GB2432486A

GB2432486A - Method for reducing the data age of image products obtained by earth observation satellites

Info

Publication number: GB2432486A
Application number: GB0622364A
Authority: GB
Inventors: Bernd Brand; Tino Zehetbauer
Original assignee: Deutsches Zentrum fuer Luft und Raumfahrt eV
Current assignee: Deutsches Zentrum fuer Luft und Raumfahrt eV
Priority date: 2005-11-22
Filing date: 2006-11-09
Publication date: 2007-05-23
Anticipated expiration: 2026-11-09
Also published as: DE102005055918B3; GB0622364D0; FR2893794B1; FR2893794A1; GB2432486B

Abstract

For reducing the data age of image products obtained using a corresponding constellation of earth observation satellites, it is provided that, using a first earth observation satellite (Sat1) of the constellation, which satellite is outside the receiving region (32) of a user ground station (2), a picture is taken of a target region (P), and the picture is stored in the form of digital image data in the first earth observation satellite (Sat1). The stored digital image data are transmitted to a mailbox ground station (1) provided in a region flown over by all earth observation satellites (Sat1 to Sat3), when the first earth observation satellite (Sat1) has reached the receiving region of the mailbox ground station (1), and the digital image data are then temporarily stored in the mailbox ground station. The image data of the target region (P) temporarily stored in the mailbox ground station (1) are transmitted to a further earth observation satellite (Sat2) moving into the receiving region (31) of the mailbox ground station (1). When the further earth observation satellite (Sat2) enters the receiving region (32) of a user ground station (2), the image data stored in its on-board memory are transferred to the user ground station (2).

Description

Method for reducing the data age of image products obtained by earth

observation satellites The invention relates to a method for reducing the data age of image products obtained by use of a corresponding constellation of earth observation satellites, and a system for performing said method.

In the use of satellites for earth observation, users frequently wish to have the fastest possible access to image data residing in a satellite. The length of time between the pick-up of the image data and the availability of the image product is referred to as the "age" of the information. The amount of this age is determined by the required time for transmitting the data obtained of a target to the user site.

In many satellite missions, it has up to now been customary that the user is located in the data receiving station of the respective satellite and has to wait until the satellite has moved into the receiving range of the antenna. Since the possibility to contact the satellite will exist only during the period when the latter is flying over the ground station, a disadvantage of this method resides in the large time gaps in which no contact is possible between the ground station and the satellite and the data age is adversely affected thereby.

Another presently available method for quicker access to image data residing in a satellite consists in the use of existing ground station receiving networks; in this method use can be made of antenna installations distributed worldwide. An example hereof is the "IGS (International Ground Stations) -Network" for the Landsat 5 and Landsat 7 satellites, which network makes it possible to receive image data on many places worldwide.

The purpose of such networks is the in-situ reception of data by geographically widely distributed users, also with the possibility of data transfers between the participating stations. The onward conveyance of the satellite data between a reception site and a user is performed by data transmission via terrestrial data lines or by communication satellites.

The disadvantages of using ground station networks of the above type become apparent in cases where no geographically widespread user groups are involved or when classified, safety-relevant (military) data have to be transmitted. The establishing of such a geographically widespread data receiving network for individual user will, however, entail considerable costs. This applies particularly to safety-relevant data of earth observation satellites wherein a tap-proof, access- protected onward transmission between a receiver station and a user must be safeguarded.

When considering, at the same time, the high data quantities which may be in the order of magnitude of several gigabits per image product, the use of public networks (Internet) has to be excluded.

As a consequence, an expensive installation of terrestrial data routes or the use of high-cost data channels via communication satellites will be necessary.

High expenses for the establishing of suitable terrestrial data connections are to be expected particularly if, due to marginal conditions related to the mechanics of satellite routes, the satellite receiving system is located at sites on the surface of the earth which are problematic regarding their geographic accessibility.

