TECHNICAL FIELD

This invention relates to a temperature differential indication apparatus and method, used to indicate thermal activity within a volume of fluid, specifically air. That information may then be used by pilots of hang-gliders or gliders as an aid to prolonging flight of those craft already airborne, or as an aid to deciding whether or not it is worthwhile to launch a hang-glider or glider and where that launch should take place.

BACKGROUND ART

It is well known to transmit sound into the atmosphere and to detect and measure the magnitude and time of an acoustic reflection of the transmitted sound so that information can be obtained as to the extent and location of atmospheric anomalies which effect the acoustic reflection. These systems are known as incoherent and Doppler acoustic sounders and are commonly designed for measuring atmospheric boundary layer structure and depth, as well as measuring winds in the atmospheric boundary layer. The systems can be either monostatic or bistatic, the relevant configuration determining the predominant scattering mechanism causing the acoustic reflection.

In the case of monostatic systems, which is the preferred configuration used in this invention, it is well known that the principal acoustic reflection mechanism is temperature differentials, which exists in the propagation direction of the transmitted sound. Bistatic systems, on the other hand, are principally designed for the purpose of detecting wind shears and have been known to be located adjacent larger airports. Bistatic systems also measure temperature differentials, but this information is embedded in the wind shear measurements.

The purpose of these existing Doppler acoustic sounders and methods used to extract data are to get specific and detailed information to enable the determination, with good resolution, of the heights of stable layers and temperature inversions. In order to achieve such detailed resolution, it has been necessary to try and generate systems with sufficient power to obtain

useful measurements throughout the depth of convective boundary layers or in non-turbulent conditions.

Therefore, it follows that such systems need to be relatively large, require significant power sources, are essentially expensive both from the apparatus required to generate the sound and effect its delivery, and are prone to interference with other acoustic sounders due to the large emission powers.

In summary, the existing acoustic sounders are designed to look for fine resolution, both spatially and temporally, use necessarily large components, require significant powers and are thus relatively expensive.

We have discovered that it is possible to build an apparatus which is adapted to use acoustic sounding techniques for gaining information that has for its purpose to establish only an indication of thermal activity within a target area or volume, by detecting temperature differentials.

Once one realises that this information is a very valuable asset indeed to some in our community it becomes possible to devise a relatively simple and economical apparatus which has a very useful purpose.

Those wishing to fly hang-gliders or ordinary gliders are very much interested in the degree, location of and temporal change in thermal activity. Specifically, those that fly the above mentioned craft rely very much on the presence of rising air currents, known as thermals, to stay aloft for any significant periods of time, or at the very least to prolong the time spent aloft. Currently, it is a matter of luck and experience to be able to determine if there are any thermals in a particular area, which can then be used as an aid to flight.

This invention has for an object an apparatus and method which are more economic to produce or use and help locate and detect thermals in the atmosphere by using acoustic sounding to determine the presence of temperature differentials which can be interpreted to indicate the presence of thermals.

The invention can not only be used on the ground to determine whether

thermals exist through a particular depth of the atmosphere but can also be mounted on a hang-glider or glider to determine the presence of thermals within a given area of use. This is a very useful feature for it enables a person wishing to fly a unpowered craft to locate an area with preferential air current conditions, without having to actually become airborne, and locally determine the conditions.

The invention may be further used as a remote sounder by including a radio transmitter that transmits information about the thermal conditions, that information being transmitted throughout a desired area. Thus, conceivably, a remote sounder may be placed at some distance away from a dwelling or temporary location enabling one to receive information about the existence or lack of thermals in the area above the remote sounder whilst at the dwelling or temporary location thereby enabling one to make a decision whether or not to launch a glider without necessarily being present at the location of the sounder by having the information gathered by the remote sounder whilst spatially separated from the remote sounder.

In addition, there well may be a number of acoustic sounders each positioned spatially apart to each other, and each of which transmits data collected by itself enabling persons to receive information about the existence of thermals throughout various areas and thus choosing perhaps the most optimal or promising location to fly. Furthermore, there well may be a receiver on the glider itself adapted to receive information from one or more of the acoustic sounders. Thus an already airborne person may be provided with sufficient information to decide to fly to a specific area so as to 'catch' the most favourable thermals. There could very well be a network of remote sounders tens of kilometres apart providing this vary valuable information indeed.

