Adaptive Measurement Correction During Remote Distance Measurement THE BACKGROUND OF THE INVENTION AND PRIOR ART The present invention relates generally to navigating crafts by means of remote distance measurement, where a remote distance meter's inclination to erroneously measure the shortest distance to the surface over which the craft moves is adjusted adaptively. Specifically, the invention relates to a method and an apparatus for navigating a craft according to the preambles of claim 1 and claim 8 respectively. The invention also relates to a computer program for performing the method according to claim 1 and a computer readable medium having such program recorded thereon.

It is generally known to utilize radar technology to determine the distance and bearing to objects (radar = radio detecting and ranging). By performing doppler detection it is also possible to determine radial velocity relationships between the radar and the registered objects.

The article"Saab NINS-An automated Navigation System", by P. Bergljung et al, 1 November, 1999, No. A, Saab Dynamics AB, describes how an aircraft can be navigated based on the appearance of the surface over which the aircraft moves. A radar altimeter in the aircraft sends signals towards the surface and determines the altitude over the surface by analyzing the signals being reflected against the surface. Information in a database pertaining to the surface's elevation in the area within which the aircraft is located is compared with a latest measured elevation of the surface. Conclusions can thereby be drawn as to the aircraft's position. A so-called measurement shell repre- sents an uncertainty volume of a predetermined size within which the craft is estimated to be positioned. By repeated measurements and measurement shell assumptions the size of

the measurement shell is reduced and the craft's position can be determined with a higher accuracy.

A so-called terrain reference navigation filter, which utilizes a radar altimeter and which has been proposed by the applicant, assumes certain characteristics of the noise being associated with the radar measurement. The noise is thereby supposed to be due to the fact that the measurement occasionally is not performed with respect to the actual ground surface, but against objects being located on the surface, such as trees, posts and masts. In order to make the terrain reference navigation filter independent of such noise, the measurement shell is selected with such thickness that the aircraft certainly can be said to be located within the measurement shell irrespective of the noise.

When performing radar altitude measurements an uncertainty arises due to the fact that the signal that is received by the radar might not have been reflected by the surface in a desired reference direction. Most commonly, this direction is set vertically with respect to the mean sea level. An uncertainty is introduced in the measurement as soon as reflections in other directions than the reference direction could be interpreted as reflections in this direction.

If, for example when performing radar altitude measurement from an aircraft, a radar echo corresponding to a reflection from a knoll in the surface, which is located closer to the aircraft than the surface vertically below, however in a direction obliquely below the aircraft, is interpreted as a vertical radar reflection the position information will be erroneous.

For reasons of precision, it is of course desirable to minimize the uncertainty area when determining the position for a craft.

Since the navigation method presupposes that all remote distance measurements (e. g. performed by means of a radar) are executed vertically below the craft it is thus desirable to reduce the influence of remote distance measurements in other

directions as much as possible, since this introduces uncertainty in the navigation method.

The yet known solutions for radar altitude measurement aided navigation fail to take into account the radar's inclination to measure a distance, which is angularly erroneous with respect to the surface over which the craft moves.

SUMMARY OF THE INVENTION The object of the present invention is therefore to alleviate the above problems and improve the precision as well as the robustness of the earlier known solutions for remote distance measurement based terrain navigation.

According to one aspect of the invention the object is achieved by a method for measurement noise modeling in a remote distance meter as initially described, wherein the remote distance measurement employs a measurement correction parameter which for each geographical position and altitude designates an estimation of a remote distance meter's inclination to measure a shortest distance between the craft and the surface in a direction which deviates from a desired measurement direction.

According to another aspect of the invention the object is achieved by a computer program, which is directly loadable into the internal memory of a computer, comprising software for performing the steps of any of the method described in the above paragraph, when said program is run on the computer.

According to yet another aspect of the invention the object is achieved by a computer readable medium, having a program recorded thereon, where the program is to make a computer perform the method proposed in the penultimate paragraph above.

According to still another aspect of the invention the object is achieved by the initially described apparatus, wherein, for each geographical position and altitude, a measurement correction parameter designates an estimation of a remote distance meter's inclination to measure a shortest distance between the craft and the surface in a direction which deviates from a desired measurement direction. A control unit in the apparatus takes the measurement correction parameter into account when calculating the craft's geographical position The proposed solution improves both the precision and the reliability during position determination by means of remote distance measurement. This, in turn, results in that the traffic security can be improved in any kind of craft traffic where such positioning is utilized, such as in aircraft navigation and marine navigation, on as well as below the water surface.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention is now to be explained more closely by means of preferred embodiments, which are disclosed as examples, and with reference to the attached drawings.

