Aerodist in Australia, 1963-64
by J. D. Lines
Introduction
Aerodist is a development from the Tellurometer system used for ground measurement of geodetic distances. It is designed for simultaneous measurement, during movement, from an airborne station to two or more stationary ground stations.
The major differences from Tellurometer equipment occur at the airborne station where: -
a) the directional parabolic reflector of the antenna system has been replaced by a small flat plate reflector which can be rotated relative to a fixed housing,
b) provision has been made for continuous simultaneous recording of distance as a function of time by the use of a 3-channel chart recorder,
c) for various reasons the carrier frequency has been changed from 3,000 mc/s (ED – megacycles per second, today is one million hertz or MHz) to the band of 1218-1298 mc/s and radiated power greatly increased by incorporation of an r.f. amplifying stage.
Aerodist is designed to be used for: -
a) long range extension of horizontal control,
b) aircraft positioning in aerial surveying.
Extension of Horizontal Control
The equipment can be used for the rapid and accurate long range extension of horizontal control either by: -
a) distance measurement between ground station and ground station using the aircraft as an intermediate station and the line-crossing technique,
b) fixing an unknown ground station from two known ground stations using an aircraft and continuous trilateration,
c) fixing two unknown ground stations from two known ground stations using an aircraft and continuous trilateration.
Fixation of Air Stations for Aerial Surveying
Aerodist provides an aid to aerial surveying by accurately positioning an aircraft during flight, either continuously or at an instant of camera exposure. The system may also be used for determining the height of an aircraft or a spot elevation. The methods are: -
a) using a two channel system to provide slant distances from two ground stations, the height component being determined by other means,
b) obtaining a "fix" in space as a function of three slant distances by use of a three channel system,
c) using a three channel system, two of which are used to measure slant distances and the third channel is used for direct height measurement over a ground station at a known height. In this latter case, the ground antenna is set vertically.
Note: - A "channel" consists of an airborne master station and a ground remote station. Each remote station is tuned to a frequency 33 mc/s higher than its master and all channels must be tuned to provide a minimum frequency spacing between them. Thus every station in a system has a unique frequency. Each channel is referred to by a colour code, i.e., red, blue, white.
Scope of Paper
In Australia, Aerodist has so far been used by the Division of National Mapping primarily for the extension of horizontal control for mapping, using a helicopter and employing the line-crossing technique to successively trilaterate quadrilaterals each 1° x 1° in arc.
All trilateration is to be contained, for adjustment purposes, within loops of the National Geodetic Survey of Australia. This survey and the adjustment have been discussed in papers by Mr B. P. Lambert (1) and Mr A. G. Bomford. (2)
In New Guinea, the Royal Australian Survey Corps has used another set of Aerodist equipment installed in a fixed wing aircraft to trilaterate quadrilaterals each 30' x 30' in arc.
This paper will report on the work carried out by the Division of National Mapping during the period 1963-64.
Preliminary Trials
After receipt of the equipment (a two channel system) from the Australian agents and the arrival of the manufacturer's engineer from South Africa, the equipment was bench checked for crystal frequencies and general performance. This check was generally satisfactory and previously prepared plans for ground and airborne trials were finalised, and field tests commenced in. June 1963.
Ground Trials
These were used to further check equipment serviceability and to compare the length of an Aerodist measurement with that of a Tellurometer when using similar ray paths.
For these trials, all the airborne equipment, except antennae, was fitted to a shock-mounted frame installed in an International 4 x 4 1-ton vehicle. The antennae were mounted on a rack on the canopy See figures 1(a) and 1(b). The 28V power source was a series of accumulators.
Figure 1(a)
Figure 1(b)
Trial 1
Four remote stations were placed in line at equal distances of approximately 0.25 mile (400m) from the master station. The resultant measurements were acceptable.
Trial 2
Was conducted over a specially selected line between Bacchus Marsh and Geelong in Victoria. See figure 2. This line is approx 36 miles (58km) and has been measured on the ground as one line and also in two parts with Tellurometer equipment. It is intersected by some roads visible from both terminals and is in an area where trials would not interfere with existing V.H.F. and U.H.F. services.
Static tests on line, and simulated line crossings using the vehicle in lieu of an aircraft, yielded measurements to within ±1.5 metre of the accepted lengths.
