Calculating Positions; Gps Positioning - Javad Prego Operator's Manual

Calculating Positions

GPS Positioning

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GPS receivers use ephemeris and almanac data to calculate accurate
positions, and the positions of your measurement points.
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 basic formula:
Speed of Light x Time = Distance
Where Speed of Light is the data baud rate 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's must lock onto at least five satellites to obtain an
absolute position.
Using only GPS or only GLONASS, the receiver must lock onto a fifth
satellite. Six satellites will provide fault tolerance in mixed
scenarious.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).
• 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.
Calculating an accurate position 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 orbit
errors, and counteracts anti-spoofing signals the US Department of
Defense transmits with GPS signals.
–The more satellites in view, the stronger the signal, the lower the
DOP number, providing more accurate positioning.
I N T R O D U C T I O N
Principles of Operation
Prego Operator's Manual 1-3