GB2091060A - Doppler navigator apparatus - Google Patents

Abstract

Doppler navigator apparatus has an antenna to radiate microwave beams at the ground at predetermined depression angles and angles to the aircraft heading. The apparatus provides navigational data from the doppler shifted ground reflections in the usual way. An additional beam is also radiated of greater width in the heading direction than the existing beams. The additional beam may have a bore sight coincident with one of the existing beams. The apparatus tracks the centre frequencies of the reflected Doppler spectra in both the existing and wider beams and employs the measured difference between these centre frequencies to apply sea bias correction to the navigator output.

Description

SPECIFICATION Doppler navigator apparatus The present invention is concerned with Doppler navigator apparatus.

The accuracy of aircraft Doppler navigator apparatus is affected by the radar back-scattering characteristics of the terrain over which the aircraft is flying. These characteristics may be described by a back-scattering co-efficient. The back-scattering co-efficient is a measure of the proportion of radar energy reflected from the ground back along the radar beam from the aircraft and is normally expressed as a function of the angle between the radar beam and the ground or sea at the point of incidence. With land, the back-scattering co-efficient is approximately constant over the range of such incidence angles involved and, as a result, the navigator apparatus can be calibrated from the depression angle from the horizontal of the "bore sight" (line of maximum transmitter power and receiver sensitivity) of the antenna generating the radar beam.

On the other hand, the back-scattering co-efficient of water varies with the incidence angle (and thus with the antenna depression angle) by an amount that depends on the surface wind speed. It can be appreciated, therefore, that the amount of radar energy reflected over the width of the radar beam is asymmetrical about the bore sight of the beam.

Where the direction of motion of the aircraft has a horizontal component in the plane of the beam, it can be seen that the Doppler shift in the reflected energy across the width of the beam in the direction of motion varies in accordance with the cosine of the angle of depression from the velocity vector. Thus, a spectrum of Doppler frequencies is received by the navigator apparatus reflected from the ground.

Normally, the navigator apparatus operates to track the frequency of peak power in the received signal, i.e. the "centre" frequency of the received Doppler spectrum. It can be seen that with the variation of back-scattering co-efficient with depression angle over the sea, the received Doppler spectrum is distorted resulting in a shift of the tracked centre frequency which, if left uncorrected, would result in an error in the navigator output.

These errors in the velocity outputs from the navigator apparatus are known as sea bias errors.

The amount of sea bias error is dependent on the variation in scattering co-efficient with the angle of depression of the radar beam from the horizontal and it is possible to determine the sea bias error and a suitable correction by comparing the amplitude of signals received in beams of different depression angles. Various arrangements have been proposed to make use of this technique to measure and correct for sea bias, but for various reasons it has proved difficult to put into practice.

It is also known to reduce sea bias error by the technique of "sequential beam lobing". In this technique, the beam can be switched between two depression angles a few degrees apart. Two overlapping Doppler spectra are received in the two beams and the Doppler tracker of the navigator apparatus is controlled to follow the frequency at which the two spectra have the same amplitude.

This method has the disadvantage of producing large fluctuations in the velocity outputs of the apparatus, especially at heights below five thousand feet, arising from the amplitude fluctuations resulting from the sequential operation. Also, the reduction of sea bias arises only in the along heading velocity and with fixed antenna systems the sea bias of the across heading velocity is not reduced. In order to reduce the large fluctuations of sequential beam lobing, a simultaneous beam lobing system has been introduced. However, even with this arrangement, design constraints preventthe sea bias from being reduced to less than thirty percent of its uncorrected value. Further, sea bias is not reduced in the across heading velocity.

In a somewhat different method for reducing sea bias, an antenna is used which has a beam shaped so that the isodops (lines of constant Doppler frequency) in the Doppler spectrum intercept the same range of incidence angles and are therefore equally affected by the scattering properties of the terrain.

An example of the antenna required for this system is described in British patent No. 1341512. However, with this system the sea bias is reduced only in the along heading channel: in the across heading channel the sea bias is greater than for a conventional Doppler antenna.