Figure 1 illustrates how an aircraft equipped with an apparatus according to the invention passes over a terrain segment whereby a first step of a per se known procedure for determining the craft's geographical position is effected, Figure 2 shows a later stage of the procedure according to figure 1, for determining the craft's geographical position, Figure 3 shows yet a later stage of the procedure according to figure 1, Figure 4 illustrates, by means of an example, the procedure according to an embodiment of the invention,

Figure 5 illustrates an embodiment of the apparatus according to the invention, and Figure 6 illustrates, by means of a flow chart, the general procedure according to the invention, which preferably is controlled by a computer program.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Figure 1 shows an aircraft 100, which is equipped with an apparatus according to the invention. The aircraft 100 passes over a terrain segment when the proposed apparatus is activated and a procedure for determining the aircraft's 100 position is instigated. Initially, a measurement shell set ; is assumed, which has a maximum size and typically is a cylindrical volume being several kilometers in width and whose height is arbitrary, within which the aircraft 100 is presumed to be located.

Figure 2 illustrates how the apparatus in the aircraft 100 in a later stage of the procedure for determining the aircraft's 100 position performs a radar altitude measurement, through which the aircraft's distance in height above the surface is determined.

The apparatus includes a radar whose transmitter-and receiver antennae have relatively large lobe angles (e. g. 40°-60°) and whose cut area, for instance, may be elliptical. The larger the lobe angle, the larger the risk of receiving radar reflections from directions which deviate from the desired direction, since any reflection within the range of the antenna lobe may be interpreted as a ground reflection in the desired reference direction irrespective of whether it in fact comes from this direction or not. A too small antenna lobe angle will, however, render the radar altimeter sensitive to changes in the direction of the aircraft, e. g. during rolling or tipping.

The transmitter antenna transmits a radar signal and the

receiver antenna, which may be identical with the transmitter antenna, receives a reflected signal. The time difference between the transmitted signal and the received signal constitutes a measure of the distance hgnd (i) between the aircraft 100 and the object that reflected the transmitted signal, since the radar signal is presumed to propagate at a constant and known speed, namely the speed of light. According to a basic assumption, it is the ground below the aircraft 100 that has reflected the transmitted signal. The aircraft 100 is thus expected to be located at a distance hgnd (i) above the ground.

By the radar altitude measurement, the measurement shell set can be reduced in height to a particular thickness. This thickness is stipulated by an estimated uncertainty in height in the radar altitude measurement. The height of any objects located on the surface (in this case the ground) places a limit as to the thickness of the measurement shell set. The precision of the radar altitude measurement is hence determined by the modeling of the surface's capability to reflect radar signals.

Normally, the lower limit for the thickness of the measurement shell ! ! set, is put equal to the height of the highest expected objects on the ground. A decrease of the measurement shell's set ; thickness below this limit jeopardizes the robustness of the measurement method, and eventually reduces the precision in the position determination.

Figure 3 shows a yet later stage of the procedure for determining the aircraft's 100 position, when another radar altitude measurement is performed. The height reduced measurement shell set, is expanded to some extent in order to take into consideration the uncertainty in position being intro- duced due to the movement of the aircraft 100. Furthermore, the measurement she ! t set ; is shifted a distance, which corresponds to the aircraft's 100 estimated movement since the previous radar altitude measurement. Sensors in the aircraft 100, for instance in the form of inertia gyros, accelerometers and pressure meters can be used to register such movement.

The renewed radar altitude measurement provides an updated value of the distance estimation hgnd (i+1) to the ground. A new measurement shell ! t set, +i based on this estimation is assumed.

When determining the aircraft's 100 position an assumption is made that on one hand the aircraft is located within the shifted and expanded measurement shell seti, and on the other hand is located within the new measurement shell setj+1, which was obtained via the latest radar altitude measurement. The aircraft 100 is thus presupposed to be located within a cut volume setîlj+1, which overlaps the shifted and expanded measurement she ! ! set ; as well as the latest measurement shell setj+1. The aircraft's 100 position can thereby be estimated with an accuracy corresponding to the cut volume setj) j+i.

A further refined position determination for the aircraft 100 can be obtained by means of repeated measurements of the aircraft's movement, repeated radar altitude measurements and comparisons with registered information pertaining to the ground's elevation htop over the mean sea level at particular geographical positions. Furthermore, an estimation of the aircraft's 100 elevation hmsi over the mean sea level 300 can be acquired according to the relationship hmsi = htop + hgnd (i+1).

Typically, the ground's actual elevation htop over the mean sea level is assumed to be registered at certain geographical positions in an orthogonal check pattern with 50 meters distance in each direction between two neighboring coordinate points.

