Archive for the ‘TCR’ Category

TCR Antenna pattern basics

Satellite control refers to Tracking Telemetry and Command (TT&C) as the operations interfaces to control the satellite throughout its mission life cycle. These links are maintained through the communications antennas system. For launch and contingency operations additional antennas are added to allow control links when the satellites Earth deck is not pointed directly at the Earth.

During launch and contingency operations it is essential to maintain command and telemetry links to the satellite. Until the satellite is placed into Geosynchronous orbit and the sensors are locked onto the earth it is spun at a target RPM to keep it stable through the orbit and the communications antennas are stowed and can not used.  Or in cases where Earth lock is lost the satellites communications antennas are not pointed at the Earth. For these conditions additional transmit and receive antennas are selected and positioned on the satellite to provide link coverage as close to 360 degrees around the satellite.

In this case horn type antennas with a 30 degree beam width are placed on the normally Earth facing deck and the opposing Aft deck. The  Earth facing horn antennas have a beam width that allows the operations team to stabilize the satellite and reestablish pointing control in the vast majority of cases before the satellite points away from the earth. For more saver contingencies Omni type antennas are selected for their toroidal radiation pattern and larger beam width +/- 35 degrees of their centerline to transmit and receive and are positioned for use as the offset increases to either side. The Aft antenna is selected for used when the satellite rotates  to an orientation with its back to the Earth. All of these antennas are measured at the 3dB or 6dB roll off point and will provide a diminishing signal level beyond the stated beam width. The RF engineering of these designs carefully take into account the link margins required to ensure complete 360 degree coverage.

TCR Telemetry

Due to the sensitive nature of this topic, I will only address this subject from a top level general process.

Data from the satellite is collected by the Flight Computer for each subsystem and passed to the CDH subsystem where it is formatted into a telemetry stream.  This stream of data is clocked out at a defined rate to the telemetry transmitter (a dual function unit). The data is then modulated onto a transmit carrier, amplified to the transmit level and is then sent to the transmit antenna. Based on the design the telemetry transmitter can produce a single telemetry carrier, 2 telemetry carriers, a ranging carrier or a combination of both. The carriers are received on the ground at the assigned telemetry frequency.

As additional units and antennas are added to this subsystem to provide redundancy and flexibility hybrid devices or switches are installed to connect the transmitters to the antenna paths. The use of switches in the transmit path could result in a potential single point failure and therefore their use is minimized.

To accommodate higher transmit power requirements  the telemetry carriers have been passed through a payload channel. This is not a typical design, when used it is related to contingency operations configuration.

TCR Command

Due to the sensitive nature of this topic, I will only address this subject from a top level general process.

TCR commanding is accomplished by transmitting the modulated commands on a RF carrier that is tuned to the command frequency. The command carrier is received at the antenna and is applied to the input of the command receiver. By design the command receiver is a dual function unit, it will lock onto a receive carrier and demodulate valid command signals to produce a digital output. Once the command receiver is locked onto the command carrier, it verifies and demodulates commands and outputs them to a Command and Data Handling subsystem or Flight Computer for execution.

As additional units and antennas are added to this subsystem to provide redundancy and flexibility hybrid devices are installed to split the received signals. The use of switches in this receive path could result in a potential single point failure and are therefore avoided. The use of hybrids allows the signal to be divided and applied to each receiver and since hybrids are passive devices the chances of failures are minimized.

TCR Ranging

During the launch and early operations phase a path through the TCR subsystem is provided to collect phase angle range data for orbit determination. This path can also be used during the mission life if desired.

TCR ranging is accomplished by transmitting the ranging signal on a RF carrier that is tuned to the command frequency and measuring the phase angle difference of the receive signal on a RF carrier tuned to the telemetry frequency. For this reason it is normally recommended that no commands are transmitted while ranging is being preformed. By design the command receiver is a dual function unit, it will lock onto a receive carrier and demodulate the signal to produce a digital or analog output signal. Once the command receiver is locked onto the ranging carrier, it demodulated the range signal and outputs it to the input of the telemetry transmitter. The telemetry transmitter (another dual function unit) then modulates the ranging signal onto a transmit carrier and amplifies it to the transmit level and is then sent to the transmit antenna. Ranging carriers are received on the ground at the assigned telemetry frequency.

To accurately process the range data all variables must be accounted for. During the integration and test phase of the satellites assembly each range path through the TCR subsystem is calibrated. To calibrate each path the subsystem is configured for ranging, the path configuration is noted and ranging is preformed. The resulting range measurement is the delay through that path and is referred to as the satellite delay.

When processing range measurements, to improve accuracy you must account for delays through the satellite and ground station. Once these delays have been accounted for then the range measurement is the distance to and from the satellite. By dividing the measurement by 2 you get the range from the ground antenna to the satellite.

Ranging can also be preformed by transmitting and receiving the range signals through a given payload channel.

Telemetry Command and Ranging subsystem of a satellite

The TCR subsystem is comprised of all the equipment needed to transmit data from the satellite to the ground in the form of telemetry.  It is the interface to receive commands from the ground and provides paths to rough ranging signals received from the ground back down to the ground for range measurements used in orbit determination.  Ranging signals received have to be converted from the receive frequency to the transmit frequency and then transmit back to the ground.  Some of the units employed by this subsystem are transmit and receive antennas, command receivers, telemetry transmitters and range signal translators.

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INFORMATION

Shining light on satellites and how they operate. Drawing from over 30 years of knowledge and experience in all phases of the life of a satellite from concept, to operations, and through end of life. You will find short topics intended to give you an understanding of how they work, the general concepts, and principals used along with information on ground systems. There is also a section dedicated to topics that can be used as basic concept training along with links to animations and 3D models I have created.