During trials 1 and 2, various minor adjustments were made and having established a degree of confidence in the Aerodist, the master stations were transferred to a helicopter.
Airborne Trials
Trial 3
These trials were conducted in various phases and over the lines shown in figure 2. The trials occupied five weeks, and included about 50 hours flying. The remaining time was spent in reducing and analyzing results, rectifying faults, adjusting signal strength and correcting other "teething troubles''.
For these trials, a Bell 47J helicopter was obtained on charter.
This type has sufficient cabin space to mount the equipment and required little modification apart from the addition of mounting frames. The configuration of the installation used was as follows: - See figures 1(c) and 1(d).
Figure 1(c) Figure 1(d)
a) 2 Channel Master Units: On an anti-vibration mounted triangular frame alongside the pilot's seat with fronts tilted up to allow operation from right hand passenger seat.
b) Pen Recorder: Anti-vibration mounted on the passenger seat behind the pilot.
c) Antennae: Bolted on a rigid platform fixed externally to the rear of the skids on each side of helicopter.
d) Triode and Junction Boxes: On the floor below the pen recorder.
e) Aerial Control Boxes: Fixed on top of their respective master units.
Most of the lines shown in figure 2 were successfully measured, but of these, only the line Shadwell / Porndon is considered unsatisfactory and this is no doubt due to terrain effect at the Porndon end. This hill has a sharp down-grade from any suitable instrument stand-point and the chart records from this station, irrespective of aircraft altitude, were consistently poor.
In all these airborne trials, line crossings were taken at various altitudes in increments of 500 feet, and the altitude where signal strength from both ground stations was at a maximum provided the best chart records.
It was also evident that the means of determining height of the aircraft and vapour pressure, in particular at the airborne station, could well be improved.
Of the longer lines, the 153 mile (245km) line was unsuccessful owing to one remote failure, but judging by signal strength from the other remote station there is little doubt that this distance is possible. The 92 mile (147km) line was completed despite bad terrain effects, and the 120 mile (192km) line was an unqualified success, as were height measurements over remote stations with the dipole in a vertical position.
The concluding test in these trials took place over the quadrilateral Bacchus Marsh, Geelong, Porndon, Monmot Hill. The six lines were satisfactorily measured in one day and all equipment functioned perfectly.
Being now satisfied that the operators, field planning and equipment had been worked up to operational standards, preparations were put in hand for the first operation, which was to fix horizontal control for 1:250,000 mapping in Queensland.
Field Operations, 1963
Field operations in the latter half of 1963 were conducted in the Bowen Basin area of Central Queensland. In these operations, the same Bell 47J type helicopter was used in the configuration as set out for the trials, and the surveying method adopted utilized the line-crossing technique to measure all the sides of a series of contiguous quadrilaterals.
Two airborne master stations and four remote ground stations were available to the field party. Each master had two remote stations available for operation with it. This proved a handicap in operations, as it demanded excessive movement of ground stations compared to that theoretically possible with a 3-channel system.
Each vehicle in the field party is equipped with a 25-Watt H.F. Transceiver which provides not only ground to ground intercommunication but also ground to aircraft facilities in the event of loss of signal from the master stations in the aircraft.
Some of the limitations of helicopter operation became evident during these operations as a combination of lines over 100 miles long, flight altitudes of 7,000 feet and hot weather were experienced. Loss of contact with the ground was also a hazard at these altitudes.
During 6 weeks of operations 13 quadrilaterals were satisfactorily measured and four comparisons were made with geodetic lengths which had been previously measured with Tellurometer equipment.
This operation continued with only one or two minor equipment troubles which were easily remedied in the field by the survey staff.
As a result of this experience, it was decided to purchase additional equipment to complete a 3-channel system. Normally one master is the "control" master and the others are "auxiliary" masters, but the third master purchased was to be modified so that it could be readily converted from "control" master to "auxiliary" master and vice-versa. This ensures continuity of operation in the event of partial equipment failure.
All new lines measured in this survey have been computed and show a mean spread of values of 33 parts per million for an average of 5 measurements per line. This spread should be compared with the comparisons against known lines in the Bowen Basin set out in Table "B".
