Permanent Committee on Tides and Mean Sea
Level
Site Selection Specifications for Tidal
Stations
Selection of appropriate sites for tidal stations is a
critical precursor to the acquisition of accurate height recordings.
The PCTMSL recommends that the following be taken into
consideration with regard to the selection of the site for tidal height
recording stations:-
1. tectonic/geological stability of the site;
2. stability of the supporting structure;
3. proximity to existing or specially installed high
stability benchmarks;
4. exposure of the site to the tidal regime of the
open ocean;
5. minimum exposure to tidal streams or currents
exceeding 0.5 knots;
6. water column stability, i.e. minimum exposure to
estuarine - river discharge effects;
7. minimum exposure to wave energy;
8. avoidance of proximity to headlands and harbours
with restricted entrances;
9. minimum subjection to siltation ;
10. minimum subjection to marine growth;
11. protection from vandalism;
12. proximity to mains power and telephone; and
13. ease of access:-
a. for servicing the
instrumentation: and,
b. to high-precision level
connection to stable benchmarks.
Attention to factors 1, 2, and 3 ensures that the
height datum will be stable over a long time providing readings that can be
used for many purposes well past the time at which they were made.
The environmental factors 4, 5, 6, 7, 8, 9, and 10 are
important. Their applic ation ensures that the recorded heights are truly
representative of the tide.
Convenience is important, factors 12, and 13.
Servicing tidal stations is an onerous task and any thing that makes it easier
for the technicians will be appreciated.
Factors 11 and 12 contribute to maintaining a high
level of serviceability from the station.
After considering all of the above factors, acceptance
of a compromise will ensure that the best possible tidal readings will be
obtained from the site selected.
The following is the statement of user requirements
upon which the selection of equipment for the Australian baseline sea level
monitoring stations was based. These specifications are provided as a guide to
the selection of high precision tidal recording equipment.
User Statement of Requirements
Instrumentation is required for the purpose of high
precision monitoring of the sea level. It is expected that the equipment,
supported by appropriate operating techniques, will be capable of measuring sea
level change attributed to the Greenhouse Effect.
The purpose of this document is to specify the
requirements of the complete Field Unit hereinafter referred to as the field
unit.
The field unit shall be a stand-alone, unattended,
data acquisition and data transmission device capable of acquiring, storing and
reporting water level measurements from remote locations. Low power consumption
and high reliability are important considerations in the design of the field
unit.
Instrument Requirements
The sensor/measurement subsystem of the field unit
provides for the measurement of water level and ancillary meteorological and
atmospheric phenomena necessary to fulfil the primary mission requirements for
tide and water level measurements.
The sensor/measurement subsystem is required to
accommodate up to eleven ancillary measurements.
Three of these are expected to be provided by sensors
that provide a digital or frequency output; the remainder will provide analogue
output.
The field unit shall be able to operate with different
site-specific combinations of sensor inputs and power sources as specified
herein. All possible sensor/power combinations shall be accommodated by on-site
actions, which include setting switches, installing cables, and entering
parameters via a temporarily connected operator's terminal or the public
switched telephone network.
The configuration of an individual field unit will
depend on the information requirements of the PCTMSL, as well as site-specific
factors such as environmental conditions. However, the minimum configuration
will consist of: -
Primary water level sensor
Seawater temperature
Air temperature
Barometric pressure
Wind speed and direction
Backup water level sensor
Resolution:
Primary Water Level: 0.0005 metres over a range of 0 -
15 metres.
Resolution: other sensors
Backup Water Level: 0.003
m over a range of 0 - 15 metres.
Temperature: 0.1
Deg. C over a range of -10 to +55 Deg. C.
Baro. Pressure: 0.01
hPa over a range of 800 to 1060 hPa.
Wind Speed: 0.5
m/s over a range of 0 to 50 m/s.
Wind Direction: 5
Deg. over a range of 0 to 360 Deg.