The above is true particularly of the near-pole regions in the area of the north and south poles which are considered as A. particularly suitable locations for the placement of receiving antennae for earth observation satellites since there, due to the inclination of the orbit normally provided for this type of satellites, a contact possibility exists virtually during each orbit cycle around the earth.

From DE 43 24 515 C2, there is known a method for extending the communication period of a spacecraft or satellite for earth observation so as to be able to transmit data to one or a plurality of ground or receiving stations also in real time, without the need for storage or intermediate storage of data on storage media aboard the spacecraft. For this purpose, use is made of auxiliary satellites arranged on the same orbit as the satellites for earth observation and transmitting the data onward to the ground or receiving stations without intermediate storage.

US-5,561,837 A describes an earth observation satellite System wherein an intermediate storage of the data in the satellite is provided whenever the satellite is in a position outside the receiving range of the ground stations. For cases when the intermediate storage in the respective satellite should fail, an additional relay satellite is provided in visual contact with the satellite; the data will then be transmitted to the relay satellite and then be intermediately stored. When the relay satellite is in the receiving range of the respective ground station, the data will be transmitted to the ground station.

Thus, it is the object of the invention, while largely avoiding the disadvantages of previous methods for the transmission of data from an earth observation satellite to the user, to design a method for reducing the data age of image products obtained by earth observation satellites in such a manner that, without the use of expensive ground station receiving networks and without installation of additional, likewise expensive data connections between the receiving site and the user, the user can nonetheless gain quick access to the satellite data.

According to the invention, the above object is achieved, in a method for reducing the data age of pixels while using corresponding constellations of earth observation satellites, by the features indicated in the characterizing part of claim 1.

Further, there is provided a system for performing the method of the invention.

According to the invention, a plurality of satellites provided for earth observation are arranged preferably on near-pole orbits, preferably with different lengths of the orbit intersections.

There will be required orbits arranged in close proximity to the poles to thus make it possible to determine a smallest possible geographic region which all of the satellites of the constellation can fly over at the smallest possible intervals, i.e. -if possible -during each orbit cycle around the earth. In case of satellites on near-pole orbits, i.e. on orbit planes with an inclination of about 900 relative to the earth's equator, such regions can be determined as sections delimited by nearpole circles of latitudes, with the north or south pole each time forming the centre.

The size of these surfaces will depend on the extent to which the inclination of the satellite orbit planes deviates from 900. In the ideal case of a polar orbit having an orbit inclination of 90 , the region flown over by all earth observation satellites on each of their orbits around the earth will be reduced to a point, notably the north and south poles. For a constellation wherein all earth observation satellites have the same orbit inclination, differing from 90 , the geometric shape which is flown over by each earth observation satellite during each orbit cycle will be a circle (circle of latitudes). If the constellation consists of earth observation satellites having different inclinations, the

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corresponding above-mentioned region will be a spherical segment of the earth's surface delimited by two circles of latitudes.

Regarding the definition of the above described regions (point, circle, spherical segment), what is to be observed is always the normal projection of the satellites onto the rotating surface of the earth. Additionally, as an effect of minor influence, it is to be noted that the orbit plane of satellite orbits will over time tend to rotate about the north/south pole axis relative to a reference system fixed in space, which effect is caused by the uneven mass distribution within the earth's globe.

A further feature of the invention resides in the arrangement, additionally to the user ground station, of a further ground station -referred to hereunder as mailbox ground station -for data connection to the earth observation satellites in the above described region which is flown over by preferably all earth observation satellites at brief intervals. By this special arrangement of the mailbox ground station, it is now made possible to establish a connection to virtually each earth observation satellite of the constellation during each orbit cycle.

In case that, for technical and/or financial reasons, it should not be possible to arrange the mailbox ground station in the described region which is flown over by all earth observation satellites at brief intervals, also other regions can be selected for the mailbox ground station. This, however, has the disadvantage that in this case not each orbit cycle performed by the earth observation satellites can be utilized for a data connection. Thus, there should always be selected a region which is suitable to be flown over by as many earth observation satellites as possible and at intervals as short as possible. For the near-pole satellite orbits as commonly used for earth observation missions, these regions are located in geographic areas with preferably high altitudes.