Adjustment of this work, and also that of the 1964 work, is awaiting the completion and final proving of the "variation of co-ordinates" programme being developed in the Division for use on the new CDC 3600 electronic computer at the Commonwealth Scientific and Industrial Research Organisation (C.S.I.R.0.), Canberra. The primary purpose of this computer programme is the adjustment of the National Geodetic Survey of Australia.
1964
Early in the year the third (white) Aerodist channel was received and delivered to the servicing agent for acceptance tests. At the same time the blue and red channels were sent for servicing and frequency change to bring the units on to suitable frequencies for the 3-channel operation.
During the servicing period the configuration in the helicopter was redesigned to improve handling. In the new configuration, a vibration-insulated two-unit mounting frame is positioned in the centre of the aircraft against the rear fire wall. The units are placed in this frame with the control panels upwards.
The three-channel recorder is mounted just above floor height immediately ahead of this frame and behind the pilot's seat. The triodes were removed from the existing three-unit box and placed in individual boxes mounted on the floor under the right hand passenger's seat to facilitate changing triodes when mounting different colour code units.
Crossover coaxial switches are placed in the antennae leads so that each unit can be operated through either colour code antennae. See figure 1(e).
Figure 1(e)
Antennae Switching
Previously, due to the mounting of the antennae abeam the helicopter, the fuselage obstructed half the circle of rotation of each antenna. This limited line crossings to one direction relative to the ground stations, doubling the amount of flying time required on a line crossing. It was also frequently impossible to maintain Aerodist contact with ground stations during the approach to a line crossing.
Placing cross-over coaxial switches in the antennae system enables each antenna to operate with either master station, where previously each antenna was directly connected to one master station. This permits line-crossing measurements to be made in rapid succession on forward and reciprocal courses relative to the ground remote stations.
With these switches it is possible to establish contact with the ground stations when the aircraft is away from the line, enabling the remote operators to follow the master station down to the line while the master operator seeks optimum height and/or position on the line. It is only necessary to give a short break in the measuring trace to advise the remote stations to hold direction at the instant of crossing.
Inter-communication
During the 1963 operations, contact between the master operator and pilot was possible only by shouting, a tedious business in a noisy helicopter. The design and construction of an intercommunication unit capable of connecting 2 master operators and the pilot and also capable of being switched through the Aerodist channels to the remote stations was commissioned this year.
Considerable difficulty was experienced with this unit when attempts were made to use it through the Aerodist and it was eventually used only as an intra-aircraft unit. Modifications will be carried out before the 1965 operations and it is expected to then have full inter-communication between all personnel through the one unit.
Airborne Psychrometry
As previously stated, the determination of vapour pressure used in 1963 left room for improvement. The problem was discussed with the Applied Physics Division, C.S.I.R.O., who agreed to design a more suitable apparatus.
The method used in the initial operations was to ascertain the depression of the wet bulb and the dry bulb temperature of the outside air by means of an Assman-Lambrecht psychrometer. As can be imagined, the difficulties of holding this equipment in the slipstream and reading a fine mercury thread in a hot cabin some seconds later, led to inaccuracy.
C.S.I.R.O. produced a prototype equipment for use in the 1964 field season. This embodied a vented and baffled venturi to reduce the velocity of air flow and prevent dynamic heating effects. The depression of the wet bulb is determined by means of thermo-couples and the output is amplified to a suitable display by means of a D.C. amplifier. This data is referenced to an insulated mercury-in-glass thermometer inside the aircraft. Readout is by means of a switched meter.
With some modifications born of experience, a fully engineered equipment will be manufactured for the 1965 operations. This new unit will be suitable for aircraft speeds up to 160 knots.
Needless to say, this unit will be far more efficient and will enable reliable meteorological soundings to be taken. It will be interesting to examine the effects of a vertical profile of the refractive index when applied to the distance computations. When considering the three main sources of error in this type of equipment, i.e., meteorological factors, ground reflection and instrumentation limitations, the critical and practical surveyor might well have reservations on the value of further complicated refinements after examination of the comparisons shown in the tables at the end of this paper.
Operations
In April (1964) field operations commenced in southern Queensland, but from the outset the equipment performed unsatisfactorily and adjustments proved to be beyond the capability of the field personnel. A technician from the servicing agency joined the party for two weeks but he also was unable to make the equipment function using only field facilities. The Aerodist was returned to Melbourne for workshop service.