It must be remembered that this invention is not concerned with obtaining a detailed resolution of temperature and/or turbulence but is concerned with indicating whether within a certain volume there are temperature differentials which then infers thermals. Furthermore, this apparatus is then used to indicate the extent to which these thermals sustain themselves.

In this application it simply doesn't matter that the thermal activity extends to 50 metres or 200 metres away (in height), but it does matter that the volume

sampled has a degree of activity and when this is sampled on a regular basis it provides information as to the extent that the activity is being sustained within the atmosphere.

The requirement for detailed resolution at the frequency used in this application has meant that existing apparatus have to transmit the sound over an extremely short period of time. In contrast, because no detailed spatial resolution is required, the transmission of sound for this apparatus can be effected over a relatively longer period, typically in the range of 100 to 250 ms.

This relatively long transmitting period is considered of no use in current acoustic sounding methods and techniques at the frequency that is used in this application (6 kHz). However, by transmitting a relatively long pulse it is found that this provides the necessary information to indicate the presence of thermals that can be used by a hang-glider or glider pilot to sustain, prolong or launch into flight.

DISCLOSURE OF THE INVENTION

The method according to one form of this invention includes transmitting a pulse of sound at a desired frequency, receiving reflected signals within a selected bandwidth about the frequency of the transmitted signal, then measuring the amplitude of the received reflected signal during a selected period subsequent to the transmitted signal and providing a further indicator signal when the amplitude of the received reflected signal is above a selected amplitude.

In preference the indicator signal is integrated over time to provide a further thermal activity signal, said thermal activity signal being a cumulative signal, dependant on the total selected number of pulses. For example, the total of pulses may be between 2 and 16; or other suitable numbers.

in a further preferred form the invention is a method for determining the presence of thermal activity in the atmosphere including the steps of;

transmitting substantially vertically a plurality of acoustical pulses into the atmosphere, each pulse being of a selected length of time, known as

the pulse time, the plurality of acoustical pulses transmitted with a selected transmitter frequency, said pulses separated from each other by periods of non-transmittal, known as sample time;

receiving back-scattered signals of the plurality of acoustical pulses from the atmosphere during at least some of the said sample time, within a selected bandwidth about the transmitter frequency;

measuring the amplitude of the received back-scattered signals;

digitising the signal so that when the amplitude is above a pre¬ determined value, called the threshold value, the received signal is given a high value, and when the amplitude of the received signal is below the threshold value it is given a low value; and

integrating the digitised signal over a desirable length of time, called the scan time, to produce a indicator signal whose value is indicative of the presence and degree of thermal activity of air from which the received signals have been back-scattered.

In a yet further preferred form of the invention there is proposed an apparatus for determining the presence of thermals in the atmosphere including ;

a transmitter means for transmitting a pulse of sound into the atmosphere at a desired frequency;

a receiver means for receiving reflected signals from the atmosphere within a selected bandwidth about the frequency of the transmitted signal and effecting an output of said received signal;

a measurement means for measuring the amplitude of said received signals during a selected period subsequent to the transmitted signal and effecting an output indicative of the amplitude of said received signals; and

an indication means providing a further indication signal when the amplitude of said received signal is above a selected amplitude.

In a yet another preferred form the invention is said to reside in an apparatus

for determining the presence of thermal activity in the atmosphere including;

a transmitter means adapted for transmitting substantially vertically a plurality of acoustical pulses into the atmosphere, each pulse being of a selected length of time, known as the pulse time, the plurality of acoustical pulses transmitted with a selected transmitter frequency, said pulses separated from each other by periods of non-transmittal, known as sample time;

a receiver means adapted for receiving back-scattered signals of the plurality of acoustical pulses from the atmosphere during at least some of the said sample time, within a selected bandwidth about the transmitter frequency, and effect an output of the received back-scattered signal;

a measurement means adapted to measure the amplitude of the received back-scattered signals and effect an output of measured signals;

a digitiser means adapted to digitise the measured signals so that when the amplitude is above a pre-determined value, called the threshold value, the measured signal is given a high value, and when the amplitude of the measured signal is below the threshold value it is given a low value said digitiser means further adapted to effect an output of the digitised signal; and

an integrator means adapted to integrate the digitised signal over a desirable length of time, called the scan time, to produce a indicator signal whose value is indicative of the presence and degree of thermal activity of air from which the received signals have been back-scattered.