This information is stored in a database in the aircraft 100. An estimation of the ground's elevation between the stored positions is preferably calculated by means of bi-linear interpolation.

Figure 4 shows an aircraft 400, which travels over a ground surface 410. The method according to an embodiment of the invention will be exemplified below with reference to the figure 4. When performing a radar altitude measurement against a terrain segment where the ground's 410 elevation varies, such

as in this figure, the radar having a lobe angle y in the aircraft 400, risk measuring a shortest radar distance hter between the aircraft 400 and the ground surface 410 in a different direction a < (p than vertically against the mean sea level. The radar distance hter is namely (roughly) registered with respect to the first received signal reflection whose power level exceeds a particular threshold level. When the aircraft 400 is positioned at a reference altitude hmslo over the mean sea level 0 at a first geographical position Xo, Yo it is therefore relatively probable that a radar signal, which has been reflected by the ground surface 410 at a second geographical position XI, Yi is the first received signal whose power level exceeds the threshold level, since at this second geographical position XI, Yi the distance between the ground surface 410 and the aircraft 400 is the shortest. When flying over the same first geographical position Xo, Yo, however at a different reference altitude h, , si, the shortest radar distance hter between the aircraft 400 and the ground surface 410 will nevertheless probably be measured against a different point beside both the first geographical position Xo, Yo and the second geographical position Xi, Y1. If, at radar altitude measurement, it is desired to compensate for the radar's inclination to measure a shortest radar distance hter between the aircraft 400 and the ground surface 410 in a different direction a than the desired direction it is thus necessary to perform such compensation not only with respect to geographical position X, Y, however also with respect to the flying altitude hmsl.

An embodiment of the invention involves calculation of a corrected radar distance hg°rdr as the sum of an expected measured radar distance hter and a measurement correction parameter ht, at particular geographical positions X, Y (typically in an orthogonal check pattern with 50 meters distance in each direction between two neighboring coordinate points) and for particular reference altitudes ohms within a certain geographical area (typically a country, a continent or the entire earth). A database in the aircraft 400 stores pre-calculated measurement

correction parameters he, for the coordinates X, Y, h,, sl together with topographical data htop.

According to another embodiment of the invention, a relevant measurement correction parameter hA is instead calculated in real time, according to the expression (1), below whenever this parameter is required.

Storing a large number of pre-calculated measurement correction parameters hA, of course, requires a relatively large database, while real time calculation of the measurement correction parameters hA demands a comparatively powerful processor. If measurement correction parameters hA are pre- calculated for particular coordinates X, Y, hums, and if a radar altitude measurement is performed in a point in space, which does not coincide with a registered coordinate X, Y, hmsl, the necessary data is interpolated from corresponding data regarding the closest registered coordinates.

Irrespective of when the measurement correction parameter hA is calculated, it is determined as a maximum difference between a correct distance hgnd at a particular geographical position and altitude, such as X0, Yo and has respectively, and an expected erroneously measured distance hter at this position and altitude.

This can be accomplished by calculating the expected erroneously measured distance hter according to the expression (1). where Xo, Yo denotes the particular geographical position, hmS denotes the particular altitude, Xi, Y1 denotes the geographical position on the surface against which the remote distance meter is expected to measure an erroneous distance, and htop (Xi, Yi) denotes the surface's vertical distance to the

mean sea level at the geographical position against which the remote distance meter is expected to measure the erroneous distance.

With information regarding the remote distance meter's expected inclination to, when performing remote distance measurement from particular space coordinates X, Y, hmsi, measure a specific shortest distance hter between the aircraft 400 and the ground surface 410 in undesired measurement directions a against the ground surface 410, it is hence possible to compensate for the anomaly by adding a corresponding measurement correction parameter ho (X, Y, hums ). A corrected distance is thus obtained by the relationship hgnd (X, Y, hmsl) hter (X, Y, hmsl) + h (X, Y, hmsl).

During aircraft navigation the aircraft's altitude ho, si (X, Y) over the mean sea level can be calculated as the sum of the ground's 410 topographical elevation htop (X, Y) over the mean sea level and the corrected distance hg°dr (X, Y, hums ). During navigation under water the craft's depth underneath the mean sea level can be calculated, in a corresponding manner, as the difference between the bottom's topographical depth dtop (X, Y) and a corrected depth d (X, Y, hms).

An embodiment of the apparatus according to the invention is shown schematically in figure 5. The apparatus includes a remote distance meter 510, which typically is constituted by a radar, however also can be a laser distance meter or an acoustical distance meter in the form of, for example a sonar.