According to the invention, the mailbox ground station is positioned in such a manner that, during fly-over by one or a plurality of earth observation satellites, it will be possible both to receive the image data residing in the earth observation satellite(s) and to transmit the data temporality stored in the mailbox ground station to selected earth observation satellites of the constellation. By the inventive arrangement of only one mailbox ground station in a region which is flown over preferably by all earth observation satellites of the constellation at brief intervals, there is no need anymore for the above-described expensive use of ground stations distributed world-wide, nor will there be the requirement to perform a likewise expensive and technically complex data transfer from these stations to the user.

Further, according to the invention, there are preferably required different node positions of the individual satellite orbit planes.

A node position describes the position of the virtual piercing points of the satellite orbits through the equator plane and, in combination with the above-described inclination, describes the position of the satellite orbit plane in space. By suitable selection of the node position when designing a satellite route, it can be determined at which times certain regions on the earth's surface will be flown over.

A further problem resides in the sometimes very long connection gaps between an earth observation satellite and the user ground station, which can be in the range of ten hours and more. These gaps are largely caused by the relative movement between the earth observation satellite on its orbit plane and the surface of the earth which due to the earth's rotation is turning under the satellite.

In case of the above described contact gaps, it may happen that, under the effect of the earth's rotation, the user ground station and consequently the spatial receiving range of this station have moved into such a far distance from the satellite orbit plane that the earth observation satellite is not capable anymore during its orbiting to get into the receiving range of the user ground station.

The influence of the drift of the satellite orbit plane caused by the uneven mass distribution of the earth's globe, which drift is relatively small in comparison to the earth's rotation, is negligible with regard to the subsequent considerations because, during the time periods of relevance here, it will lead to merely insignificant shiftings between an earth observation satellite and a receiving range of the ground station.

By suitable selection of the orbit nodes for the individual earth observation satellites of the constellation, the individual orbit planes can be arranged to the effect that, provided that the number of satellites is sufficient, in the ideal case there will at all times one or a plurality of earth observation satellites flying through the receiving range of the user ground station during their orbiting movement. Should the number of satellites be too small, it is at least possible to reduce the time periods during which none of the earth observation satellites in orbit can contact the user ground station.

According to the invention, the mailbox ground station is used for intermediate storage of the image data of an earth observation satellite of the constellation which currently has a reduced possibility to contact the user ground station; thus, these image data can then be transmitted to another earth observation satellite of the constellation which, when next having the occasion to contact the user ground station, will provide an image whose data age is satisfactory to the user.

In the invention, in contrast to previously employed methods performed by using only one (user) ground station or using worldwide networks of ground stations, the long connection gaps occurring when using only one (user) ground station, are avoided along with their negative influence on the age of the image information. Further, the need for relay stations or special terrestrial data connections between the ground stations or the worldwide network and the user ground station is obviated.

Further, also the confidentiality of the data is safeguarded because the data do not have to be transmitted via public networks.

A central component according to the invention is the mailbox ground station which is located on the surface of the earth and serves for intermediate storage of the data received from earth observation satellites flying across the respective region. The intermediate storage of the data has to be maintained until the earth observation satellite selected for onward transmission of the data will fly over the mailbox ground station.

Apart from the data store, there is provided a data receiving unit and a data transmission unit, consisting of receiving hardware and transmission hardware, respectively, and preferably a common antenna. The receiving and the transmitting of the data is started respectively in response to commands predetermined for use between the mailbox ground station and the earth observation satellites and can be realized in a fully automated manner, thus offering also the option to use an unmanned and thus economically advantageous mailbox ground station. To avoid costs for setting up the mailbox ground station particularly in polar regions, one can use the existing infrastructure of ground stations in these geographic regions.

In the above described variant of a fully automated mailbox ground station, the required hardware can be stored also in a buoy anchored in polar waters. For applications where the use of satellite data with short age of information is required only for a relatively restricted period of time, i.e. for instance in satellite-based recognition for military purposes in local conflict areas, the hardware of the mailbox ground station can also be arranged on a ship.