According to the present invention, Doppler navigator apparatus has antenna means arranged for radiating and receiving microwave electromagnetic energy in at least one beam directed towards the ground to provide navigational data and at least one additional beam with a different beam width from the beam or beams providing the navigational data, respective Doppler frequency trackers for tracking the centre frequencies of the received Doppler spectra from said beams, and correction means for comparing said respective centre fre quenciesto calculate a sea bias correction for the navigator apparatus, and applying said sea bias correction to the apparatus.

The basic principles of operation of the present invention can be understood by considering a simplified Doppler navigator apparatus as illustrated in FIGURE 1 of the accompanying drawings.

FIGURE 1 shows a "single ended" system where Al and A2 are the transmitting antennas of the two beams of the apparatus and A3 and A4 are the receiving antennasforthe respective two beams.

The beams on reception and transmission are drawn separately in the figure although of course, common transmitting/receiving antennas may be used.

In this ideal example, all the antennas have the same depression angle yO. The beams are all in the heading plane of the aircraft and the beam width in the along heading plane of antennas Al and A3 is Ay, and that of aerials A2 and A4 is bx. A transmitter 1 and receiver 2,3 are connected sequentially first to serials A1 and A3 and then to aerials A2 and A4. The receiver comprises a mixer 2 which is fed with a reference signal from the transmitter and a Doppler amplifier 3 which presents at its output the Doppler spectrum in the received signals. The output from the Doppler amplifier is switched between Doppler frequency trackers 4 and 5 simultaneously with switching of the transmitter and receiver between the two sets of aerials.The output frequency of the tracker 4 is frequency f, which is the tracked centre frequency of aerials Al and A3 with beam width A and the output of tracker 5 is the frequency f2 which is the tracked centre frequency of the transmitter and receiverwith the aerials A2 and A4 of beam width ay,. A sea bias correction unit 6 computes Af, from the equation

As shown later, Af, is the sea bias correction to be applied to the measured centre frequency f, using the aerials Al and A3.

The distribution of transmitted energy, and receiver sensitivity, in the along heading plane of the beam of each of antennas Al and A3 is given to a good approximation by the expression Exp

Where the beam width Ay, is taken as the angular width between the three dB points of the beam.

Similarly for the beam of each of antennas 2 and 4 the distribution is Exp

When the aircraft is travelling over sea, the scattering co-efficient, or the proportion of energy reflected back along a beam, varies with the depression angle, for an aircraftflying level, in a manner given bya function F(y). This function typically has a reducing value with reduced values of depression angle with a maximum value at a depression angle of ninety degrees, i.e. a vertical beam.

Thus, the distribution of received energy in the beam of antenna A3, [D(y)j, is given by the equation

It will be appreciated that the Doppler spectrum in the received beam has a corresponding distribution since the Doppler frequency is dependent on the depression angle.

FIGURE 2 of the enclosed drawings shows graphically F(y) against yfor land and water. As can be seen from the drawing, for land, F(y) is substantially constant against y but for water it increases as the depression angle increases.

FIGURE 3 of the enclosed drawings illustrates the distribution of received energy, D(y), with respect to depression angle again for land and water. For land, the distribution D(y) is substantially unaffected by the scattering co-efficient and is represented by a curve Exp

This distribution has a maximum at the centre ofthe beam, i.e. at yo, so that over land the aircraft velocity can be determined from the depression angle of the bore sight of the radar beam. However, over sea, the distribution is as in equation (1) above which is represented by the dotted line in FIGURE 3.The maximum of this curve is shifted from a depression angle of to by an angle q. The value of e" which can be regarded as the angular sea bias, can be found by differentiating equation (1) above and making it equal to zero i.e.

where F'(y) is the derivative of F (y) with respect to y.

Hence,

Similarly the angular sea bias with the beams from antennas 2 and 4, E2, is given by the equation

Thus,

This relationship depends only on the antenna beam widths and is independent of the scattering characteristics of the terrain.

The tracked centre frequencies f, and f2 i.e. the frequencies corresponding to the depression angles at the peaks of the dotted curve of FIGURE 3, are given by the equations

where A is the transmission wavelength of the apparatus. Thus,

where E1 and E2 are small and expressed in radians.

