Javad Maxor - Calculating Positions; GPS Positioning

1-4 Maxor Operator’s Manual www.javad.com
INTRODUCTION
Principles of Operation
1
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) satellites broadcast. To calculate a position,
receivers use the following formula:
Velocity x Time = Distance
Where Velocity is the speed at which radio wa
v
e
s
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
receiv
er 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
recei
ver 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 recei
ver collects ephemerides and
almanacs, sa
ving this information to its NVRAM (Non-Volatile RAM).
• GPS and GLONASS satellites broadcast ephemeris data c
yclically,
w
ith a period of 30 seconds.
• GPS satellites broadcast almanac data c
yclically with a period of
12.5 minutes; GLON
ASS 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 inte
grity, and placement (also know
n as Dilution of
Precision, or DOP) of satellites.
–Differential GPS (DGPS) strongly mitigates atmospheric and
orbita
l 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 a
vailable, the more valid
and accurate the position. Natural and man-made objects can block,