Calculating Positions; Gps Positioning - Javad Maxor User Manual

I N T R O D U C T I O N
1
Principles of Operation

Calculating Positions

Once the receiver locks on to a satellite, it starts recording measurements and receiving
the various digital information (ephemeris, almanac, and so on) the satellites broadcast.
To calculate a position, receivers use the following formula:
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 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.

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, and
counteracts antispoofing 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.
• 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, interrupt, 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.
–Wide Area Augmentation System (WAAS) creates and transmits DGPS correction
messages.
1-4 Maxor User's Manual
Velocity x Time = Distance
www.javad.com