Substituting the value of e2 given in equation (2) gives

With an angular sea bias shift of s, the corresponding sea bias shift in frequency Af, is given by

and substituting the value for e, given in equation (3) gives

Equation (4) shows that the sea bias shift in the received Doppler frequency produced with the beams of antennas 1 and 3 is equal to the difference in the Doppler frequencies of the two antenna systems with different beam widths multiplied by a constant dependent only on the beam widths.For example, with typical beam widths A71 =60 and Ay2 =80, Af1 =1.29 (f1 -f2) Hz It should be noted that movement of reflecting facets on wind generated waves which is respons ible for the so-called wave motion error affects the two tracked frequencies f, and f,similarly and hence does not affect the sea bias frequency shift given by equation 4.

In the present invention, the tracked centre frequencies of the two beams are compared to calcu late a sea bias correction and then applied in the apparatus to correct sea bias.

Preferably, in the invention, the depression angles relative to the aircraft of the two beams are equal.

However, it is not essential for the depression angle to be equal and in the above-described explanation of the invention, the various beams need not all be equal to y,. In this case, the value of (f1 -f2) is not directly related to the sea bias correction. However, a proper correction can still be established by noting the difference between the centre frequencies of the two beams when flying over land, (f1 -f2)land. The sea bias correction for the centre frequency f1 is then given by the equation

Where (fl -2)sea is the difference between the centre frequencies over sea.

Nevertheless, it is most convenient for the depression angles of the two beams to be equal and in a preferred arrangement the bore sights of the two beams are coincident.

It is also convenient for the beam widths of the two beams, in the invention, to be the same on transmission and reception; i.e. in the above explanation of the invention for the beam width of antennas Al and A3 to be equal and of A2 and A4 to be equal.

The antennas Al and A3 may be constituted by the same antenna used for both transmission and reception and similarly for antennas A2 and A4. However, it is not essential for any of aerials Al to A4 to have the same beam width. For the general case where the beam widths are A71, Ay2, Hay,, 74, for the antennas A1,A2, A3 and A4 respectively.

Normally, the Doppler navigator apparatus of the present invention is arranged with said antenna means to radiate and receive, to provide navigational data, with beams in at least three different bore sight directions with predetermined forward and lateral depression angles and distributed in two separate planes, and to generate from the tracked Doppler centre frequencies from the different beam directions data defining aircraft velocity in three component directions of the aircraft reference frame.

It will be appreciated that the velocity values deter mined by the Doppler navigator apparatus in the aircraft reference frame can be converted to corresponding component velocities in the along heading, across heading and vertical directions by taking into account the aircraft pitch and roll angles at any time.

In one possible arrangement, there are three said beam directions for providing navigational data with beams of equal width in the forward direction of the antenna in the aircraft, one of the three directions being towards the rear of the aircraft and the other two being directed forwardly symmetrically on either side of the plane containing therefore and aft axis of the antenna in the aircraft and in one or more of said directions the apparatus being arranged to transmit and receive at least one said additional beam with a different beam width in the forward direction. This arrangement with three beam directions is one of the standard Doppler navigator beam arrangements and in such an arrangement, the present invention is embodied simply by providing a further slightly wider beam or beams coincident with one or more of the three directions.It will be appreciated that sea bias frequency correction may be obtained directly for each direction which has an additional beam or beams with a different beam width. Alternatively additional beam or beams can be provided in one selected direction and frequency corrections for the other directions can be calculated from the sea bias frequency correction of the selected direction by taking into account pitch and roll angle ofthe aircraft at any particular time.

Conveniently, said correction means is arranged to receive data defining the pitch and roll angles of the aircraft and includes means for calculating from said pitch and roll angles, the difference in the centre fre quencies from said beams of different beam width, and the predetermined angular distribution of said three or more different beam directions, sea bias corrections for the tracked centre frequencies from each of said beam directions, and means for apply ing said corrections to said centre frequencies.

