A.2.2.2 Telecommunications Analysis1

A.2.2.2 Telecommunications Analysis

One of the primary goals of the telecommunication equipment on our launch vehicles is to allow the vehicle to ‘talk’ with the ground. The telecommunications system keeps mission controllers informed of the vehicle’s health and its current position as it rises to its destination. The selection of a signal frequency for the telecommunications system dictates how far the signal can travel, how much the signal is altered by outside forces, what hardware is needed to build the system, and the rate at which the system can transmit data. We iteratively evaluated these factorsto determine the signal frequency for our design.

The signal frequency is speed at which the radio waves in the signal will travel. Most communications frequencies in modern communication exist in the ultra high frequency (UHF) or the super high frequency (SHF) ranges. The UHF band consists of signal wavelengths 100 millimeters to 1 meter in length and frequencies ranging from 300 Megahertz (MHz) to 3 Gigahertz (GHz). We chose to primarily investigate this frequency range because of its common use in today’s society for communication. Mobile phones, television signals, two-way radios, and GPS navigation systems all run on signals in the UHF band, making the technology to support it common and well-developed.1

We began our investigation by examining the frequency allocations put in place by various political powers throughout the world. In order to prevent a sort of radio chaos, it is common for governments and world organizations to divide up the frequency spectrum into segments, allocating certain divisions for specific uses such as the FM radio range or restricted military frequencies. In the United States, the Federal Communications Commission (FCC) is in charge of regulating the use of the radio spectrum. By examining the allocations in use by the FCC, we were able to select several potential signal frequencies. A listing of some of our options appears in Table A.2.2.2.1, below.

Table A.2.2.2.1 Potential Signal Frequencies2
Signal Frequency (MHz) / Current Use
15.005-15.010 / space research
18.068-18.168 / amateur satellite
30.005-30.010 / satellite identification and space research
137.00-137.025 / space operation
272-273 / space operation
400.15-401.00 / space operation
1427-1429 / space operation, telemetry
1525-1530 / space operation, mobile satellite
1755-1850 / space operation
2025-2110 / space operation
2200-2290 / space operation
2290-2300 / space operation

As the table above shows, there are many potential frequencies to choose from. To compare the available frequencies, we prepared a link budget using data from Purdue University’s Cube-Sat project and varied different parameters to see their effect on the signal strength’s final margin. In order for the signal make it to its destination, the final margin must be greater than 3 decibels (dB). The process of constructing a link budget and calculating the system’s final margin is described in Section A.2.2.3.

The first signal frequency analysis we performed looked at changing the maximum path length the signal would have to travel and the resulting link budget’s final margin. A representative set of frequencies was run through this analysis to determine how far we could send the signal before it would no longer be strong enough to read. Figure A.2.2.2.1, below, shows this process.

Fig. A.2.2.2.1 shows the final link margin as the path length and signal frequencies change.

Figure A.2.2.2.1 shows that as the path length the signal travels increases, the final link margin decreases. The figure also indicates that as the signal frequency increases, the maximum path length of a usable signal decreases. Higher frequency signals are more susceptible to interference and disruption as they pass through space. This decreases the distance they can successfully travel, suggesting that we can achieve better distance performance by using a lower frequency signal.

One of the properties of signal frequency is how fast the signal travels through space. High frequency signals travel very fast, whereas low frequencies move much slower. Due this signal velocity, a signal being transmitted from a moving vehicle experiences a phenomenon known as the Doppler shift. To an observer on the ground, the signal will appear to be travelling at a different speed. This effectively causes the signal to have a different frequency than intended. The change in the frequency at a given velocity is calculated using Eq. A.2.2.2.1 on the following page.

(A.2.2.2.1)

where is the observed Doppler shift in kHz, f is the signal’s original frequency in kHz, v is the velocity of the vehicle in meters per second, and c is the speed of light in meters per second.

By varying the vehicle’s velocity, we predicted the Doppler shift for our representative set of potential signal frequencies, as Fig. A.2.2.2.2, below, shows.

Fig. A.2.2.2.2 shows the Doppler shift effect as the vehicle velocity and original signal frequency change.

In the early design phases of our launch vehicles, it was suggest that the vehicle may reach up to over 18,000 meters per second during its ascent. As Fig. A.2.2.2.1 shows, at this velocity a high frequency signal can shift by almost 180 kHz, whereas the 401 MHz signal barely changed 20 kHz. Even at the more moderate velocities, the Doppler shift can mutate the signal by a large margin. This shift in the frequency will have to be accounted for in ground tracking hardware, suggesting that a radio system running on a lower frequency may be easier to construct.

Radio communications equipment designed to operate at high frequencies are generally more complex than their counterparts using lower frequencies. This complexity is caused by the need to more carefully construct, transmit, and receive the signal to account for higher frequency signals being more delicate. Factors such as the Doppler shift and the signal degradation as it travels through space must be accounted for in the hardware. This causes high frequency radio systems to become more costly.3 We are primarily concerned with cost in the design of our launch vehicles, so we again find an advantage in using a low frequency signal.

The rate at which the telecommunications system can transmit data from the vehicle to the ground is also a function of the signal frequency. There are many ways in which a signal can be changed such that data can be transmitted, and the exact method of data transmission is beyond the scope of this project. The data rate plays a role in establishing the final link margin of the system, and we can use it to evaluate the ability of different frequency signals to support the amount of data the vehicle will need to transmit. Using the same iterative approach as the path length analysis, the link margin at a range of data rates was calculated for a set of signal frequencies, as shown in Fig. A.2.2.2.3, below.

Fig. A.2.2.2.3 showsthe change in final link margin as more bits per second are
transmitted across a set of different frequencies.

It can be seen that as more bits per second are pushed through the telecommunications link, the link margin falls. At a set data rate, as the signal frequency rises, the link margin again falls. This suggests two things. First, the lower the data rate, the stronger the link margin will be. The Purdue Cube-Sat project is designed to support data rates up to 8x107 bps. If we can limit the amount of data our vehicle needs to transmit, we can greatly increase the strength of our telecommunications system. Second, lower frequencies are able to sustain better link margins at higher data rates than higher frequencies. While similar performance may be possible at extremely lower data rates, as the data rate requirements increase, lower signal frequencies will enable better signal margins for the system. We note that it is common for systems that require high data rate capacity, such as communications satellites, to use higher frequency signals. These systems must support different capacities than our vehicle will need, thus the logic that the lower frequency signal will be more robust as the data rate increases holds true.

In each parameter of the telecommunications system’s performance, we find that the lower frequency signals will better support our final link margin. From this analysis, we choose the 401 MHz frequency to support our vehicle telecommunications system. By selecting the signal frequency, we can then begin a much more detailed link budget analysis and establish other system design specifications.

References

1. Pozar, David. Microwave Engineering, Wiley, New York, NY, 2004.

2. "FCC Radio Spectrum Homepage." Federal Communications Commission [online], 2007. URL: [cited 26 February 2008].

3. Filmer, David. “Link Budget Analysis,” AAE 450 Lecture, Department of Aeronautical and Astronautical Engineering, Purdue University. 23 January, 2008.

Author: Justin Rhodes