Syntonization of Signals Between Satellites

There are many space applications in which a number of satellites cooperate, for example in radio astronomy (e.g., Very Long Baseline Interferometry), remote sensing (e.g., Synthetic Aperture Radar Interferometry or Passive Aperture

Synthesis) or to generate navigation signals (e.g., Global Navigation Satellite System).

In such cooperative space applications, there may be need for a common clock amongst the satellites, which clock may be used for various purposes, e.g., for timing or control. For that purpose, each satellite may comprise an onboard oscillator which may be configured to oscillate at a nominal frequency and which nominal frequency may be chosen to be the same for all satellites. However, the onboard oscillator may be unable to oscillate at precisely the nominal frequency, e.g., due to aging or due to tolerances in its internal components. Nevertheless, many space applications require the oscillator signals generated at each satellite to have a very similar frequency.

Trying to reach such similarity in frequency is also referred to as syntonisation. 

The present invention consists of a method to generate extremely similar frequencies aboard two satellites in relative motion with respect to each other. Under this method:

• The syntonization is kept over a very large range of time scales (from a small fraction of a second to years)
• The frequencies are generated from independent local oscillators at each of the satellites using an inter-satellite link
• The absolute stability of the frequencies results from the combined absolute stability of the independent local oscillators 

The general block diagram of the concept with 2 satellites is shown in Figure 1;

• Sat-2 is symmetric to Sat-1.
• On Sat-1 a master oscillator at F1 is sent over an inter-satellite link (ISL) to Sat-2 and also locally P-frequency multiplied into P×F1, with P>1.
• The P×F1 tone is used to up-convert the received F2’ from Sat-2 in a first mixer (M-A), where the prime denotes ‘Doppler shifted’.
• The Upper Side Band (USB) at P×F1+F2’ is selected by an USB filter (USB-A) at the mixer output and used to up-convert, in a second mixer (M-B), a (P−1)-frequency multiplied version of the received F2’.
• The USB at P×(F1+F2’) available at the output of the second mixer is selected by an USB filter (USB-B) as desired frequency.
• Syntonisation:

The frequency difference between the output frequencies of the two satellites is β×P×(F2−F1), with β=v/c being the ratio of the radial velocity between the two spacecraft v and the speed of light c. 

The invention can be extended to the case of N satellites.

Innovations and advantages:

The invention is a low cost approach to syntonize two satellites. It does not require flying atomic clocks for example if precise syntonisation is the only requirement for a particular application. Moreover, it is the only way to achieve coherence for long enough periods of time for the specific application of the black hole Event Horizon Imager.

It can be implemented with standard microwave components and existing inter-satellite link equipment.

Domain of application:

This concept could be used in any application where two satellites have to operate in a syntonised manner, that is, using the same frequency at any time, as for example: Very Long Baseline Interferometry from space, remote sensing from cooperative satellites (Synthetic Aperture Radar Interferometry and Passive Aperture Synthesis) or the generation of satellite navigation signals (like those from GNSS). 

IP Status

An International patent application (PCT) has been filed.