Preferably, the transmitter frequency is approximately 6 kHz. However, other frequencies may well be used. A frequency of 6 kHz is preferred since at that frequency there is relatively little ambient noise.

In a preferential form of the invention, the number of acoustical pulses transmitted into the atmosphere is at least 2 and no more than 16.

In a preferential form of the invention, the pulse time is between 100 and 200 milliseconds in duration.

SUBSTΓΓUTE SHEET (Rule 26

In a preferential form of the invention, the sample time is determined by the height and depth of the atmosphere from which back-scattering occurs, and is typically of several seconds duration, and in particular between 2-3 seconds.

In a preferential form of the invention, the scan time is in the range of 5 to 40 seconds.

In a preferential form of the invention, the digitised low value is a voltage signal of 0 volts.

In a preferential form of the invention, the indicator signal is reset after each scan time.

In a preferential form of the invention, a plurality of the latest indicator signals are stored in a memory means.

In a preferential form of the invention, the indicator signal is provided as a visual display, such as a LCD or LED display.

In a preferential form of the invention, the visual display shows a plurality of indicator signals showing the temporal evolution of thermal activity.

In a preferential form of the invention, the indicator signal is provided as an audio signal.

In a preferential form of the invention, before each most recent indicator signal is provided as an audio signal a plurality of previous indicator signals are provided as an audio signal, thus indicating the temporal evolution of thermal activity.

In a preferential form of the invention, the received back-scattered signals are transmitted to a spatially apart location providing a method of remote determination of thermal activity in the atmosphere.

In a preferential form of the invention, the transmission may be by any one of but not limited to, electromagnetic wave transmission, telephone line, fibre- optic.

In a preferential form of the invention, the transmitter and the receiver means is a horn antenna.

In a preferential form of the invention, the apparatus is adapted to be mounted on an aircraft, such as a glider, or hang-glider.

In a preferential form of the invention, the direction of the transmitted acoustical pulses, and thus the received back-scattered signals may be variable providing the pilot of the aircraft with information as to the presence of thermals in multi-directions.

In a preferential form of the invention, the aircraft includes an air speed indicator means and which effects an output that is used by the thermal activity apparatus to take into account the relative motion of the aircraft.

An apparatus using this method has the result that the thermal activity signal is used as a gauge to estimate the extent of any temperature differentials, and hence thermal activity, within the target volume. This information is of extreme value to those looking for such thermal activity, for instance hang-gliders, so that readings taken integrated over time can provide information as the potential usefulness of thermals within the atmosphere.

Current techniques used by glider pilots include observing the terrain which can produce thermals. For example, since thermal production is closely related to sun-shadow spots, pilots can follow any sun-spots hoping that the produced thermals have not been diffused by winds and turbulence. Another example of current techniques used is that it is well known amongst glider pilots that rocky out-crops can also produce thermals, but again, atmospheric turbulence and winds can diffuse such activity, the degree of diffusion usually related to the height above ground level.

The important point here is that the information obtained is not used for and nor can it be used to obtain detailed information of the temporal and spatial extent of thermal activity, for such information would be of little value to a hang-glider pilot. The information is by the very requirement needed, coarse indeed.

The requirement for such coarse resolution is that an apparatus can be manufactured which is very portable and which can operate for long periods at a time (several hours or more), and requires only a small power supply.

Such a portable apparatus can be further adopted and can be carried directly either by a hang-glider or glider, and can be used by a pilot who is already airborne to detect the location of any temperature differentials and thus thermal activity. It is envisaged that a pilot will be able to execute a type of search and locate pattern allowing the total flight time in the air to be greatly extended and not just rely on luck and guess work by the pilot.

Such an airborne apparatus can be further adopted to allow for the Doppler shifting of the transmitted and reflected signal caused by the relative motion of the craft with respect to the air. This can be achieved by coupling an air¬ speed indicator with the electronics of the sounder so as to allow for the Doppler shifting.

