Radio Positioning Accuracies
by Bruce Calderbank, Hydrographic Survey Consultants Intl. Ltd., Calgary, Alberta
1. Introduction
Offshore radio positioning came of age during World War II and remained a staple of the offshore
industry, until early 1994 with the full implementation of the Global Positioning System (GPS). The
positioning accuracies of various offshore surveys, structures and platforms, relied on radio
positioning during that time. The author provides information on the accuracies of various systems
used in that period to facilitate reconciliation between new and old offshore survey data, and for
historical purposes.
2. Fixed Electronic Radio Navigation and Positioning Systems
Tabulated below are the fixed electronic radio navigation and positioning systems, available prior to
the widespread use of GPS. They are grouped by transmission frequency and sorted within each
group alphabetically by system and manufacturer.
The principle methods of transmission were pulsed and phase comparison systems with various
combinations employed. Over the years a tremendous variety of radio positioning systems were
developed.
Pulsed systems consisted of a master and two or more slave stations. Positions were determined by
the time relationship between the pulses transmitted by the master station and those re-transmitted
by the slave stations.
Phase comparison systems also consisted of a master and two or more slave stations. However,
positions were determined by the phase relationship between the constant radio transmission from
the master and each of the slaves.
Super high frequency (SHF or microwave at 1 to 10 GHz) systems included such standards as
Miniranger and Trisponder. At these high frequencies, the signals travelled in nearly a straight line,
with minimal reflection or refraction, and hence were generally known as line of sight sytems.
Although extremely accurate, these were fairly short range systems. Maximum range was dependent
on the elevation of the master and slave antennae. Also any object in the direct transmission path
would block the signals.
Ultra high frequency (UHF at 400 to 450 MHz) systems included such standards as Syledis and
Maxiran. At these frequencies, the signals tended to follow the curvature of the earth to a slight
degree, resulting in fairly accurate results at over the horizon distances. These systems were
generally very accurate within the line of sight. A considerable amount of both reflection and
refraction by the tropospheric layers of the atmosphere resulted in a greater degree of ambiguity in
the position as the distance increased beyond the horizon.

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Medium frequency (MF at 1.6 to 3.4 MHz) systems included such standards as Argo and HyperFix
(including its predecessors HiFix and HiFix 6). Low frequency (LF at 70 to 130 kHz) systems
included such standards as Decca Navigator, Pulse 8, and Loran C. At these frequencies, the signals
relied on reflecting off the ionospheric layers of the atmosphere to obtain the larger than beyond the
horizon distances. However these systems were effected by skywave signals which were propagated
by way of the ionisphere.
During day light hours ultraviolet rays from the sun ionised the denser atmospheric gases closer to
the earth, creating the D-layer. Radio waves entering this layer are scattered and absorbed.
Consequently the ground wave signal was usually stronger than the reflected signal. At night the
electrons in the ionosphere reunite with the ionised gases and become polarised, and the D-layer
disappears. The radio waves are then free to travel upward, where they are reflected by higher layers.
Consequently the reflected signals could be stronger than the ground wave signal at over the horizon
ranges.
The high frequency systems (SHF and UHF) were effected by atmospheric conditions which
included humidity, temperature and pressure.
The conductivity and permittivity (dielectic constant) of the surfaces over which the ground wave
travelled effected the ground waves used by low frequency systems (MF and LF). To improve the
efficiency of these systems a ground mat was laid at the shore antennae. To further improve the
signal reception some of these systems employed phase locking at the master base station where the
receiver monitored the transmission of the other slave stations. Whenever the phases started to drift,
the receiving station adjusted the phase of its own transmissions to match that of the incoming
transmissions.
3. Processing Principles
The commonly used classification processing principles employed by the radio navigation and
positioning systems are described below.
Processing Principle Description
Phase Comparison Measured the received signal relative to that of another reference carrier wave which could

have been generated by one of the following methods: a carrier wave received from another station; a
carrier wave transmitted from the vessel; or an internally generated carrier wave Pulse - Match (or PRN Code)
The receiver replicated exactly the pseudo random noise (PRN) code
which was cross correlated with the received signal to determine the
appropriate time shift
Pulse - Time Difference The timing differences in the transmitted and received pulses were
established
Pulse - Wavefront The pulse wavefronts in the transmitted and received pulses were
compared and aligned so that they remained perpendicular to the line
between the vessel and the fixed station..