In a different arrangement, said correction means includes means to calculate a sea bias correction co-efficient from a comparison of said respective centre frequencies, normalisation means, responsive to said co-efficient and to data defining the pitch and roll angles of the aircraft and the aircraft forward and sideways drift velocities, to calculate a normalised sea bias correction co-efficient corresponding to the aircraft flying straight and level, means to smooth the normalised sea bias correction coefficient and means to generate from the smoothed coefficient sea bias corrections to be applied to the tracked centre frequencies ortho the aircraft velocities measured by the apparatus.With this arrange ment relatively long time constant smoothing can be applied to the normalised sea bias correction co-efficient since variations in this due to variations in the pitch and roll angles of the aircraft are removed and need no longer be preserved in the normalised correction co-efficient. It is important to smooth the correction co-efficient as much as possibleto remove fluctuations which tend to arise in these co-efficients from the inherent noise-like character of received Doppler signals.

Another source of error in Doppler navigator systems over the sea is due to wave motion which tends to produce a vector error in the resulting aircraft velocities measured. The direction and speed of travel of wavelets on the sea surface is substantially dependent on the wind speed and direction and established corrections can be applied to the measured aircraft velocities from the estimated speed and direction ofthe surface wind. In typical systems, this wave motion error is corrected to some extent by estimating the surface wind speed from meteoroligical data available during the aircraft flight. However, it will be appreciated that these corrections are dependent for their accuracy on the accuracy of the available meteorological data. Furthermore, the estimated surface wind speed must be continually updated usually manually making use ofthe latest weather information.

In a preferred embodiment of the present invention, the apparatus includes surface wind speed calculation means responsive to data from said correction means defining a sea bias correction co-efficient and to data defining the geometry of said antenna beams to calculate a value for the wind speed at the sea surface, and wave motion correction means responsive to the calculated surface wind speed to apply a cprrection for wave motion to the navigator output signals. It can be shown that the sea bias error at any time is directly related to the surface wind speed and the beam geometry of the navigator apparatus. Expressions relating to these parameters are known in the art.In one example, for a beam, constituting one of the navigation beams of the apparatus, with an along heading depression angle of 67 , a beam width in the along heading plane of 5.8 , an across heading broadside depression angle of 78.7" and a beam width across heading of 11 , the surface wind speed Ws is given by the equation Ws = 24.4 (-loglo (58.82 ah-.76) )knots where ah is the sea bias correction coefficient.

The wave motion correction means may include wind direction calculation means responsive to data defining the along and across heading velocities of the aircraft and true air speed to calculate the wind direction at the aircraft altitude and thence to calculate the wind direction at sea level. The variation of wind direction with altitude can be calculated from expressions which are known in the art. From empirical measurements, it can be shown that normally in the Northern Hemisphere and for heights up to six thousand feet, the wind direction veers (i.e. turns clockwise) with increase of height at the rate of 1.2 sin x per 100 feet, where xis the latitude in degrees.

In the Southern Hemisphere the direction of rotation is reversed. Accordingly, the wind direction calculation means is arranged to calculate an estimated wind direction at sea level using this relationship from the measured wind direction at altitude. These determined values for surface wind speed and direction are then used to introduce wave motion correction to the output velocities of the navigator apparatus.

The present invention also provides a method of correcting sea bias errors in Doppler navigator apparatus comprising radiating and receiving first and second radar beams towards the ground with components along the direction of heading of the aircraft and with different beam widths in said direction of heading on at least one of transmission and reception, tracking the respective centre frequency of the received Doppler spectrum of each said beam and correcting for sea bias in the output velocity of the apparatus by comparing said centre frequencies to calculate a correction and applying the correction.

Further more detailed examples of the present invention will now be described with reference to FIGURES 4 to 7 of the accompanying drawings.

FIGURE 4 is a block diagrammatic representation of an embodiment of the invention in which the tracked centre frequencies of three Doppler navigator beams are corrected. In this arrangement the antenna is fixed in azimuth but the invention can equally well be applied to an equipment with a drift stabilised antenna.