Accuracy of the Primary Water Level Sensor:
Height +/-0.005
metres.
Time +/-1
minute per year.
Datum +/-0.001
metres. This must be maintained for a minimum period of twelve
months between re-levelling.
Sample interval:
User selectable within the range 1 to 60 per hour.
The default value will be 12 per hour for the Primary
Water Level Sensor.
It is required that each sensor be capable of being
sampled at its own individual sample rate.
Integration time:
User selectable within the range 1 to 10 minutes, at a
nominal rate of two samples per second with a default value of one sample per
second. This must also be settable for each individual sensor.
Calibration:
Capable of regular calibration in the field, automatic
self-calibration is required for the Primary Water Level Sensor.
Timing:
The sensor/measurement subsystem must contain a
calendar clock capable of generating year, month, day, minute and second with
leap year correction. Time shall be resolved to at least one second. The
sensor/measurement subsystem shall, at least once a day, re-initialise its
calendar clock with time and date from the satellite telemetry module, (where
fitted) which is required to be more accurate.
The sensor/measurement subsystem must be capable of
recording a sample on the integral hour.
Watchdog Timer:
The sensor/measurement subsystem shall contain a
watchdog timer that can restart the field unit without loss of current system
parameters or stored data.
This timer shall start the field unit after a fixed
time delay unless the timer is itself reset by a software controlled signal.
The presence of the timer shall ensure that the field unit will always operate
when power is available, and there is no damage to the field unit itself.
Communications:
Five communication ports (four serial and one
telephone) shall be provided, one shall be dedicated to the Backup water level
sensor and one dedicated to satellite telemetry. The other two serial ports
will be available for parameter set-up, data collection and external modem.
A common protocol shall be used for the latter two
serial ports and the telephone port. This protocol shall be selected from
protocols currently in use and supported by multiple users in industry and the
government.
Telemetry:
Telemetry will be by telephone modem and/or satellite
transmission. Satellite transmission will be by use of one or more of the
following systems: GOES, GMS, ARGOS and AUSSAT.
Transmitter and telephone modem must conform to the
standards required by the appropriate licensing authority.
Data Storage:
On board back-up:
Type: Digital.
Medium: Solid-state
memory.
Output: RS232C
compatible with downloading facility, software to be provided.
Memory retention:
Thirty (30) days minimum, obtained from the following
representative configuration of sensors and measurement rates:
Primary water level sensor 12 measurements/hour
Six of the eleven ancillary sensors. 1 measurement/hour
Backup water level sensor 12 measurements/hour
All data acquired by the field unit through automatic
sensor sampling shall be stored as a sequence of individually identified data
entries. These data entries shall be stored in chronological order as they are
acquired. The acquisition date and time of each data entry, to nearest minute,
shall be available for each data entry. This date and time may either be stored
with the entry or obtained by computation. All memory not used for other
purposes shall be available for storage. When the storage has consumed all
available memory, the oldest data entries shall be discarded to provide space
for new entries.
Other types of data entries that shall be stored
include, primarily:
Field unit system
parameters.
Field unit performance
status.
The field unit data storage capacity shall be expandable,
on-site, by field maintenance personnel. The capacity shall be expandable by at
least 50 per cent.
Power:
The complete field unit must be run from an internal
rechargeable sealed battery. The unit must also contain a power conversion
module capable of powering the field unit from any one of the following
sources:
External 12v batteries.
Solar panels.
Wind generators
Single connection to a
commercial power service (216 - 288v, 40 Hz. to 60 Hz.).
This module must also be capable of charging the
internal batteries when connected to any of the above sources. The internal
battery compartment, whilst it may be contained within the same environmental
enclosure as the rest of the field unit, must have its own sealed enclosure,
which is vented directly to the outside atmosphere. The batteries shall be
sealed, shall not emit corrosive fumes, and shall be resistant to explosion.
They shall also be compatible with locally available types for replacement
purposes.
