User's Manual Part 1

Introduction
HiPer XT Operator’s Manual
1-4
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 elements:
• Accuracy – The accuracy of a position primarily depends upon
the satellite geometry (Geometric Dilution of Precision, or
GDOP) and the measurement (ranging) errors.
– Differential positioning (DGPS and RTK) strongly mitigates
atmospheric and orbital 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, the higher positioning accuracy.
• 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 distort signals, lowering the number of
available satellites and adversely affecting signal reception.
• 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.
– Five or more visible satellites for only GPS or only
GLONASS; six or more satellites for mixed scenarios.