Topcon HiPer Plus - GPS Positioning; Chapter 1 Introduction

Introduction
Topcon HiPer+ Operator’s Manual
1-4
mixed, GPS and GLONASS scenario, receiver’s 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
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 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, providing more accurate
positioning.
• Availability – The availability of satellites affects the
calculation of valid positions. The more visible