Gnss Positioning - Javad LGG100-GG Operator's Manual

I N T R O D U C T I O N
1
Principles of Operation
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 recep-
tion 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 sce-
narios. 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 almanacs,
saving this information to its NVRAM (Non-Volatile RAM).

GNSS Positioning

Achieving quality position results requires the following three elements:
18 LGG100-GG Operator's Manual
• 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.
• 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
accurate the position. Natural and man-made objects can block, inter-
rupt, and weaken signals, lowering the number of available satellites.
• Integrity – Fault tolerance allows a position to have greater integrity,
increasing accuracy. Several factors combine to provide fault tolerance,
including:
–Receiver Autonomous Integrity Monitoring (RAIM) detects faulty
GPS and GLONASS satellites and removes them from the position
calculation.
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