Primary Water Level Sensor
The Primary Water Level Sensor shall consist of an Air
Acoustic Sensor and two Correction Air Temperature Sensors.
Air Acoustic Water Level Sensor.
The air acoustic water level sensor shall be an
Aquatrak Model 2010-10-C or equivalent. This sensor shall use an active,
air-acoustic, ranging technique to measure water level within a Government
furnished protective well. The sensor shall have no active contact with the
water e.g. (float). Passive (static) contact, such as a protective tubing or a
signal duct, is acceptable. The sensor shall perform in the presence of
disturbances or other biological fouling, joints in the protective well, and
ambient acoustic noise.
The sensor shall measure water level in a protective
well that:
Will be a 0.1 to 2 m
diameter pipe open at the top and bottom.
May be fabricated from one
or more lengths of pipe to the length required by the station
conditions.
Will be vertical to within
5 degrees.
May not have smooth walls
or specially fabricated joints.
The sensor shall sample the water level at a nominal
rate of two samples per second and have a digital
output proportional to the water level. A sample is
defined as a single observation of water level. The
sensor shall have a dynamic response capable of
measuring to the specified accuracy with a water level
change rates up to 0.3 m/sec.
The field unit shall measure water levels via the
Primary water level sensor at selected times and
selectable rates as defined above. Each measurement
shall be associated with an acquisition date and
time (time-tag) computed to the nearest full minute.
A measurement shall consist of a mean water level
value, two data quality assurance parameters, and
two air temperatures for correcting measurement errors
caused by temperature profile variations along
the acoustic path.
The mean water level and data quality assurance
parameters values shall be computed from a
selectable fixed number of sequential samples acquired
from the sensor, acquisition of this sequence of
samples shall start before the time indicated by the
time-tag, and shall be such that the middle of the
sequence occurs at the time indicated. Timing shall be
synchronized so that one of the time tags will be
on the integral hour.
To allow for the most flexible operation of these
instruments over their expected life, as much as
possible of the operation and computation should be
done under software control. As an example, the
mean water level computations could be made as
follows:
1. Compute the mean of the acquired sample sequence.
2. Compute the standard deviation about the mean.
3. Count all samples (outliers) that differ from the
mean by more than three times the standard
deviation.
4. Remove the outliers from the sample sequence.
5. Recompute the mean water level.
6. Recompute the standard deviation.
The value computed in step 5 shall be stored as the
mean water level. The standard deviation computed
in step 6 and the outlier count in step 3 shall be
stored as the data quality assurance parameters values.
The contractor will be required to define the
algorithm that is proposed to compute the mean water
level.
The Primary water level sensor must have been
designed, manufactured, assembled and tested for the
marine environment.
The Primary Water Sensor must have a demonstrated fie
ld history of satisfactory performance of at
least 12 months in use in a national/international
monitoring program.
Redundancy of Water Level Sensor and Data Logger
The field unit shall also record water level by
acquiring serial data from a Backup water level logger,
which shall use a pressure transducer to determine
water level. This sensor will be mounted separately
from the Primary water level sensor.
The Backup water level logger shall be capable of
operating independently of any other component of
the field unit. There shall be no other connection
between it and the rest of the field unit.
The unit shall meet the following specifications:
Measure water level by
sampling and processing water pressure.
Compute water level and
data quality assurance parameters data by the same method as that used for the
Primary water level sensor.
Automatically compensate
for local barometric pressure.
Transmit water level and
data quality assurance parameters data periodically or upon demand
serially.
Contain a calendar clock
with equal resolution and accuracy as that in the sensor/measurement
subsystem of the field
unit.
Range: 0 to 15m.
Accuracy: +/- 0.015m.
Resolution: 0.003m.
Operate in the same
environment as that defined for the Field Unit.
Pressure sensor shall
withstand submersion to a depth of 30m without damage.
Operate for one year
without service
Provide internal solid
state data storage for the most recent 90 days of data.
