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Efficient Design and Analysis of Airborne Radomes
Creating the complete transmission path of a satellite in training, development, troubleshooting, or system set-up applications requires three components: two earth terminals and a spacecraft. While terminals are generally accessible, satellite accessibility is more problematic, requiring costly leased time on an orbiting spacecraft and offering no control flexibility. Often, a spacecraft may not be available at all. The LTT-03-W-198 Satellite Simulator (Satsim) offers a comprehensive, economical alternative solution. It can accept input signals from uplink terminals at C-, X-, Ku- and Ka-bands individually or simultaneously, translate them to corresponding downlink frequencies, and retransmit them for reception by the other earth terminal.
The Satsim was developed to meet the need for a simple system with the ability to provide all of the frequency translation and retransmission capabilities of an actual satellite. It has several applications, the first of which is the training of people who must rapidly set up satcom systems in the field. This is generally conducted so that military personnel can become familiar with the set-up procedure to minimize deployment time under battlefield conditions. Similar commercial applications exist as well, such as simulation of system set-up for satellite news gathering (SNG). In each case, the ability of a Satsim to operate on multiple bands simultaneously allows several training or set-up groups to use it concurrently.
In system development and troubleshooting environments, a Satsim can provide the “missing link” in a complete transmission path for earth terminal performance evaluation. Loop-back testing with a Satsim allows the complete terminal to be tested instead of a partial path as with traditional wired loop test translators. It can be set up outside the facilities of a system integrator so that the earth terminals can be evaluated without being moved from protective cover.
By deploying a Satsim system on an aircraft, remote-piloted vehicle (RPV), or tethered balloon, it may also be utilized for establishing temporary short-range battlefield communications, or installed on a tower or pole at a seaport to provide a convenient way of checking shipboard satcom equipment on departure and arrival.
A Satsim system consists of a main unit (containing frequency translators, attenuators, antennas, control circuits and peripheral hardware) and a control unit (wired or wireless) that serves as the operator’s interface. The main unit is deployed on a tower, tripod, or other elevated structure where it can be accessed by the earth terminals (earning its nickname “satellite on a stick”). The main unit is very compact, measuring 6 in. x 14 in. x 12 in., and weighs less than 35 lb. The only required connection is for AC power from a 100 to 240 VAC source at 47 to 63 Hz; it can optionally be specified to operate from a DC source. The system will operate over a temperature range of 0° to +50°C.
A total of seven bands of frequency conversion are provided (as listed in Table 1), and additional beacon frequencies can be added to the unit as well. The main unit and remote control (RCU) are ruggedly constructed to function under outdoor environmental conditions.
Operation of the system is straightforward. The operator merely connects the power and checks the remote control display to see that all four bands are powered and locked in frequency. One or more bands are enabled via the remote control, and if operating at Ku-band, the appropriate subband is selected.
The compact enclosure includes separate transmit and receive antennas for each band. Waveguide antennas with low gain (about 6 dBi) are employed to provide wide beamwidths, so that multiple terminals in a nominal 90° quadrant can simultaneously access the Satsim system.
In a simple representation, a Satsim can best be described as a local oscillator (LO), mixer and antennas for the appropriate uplink and downlink frequencies (see Figure 1). Frequency translation is accomplished by mixing the received signal transmitted by the earth terminal with a phase-locked LO. For example, in the case of a Ku-band system for US operation, the input antenna receives a signal between 14.0 and 14.5 GHz. The mixer translates the input frequency by 2.3 GHz to between 11.7 and 12.2 GHz and retransmits it. The frequency coverage of the unit ensures that virtually any commercial or military satcom system operating in C-, X-, Ku- and Ka-bands can be accommodated.
In order to simultaneously operate on all four satcom bands, the Satsim incorporates considerable isolation in its design, including filtering of the input and output frequency bands in addition to orthogonal waveguide feeds. As a result, multiband operation can be conducted without worry of spurious responses from lower frequency translators in the Satsim system corrupting the operation of higher frequency translators. In addition, since power levels are low, intermodulation distortion is not a consideration. The system can operate with any modulation type as well.
A standard Satsim unit includes manual variable attenuators that can be adjusted to control the output level based on a chart supplied with the unit. In practice, attenuation is initially set to maximum and is slowly reduced to the level at which optimum performance is achieved. Control over the attenuators can also be provided with the wireless remote control option. The wireless Satsim option incorporates a microcontroller that loads data for frequency of operation, reports the power setting, and indicates the lock status of the oscillators in each translator.
The wireless remote control allows frequencies, attenuation values and other parameters to be changed at distances up to 1800 ft. from the main Satsim unit via an integral transceiver operating at 900 MHz. All major functions are orchestrated via a keypad and LCD display. The remote control will operate continuously for up to 40 hours on a pair of AA batteries, and powers down one minute after the last keypad entry to conserve power. The remote receives messages from the controller in Satsim and displays them on the LCD panel. The display shows the band or bands of operation, converter operation (locked or unlocked), attenuator setting and other status messages. An alarm message will be shown if one or more converters become unlocked. The operator can then step through the band status displays to find the unlocked converter.
Dedicated keys are provided on the remote control to enable and change frequency bands and adjust the power output in 1 dB steps over a 30 dB dynamic range. While normally operated with the wireless remote, the simulator can also be supplied with a wired remote which enables band selection and status. In this case the attenuators are adjustable by opening the cover of the Satsim. Operation is automatically transferred from wireless to wired when the units are connected.
A signal level of –15 dBm (up to +15 dBm can be accepted) is typically input to the main unit, and output midband gain is –30 dB (loss plus the attenuation setting). Gain can be adjusted continuously over a 30 dB span, and gain ripple is less than ±1.5 dB. Phase linearity per 5 MHz is ±10° or less, frequency translation accuracy is ±5 kHz, rejection of spurious in-band signals is –50 dBc and image rejection is greater than 50 dB. The system will operate with vertically or circularly-polarized transmitters and horizontal or circularly-polarized receivers.
The standard configuration of the Satsim system can be customized in several ways to accommodate specific requirements. It can also be complemented with instruments such as a satellite link emulator and carrier-to-noise generator to provide signal impairments.
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