Distance
Where Velocity is the speed at which radio waves travel (i.e., the speed of
light) and Time is the difference between the signal transmission time and
signal reception time.
To calculate absolute 3-D positions – latitude, longitude, altitude – the
receiver must lock on to four satellites. In a mixed, GPS and GLONASS
scenario, receiver must lock onto at least five satellites to obtain an absolute
position.
To provide fault tolerance using only GPS or only GLONASS, the receiver
must lock onto a fifth satellite. Six satellites will provide fault tolerance in
mixed scenarios. Usually, the number of GPS and GLONASS satellites in
view does not exceed twenty (20).
Once locked on to a satellite, the receiver collects ephemerides and alma-
nacs, saving this information to its NVRAM (Non-Volatile RAM).
• GPS and GLONASS satellites broadcast ephemeris data cyclically, with
a period of 30 seconds.
• GPS satellites broadcast almanac data cyclically with a period of 12.5
minutes; GLONASS satellites broadcast almanac data cyclically with a
period of 2.5 minutes.
GPS Positioning
Achieving quality position results requires the following three elements:
• Accuracy – The accuracy of a position depends upon the number, signal
integrity, and placement (also known as Dilution of Precision, or DOP)
of satellites.
–Differential GPS (DGPS) strongly mitigates atmospheric and orbital
errors
.
–The more satellites in view, the stronger the signal, the lower the DOP
number, providing more accurate positioning.
• Availability – The availability of satellites affects the calculation of
valid positions. The more visible satellites available, the more valid and