Performance Monitoring
The field unit shall monitor its performance,
non-invasively, shall encode the results into a status data
set at the beginning of each hour, and shall store
that set along with a time-tag.
Performance monitoring shall be done by a dedicated
software task that is executed hourly and upon
operator request. As a minimum, the following tests
shall be performed:
Program memory check.
Random access memory check.
CPU check.
Telemetry interface check.
Battery voltage.
Charge rate (if external
power is in use).
AC power-interruption (if
AC power is used).
Other hardware/software
checks, as required by system design.
The above tests shall also identify and record fault
location, such as memory addresses and
communication ports.
Interchangeability
All circuit boards, modules and sensors providing the
same function(s) shall be completely
interchangeable among the various modules and units
without modification or adjustment.
Maintenance
The field unit shall be designed and constructed for
reliable and simple maintenance and operation in
the specified environment.
The design of the field unit and the maintenance program
shall minimise the use of special facilities
and test equipment. The contractor shall provide
specialised test equipment which is designed to assist
in the installation and check-out of the field units
if they determine it is necessary to meet the required
specifications, and is advantageous to the program.
This equipment shall obtain power primarily from
its own internal batteries or optionally from the
power conversion module of the field unit.
Minimum Operating Periods
The field unit shall be capable of fully automatic and
unattended operation at a remote site for an
operating period of not less than one year without
replenishment of expendables or preventative
maintenance.
The internal battery shall operate the field unit
without loss of performance or data during interruption
and re-establishment of commercial AC power for a
period of at least 10 days with the following
representative configuration of sensors and
measurement rates:
Primary Water Level Sensor
12 measurements/hour.
(includes level, data quality assurance parameters, and two
correction air
temperatures)
Backup Water Level Sensor
12 measurements/hour.
(including level and data quality assurance parameters)
Six of the eleven ancillary
sensors
1 measurement/ hour.
The field unit shall be able to repeat the ten day,
no-power operation after recharging its internal
batteries for a period no greater than 24 hours.
Furthermore, the field unit shall be able to operate on
this 10 day, no power, 24 hour recharge cycle with a
full complement of 15 sensors (primary water,
backup water, and eleven ancillaries) by adding more
internal batteries in the field. The number of
additional batteries required shall be determined by
the contractor.
Operating Environment
The field unit will operate in the following climatic
conditions:
Ambient Air Temperature: -10 to +55 deg. Celsius
Air Temperature Change: 20 deg. Celsius per hour
Water Temperature: -2 to +35 deg. Celsius
Salinity: 0 to 50 parts
per 1000
Humidity: 0
to 100% condensing
Wind Speed: 0
to 50m/sec, with gusts to 60m/sec
Water Currents: 0
to 2.5m/sec
Shock/Vibration Resistance: Resistant to 2g at 5 to 50 Hz.
Moderate sand and dust, salt spray and humid salt air:
Electromagnetic:
Capable of operating in an
electromagnetic environment that includes commercial radio and
television broadcast
signals, UHF and marine radio transmission and reception and marine radar
transmissions and stronger interference up to one volt per meter from several
sources:
UHF Radio: 400 to 420 MHz.
VHF Radio: 30 to 170 MHz.
Radio Beacons: 280 to 300
KHz.
HF Radio: 0.5 to 30 MHz.
Radar: S-band 2 to 4 GHz.
C-band 4 to 8 GHz.
X-Band 8 to 12 Ghz.
The field unit must have adequate protection against
damage caused by indirect lightning strikes and
static discharges, with particular attention to sensor
lines, antenna leads and any power lines.
Techniques such as opto-isolation, transformer
coupling, surge diverters or combinations of these, in
addition to adequate earthing and shielding
techniques, shall be employed. An earth stud or terminal of
adequate current carrying capacity shall be provided
on the outside of the field unit for connection to
an earthing system. All surge protection circuits
should be directed to this point.
