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Management
The Rocket Scientist sat with Dany Harel, Vice President, Space Systems & Operations, to take a look at how a satellite is monitored and controlled:
a. How Does Americom Monitor and Track its Fleet?
1) Just what is it you monitor and control?
What we do, is continuously monitor the health of the spacecraft and keep the spacecraft in proper trim through precise mission planning and maneuver implementations.
As you can imagine, this "flying" of a satellite is a very complicated procedure. It requires a combination of the very best technical facilities and the very best people, each of whom possesses special technical and management skills seasoned by experience.
2) How is this done?
The laws of physics are constantly affecting a spacecraft, pushing it here or pulling it there. We use special techniques to determine these actions and use various satellite components to counteract these effects. In addition, a satellite flies in a pretty harsh environment. To monitor the health of our spacecraft in this environment we rely on nearly 4,000 on-board sensors to monitor all sorts of pressures, temperatures and voltages. The information from all these sensors are then channeled together and transmitted to our telemetry and control earth stations - every two seconds.
On the ground, our computer systems translate this data into engineering units - like volts or degrees -that are checked against predetermined value limits set for preferred performance. Should they not fall within these limits, our Spacecraft Controllers are alerted instantly through visual and audible alarms. The Controllers also are able to take corrective actions based on established procedures by sending appropriate spacecraft commands and reporting the results.
In addition, we also utilize an optical disc data retrieval system where all telemetry data is archived from the time the satellite becomes operational until it is retired. This data can then be reviewed at any time by our Spacecraft Analysts and Engineers and used to improve designs for new satellites.
3) Where does all this occur, is there one "super" earth station?
No, actually our fleet is monitored and controlled from three geographically dispersed manned earth stations and one remotely controlled earth station. In addition, we have the ability to monitor the satellites in a lab at headquarters. While each station could control the whole fleet, we typically assign each earth station several satellites for which they act as either primary or secondary control. I should add that our Spacecraft Analysts at these earth stations have the extremely important function of intimately knowing the satellites they have been assigned and building the daily operations plans for every moment of each satellite's life.
b. What Happens When a Satellite Gets Old?
1) What is an inclined orbit and why does a satellite go into one?
A geosynchronous satellite (geosat) in its orbital location has normal pendulum-like swings daily on the order of 0.05° in the East-West and North-South directions. This allows stationary ground antennas to continuously access the satellite. For a geosat in an inclined orbit, the East-West swing is the same, but the North-South swing is equal to its inclination angle (i.e. 0.8°). This means an inclined geosat will reach 0.8° North of center, then 12 hours later reach 0.8° South. To maintain the size of daily swings, periodic rocket firings are performed using on-board fuel. Thus, by allowing a satellite to go inclined, less fuel is used and lifetime extension can be realized.
2) What type of communication capabilities can you expect from a satellite in an inclined orbit?
For users that have tracking antennas or short-term communications needs, communications performance of an inclined satellite is comparable to that of a fully station-kept satellite. For news gathering where multiple ground equipment setups, knock-downs and short feeds are standard operating procedure, an inclined satellite is ideal.
3) How long can a satellite in an inclined orbit last?
Whether fully station-kept or inclined, on-board fuel is the primary lifetime limiter.
c. When and How is a Satellite Retired?
The Rocket Scientist asked John Bailey, Lead Satellite Analyst at our Vernon Valley, NJ, Tracking, Telemetry and Control facility, why and how a satellite is retired:
1) What causes a satellite to be retired? Electronic failures? Fuel depletion?
In the past, retirement of a satellite was often a function of the satellite's on-board electronics failing. But these days, since a satellite's electronics have been so improved and since so much redundancy has been built into the electronics package, when a satellite is retired today it is almost always because of fuel depletion.
2) How do you measure the remaining fuel, after all geosynchronous orbit is 22,300 miles in space?
Modern day satellite fuel sensors, which measure fuel pressure, are very sophisticated, but just like the fuel gauge on your car they still possess a degree of uncertainty. To determine remaining fuel levels, we consider fuel pressure readings; the satellite's "fuel budget", or checkbook, where we debit the fuel burn of each maneuver; and factor in our experience in flying satellites for over 26 years.
3) What does a typical fuel budget include?
There is a line item for each type of maneuver planned during a satellite's mission. These maneuvers include: north-south stationkeeping, east-west stationkeeping, momentum control, eccentricity control; retirement, etc.
4) What do you mean by "retirement"?
It is the decommissioning of a satellite at the end of its useful mission; fuel is allocated to place it into an orbital location away from other active satellites that are in geosynchrononous orbit.
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