FIGURE 5 illustrates a typical beam geometry for a navigator apparatus. When separate transmitter and receiver antennas are used, beam 22 represents both the transmitting and receiving beams and so on.

FIGURE 6 is a block schematic diagram illustrating a different embodiment of the invention which cor rects the aircraft forward and sideways velocity val- ues.

FIGURE 7 is a block schematic diagram illustrating the arrangement of FIGURE 6 with the addition of wave motion correction.

Referring to FIGURES 4 and 5, a Doppler navigator sensor 7 has the beam configuration illustrated in FIGURE 5 which show the beams from the aircraft when the aircraft is heading in the direction of arrow 24. Beams 21, 22 and 23 are the basic beams of the apparatus used to provide the normal Droppler frequency outputs, f1, f2 and f3. An additional beam 21A is an auxiliary beam used to give an additional Doppler centre frequency f,A for use in correction of sea bias. The beams 21, 22 and 23 have the same beam width Ay0 and the auxiliary beam 21A has a greater beam width A. Seperate or common aerials may be used for transmission and reception.The frequencies f., f2, f3 and fin are produced by the Doppler sensor 7 in the usual way employing Doppler trackers. The frequencies f, and f,A are fed to a subtraction device 25 from which the difference frequency fi - f,A is fed to a correction unit 8. The correction unit 8 also receives data defining the pitch and roll angles of the aircraft at any time on lines 26 and 27 respectively.

The frequencies f, and f2 are also fed to the unit 8.

The unit 8 calculates the sea bias corrections to be applied to each of frequencies f1, f2 and f3. The correction for f1 is given directly by the expression

The corrections for f' and f3 are different by amounts which depend on the aircraft pitch and roll and on the across heading velocity of the aircraft, which is proportional to f2-f1. It is a straight-forward matter to produce expressions for calculating the frequency corrections to be applied to f2 and f3 from the beam geometry of the navigator apparatus and scattering co-efficient data.

The correction frequencies are supplied from the unit 8 on lines 28 to subtractors 29 and the corrected frequencies f1,, f2, and f3, are supplied from the subtractors 29 to a velocity conversion unit 9 for calculation oftheforwards sideways and up and down velocities, X, Y and Z in the aircraft reference frame.

These velocities in the aircraft reference frame can be calculated from the expressions

Where 80 is the sideways depression angle of beams 21,22 and 21A.

The X, Y and Z velocities in the aircraft frame are supplied from the unit 9 to a normalising unit 10 which also receives pitch and roll angle data and converts the velocities to corresponding velocities horizontally along the aircraft heading direction X, horizontally across the heading direction Yh and vertically Zh. These normalised velocity components are supplied on lines 30 from the unit 10 to the navigation computer for use in the normal way.

FIGURE 6 shows an alternative arrangement also using the beam configuration of FIGURES. In this arrangement, the tracked frequencies f1, f2, f3 and flA are supplied from a Doppler sensor 11 to a velocity calculating unit 12 which calculates the corresponding velocities X, Y and Z in the aircraft reference frames. In the present case, the values X, Y and Z are uncorrected for sea bias and the unit 12 employs the expressions given above but with the uncorrected tracked frequencies.The unit 12 also calculates a sea bias co-efficient a which is derived from the expression

This is simply the ratio of the sea bias correction frequency for f1 to the value of the frequency f1. The sea bias co-efficient a is fed from the unit 12 to a normalising unit 13 which receives data defining the pitch and roll angle of the aircraft and also the X and Y velocity components from the unit 12. The unit 13 converts the sea bias co-efficient a, which is dependent, inter alia on the instanteous pitch and roll ang les of the aircraft and the drift velocity at any time, to a normalised value ah which is independent of pitch and roll and drift. The expressions necessary for this conversion can readily be calculated from the beam geometry of the system.The resulting normalised bias co-efficient ah is thus independent of short-term variations resulting from changes in the pitch and roll angles of the aircraft and the drift. The normalised sea bias co-efficient ah is then smoothed in a filter 14 which has a relatively long time constant, typically of the order of one to five minutes, thereby removing the fluctuations in the sea bias which can arise from the inherent noise-like character of the Doppler signals. It will be appreciated that the normalised sea bias itself changes only in response to variations in sea conditions and therefore rapid changes are not expected. On the other hand the unnormalised sea bias changes in response to variations in pitch and roll and also drift, and long time constant smoothing cannot therefore be applied to the unnormalised sea bias or else these ancillary effects will be lost.