A complete realignment of the equipment was carried out in Melbourne and it was again operational by early July. Figure 1(f).
Figure 1(f)
The Surat Basin control was recommenced at Bourke, N.S.W., in early July and concluded in September, with only a few minor interruptions. The equipment, to date, has provided good serviceability provided it receives first class workshop attention followed immediately by a local air test and calibration on a test range.
Lines on which a direct comparison to Tellurometer traverse distance was possible were available on the southern, south-western and eastern boundaries of the survey area and these provided good calibration and comparison distances for the first half of the survey.
Additionally, the final line measurement was of known length. The northern and western boundaries were each connected to first order stations at two points.
The scheme was made up of approximately one degree grids with three additional points required for oil lease survey control and the figures built up were braced quadrilaterals. The line-crossing technique was used throughout with an average of 7 crossings to each line. Where possible, crossings were made at two or three different heights on each line to obtain alternative ray paths and to minimise errors due to terrain effects.
Due to shortage of personnel it was not possible to use the new white channel operationally at all times, but it was used on three lines and proved successful. The white master unit is so designed that it can be used as either a control master, or with the measuring oscillator, tone oscillator, gating and speech cards removed, as an auxiliary master. It was used in both roles. An attempt to use the red and white units as independent control masters was unsuccessful due to the excessive cross-coupling effects between the signals.
No major equipment failures were experienced during this survey although the unreliability of the thermal writing recorder pens was a source of delay. Early in the survey, the pattern switching levels of all remote units was increased to a maximum, and to a large extent cured the intermittent B, C and D pattern failures associated with low signal strengths.
Note: - In Aerodist, the A pattern is the primary measuring wave and is the wave form almost continuously recorded on the chart. This indicates the final two digits of the slant distance measured to a ground remote station. All distances are measured in metres. This A pattern is switched out at approx. 6 second intervals, and the three remaining patterns B, C and D, are switched in for a fraction of a second. The B, C and D patterns provide the "coarse" readings, i.e., the initial digits of the measurements.
During a visit to the field party by an engineer from Tellurometer Ltd, the modulation level of the red master was increased with a significant improvement in quality of trace.
In all, the survey provided control for about 90,000 sq miles; 18 new stations were established and 110 lines were measured. The duration of the survey was 9 weeks excluding station establishment.
The limited load capacity and endurance of the helicopter was an inconvenience although this was alleviated by using only one operator on the airborne equipment. With one operator, the activity during a line-crossing is hectic and makes blunders likely. This is a practice which should be avoided if possible. It is planned to carry out future operations in a twin-engined fixed wing aircraft with the three channel Aerodist installation and camera for spot photography.
The aircraft to be used will probably be an Aero Commander 680E. This is a modern high performance aircraft and will permit meteorological soundings and many more Aerodist height checks to be made, in addition to providing more data for a given financial outlay than previously. Fixed wing aircraft pose fewer logistic problems than helicopters and the operations can be conducted with a greater margin of safety, and a survey crew adequate for all contingencies.
Aero Commander’s 680E VH-EXY (1965) and 680FL VH-EXZ (1966-74)
The 1963-64 survey is illustrated in figure 3 and the operational statistics in figure 4.
Figure 4
Establishment of Aerodist Ground Stations
From the table of Aerodist statistics, it will be seen that reconnaissance, marking, air photo identification and heighting absorb something like 40 per cent of the manpower requirement in the field operations.
Aerodist stations, unlike classical trigonometrical survey stations, can be sited in locations convenient to communications and developing areas. In practice, all weather vehicle access is desirable as the rate of movement on the around generally controls the pace of the survey.
Restrictions on station sitings are few, a very shallow angle of signal "get-away" being probably the most important, particularly in timbered areas. The chain saw is used very sparingly.
Air photo identifications are made by the reconnaissance parties and these are confirmed by spot photography from the aircraft where possible. (3)
Heighting of the ground stations has been carried out in two ways, either by instrumental levelling with automatic levels or by barometric levelling using barometers of the type fitted with a cathode ray indicator.
Most of the stations established to date have been instrumentally leveled from convenient bench marks of the National Levelling Network, which is rapidly being extended over the Australian Continent. Progress to date is shown in a paper by Mr K. Leppert. (4)
Reduction of Data
Data from the airborne station consists of the paper trace of the ranges to each ground station recorded by the automatic chart recorder which has provision for up to three simultaneous records. A sample of a chart is shown in figure 5.
