GB2380626A

GB2380626A - Navigation satellite system

Info

Publication number: GB2380626A
Application number: GB0123893A
Authority: GB
Inventors: Barry James Darby; Pierre Diederich
Original assignee: Thales Research and Technology UK Ltd
Current assignee: Thales Research and Technology UK Ltd
Priority date: 2001-10-04
Filing date: 2001-10-04
Publication date: 2003-04-09
Anticipated expiration: 2021-10-04
Also published as: GB2380626B; GB0123893D0

Abstract

To remove the requirement for accurate atomic clocks in navigation satellite systems, such as Global Positioning System (GPS, GLONASS and GALILEO), ground stations 14 transmit timing signals via a communications uplink 16 to a plurality of medium earth orbit navigation satellites 2 via communication satellites 18 in either a medium earth, or geostationary orbit. This enables the navigation satellites to use simpler clocks such as a crystal oscillator. Long term drift of these clocks is compensated by the timing pulses transmitted via the communications satellite. The navigation satellites 2 transmit the timing signals 10 to a ground based monitoring and control station 8 to carry out any corrections, these corrections 12 are passed to the ground station 14 to complete a feedback loop of the corrected timing signals.

Description

NAVIGATION SATELLITE SYSTEM The present invention relates to a navigation satellite system. In particular, the present invention relates to a method and apparatus for providing accurate onboard clocks for the satellites in such a system.

The most well known position fixing system is the satellite navigation GPS system owned and operated by the USA Department of Defense. Russia has a similar system called Glonass and the European Union is also developing a system called Galileo. All of these systems work on a similar principle whereby the reference transmitters are onboard satellites and in this application the general term global navigation satellite system (GNSS) will be used to refer to all such systems.

Taking the GPS system as an example, the basic system consists of a number of satellites orbiting the earth in medium earth orbits (MEOs). A receiver station can determine its position by receiving and interpreting signals transmitted by a number of the satellites.

In principle, the GPS system operates by the receiving station determining the time taken for a given signal to reach it from the given satellite. From this information, the distance to the satellite in question can be determined. If the receiving station can receive substantially simultaneous data from a sufficient number of satellites, and it knows the position of the relevant satellites, then the position of the receiving station can be determined.

The accuracy of the system depends in part on the accurate measurement of the time taken for the signal to reach the receiving station and so depends on any relative error between a clock on board the satellite and a clock associated with the receiving station.

Accordingly, highly accurate atomic clocks are used in GPS satellites to provide the necessary precise time and

the GPS ground control system monitors the satellite signals and transmits commands as needed to the GPS satellites in order to ensure satellite clock accuracy.

It is not unknown for such clocks to fail, thereby rendering useless an otherwise healthy satellite; for that reason each satellite carries three clocks.

However, such atomic clocks are expensive.

An alternative exists, whereby an ensemble of clocks on the ground produce a highly accurate time signal which is then uploaded to the navigation satellites. Closed loop feedback ensures that each satellite transmits signals accurately synchronised to their system time.

The closed loop increases systems integrity and building critical components into ground stations makes repair or replacement possible.

However, applying this alternative method to a complete constellation of an MEO navigation satellite system (which comprises twenty-four or more satellites) requires continuous uplinks with handovers from an array of steerable dishes. The dishes need to be large in

order to reduce the risk of signal disruption from manmade interference and for global coverage around two hundred dishes would be required, appropriately distributed around the globe.

The present invention aims to provide a system that reduces some or all of the above problems.

Accordingly, in a first aspect the present invention provides a navigation satellite system including: a plurality of navigation satellites, each having means for receiving a time signal from a communications satellite.

In this way, a single ground station can communicate with a single communications satellite which in turn can communicate the time signal to a plurality of the navigation satellites (which may be some or all of the navigation satellites in the particular GNSS).

The system may include at least one communications satellite, which will include means for transmitting the time signal. The system may also include one or more

ground stations for transmitting the time signal (or a signal including time information) to the communications satellite (s).

Preferably the navigation satellites are MEO satellites. Also the communications satellite may be a geostationary (GEO) satellite, which provides the advantage that the ground station can be simpler e. g. need not include a steerable dish.

In one embodiment, it is proposed to uplink the ground generated time signals through conventional communications channels of e. g. transparent transponders on board one or more GEO satellites. The system may include a plurality of communications satellites e. g. in order to provide greater coverage or more frequent communication with the navigation satellites. For example twenty channels on at least three evenly distributed GEOs (e. g. as currently provided by the Inmarsat system) could provide sufficient redundancy of downlinks to each navigation satellite.

In one embodiment, each channel of the communications satellite system could be used to transmit ground generated time signals to each of the MEO satellites in the constellation. The time transmitted by each of the MEO satellites could be individually adjusted e. g. by sampled closed loop control.

