Determination of frequency offsets in communication systems

Apparatus for determining frequency offsets from a desired communication signal center frequency in at least one of a plurality of user terminals in a communication system having at least one fixed signal transceiver for transferring communication signals to and from user terminals, comprising:

• means for transmitting a forward link communication signal from the at least one fixed signal transceiver using a first predetermined carrier reference frequency and a first Doppler effect pre-correction value for communication link Doppler, when known;
• means for receiving said forward link communication signal at said user terminal, and determining a first offset for associated carrier frequency relative to the first predetermined carrier reference frequency;
• means for generating a reverse link signal at a second predetermined carrier reference frequency at said user terminal also incorporating any user terminal reference frequency source error, scaled to the second predetermined carrier reference frequency;
• means for applying a frequency pre-correction to said reverse link signal based on said determined first offset;
• means for transmitting said reverse link signal to said fixed signal transceiver;
• means for receiving said reverse link signal at said fixed signal transceiver, and determining a second offset for associated carrier frequency relative to said second predetermined carrier reference frequency associated with said user terminal communication, after again applying said first Doppler effect pre-correction value for communication link Doppler when known;
• and
• means for dividing the resulting second offset in half to produce a user terminal communication link frequency offset that is to be compensated for during communications.

Apparatus for determining oscillator error in at least one of a plurality of user terminals used in a satellite communication system having at least one gateway and at least one satellite for transferring communication signals between gateways and user terminals, comprising:

• means for transmitting a forward link communication signal from at least one gateway through said satellite using a first predetermined carrier reference frequency and a first Doppler pre-correction value for gateway-to-satellite communication link Doppler;
• means for receiving said forward link communication signal at said user terminal, and determining a first offset for associated carrier frequency relative to the first predetermined carrier reference frequency;
• means for generating a reverse link signal at a second predetermined carrier reference frequency at said user terminal also incorporating any oscillator error, scaled to the second predetermined carrier reference frequency;
• means for applying a frequency pre-correction to said reverse link signal for a second Doppler value for user terminal-to-satellite Doppler, based on said determined first offset;
• means for transmitting said reverse link signal to said at least one gateway through said satellite;
• means for receiving said reverse link signal at said gateway, and determining a second offset for associated carrier frequency relative to said second predetermined carrier reference frequency associated with said user terminal communication, after compensating for said first Doppler pre-correction value;
• and
• means of dividing the resulting second offset in half and scaling to the user terminal oscillator frequency to produce a user terminal oscillator error value.

A method for determining oscillator error in at least one of a plurality of user terminals used in a satellite communication system having at least one gateway and at least one satellite for transferring communication signals between gateways and user terminals, comprising the steps of:

• transmitting a forward link communication signal from at least one gateway using a first predetermined carrier reference frequency and a first Doppler pre-correction value for gateway-to-satellite communication link Doppler;
• receiving said forward link communication signal at said user terminal, and determining an offset for associated carrier frequency relative to the first predetermined carrier reference frequency;
• generating a reverse link signal at a second predetermined carrier reference frequency at said user terminal also incorporating any oscillator error, scaled to the second predetermined carrier reference frequency;
• applying a frequency pre-correction to said reverse link signal for a second Doppler value for user terminal-to-satellite Doppler, based on said determined first offset;
• transferring said reverse link signal through said satellite to said gateway;
• receiving said reversed link signal at said gateway, and determining a second offset for associated carrier frequency relative to said second predetermined carrier reference frequency associated with said user terminal communication, after compensating for said first Doppler pre-correction value;
• and
• dividing the resulting second offset in half and scaling to the user terminal oscillator frequency to produce a user terminal oscillator error value.