FIELD

A multimodal approach to achieve improved efficiency and resiliency for In-Route (IR) Timing synchronization. Ephemeris based timing (EBT) uses the satellite ephemeris data to achieve inroute timing synchronization. When the ephemeris data is not available, the system transitions via a Temporary Outage Mode (TOM) and a Short-Term Outage Mode (STOM) to an EBT-CLT mode where the satellite location is assumed fixed at the center of box. The aperture size for each of these modes may be the same. In the EBT-CLT mode, gateway feedback to a terminal helps keep the timing synchronization. Prediction of TROat the terminal may reduce the number of CLT messages required by up to 75%.

BACKGROUND

Each gateway and its terminals are interconnected in a star topology with the gateway at the center of the star and the terminals at the points of the star. Each gateway broadcasts multiple outroute carriers to each of its beams. The terminals use TDMA to access shared inroute channels for transmissions through the satellite to the gateway. For the purposes of inroute timing synchronization, each gateway-beam pair can be treated as an isolated and independent TDMA network, which manages its inroute timing synchronization independently of the other gateways and beams.

An ephemeris file is periodically distributed by the Satellite control facility to the network components. The gateways interpolate the coarse data to achieve a finer resolution before sending the current satellite location to the terminals via the SFNP message. The terminals use their own location, the GW location and the satellite location to achieve timing synchronization.

TDMA requires that each terminal transmit its data bursts to the satellite for relay to the gateway such that the bursts start within a narrow window of time, the aperture, at the gateway. For different terminal locations within a beam, the propagation time from a terminal to the gateway through the satellite can vary by several milliseconds (ms). Further, due to the satellite movement in its orbit and/or movement of the terminal, the propagation time between the terminal and the gateway via the satellite varies continuously. These variations differ for different terminal locations. These variations require that each gateway and terminal execute procedures to determine exactly when the terminal should transmit a data burst, so that it will arrive at the gateway at the assigned times (i.e., within the aperture).

The inroute time axis is divided into units of superframes, frames and slots. An exemplary Superframe duration (TSF) is 360 ms and an exemplary frame duration (TFRM) is 45 ms for frames in the Superframe. The frame duration is such that a superframe will contain an integer number (M) of frames. Each frame contains an integer number of slots (NSL), and each slot contains an integer number of symbols (NSYM).FIG. 1illustrates a timing relationship between a gateway and a terminal. A vertical line on the figure shows what is happening at the gateway (at the top) and at the terminal (at the bottom) at the same time. On the inroute, the horizontal (time) axis is marked in units equal to the length of an inroute frame. Terminals access the inroute by transmitting in bursts, each of which occupies multiple slots. A unique word (UW) is placed at the start of the burst in order to allow burst detection at the gateway. In some embodiments, a burst can be detected when at least a last symbol of the UW arrives within the aperture. In other embodiments, the UW need not arrive within the aperture,

SUMMARY

This Summary is provided to introduce a selection of concepts in a simplified form that is further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

The present teachings disclose an efficient and resilient system to determine the timing offset TRO to compensate for time varying gateway-satellite-terminal propagation delay.

A system of one or more computers can be configured to perform particular operations or actions by virtue of having software, firmware, hardware, or a combination of them installed on the system that in operation causes or cause the system to perform the actions. One or more computer programs can be configured to perform particular operations or actions by virtue of including instructions that, when executed by data processing apparatus, cause the apparatus to perform the actions. One general aspect includes a method for determining inroute frame timing for a Very Small Aperture Terminal (VSAT). The method includes receiving an appointment to transmit, on an inroute, at a start of a slot X of a frame number M; establishing, at a VSAT, an arrival time of a super frame numbering packet (SFNP) including a satellite ephemeris vector and a frame number N; calculating, at the VSAT, a timing offset (TRO) to be applied to the arrival time to compensate for a time varying gateway-satellite-terminal propagation delay (THS+TSR); setting a transmit instant as an end of the TRO after the arrival time; adding to the transmit instant a duration of X slots and a duration of (M−N) frames; and transmitting a burst, on the inroute from the VSAT, at the transmit instant. In the method, the calculating is based on computing THS+TSR from the satellite ephemeris vector, a gateway transmits the SFNP and receives the burst in the slot X within the frame number M of the inroute, and N is greater than or equal to M. Other embodiments of this aspect include corresponding computer systems, apparatus, and computer programs recorded on one or more computer storage devices, each configured to perform the actions of the methods.

Implementations may include one or more of the following features. The method where the transmit instant has an aperture size of 15 microseconds or less. The method may include determining a TRO reference (TRO-ref) by requesting timing information from the gateway via an asynchronous burst, where the TRO-ref is used for the calculating. The method may include determining an estimated TRO (TRO-est) via a ranging ALOHA session, where the TRO-est is used for the calculating. The method where the calculating updates the TRO based on a new satellite coordinate in a new SFNP. The method where the calculating is based on a timing feedback include an inroute burst timing offset. The method may include requesting a timing feedback when the VSAT has been idle for longer than a timer threshold or when the SFNP indicates that the gateway requests a timing reacquisition. The method where the satellite ephemeris vector includes an estimated center-of-box vector for a satellite location. The method may include predicting, a next inroute burst timing offset, based on previous timing feedbacks. The method where the appointment is received by the VSAT via a bandwidth allocation packet. Implementations of the described techniques may include hardware, a method or process, or computer software on a computer-accessible medium.

One general aspect includes a method for using ephemeris data for inroute timing. The method includes receiving ephemeris prediction data may include ephemeris vectors over a usable period; validating the ephemeris prediction data; calculating satellite-to-gateway delays for transmitting RF to the ephemeris vectors; and sending, to a VSAT, a SFNP including one of the ephemeris vectors and an associated satellite-to-gateway delay. Other embodiments of this aspect include corresponding computer systems, apparatus, and computer programs recorded on one or more computer storage devices, each configured to perform the actions of the methods.

Implementations may include one or more of the following features. The method may include interpolating the ephemeris prediction data over the usable period to obtain ephemeris vectors at a fine resolution, where the calculating calculates the satellite-to-gateway delays for the ephemeris vectors at the fine resolution, the ephemeris prediction data is at a gross resolution, and the fine resolution has a smaller duration than the gross resolution. The interpolating may include linear interpolation using two adjacent in time datapoints from the ephemeris prediction data. The interpolating may include smoothing the ephemeris vectors at the fine resolution when a round-trip time difference between two adjacent in time datapoints in the ephemeris prediction data exceeds a threshold. The validating may include a time-interval validation. The validating may include a data validation. The validating may include a file transition validation. The validating may include smoothing a transition, using a weighted average, from the ephemeris predication data to a previous ephemeris predication data. The method may include determining the ephemeris vectors and associated satellite-to-gateway delays based on a center-of-box location of a satellite when the validating indicates a failure in the ephemeris predication data. The method may include acquiring valid ephemeris prediction data; and smoothing a transition, using a weighted average, from using the ephemeris vectors based on the determining to valid vectors calculated based on the valid ephemeris predication data. Implementations of the described techniques may include hardware, a method or process, or computer software on a computer-accessible medium.

Additional features will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of what is described.