Additional data available includes wet and dry bulb temperatures of the outside air, altimeter readings, aircraft heading, line identification, time of day, and other manually recorded data.
At the ground station data recorded includes altimeter or barometer readings wet and dry bulb temperatures, height of dipole, station eccentricity and reference mark data.
In line-crossing reductions, the charts are examined and pairs of distances, one from each ground station, are extracted and summed for the same number of intervals on either side of the actual line-crossing. As the sums of the distances measured on a line-crossing, graph as a parabola, twenty-one sums, ten on either side of the adjusted minimum are graphed and then computed by the method of least-squares to obtain the theoretical minimum. This may vary from the minimum shown on the graph of the twenty-one sums but the mathematical minimum is considered the better value primarily due to the averaging of possible terrain effects. See Figs 6 & 7.
Figure 6
Corrections must be made for aircraft height, ground station height and eccentricity, refractive index and reduction to sea level arc distance.
Computation Statistics
Computations for the determination of distance by the line-crossing technique have been fully programmed for electronic computing.
Although this is a great help, a large volume of preparatory work remains in breaking-out the charts and checking the extraction of results prior to writing up the data for the computer.
Office work required for reducing heights, computing eccentric corrections and assembling co-ordinate data for the 1964 survey amounted to approx. 40 man-days, and approx. 320 man-days have been necessary to obtain and write up the data for the computer.
Results
At the time of writing, the processing and examination of the complete results of the 1964 survey is in progress. A summary of distance measurements should be available by April 1965. The figure adjustment and preliminary co-ordinate values will be available as soon as possible after the final proving of the variation of co-ordinates programme which is being developed for the adjustment of the National Geodetic Survey. This latter adjustment will introduce small corrections to existing co-ordinate values of the geodetic stations used to contain the Aerodist work.
Preliminary Assessment of Results
In assessing the results of the work done to date, it is considered that valid comparisons could only be made between lines directly measured both by Aerodist and by Tellurometer traverses of the National Geodetic Survey. Such comparisons are set out in the following tables "B" and "C".
Table "A" concerns the initial trials in Victoria. The distances marked with an asterisk are direct Tellurometer measurements, and the other distances in this table are derived from a mixture of first, second and third order stations. The Tellurometer distance, Porndon-Monmot Hill, was measured with a model MRA2 fitted with 48 inch diameter reflectors.
All the distances in tables "A" "B" and "C" have been derived by inverse solution from the geographical co-ordinates of the terminal points using Sodano's long line formula.
From the results so far achieved it would appear that lines 100-150 kilometres in length can be measured with a standard deviation of ± 1 metre.
Conclusion
The work carried out to date encourages the hope that first order distances can be obtained with this equipment. Canada (5) has reported that tests in that country indicate that Aerodist is capable of producing first order length measurements.
Future operations are to be conducted in a twin-engine fixed wing aircraft fitted with improved ancillary instrumentation for more precise determination of refractive index and altitude. Given this greater mobility, additional Aerodist height checks and the improvement in technique which comes with experience, there seems to be no reason why with observations carried out on two different days (as with first order ground Tellurometer work), first order standards should not be reached.
Future tests will have as their object the determination of the optimum amount of flying and number of observations required to establish detailed mapping control.
References
(1) B. P. LAMBERT, "Laplace Observations in Geodetic Survey", Australian Surveyor,
Vol. 20, No. 2, 1964.
(2) G. BOMFORD, "Geodetic Adjustment of Australia", Australian Surveyor, Vol. 20,
No. 2, 1964.
(3) J. D. LINES, "Spot Photography for Map Revision", Cartography, Vol. 4, No. 4, 1962.
(4) K. LEPPERT, "Procedures and Instruments for Third Order Levelling", Australian
Surveyor, Vol. 20, No. 8, 1965.
(5) S. A. YASKOWICH, Department of Mines and Technical Surveys, Canada.
(Unpublished.)
A paper presented to the 8th Survey Congress, Canberra, by J. D. Lines, M.I.S.
Aust., M.A.1.C.
THE AUSTRALIAN SURVEYOR, June, 1966 733-751