In addition the water level sensors, which are part of
this field unit specification, will be exposed to
turbidity, biological fouling and water borne marine
life expected in the tidal areas of Australia and
surrounding territories.
Construction of Field Unit
The field unit must be an integrated unit of modular
construction and housed in an environmentally
protective cabinet.
Integration need not extend to the sensors which may
be remote from the field unit itself, however all
sensor and communication signals shall enter the field
unit through an interconnection sub-unit, which
shall consist of four components:
Weather-proof entry.
Terminal block
Suppression circuits
Connector for cable to the
sensor/measurement subsystem.
Water-proof plugs or water-proof cable glands shall be
used to ensure the waterproof integrity of the
unit. The terminal block shall have sufficient
capacity ( terminals and space ) to connect cables from a
full complement of sensors plus 20 per cent spare
capacity. Suppression circuitry shall be provided for
all terminals on this terminal block.
The connector shall provide connections for a full set
of sensors plus 20 per cent spare capacity, all
wiring between the connector and the terminal block
shall be installed.
If the field unit has provision for cable connection
or entry through its bottom surface, then feet shall
be attached to that surface so that cables can connect
or enter without undue bending when the
enclosure is placed on a work-bench or floor.
Each component located within the enclosure shall be
easy to install and remove. Access shall be
provided for the connection of all cables internal to
each enclosure. All internal controls and displays
shall be visible, accessible and clearly marked when
the cover is open.
Each enclosure shall be weather-proof when permanently
attached covers, caps, or lids are closed. All
spaces containing electronics shall be at least
weather resistant when operating with the covers open.
Each enclosure shall have tabs or other provision for
wall mounting.
The Barometric Pressure sensor if housed in the
environmental cabinet must be vented to the outside
by means of a static head connection.
Vertical reference
The Primary water level and Back up water level
sensors will be referenced to a set of high stability
benchmarks by direct levelling. Each sensor shall have
a permanent reference point suitable for use
with surveyor's levelling instruments. This point
shall be both visibly distinct and physically accessible
so that a high precision levelling staff may be held
against it.
This point shall be located at a fixed, known distance
from the sensor mounting holes (or pins). This
fixed distance shall be the same for all sensors;
thus, sensors can be replaced, on-site, without resurvey
(levelling) to the local benchmark.
Ancillary Sensors
The field unit shall be capable of acquiring and
storing input signals from a minimum of eleven
sensors simultaneously. Eight input ports shall be
provided for analog voltage signals, and three input
ports shall be provided for pulse stream of frequency
signals. As well as the mandatory sensors already
listed several other types of oceanographic and
meteorological sensors may be utilised, typically:
Humidity Water Current
Speed
Rainfall (Tipping Bucket) Water Current Direction
Water Conductivity Water Density
Not withstanding the specifications already stated,
the Ancillary sensors will, where possible be
selected from a range in accordance with Bureau of
Meteorology specifications AS2659.
All cables necessary to interface the sensors,
external power sources and antenna to the field unit are to
be supplied.
Transport
The field unit will be subjected to the following
conditions during transport to and from sites:
Temperatures: -40C to +60C.
Humidity: 0-100% condensing.
Shock: 1 meter drop.
Vibration: 2g, 5 to 50 Hz.
As these field units will be manually handled and
located in remote areas, they could be transported by
one or more of the following means: air freight,
truck, four wheel drive vehicles, all terrain vehicles,
car, survey ship, launch, small boat, light aircraft,
helicopter and human carrying. They should
therefore be of a size and weight to meet any such
carrying restrictions and be easily handled by
preferably one but no more than two persons.
Mandatory Documentation
Software/Firmware documentation:
Program specifications:
User Manuals: and
Program Maintenance
Manuals.
Technical Manuals ( including circuit diagrams).
Field Unit;
Backup Data Logger: and
Each additional
environmental sensor.
User Manuals
Field Unit;
Backup Data Logger; and
Each additional
environmental sensor.
Test Reports for each component must be supplied.