BRIEF DESCRIPTION OF THE DRAWINGS

To aid in the description of the invention one should refer to the following drawings wherein;

FIG 1 is a typical scenario indicating the use of the invention;

FIG 2 is one preferred embodiment of schematic circuit diagram that may be used to implement the thermal activity apparatus;

FIG 3 is a schematic embodiment of a remote transmitter to provide thermal activity information from a remote site;

FIG 4 is a schematic embodiment of a receiver that may be used to receive information transmitted by the transmitter of FIG 3;

FIG's 5A-5E show the various signals and the signal analysis used to provide the thermal information;

FIG 6 is the schematic of the power output and receiver circuit;

FIG 7 shows a typical example of a typical display;

FIG 8 is a cross-sectional view of the transmit-receive born antenna;

FIG 9 is a cross-sectional view of the apparatus when mounted on the nose of a hang-glider;

FIG 10 is a block diagram showing the operation of the apparatus;

FIG 11 is a graph of the frequency selectors;

FIG 12 is a graph of the transmission frequency as composed with the relative air speed;

FIG 13 shows the signal analysis of multi-sampling; and

FIG 14 shows the multi-sampling of FIG 13 when centred around the central frequency.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the drawings in more detail, it is to be understood that the particular representations are only used for illustration purposes of the invention and are not meant to limit the invention. There may be many embodiments used to perform the invention without deviating from the spirit of the invention.

FIG 1 shows a typical use of the invention. There is a thermal activity apparatus 10 used at ground level 11 to sample a volume, termed a glider release volume 12 to determine if there is any thermal activity within that volume that causes air to move upwards in direction 13. A tow car 14 is used to pull along, via rope 15, a hang-glider 16 which includes a pilot 17 so that the hang-glider 16 is released when it reaches the glider release volume 12 (or when it is sufficiently close to it). An observer (not shown) may operate the thermal activity apparatus 10 and relay by either a radio link (not shown) or some other method, information as to the extent of thermal activity in the glider release volume 12, to an operator (not shown) in the tow car 14 so that they may make a decision as to the value of launching (releasing) the hang-glider 16. However, the thermal activity apparatus 10 may include a transmitter (not shown) which could relay information to the operator in the car directly obviating the need for an observer, or may even relay the information directly to the pilot 17 thus obviating the need for an operator in the tow car. Other variations may equally well be successfully used, such as using a stationary catapult or engine to launch the hang-glider and not use a tow car or tow plane.

FIG 2 is a typical block diagram of the thermal activity apparatus 10 and the various electronics that may be used to carry out the invention. Thus there is a transmitting and receiving horn antenna 20 controlled by a transmitting oscillator and amplifier 21 , with transmission and receiving logic circuit 22.

The transmit frequency' is generated by the transmitting oscillator 21 and is gated to the special power output and receiver circuit (details of which can be seen in FIG 6). The transmitting and receiving logic circuit 22 also provides synchronisation signals 23 to the display circuit 24 and receiver

circuit 25. . The amplified signal 26 is then applied to the transmitting and receiving horn antenna 20 which produces a narrow beam 27 of acoustic power.

By changing the configuration of the output stage from series resonance in transmit mode to parallel resonance in receive mode, a relatively high return signal 28 is received. This reduces the overall gain required in the following stages and improves the S/N ratio. The return signal 28 is controlled by diode limiters 29 protecting the sensitive pre-amplifier input circuit 30. After passing through the pre-amplifier 30 the signal 31 is gated to a first band pass (BP) filer 32 to remove any out-of-band noise. A time- varying gain amplifier (voltage controlled amplifier) 33 with minimum gain adjustment 34 is used to ensure that the following stages are not overdriven because of the high return signals received from low altitude levels shortly after opening the receiver gate. Together with the minimum gain adjustment 34 the overall sensitivity of the circuit is controlled by changing the slope of the ramp generator 35, which itself is controlled by ramp generator gain control 36.. Low level temperature differentials can be tuned out this way and do not appear on the display.

After passing through the gain amplifier 33 the signal 37 passes through a second band pass filter 38 and is then rectified by a precision rectifier 39, the now rectified signal being a DC voltage 40 then applied to the input of Schmidt trigger 41. The output signal 42 of this stage, logic one, then switches in a selected constant current source 42 to linearly charge capacitor 43. Depending on the time resolution sensor 44, up to 16 return signals may be combined. Thus 44a may be the selector for 5 seconds, whilst 44b may be the selector for a time resolution of 40 seconds (or update time). Thus the total voltage level signal 45 represents the total receiving time of the returned signal 28. Before the total voltage level signal 45 is reset by gate 46 it is sampled, stored and displayed on the display 47. At the same time the entire display content is shifted one column to the left

(although other directions may well be used). A time recording facility 48 and channel selector 49 may be used to enable selective viewing of the recorded signals in replay mode.