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Some systems used a combination of pulse matching and phase comparison to achieve better
positioning.
The mode or method of computation depended on the type of lines of position (LOP) generated.
Some systems provided a compound mode, which allowed a combination of range/range and
hyperbolic patterns to be combined within the system receiver to generate a position.
The accuracy quoted were dependent on proper calibrations in accordance with good survey
practices before, during, and after the survey. The range quoted is the maximum effective range at
which acceptable positioning would have been achieved under typical circumstances.
4. Determining the Optimum System
The main points considered when determining the optimum positioning system for marine
geophysical, site surveys, rig positioning, and offshore construction surveys were as follows:
· availability
· frequency licences required
· maximum expected range
· required accuracy
· coastline configuration
· availability of adequate geodetic control
· number of users required
· cost
· logistics and related expenses
5. Transit Accuracy
The Transit system was the predecessor to GPS. However the system did not provide 24 hour
coverage and there were only 5 (in 1989) or 6 (by 1990) satellites available in the constellation
deployed. All were in polar orbits with heights between 1100 and 1200 kilometre, and with a period
of between 105 and 110 minutes. Due large precession of the satellites in their orbits, the orbits
planes moved with respect to each other and regularly crossed.
For instance, an observer in off Nova Scotia would have expected on average 14 to 18 useful passes
per day. Each pass lasted for up to 18 minutes and at least 15 satellite passes would be needed to
achieve acceptable final positioning, which might take 2 days or more to collect. This allowed the
point positions collected to be averaged with a good degree of certainty in the final co-ordinates.
Offshore positioning for a moving vessel was complicated by the north / south component of the
ship's velocity, which effected the relative radial velocity between the Transit receiver and the
satellite..

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If Transit was used, there would be the issue of the datum transformation values used by the
positioning contractor to go from WGS72 (a global datum in use from 1974) to the local datum.
There is also the possibility that a variant of WGS72 was used, such as NWL-9D (Naval Surface
Weapons Laboratory) or NWL-10F. These were slight improvements over WGS72. For most
offshore applications the datum transformation values for any of these systems would be considered
the same as for WGS72.
Doppler Technique Horizontal Positioning Minimum Number of Passes Required
Accuracy (1s s)
Point Positioning ±10 metres at least 15 acceptable Transit passes
Translocation ± 1 metre at least 4 acceptable passes common to each
Transit receiver with one mounted on the jack
up drilling rig and the other on a 1 st order
geodetic point (not suitable if vessel was
floating)
Translocation between ± 0.5 and ± 1 metre at least 17 acceptable passes common to each
Transit receiver.
After 'Doppler Positioning Techniques Applicable to Different Position Accuracy Requirements' in
Gregory J. Hoar, "Satellite Surveying", Magnavox, September, 1982, pages 4 to 14.
6. Some Common Radio Positioning Systems
The following information was compiled from a variety of sources and extracted from the
manufacturer's specification sheets. Some of the main sources were as follows:
- "Positioning and Mapping at Sea" incorporated into IPMS, the E&P Knowledge, and Learning
System produced by IHRDC of Boston, by Pieter G. Sluiter, with E.S. Sodbinow, Editor and
N.A. Arstey, Series Editor in 1989, pages 217 to 219.
- "Fixed Electronic Radio Navigation and Positioning Systems" by Offshore Navigation Inc.
(ONI) published in 1986.
About the Author
Bruce Calderbank has been involved in the offshore since 1978 after receiving his undergraduate
degree in surveying from University of New South Wales. After obtaining a postgraduate Diploma
in Hydrographic Surveying from Plymouth Polytechnic in 1980, Bruce worked for a couple of
companies before returning to Canada and setting up Hydrographic Survey Consultants Intl. in 1983.
He has worked in over 25 countries worldwide and can be reached at
bruce_calderbank@nucleus.com.