However, in an alternative embodiment each of the navigation satellites could carry an onboard clock (albeit preferably a simpler and less expensive clock than an atomic clock) and the time signal sent by the communications satellite would in that case be a time correction signal to correct the onboard clocks. The onboard clocks could then be controlled (possibly continuously) from the ground through relatively narrow band channels (e. g. lkbps) through one or more communications satellites described above.

One example of a suitable onboard clock is a crystal oscillator, which generally has superior short term characteristics to an atomic clock. Many other suitable oscillators or resonators may be used e. g. a d. r. o. or

even a tuning fork provided it has sufficient stability for this application.

Crystal oscillators usually suffer from long term drift but this would be substantially eliminated in real time using the present invention by comparison of the crystal oscillator with an accurate clock (e. g. an ensemble of atomic clocks) based on the ground. Preferably the system of the present invention also includes ground based monitoring and control means for receiving time signals from the navigation satellites and producing the time signal or time correction signal as appropriate for transmission by the ground station.

Preferably some or all of the navigation satellites are provided with an antenna each for receiving the time signals or time correction signals. Preferably this wide beam antenna points to the communications satellite and in use is usually directed away from the earth. In this embodiment, it is anticipated that the communications satellite will have a higher orbit than the navigation satellites (i. e. be further away from the earth) and this

arrangement will reduce the effect of any ground generated interference.

In a further aspect, the present invention provides a navigation satellite including a clock other then an atomic clock (e. g. a crystal oscillator) and the means for receiving from a communications satellite a time correction signal for correction of the onboard clock.

In a further aspect, the present invention provides a method of operating a navigation satellite system including the steps of 1) transmitting a time signal from a ground station to a communications satellite; and 2) transmitting the time signal from the communications satellite to a plurality of navigation satellites.

As mentioned above, each navigation satellite may include an onboard clock, in which case the time signal is a time correction signal for correction of the onboard clocks.

An embodiment of the present invention will now be described with reference to the accompanying drawing.

Fig. 1 is a schematic diagram of a navigation satellite system according to an embodiment of the present invention.

In Fig. 1, a navigation satellite system includes a plurality of MEO navigation satellites 2. The MEOs transmit signals 4 to users 6, whereby the users can calculate their position.

The system also includes a monitor and control station 8 which produces a time signal 12 which is transmitted to a ground station 14 and then further transmitted via a multiplexed uplink 16 to a communications satellite 18, which may be a geostationary satellite. In turn the geostationary satellite 18 transmits time information to each of the navigation satellites 2. The feedback loop is completed by the monitor and control station 8 receiving time signals from the navigation satellites 2.

As explained previously, the system may include more than one geostationary satellite 18 in order to provide a

further or better coverage to some or all of the navigation satellites 2.

Also as explained previously, each of the navigation satellites 2 may include an onboard clock, in which case the time signal 12 is a time correction signal for correction of the onboard clocks.

This invention has been described by way of example only and modifications will be apparent to those skilled in the art.

Claims

CLAIMS 1. A navigation satellite system including: a communications satellite, which includes means for transmitting a time signal, a plurality of navigation satellites, each having means for receiving a time signal from the communications satellite and for transmitting a time signal, a ground station including control means for comparing time signals received from the navigation satellites with a reference time source and for transmitting a time signal to the communications satellite, wherein the communications satellite system is usable to transmit ground generated time signals to each of the navigation satellites.
2. A system according to claim 1 wherein the communications satellite is a geostationary (GEO) satellite.
3. A system according to any of claims 1-2 in which each of the navigation satellites includes an onboard clock and the time signal sent by the communications

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satellite is a time correction signal to correct the onboard clocks.
4. A system according to claim 3 wherein the onboard clocks are crystal oscillators and the navigation satellites each include means for receiving from a communications satellite a time correction signal for correction of the onboard clock.
5. A navigation satellite including a clock other than an atomic clock and means for receiving from a communications satellite a time correction signal for correction of the onboard clock.
6. A method of operating a navigation satellite system including the steps of i) transmitting a time signal from a ground station to a communications satellite; and ii) transmitting the time signal from the communications satellite to a plurality of navigation satellites.

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7. A method according to claim 6 wherein the communications satellite is a geostationary (GEO) satellite.
8. A method according to claim 6 or claim 7 in which each of the navigation satellites includes an onboard clock and the time signal sent by the communications satellite is a time correction signal to correct the onboard clocks.
9. A method according to claim 8 wherein the onboard clocks are crystal oscillators.
10. A method according to any of claims 6-9 wherein control means are used for receiving time signals from the navigation satellites and producing the time signal or time correction signal for transmission by the ground station.