What are military communication satellites and their functions?

What are military communication satellites and their functions?

Published literature indicates that there are about 30 simple spinning satellites launched in the United States that are placed near geosynchronous orbits for special reasons. Defense Satellite Communications Systems (DSCS), such as DSCS I, DSCS II, DSCS III, DSCS IV, are deployed to provide communications and telecommunications services to the U.S. military. The satellite’s design configuration incorporates continuous improvement in state-of-the-art antenna and sensor technology. This results in significant improvements in performance, weight, reliability and power consumption. MILSATCOM (Military Satellite Communications) is a communications satellite system capable of meeting military communications and telecommunications needs.

1. DSCS III communication satellite system
DSCS III satellites provide 6 covert communication channels and are equipped with space user interference discrimination capability, sometimes referred to as antenna nulling capability, for high discrimination performance. According to the published literature, these two channels of the satellite system deploy high-efficiency 40W TWTA, and the remaining 4 channels use 10W TWTA each, which means that the total RF output of the TWTA is 120W. Assuming 40% efficiency per DC-to-RF, the DC input power required for the TWTA will be close to 300W. The battery must provide this amount of DC power. Additionally, additional DC power is required to run the antenna stabilization mechanisms and various electronic, electro-optical and microwave sensors on the satellite. The solar panel must be able to charge the battery to meet all the DC power consumption.

What are military communication satellites and their functions?
DSCS III communication satellite system

In the case of multi-carrier operation, back-off power is required, which reduces the DC-to-RF efficiency of the TWTA unit to about 25%. Under such operation, the secondary battery must be able to supply more than 1,200 W of DC power to the TWTA alone. Because of the integration of advanced technology in the high gain antenna and receiver, the low power TWTA is deployed in the DSCS III system.

2. Electricity generation, temperature regulation and storage requirements
Determine specified performance requirements for power generation, PC and storage equipment, including reliability of DSCS III satellites. The specific details of these requirements are clearly defined under each category.

What are military communication satellites and their functions?
Defense Satellite Communications System

1) Power generation
Regardless of the satellite category, the output power of the solar array decreases after each year of operation in space. To account for the power degradation of the solar array, assume that the output power of a communication satellite solar array with an operating life of 7 years is 650W in the first year of operation. After one year of operation, this output was reduced to 610W, 570W after 2 years of operation, 548W after 3 years of operation, 525W after 4 years of operation, 510W after 5 years of operation, 480W after 6 years of operation, and finally 450W after 7 years of operation.
A study of solar cells by the author [6] has shown that electron and proton bombardment in the space environment causes defects in the semiconductor structure, causing the degradation of silicon solar cells. The study further showed that a substantial reduction in output power occurred in geosynchronous orbit for protons from solar flares. It is estimated that the normal increase in the output power of a solar array is about 9%, at either equinox of the biannual equinox, March 21 and September 23 of each year. A solar eclipse occurs during the time when the sun crosses the equator (day and night), so more power is needed to charge the batteries, which comes from the solar array. Battery charging power may increase to 25% of the equinox or equinox sunlight power requirements, so a 9% increase in normal power may not be enough. In these cases, additional solar cell area is necessary to meet the power requirements for recharging the battery. Simply put, the vernal or autumnal equinox power conditions become the most important solar array design. Space scientists believe that the degradation of the output of the solar array can be matched by the degradation of the payload capacity, which can limit the channels to be deployed.

2) PC function
Regardless of the spacecraft category, PC functionality plays a key role in all spacecraft. The power available from a solar array depends on the array area of ​​normally incident solar radiation and the degree of degradation of the illumination. In addition, the temperature of the solar cell is strictly dependent on the temperature of the array, ranging from -180 to +60 °C during a solar eclipse, which may exceed the normal operating voltage to 2.5 times the normal value. Therefore, regulation of the array voltage is necessary to avoid overvoltage conditions. The PC can keep the voltage variation within 1%~2% through the proper use of voltage regulation and uncontrolled bus configuration.

3) Energy storage device
A short-term study conducted by the author shows that a geostationary satellite has experienced 90 solar eclipses within a year of the station, with the largest solar eclipse lasting 72 minutes per day. This will result in a relatively low number of charge-discharge cycles for the energy storage system (ie the battery). This allows high depth of discharge applications. This results in 50% to 70% of the battery capacity being used in each discharge cycle, compared to 10% to 20% for low-orbit satellites, with thousands of cycles per year. Under these operating conditions, energy storage devices or batteries must have high reliability and long life compared to solar cell devices. Nickel-cadmium batteries have demonstrated high reliability and long life in space applications. Nickel-cadmium batteries are widely used in satellite applications due to the overall improvement in battery performance and the use of advanced electrode materials. The battery power requirement is at least 3.8kW.