Launches and launch vehicles
A satellite cannot be placed into a stable orbit unless two parameters that are uniquely coupled together the velocity vector and the orbital height are simultaneously correct. There is little point in orbiting the correct height and not having the appropriate velocity component in the correct direction to achieve the desired orbit. A geostationary satellite for example must be in an orbit at height 35,786.03 km above the surface of the earth with an inclination of zero degrees an ellipticity of zero, and a velocity of 3074.7 m/s tangential to the earth in the plane of the orbit, which is the earths equatorial plane. The further out from the earth the orbit is greater the energy required from the launch vehicle to reach that orbit. In any earth satellite launch, the largest fraction of the energy expanded by the rocket is used to accelerate the vehicle from rest until it is about 20 miles (32 km) above the earth.
To make the most efficient use of the fuel, it is common to shed excess mass from the launcher as it moves upward on launch; this is called staging.
Most launch vehicles have multiple stage and as each stage is completed that portion of the launcher is expended until the final stage places the satellite into the desired trajectory. Hence the term: expandable launch vehicle(ELV). The space shuttle , called the space transportation system (STS)by NASA, is partially reusable. The solid rocket boosters are recovered and refurbished for future mission and the shuttle vehicle itself is flown back to earth for refurbishment and reuse. Hence the term:reusable launch vehicle(RLV) for such launchers.
Launch vehicle selection factor
- Price/cost
- Reliability-Recent launch success/failure history
- Dependable launch schedule- Urgency of the customer
- Performance
- Spacecraft fit
- Safety issues
- Launch site location
- Availability-launch site; vehicle; schedule;
- Market conditions-what the market will bear
Launching orbits
Low Earth Orbiting satellites are directly injected into their orbits. This cannot be done in case of GEOs as they have to be positioned 36,000 kms above the Earth's surface. Launch vehicles are hence used to set these satellites in their orbits. These vehicles are reusable. They are also known as “Space Transportation System” (STS).
When the orbital altitude is greater than 1,200 km it becomes expensive to directly inject the satellite in its orbit. For this purpose, a satellite must be placed in to a transfer orbit between the initial lower orbit and destination orbit. The transfer orbit is commonly known as *Hohmann - Transfer Orbit.
(*About Hohmann Transfer Orbit: This manoeuvre is named for the German civil engineer who first proposed it, Walter Hohmann, who was born in 1880. He didn't work in rocketry professionally (and wasn't associated with military rocketry), but was a key member of Germany's pioneering Society for Space Travel that included people such as Willy Ley, Hermann, and Werner von Braun. He published his concept of how to transfer between orbits in his 1925 book, The Attainability of Celestial Bodies.)
The transfer orbit is selected to minimize the energy required for the transfer. This orbit forms a tangent to the low attitude orbit at the point of its perigee and tangent to high altitude orbit at the point of its apogee.
The rocket injects the satellite with the required thrust** into the transfer orbit. With the STS, the satellite carries a perigee kick motor*** which imparts the required thrust to inject the satellite in its transfer orbit. Similarly, an apogee kick motor (AKM) is used to inject the satellite in its destination orbit.
Generally it takes 1-2 months for the satellite to become fully functional. The Earth Station performs the Telemetry Tracking and Command**** function to control the satellite transits and functionalities.
(**Thrust: It is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system.) (***Kick Motor refers to a rocket motor that is regularly employed on artificial satellites destined for a geostationary orbit. As the vast majority of geostationary satellite launches are carried out from spaceports at a significant distance away from Earth's equator, the carrier rocket would only be able to launch the satellite into an elliptical orbit of maximum apogee 35,784 kilometres and with a non-zero inclination approximately equal to the latitude of the launch site.) (****TT&C: it's a sub-system where the functions performed by the satellite control network to maintain health and status, measure specific mission parameters and processing over time a sequence of these measurement to refine parameter knowledge, and transmit mission commands to the satellite. Detailed study of TT&C in the upcoming units.) It is better to launch rockets closer to the equator because the Earth rotates at a greater speed here than that at either pole. This extra speed at the equator means a rocket needs less thrust (and therefore less fuel) to launch into orbit. In addition, launching at the equator provides an additional 1,036 mph (1,667 km/h) of speed once the vehicle reaches orbit. This speed bonus means the vehicle needs less fuel, and that freed space can be used to carry more pay load.
Orbital effects in communication systems performance
There are a number of perbuting forces that cause an orbit to depart from ideal Keplerian orbit. The most effecting ones are gravitational fields of sun and moon, non-spherical shape of the Earth, reaction of the satellite itself to motor movements within the satellites.
Thus the earth station keeps manoeuvring the satellite to maintain its position. Within a set of nominal geostationary coordinates. Thus the exact GEO is not attainable in practice and the orbital parameters vary with time. Hence these satellites are called “Geosynchronous” satellites or “Near-Geostationary satellites”.
Doppler Effect
To a stationary observer, the frequency of a moving radio transmitter varies with the transmitter’s velocity relative to the observer. If the true transmitter frequency (i.e., the frequency that the transmitter would send when at rest) is \(f_T\), the received frequency \(f_R\) is higher than \(f_T\) when the transmitter is moving toward the receiver and lower than \(f_T\) when the transmitter is moving away from the receiver.
Range variations
Even with the best station keeping systems available for geostationary satellites, the position of a satellite with respect to earth exhibits a cyclic daily variation. The variation in position will lead to a variation in range between the satellite and user terminals. If time division multiple access(TDMA) is being used, careful attention must be paid to the timing of the frames within the TDMA bursts so that the individual user frames arrive at the satellite in the correct sequence and at the correct time.
Earth Eclipse of A Satellite
It occurs when Earth's equatorial plane coincides with the plane f he Earth's orbit around the sun. Near the time of spring and autumnal equinoxes, when the sun is crossing the equator, the satellite passes into sun's shadow. This happens for some duration of time every day. These eclipses begin 23 days before the equinox and end 23 days after the equinox. They last for almost 10 minutes at the beginning and end of equinox and increase for a maximum period of 72 minutes at a full eclipse. The solar cells of the satellite become non-functional during the eclipse period and the satellite is made to operate with the help of power supplied from the batteries.
A satellite will have the eclipse duration symmetric around the time t=Satellite Longitude/15 • 12 hours. A satellite at Greenwich longitude 0 will have the eclipse duration symmetric around 0/15 UTC + 12 hours = 00:00 UTC. The eclipse will happen at night but for satellites in the east it will happen late evening local time. For satellites in the west eclipse will happen in the early morning hour‟s local time. An earth caused eclipse will normally not happen during peak viewing hours if the satellite is located near the longitude of the coverage area. Modern satellites are well equipped with batteries for operation during eclipse.
Sun Transit Outage
Sun transit outage is an interruption in or distortion of geostationary satellite signals caused by interference from solar radiation. Sun appears to be an extremely noisy source which completely blanks out the signal from satellite. This effect lasts for 6 days around the equinoxes. They occur for a maximum period of 10 minutes.
Generally, sun outages occur in February, March, September and October, that is, around the time of the equinoxes. At these times, the apparent path of the sun across the sky takes it directly behind the line of sight between an earth station and a satellite. As the sun radiates strongly at the microwave frequencies used to communicate with satellites (C-band, Ka band and Ku band) the sun swamps the signal from the satellite.
The effects of a sun outage can include partial degradation, that is, an increase in the error rate, or total destruction of the signal.
Previous lecture: Satellite Communication - Orbital Mechanics.