The remote distance meter 510, in turn, includes at least one means 511 for transmission and reception of signals. The means 511 is thus at least one radar antenna, a laser including a sensor or corresponding sonar sensors. According to a preferred embodiment of the invention, the remote distance meter 510 is equipped with two separate means 511 for transmission and reception respectively. The remote distance meter 510 delivers

values of measured distances hgnd between the craft and the surface over which it moves to a control unit 540. The control unit 540 also receives signals hsh AX and AY from a movement meter 520, which represent estimations of the craft's relative movement in space.

In those cases when the craft travels above the sea level the movement meter 520 normally includes a barometric sensor, inertia gyros, accelerometers and/or a GPS-receiver (GPS = Global Positioning System). When navigating under water, the movement meter 520 instead preferably includes a pressure meter designed to measure the pressure of the water that surrounds the craft plus, for example, inertia gyros and accelerometers. When navigating a craft at sea level (i. e. typically a surface ship) it is normally sufficient if the movement meter 520 includes sensors for registering horizontal move- ments, i. e. two-dimensional inertial gyros and accelerometers for example. In addition to the signals Ahmsh AX and AY from the movement meter 520, the control unit 540 retrieves topo- graphical data htop (X, Y) and measurement models Ngnd (X, Y) for relevant geographical positions from a database 530.

According to an embodiment of the invention, the control unit 540 moreover retrieves pre-calculated measurement correction parameters hN (XXYXhms) for relevant geographical positions X, Y from the database 530. According to another embodiment of the invention, the control unit 540 instead calculates a measurement correction parameter h, (X, Y, hmsi) in real time for relevant geographical positions X, Y.

Based on a corrected distance h°rd (X, Y, has), the signals Ahmsl, AX and AY, and the information htop (X, Y) from the database 530 (alternatively : real time calculation in the control unit 540), the movement meter 520 respective the database 530, the control unit then calculates the craft's position according to the procedure described with reference to the figures 1-3 above.

Figure 6 shows a flow chart, which illustrates the general method according to the invention that is employed when a craft navigates with aid from remote distance measurement against a surface. A first step 600 assumes a measurement shell of maximum size as in the first procedure step described with reference to figure 1. A following step 610, performs a remote distance measurement whereby a measured distance between the craft and the surface is obtained. A step 620 thereafter models the remote distance meter's inclination to measure a shortest distance between the craft and the surface over which it moves in a direction that deviates from a desired measurement direction. The distance measured in the step 610 is thereby adjusted by addition of a measurement correction parameter as described above. The measurement correction parameter can either be calculated in real time or be pre-calculated for particular coordinate points between which a relevant value is interpolated when necessary. In a next step 630, a time update takes place due to the movement of the craft whereby the measurement shell is updated. A step 640 after that, performs a renewed remote distance measurement and a new distance to the surface is obtained. A following step 650 updates also the model pertaining to the remote distance meter's inclination to measure the shortest distance between the craft and the surface over which it moves in a direction which deviates from a desired measurement direction. This update is necessary because the surface beneath the craft has changed since the previous remote distance measurement. The distance measured in the step 640 is thereby adjusted by addition of a new measurement correction parameter, which in similarity with the previous measurement correction parameter, either is calculated in real time or has been pre-calculated. Subsequently, the measure- ment shell is reduced according to the procedure earlier described with reference to figure 3. A following step 660 investigates whether the measurement shell has been reduced to an empty volume, i. e. whether there is an overlap between the previous measurement shell and the latest measurement

shell. Provided that these measurement shells overlap the position can be determined with an increased accuracy and the procedure is returned to the step 630 for a time update.

Otherwise, the procedure is returned to the first step 600, where a maximum sized measurement shell again is assumed.

The above-described method steps are preferably controlled by a computer and can thus be realized in software code. Naturally, such code can be stored on arbitrary computer readable medium, be transferred via any kind of transmission medium and format and be stored into the primary memory of a general computer.

Naturally, the present invention does not preclude that parameters in addition to the suggested measurement correction parameter and the corrected distance are used in order to describe the measurement errors, which are expected to occur during remote distance measurement. In particular, it is not precluded that the invention be combined with a solution where a measurement noise parameter is utilized, which for particular geographical positions designates an estimation of the surface's inclination to cause measurement noise when measuring a distance against it.

The term"comprises/comprising"when used in this specification is taken to specify the presence of stated features, integers, steps or components. However, the term does not preclude the presence or addition of one or more additional features, integers, steps or components or groups thereof.

The invention is not restricted to the described embodiments in the figures, but may be varied freely within the scope of the claims.