Spectracom Corporation
PathAlign-R
2200 / 2240 / 2241 PathAlign-R Manual
4-39
program selected. The file can then be printed, formatted, modified, saved under a different file
name, copied, exported, etc. from the selected application. The Log View-R utility saves data
records in the ’.CSV’ (Comma Separated Variable) file format, which allows them to be imported
into a spreadsheet such as Microsoft Excel. Each portion of a data record will be placed in a
separate cell of the active worksheet.
4.7
GPS, Global Positioning System
The Record-R data logger, within the 2241, contains an imbedded GPS receiver that, along with
the front panel GPS antenna, provides position/date/time data for incorporation into a data
record. The following information is provided to aid in understanding how the GPS system
operates.
4.7.1 NAVSTAR Global Positioning System (GPS)
The Global Positioning System is designed and deployed by the U.S. Department of Defense as
an all-conditions, world-wide, 24-hour, three-dimensional navigation system. The system is
composed of a constellation of at least 24 operational satellites, in six different orbital planes at
an altitude 10,900 nautical miles, circling the earth twice a day, broadcasting radio navigation
signals. This is sufficient to provide a clear view of at least four satellites at all times. One
satellite is required to acquire time (once position is established), and four satellites are required
for a complete four-dimensional solution latitude, longitude, altitude, and time. The GPS land
based control stations track the satellites and upload orbital information (ephemera) and clock
corrections. There are five monitoring stations, three up-link stations, and one master control
station. The U.S. Air Force has the responsibility for this segment of the system.
The satellites transmit two spread-spectrum modulated carriers: L1 (1575.42 MHz) and L2
(1227.6 MHz). There are two 20 MHz wide spread spectrum PRN strings transmitted: Course
Acquisition (C/A), broadcast on the L1 carrier, and Precision (P), broadcast on both the L1 and
L2 Carriers (P code is encrypted and for military use only). At least four satellites are within sight
of any ground station, 24 hours a day. Satellite position and time are derived from on-board
celestial navigation equipment and atomic clocks, accurate to one second in 300,000 years
however, the heart of the GPS system is ranging.
The GPS receiver determines time and position with a process that starts with measuring the
time of arrival of the satellite signals. Each GPS satellite transmits its own position, its time, and
a long pseudorandom noise code (PRN). The noise code is used by the receiver to calculate the
range (precise travel time of the signal) from satellite to receiver. The receiver’s time and
position are calculated using this range data, the satellite’s position and current time, and a
model of the transmission path characteristics. Since the receiver knows the precise position of
the satellite, and its range from the receiver, a simple triangulation calculation can give two-
dimensional position (latitude & longitude) from three satellites and additional elevation
information from a fourth satellite. The final solution yields an exact three-dimensional position
of the receiver’s antenna, as well as time, as determined on the UTC time scale (note: the
Record-R does not record altitude information).
Note:
An accuracy of one (1) second in 300,000 years is approximately the equivalent of
an oscillator aging specification of 1 part in 10
-13
/year. There are approximately 9.467 x
10
12
seconds in 300,000 years.
Summary of Contents for Pendulum PathAlign-R 2200
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