THE PROBLEMS OF RELIABLE LONG-RANGE TRANSMISSION 
OF REMOTE OCEANOGRAPHIC MEASUREMENTS 
by C. McLOON, Member Technical Staff 
Hughes Aircraft Company 
Los Angeles, California 
PRECIS 
Data telemetry transmission falls 
basically into two categories: line-of-sight and 
beyond-line-of-sight. When the transition to long 
range is made, entirely new telemetry techniques 
are required. This paper describes the problems 
involved with long range telemetry, together with 
some state-of-the-art solutions. 
INTRODUCTION 
With the advent of mass oceanographic 
measurements brought about by the emphasis the 
Government is placing on oceanography, tech- 
niques must change to handle the increased volume 
of data. One of the principal changes will be in 
the manner in which measurements are taken. 
Thetrend will be toward full automatic electronic 
systems wherever possible. This, then, will 
mean an expansion in the field of data telemetry. 
Because of the ''remote nature" of the 
oceans, remote measurements will be required. 
For year-round data collection automatic telem- 
etry systems will be used. At the field location 
this will entail an automatic examination of the 
sensor, coding of the information, and trans- 
mission of this coded information. At the receiv- 
ing station these data will be received, detected, 
decoded, and presentedor stored in some manner. 
These techniques are not new to the telemetry and 
communication industries but, considering the 
ranges involved, the application to oceanography 
is unique. The problems involved in the long 
range transmission of these measurements are 
discussed below. 
TELEMETRY SYSTEMS 
Radio telemetry appears to have had its 
inception! about 1930 by telemetering data from 
weather balloons in Germany. Since that day, 
the wordtelemetryhas become practically synon- 
ymous with aircraft data, missile data, and space 
data whereas, in reality, the word is synonymous 
with data transmission. In reading telemetry 
texts, published articles, and papers on the sub- 
ject, it would seem reasonable to guess that about 
99 percent of them make reference to radio 
telemetry application in one _ of the fields of 
science listed above. 
The work inthe above areas has advanced 
the field of telemetry using techniques such as 
AM-FM, PPM, PAM, and PDM, to name but a 
few. For reasons to be explained later, these 
systems are limited to use in line-of-sight VHF 
and UHF systems and certain scatter systems, 
but are not usable in HF systems. Beyond-line- 
of-sighttelemetryhas somewhat lost its associa- 
tion with the telemetry industry and is now termed 
data transmission systems. Unfortunately, this 
field has not received the attention given to line- 
of-sight telemetry and therefore is open for 
improvement. In general, there are presently 
three means of propagation that are used to one 
degree or another for beyond-line-of-sight radio 
telemetry: high frequency radio, ionospheric 
scatter, and tropospheric scatter. Each of these 
methods has certain disadyantages such as the 
extreme multipath problems® experienced in HF 
radio, a degree of multipath3 in the scatter sys- 
tems, and high power requirements of the scatter 
systems. 
It will be assumed that for remote 
oceanographic measurements an unmanned buoy 
of small size is to be used. Therefore, large 
power sources are out of the question from a 
physical standpoint, thereby eliminating scatter 
systems. This paper will detail the HF radio 
technique which is judged to show the greatest 
promise. 
HF TELEMETRY PROBLEMS 
Long range operationin the HF spectrum 
depends wholly on the ionosphere. The ionosphere 
functions to reflect radio waves of certain fre- 
quencies. Characteristics affecting the use of the 
ionosphere are: time of day (or night), season, 
ll-year cycle sunspot activity, frequency, and 
geographical location. Unexplained anomalies 
Superior numbers refer to similarly numbered references at the end of this paper. 
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