While the above described device is located on the ground as part of the mailbox ground station, the performing of the inventive method can also be favourably influenced by a suitable design of the earth observation satellite. To avoid a disturbance of the pick-up of target data performed by the earth observation satellite, the transmitting and receiving unit required on the satellite side is, according to the invention, to be configured to allow the inventive method to be performed via the mailbox ground station independently of the actual target observation function of the earth observation satellite. Particularly, the antenna required for exchange of data with the mailbox ground station is to be arranged on the earth observation satellite in such a manner that the antenna can be oriented towards the mailbox ground station independently of the current spatial position of the earth observation satellite.

The present invention will be described in greater detail hereunder with reference to the accompanying drawings wherein: Fig. 1 is a schematic representation of the geometric conditions and the participating components for a constellation comprising three earth observation satellites on different orbit levels, a user ground station and a mailbox ground station; Fig. 2 is a schematic representation illustrating the pick-up of target data of a target region by an earth observation satellite; Fig. 3 is a schematic representation illustrating the transfer of the target data from an earth observation satellite to the mailbox ground station; Fig. 4 is a schematic representation illustrating the transfer of the target data from the mailbox ground station to a further earth observation satellite, and Fig. 5 is a schematic representation illustrating the delivery of the target data to a user ground station.

In Fig. 1, the geometric conditions and the participating components for a base station are illustrated. Three earth observation satellites Satl to Sat3 are each arranged on near-pole orbits, while the orientation (node position) of the orbit plane, not shown in greater detail, is different for each earth observation satellite. Further illustrated are a geographic region around the North Pole NP, which region is observed by the three earth observation satellites Satl to Sat3 during each orbit cycle, and an additional mailbox ground station 1 arranged in this geographic region.

Due to the identical orbit inclination of the three earth observation satellites Sati to Sat3 in the illustrated example, the respective region flown over by the three earth observation satellites during each orbit cycle presents itself as a circle of latitudes. In Fig. 1, the borderline 3 of the receiving region of the mailbox ground station 1 is schematically indicated by a chain-dotted line which is represented by the projection of an edge curve onto the surface of the earth. The borderline 32 of the receiving region of a user ground station 2 is indicated by an interrupted line. I'

Next, a step-wise description will be given of an image pick-up process performed by the earth observation satellite Sati along with the subsequent transfer of the image data by the mailbox ground station 1 to the earth observation satellite Sat2.

Step 1: By the earth observation satellite Sati, a picture is taken of a target region P. In Fig. 2, it can be seen that the earth observation satellite Sati, while taking the picture of the target region P, is located outside the receiving region 32 of the user ground station 2 so that the image data residing in the earth observation satellite Sati cannot be transmitted to the user ground station 2. Further, Fig. 2 shows that, due to the earth's rotation indicated by an arrow, the receiving region of the user ground station 2 will move ever farther away (to the right-hand side) from the earth observation satellite Sati and thus for a longer period of time will not be flown over by the earth observation satellite Sati anymore.

Step 2: As depicted in Fig. 3, the earth observation satellite Sati has reached the receiving region 3 of the mailbox ground station 1 and is transmitting the image data from its on-board memory to the mailbox ground station 1 where the image data are temporarily stored. Since it is safeguarded that a possibility for contact to the mailbox ground station 1 will exist for each orbit cycle of the earth observation satellite Sati, the above process can be carried out without a larger delay.

Step 3: As evident from Fig. 4, the image data of the target region P which have been temporarily stored in the mailbox ground station 1 are transmitted to the earth observation satellite Sat2 which, lust as the earth observation satellite Sati, can utilize a contact possibility to the mailbox ground station 1 during each of its orbit cycles.