Other parts of the circuit that are used for their normal functions are the fast replay clock 50, the display-memory timing circuit 51 , the record/play switch

52, the timer selector 53, a multi-level (about 10) bar output 54 with a vertical sensitivity control 55, a D type octo-latch 56, memory 57, and serial in/parallel out shift register 58.

It is to be understood that this circuit diagram is but one that may be used to perform the invention. Other electronic circuits may equally well be employed, and in fact most of the electronic circuitry can be performed by a microprocessor controlled with a LCD display.

FIG 3 is a block diagram that shows the use of a remote transmitter that may be employed in combination with the circuit of FIG 2 to enable one to obtain information when one is spatially apart from the thermal activity indicator apparatus. Thus, in FIG 3 three signals, being the transmit synchronisation signal 60, signal 61 which is the received signal after passing through the Schmidt trigger 41 in FIG 1 , and a Light Dependant Resistor (LDR) signal 62 are combined into data signal 64 in a tone encoder 63 and passed to the modulator 65, the RF transmitter 66 and transmitted through antenna

67. The data signal 64 could also or otherwise be transmitted via a two-way radio link or phone line to produce audio output through speaker 68.

FIG 4 shows a receiver that is adapted to receive the data signal which is either transmitted through antenna 67 or sent via speaker 68. In the first case a receiving antenna 70 and the RF receiver detector 71 feed the received signal to a tone decoder 72, whereupon the combined signals are subsequently separated into the three original signals, signals 60,61 and 62. Alternatively, the signal may have been received by appropriate microphone 73 which is then fed to tone encoder 72. The three individual signals can then be used to provide the necessary information regarding the activity of thermals.

If the apparatus is controlled by a microprocessor it may have a printer output port and modem connection for remotely displaying the stored information on a computer. There could also be provided an automatic voice output of the last three sampled inputs. This may have a threshold level such that it is only activated of say 2 of the three samples exceed a preset value. This output may also be connected to the microphone input of any two-way radio or other communication system for remote sampling and gathering of information.

FIG 5A to 5E is an example of the typical signals that occur during the operation of the thermal activity indication apparatus. Thus there is a transmitted signal 80 consisting of sound pulses between 100 and 200 milliseconds duration 81, each separated by up to several seconds 82. The receiver gate signal 83 is adapted to sample any received acoustic signal during the time of a non-transmission of the transmitter pulse 80. A typical returned signal 84 is shown, in this case slowly decaying over time, whilst the receiver gate 83 is open. A detector threshold level 85 is set at some predetermined value and the received signal 84 is digitised according the threshold level 85 to produce a digitised return signal 86. Here it can be seen that any return signal 84 above the threshold value 85 results in a high value 87 and any signal below the threshold level 85 results in a digitised low value 88.

The digitised signal 86 is then integrated over time to produce a voltage signal which accumulatively adds all digitised signals 86 to produce a final voltage output 89.

FIG 6 is a schematic diagram of the power driver and transmitter receiver changer over. In the transmit mode the network T1-T4 (H-shaped network) drive C1 (active transmit element) and L1 at resonant frequency, being a series resonance, resulting in a high driving voltage to transducer element

C1 at a low supply voltage. In the receiver mode, T1 and T2 are open whilst T3 and T4 are closed, thus effectively grounding A and B resulting in a parallel resonance receiving circuit with several times the received signal level. The output 90 is sent to the pre-amplifier

FIG 7 shows the typical example of a display 47 which may be used to indicate thermal activity. The display 47 consists of a number of light emitting diodes, LED'S 100, where thermal intensity is displayed in the vertical direction whereas the time factor is displayed in the horizontal direction. The apparatus can be further used whereas a top row of LED'S 101 are used to indicate that the test site is in the sun shadow. The display shown in this instance, is typical of a partly cloudy day with light winds showing good thermal conditions. The light winds can be judged from the slow moving of thermal intensity across the horizontal axis whereas the large thermal intensity amplitude indicated in the middle of the display shows good thermal activity. The apparatus can include various

resolutions selectors 102 to adjust both the thermal intensity and the display speed. It is to be understood that instead of a LED display other displays may be used such as a LCD screen display.

FIG 8 is a schematic diagram of a typical horn antenna used for both transmitting and receiving the sound. The horn antenna 110 can be mounted in a container 111 open at one end 112, the sides of the container having sound dampening material 113 within its construction. The horn antenna works by using a piezzo electric transducer 114 which acts so as to transmit and receive acoustic signals. The horn antenna 110 is usually of a concave shape so as to assist in the amplification and directivity of the sound pressure wave.