The normalised and smoothed sea bias correction ah is fed to a velocity correction calculation unit 15 which also receives data defining the pitch and roll angles of the aircraft and the velocity values X and Y from the unit 12. Unit 15 calculates velocity correc- tions to be applied to the values X and Y to correct the sea bias. These corrections AX and AY can be expressed by the equations AX = a'X + b'Y AY = e'Y + d'X where a', b', e' and d' are the sea bias co-efficients that take into account aircraft pitch and roll. These co-efficients are functions of the along heading sea bias co-efficient for straight and level flight, ah, which is calculated in the apparatus, and can be computed from the beam geometry, the known water scattering characteristics and the aircraft pitch and roll angles.The unit 15 computes these co-efficients and therefrom the corrections AX and AY. The corrections are fed from the unit 15 on lines 31 to subtractors 32 where they are subtracted from the uncorrected velocity values X and Yto produce corrected velocity values X' and Y'. With most typical Doppler apparatus, the sea bias effect on the Z velocity, i.e.

up and down relative to the aircraft reference frame, is very small and correction is not required. However, if correction is required it is possible to derive a correction term AZto produce a corrected Z'. The corrected velocity values are fed from the subtractors 32 together with the velocity value Z to a transform unit 16 which also receives data defining the pitch and roll angles and operates to convert the velocity values X', Y' and Z which are in the aircraft reference frame to values X'h, Y'h and Zh representing velocities horizontally along and across the aircraft heading and vertically. As before, the corrected velocity components from the unit 16 are supplied to the usual navigation computer.

Instead of calculating the corrections for the X and Y velocity channels, corrections for the Doppler frequencies forthethree boresight directions may be calculated from the available data and introduced into the frequency channels.

Referring now to FIGURE 7, all the components of FIGURE 6 are shown as before. The normalised and smoothed sea bias correction coefficient ah is fed also to a wind speed calculation unit 17. It is known in the art that the sea bias coefficient is related by expressions determined empirically to the surface wind speed. Thus, a value for the surface wind speed can be calculated from the sea bias co-efficient and data defining the beam geometry ofthe apparatus.

An example of a relationship between wind speed W0 and normalised sea bias co-efficient ah is given earlier herein. Unit 17 calculates a value for surface wind speed Ws from such an expression. The surface wind speed value is supplied from the unit 17 on a line 33 to a wave motion correction unit 20. The unit 20 also receives a value t5 representing the surface wind direction. From surface wind speed and direction Ws and t5 the unit 20 calculates wave motion corrections for the horizontally along and across heading velocities Xh' and Yhtfrom the unit 16.These corrections are directly related to the estimated velocity of wavelets on the sea surface which are assumed to travel in the same direction as the surface wind with a speed Sw which can be calculated from the express Sw =4.5(1 - Exp( - W,I1 1.7) ) + 3was/100 The components of the wave speed in the directions X'h and Y'h are calculated from the surface wind speed direction t5. The wave motion corrections from the unit 20 are added (or subtracted) to the values Xh' and Yh' in subtractors 34to provide the velocity outputs Xh" and Yh" corrected for both sea bias and wave motion together with uncorrected Zh.

These velocities are supplied on lines 35 to the navigation computer in the usual way. The surface direction is estimated by the apparatus automatically using the corrected velocity values Xh" and Yh" supplied to a wind direction calculator 18. The unit 18 also receives true air speed on a line 36 measured in the aircraft in the usual way. From these inputs it is a simple matterforthe unit 18 to calculate the direction of the wind vector relative to the aircraft heading, 8, at the aircraft's altitude, using the equation

where V is the air speed. The calculated wind direction tisfed from the unit 18to a further unit 19 which also receives data on line 37 defining the aircraft height. As explained previously herein there is a empirical relationship between wind direction and height above sea level and the unit 19 uses this relationship to calculate the estimated wind direction at sea level t5 which is fed to the unit 20.