Step 4: After the image data have now been stored in the on-board memory of the earth observation satellite Sat2 and the latter is located in the receiving region 32 of the user ground station 2, the data can be transmitted to the user ground station 2 still during the same orbit cycle. At this time, as can be gathered from Fig. 5, there would still have been a long waiting period for a contact possibility between the earth observation satellite Sati and the user ground station 2. Thus, without using the earth observation satellite Sat2, the data age of the image product of the target region P would be correspondingly unfavourable.

Claims

Claims 1. A method for reducing the data age of image products obtained

at a corresponding constellation of earth observation satellites, characterized in that a) using a first earth observation satellite (Sati) of the constellation of earth observation satellites (Sati to Sat3), which satellite is outside the receiving region (32) of a user ground station (2), a picture is taken of a target region (P) and the picture is stored in the form of digital image data in the first earth observation satellite (Sati); b) the digital image data stored in the first earth observation satellite (Satl) are transmitted to a mailbox ground station (1) provided in a region flown over by all earth observation satellites (Sati to Sat3) at the smallest possible time intervals, said transmission being performed when the first earth observation satellite (Sati) has reached the receiving region of the mailbox ground station (1), and the digital image data are then temporarily stored in the mailbox ground station (1);

c) the image data of the target region (P) temporarily stored in the mailbox ground station (1) are transmitted to a further earth observation satellite (Sat2) of the satellite constellation, which during its orbit cycle moves into the receiving region (3) of the mailbox ground station (1), and d) when said further earth observation satellite (Sat2) enters the receiving region (32) of a user ground station p..

(2), the image data stored in its on-board memory are transferred to the user ground station (2)
2. A system for performing the method according to claim 1, characterized in that the earth observation satellites (Sati to Sat3) of a satellite constellation are arranged on selected orbit planes such that, to make it possible that data taken from a target region (P) can be conveyed onward from an earth observation satellite (Sati to Sat3) of the satellite constellation via the mailbox ground station (1) to the user ground station (2) within the shortest possible time, the user ground station (2), by corresponding selection of orbit nodes, is flown over by at least one of the earth observation satellites (Sati to Sat3) of the satellite constellation at substantially equal time intervals.

3. The system according to claim 2, characterized in that all earth observation satellites (Sati to Sat3) of the satellite constellation are arranged on near-pole orbits having an orbit-plane inclination relative to the equator of about 90 , whereby geographic regions near the north and south poles are determined which are flown over by all earth observation satellites (Sati to Sat3) of the satellite constellation during each orbit cycle, and that the mailbox ground station (1) is arranged in one of these geographic regions.

4. The system according to claim 2, characterized in that the mailbox ground station (1) is at least arranged in geographically northern or southern regions located at the highest possible elevations so that the receiving region of the mailbox ground station (1) is flown over by the earth observation satellites (Sati to Sat3) of a satellite constellation with sufficient frequency.

5. The system according to claim 2, characterized in that the mailbox ground station (1) comprises a data storage unit, a data transmitting and receiving unit, and an antenna to be used for transmitting as well as receiving.

6. The system according to claim 2, characterized in that, in fully automated operation, the mailbox ground station (1) can be arranged on a buoy anchored in polar waters.

7. The system according to claim 2, characterized in that the mailbox ground station (1) is arranged on a ship.

8. The system according to claim 2, characterized in that an antenna provided on earth observation satellites of the satellite constellation for exchange of data with the mailbox ground station (1) can be oriented towards the mailbox ground station (1) independently of the current spatial position of the respective earth observation satellite.

9. A method of acquiring an image of a terrestrial target region by way of a constellation of earth observation satellites which are remotely controlled from a user ground station and which all pass over a mailbox ground station at smallest possible time intervals, comprising the steps of controlling a first one of the satellites while outside a given receiving range of the user ground station to take an image of the target region and store the image in the form of digital data and to then transmit the stored data to the mailbox ground station when in a given receiving range thereof, the mailbox ground station being arranged to store the data transmitted from the first satellite and to then transmit the stored data to a second one of the satellites for storage therein when in the receiving range of the mailbox ground station, and controlling the second satellite to transmit the stored data to the user ground station when in the receiving range thereof.