FIG 9 is a schematic view of the horn antenna 110 mounted on the nose

120 of a hang-glider on a suitable extension 121 and fixed to that extension

121 by suitable brackets 122. The horn antenna can be shrouded in suitable material 123 which allows for the propagation of sound and yet protects the horn antenna from such things as insects. In this case the protective shroud 123 can be shaped as to minimise air drag.

FIG 10 is a block diagram showing the operation of the thermal activity indication apparatus when mounted on a hang-glider, and is basically a simplification of FIG 2. However, there is the additional feature of an air¬ speed indicator 130 whose signal is used with appropriate electronics 131 to a VCO 132 which receives the signal from the transmitter/receiver logic circuit 22 and outputs a signal to the amplifier 21.

FIG 11 is a graph showing the transmitted frequency 140 which may be adjusted by the air speed sensor within range 142. The receiver may have a fixed narrow frequency range 142 due to the band-passing. The band¬ pass receiver can be chosen to account for the Doppler shifting caused by the craft movement.

FIG 12 is a simple graph showing the relationship between the transmitted frequency (horizontal axis) and air speed (vertical axis) wherein the air speed may be within the range 150 and the transmitter frequency may be within the range 151.

FIG 13 is a schematic diagram showing the averaging of the various individual band-passed signals so as to provide one display signal whilst, in FIG 14 the band-passed signals of FIG 11 are shown centred around central frequency Fc.

The essence of this invention is to enable persons such as hand-glider or a glider pilots to locate thermal lifts within a volume of the atmosphere as well as to gauge the intensity of the thermal lift before launch at ground level occurs. This enables the launch to be undertaken at a time when the release volume and thermal lift will coincide. However, it may equally well be used by other people, such as those that launch kites or model aircraft, whether powered or unpowered.

In one preferred embodiment a sound pulse at a frequency of approximately 6 kilohertz is transmitted in pulses of 100 to 200 millisecond durations through a directional hom with a directivity up to 30°. During periods of non-transmission the horn is adapted to receive the reflected echo's and used to indicate the thermal activity over a particular test site by displaying the information on a LED display matrix.

The apparatus is further adapted to display any sun-shadow areas by having a suitable means such as a photoelectric sensor or solar cell or other applicable means. The collected data can be further stored in memory by suitable memory means for later evaluation with the ability to have a fast replay function enabling the viewer to examine the days thermal signature.

The averaging of the sample received echo's may be adjusted to suit particular wind conditions. For example, in strong winds a fast display update setting, say of 5 seconds is used while in calm and light conditions, a slow setting say around 40 seconds, is used. Therefore, the fast setting also can be used to get indication of the temperature lapse rate before strong thermal activity starts when the sun is at its highest position, i.e around noon.

The apparatus may be further adapted to be mounted on a hang-glider or a glider pilot so as to allow a pilot to scan a volume of air up to several hundreds of metres in front in the direction of the craft flight. This is only

dependent on the total emitted power. This allows a pilot to locate and use any thermal lifts to stay aloft for longer periods of time. The display apparatus used by an airborne pilot can be either a visual display providing instantaneous information and it could be a display mounted in glasses especially adapted to display that information required. However, it is not limited to a visual display, it could be an audio signal adapted to convey the necessary information.

Further to the scenario where there is multiple stations, there is envisaged the scenario that a hang-glider pilot may be able to choose which station they receive information from. Thus in a scenario where a pilot is already in the air, they could select particular remote stations to receive data from, and then decide to which area they wish to fly to on the basis of that information. Of course, this would necessitate having a separate identification signal fro each remote station, but that would not be too difficult to do.

Furthermore, as already discussed, the apparatus could include the feature that several previous measurements could be relayed at the same time as the new measurement. Thus a person receiving information would now what change is occurring and how rapidly it is occurring without necessitating the use of a complex display which may not always be the preferred option. The information may in its simplest form be relayed by audio means, whereby a synthesised voice could simply read the last three values that have been measured. This could be especially useful with a pilot who is in radio contact with the ground and has thus already some means of audio communication.

It is to be understood that this specification and the embodiments described above describe the spirit of the invention and not limit it to those specific embodiments. Other variations may be evident to persons skilled in the art without deviating from the invention.