Claims (12)

1. Doppler navigator apparatus having antenna means arranged for radiating and receiving microwave electromagnetic energy in at least one beam directed towards the ground to provide navigational data and at least one additional beam with a different beam width from the beam or beams providing the navigational data, respective Doppler frequency trackers for tracking the centre frequencies of the received Doppler spectra from said beams, and correction means for comparing said respective centre frequencies to calculate a sea bias correction for the navigator apparatus, and applying said sea bias correction to the apparatus.

2. Apparatus as claimed in claim 1 wherein the beam widths differ in the forward direction.

3. Apparatus as claimed in claim 1 or claim 2 wherein the forward and broadside depression ang les of the or one of the additional beams is the same as that of the or one of the beams providing navigational data.

4. Apparatus as claimed in any preceding claim wherein for each of said beams the beam widths on transmission and reception are equal.

5. Apparatus as claimed in claim 4 wherein a common antenna is provided for both transmission and reception of a respective beam.

6. Apparatus as claimed in any preceding claim and arranged with said antenna means to radiate and receive, to provide navigational data, with beams in at leastthree different boresight directions with predetermined forward and lateral depression angles and distributed in two separate planes, and to generate from the tracked Doppler centre frequencies from the different beam directions data defining aircraft velocity in three component directions of the aircraft reference frame.

7. Apparatus as claimed in claim 6 as dependent (directly or indirectly) on claim 3, wherein there are three said beam directions for providing navigational data with beams of equal width in the forward direction of the antenna in the aircraft, one of the three directions being towards the rear of the aircraft and the other two being directed forwardly symmetrically on either side of the plane containing the fore and aft axis of the antenna in the aircraft and in one or more of said directions the apparatus being arranged to transmit and receive at least one said additional beam with a different beam width in the forward direction.

8. Apparatus as claimed in claim 6 or claim 7 wherein said correction means is arranged to receive data defining the pitch and roll angles of the aircraft and includes means for calculating, from said pitch and roll angles, the difference in the centre frequencites from said beams of different beam width, and the predetermined angular distribution of said three or more different beam directions, sea bias corrections for the tracked centre frequencies from each of said beam directions and means for applying said corrections to said centre frequencies.

9. Apparatus as claimed in claim 6 or claim 7 wherein said correction means includes means to calculate a sea bias correction co-efficient from a comparison of said respective centre frequencies, normalisation means, responsive to said co-efficient and to data defining the pitch and roll angles of the aircraft and the aircraft forward and sideways drift velocity, to calculate a normalised sea bias correction co-efficient corresponding to the aircraft flying straight and level, means to smooth the normalised sea bias correction co-efficient and means to generate from the smoothed co-efficient sea bias corrections to be applied to the tracked centre frequencies of each boresight direction or to the aircraft velocities measured by the apparatus.

9. Apparatus as claimed in any preceding claim and including surface wind speed calculation means responsive to data from said correction means defining a sea bias correction co-efficient and to data defining the geometry of said first and second beams to calculate a value for the wind speed at the sea surface, and wave motion correction means responsive to the calculated surface wind speed to apply a correction for wave motion to the navigator output signals.

10. Apparatus as claimed in claim 9 wherein said wave motion correction means includes wind direction calculation means responsive to data defining the along and across heading velocities and true air speed to calculate the wind direction at the aircraft altitude and thence to calculate the wind direction at sea level.

11. Doppler navigator apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

12. A method of correcting sea bias errors in Doppler navigator apparatus comprising radiating and receiving microwave electro-magnetic energy in at least one beam directed towards the ground to provide navigational data and at least one additional beam with a different beam width from the beam or beams providing the navigational data, tracking the centre frequencies of the received Doppler spectra from said beams, comparing said respective centre frequencies to calculate a sea bias correction for the navigator apparatus, and applying said sea bias correction to the apparatus.