Satellite Simulator Speeds Training and Troubleshooting
This programmable satellite emulator/simulator essentially takes the place of an orbiting earth satellite and allows testers and trainers to understand the effects of signal impairments on satcom system performance.
By Rick Walsh, VP Engineering, Tampa Microwave

Satellite communications (satcom) system testing poses many unique challenges compared to evaluating the performance of terrestrial communications systems, mainly because a key component in the system – the satellite – is out of reach. Fortunately, with the help of sophisticated digital-signal-processing (DSP) capabilities, modern satellite simulators and emulators can effectively provide all the functionality of the orbiting satellite in testing the end-to-end performance of a satcom system. Advanced test solutions such as the Satellite Emulator System (SES) from Tampa Microwave (Tampa, FL) can simulate the many different RF channel characteristics found in satellite uplink and downlink propagation paths, including rainfall attenuation, Doppler shifts, path fading, and propagation delays, for all of the standard satcom frequency bands, including C-, X-, Ku-, and Ka-band frequencies. These same systems are also invaluable for training personnel on installing and bringing online fixed and portable satcom systems in commercial, industrial, and military applications.

Satcom systems employ space vehicles at a variety of orbital heights, including constellations with low-earth-orbit satellites (LEOS), medium-earth-orbit satellites (MEOS), and geosynchronous satellites. In order to test a satcom system, representative signals must be generated and analyzed. Because of the distances between satellites and ground stations, signals can suffer from path loss, noise, and propagation delays. These signal impairments must also be included in precisely controlled amounts as part of any test signals used for evaluating satcom system performance.

One of the key functions of a satellite transponder is translation of uplink frequencies transmitted by a satcom ground station to the downlink frequency band for retransmission to a receiver at a second satcom ground station. In cases where uplink and downlink terminals are physically close, a traditional loop test translator (LTT) such as the LTT line of products from Tampa Microwave can be used for evaluating performance. Microwave LTTs are available in L-, C-, X-, Ku-, and Ka-band frequencies and are housed in 1U rack-mount enclosures for ease of installation in a satcom system (Figure 1). They rely on a wired connection between the uplink terminal and the receive terminal, bypassing the system’s antennas as well as the effects of over-the-air transmission, such as propagation delays and path fading.

An LTT uses directional couplers to extract some of the RF energy from the uplink transmitter and inject it into the downlink receiver. Attenuators are used for level control and to ensure that the receiver is not overloaded and is operating under linear conditions. Frequency converters in the LTT, based on superheterodyne mixing, perform the frequency translation of uplink signals to required downlink signals, allowing the link to be tested for bit-error rate (BER) and other performance parameters. Where wired connections can be made, an LTT is an effective test tool. Lack of wireless functionality, however, makes them less than ideal for full in-field testing and/or training.

In contrast to an LTT, a satellite emulation or simulation system such as the SES can be used to establish an end-to-end satcom path link using wireless rather than wired communication between the uplink and downlink terminals. An SES enables real-time, hardware-in-the-loop emulation of satcom system performance under changing signal-impairment conditions, rather than performing off-line, computer-based simulations. It is a cost-effective alternative to leasing satellite transponder time in order to perform ground terminal testing.

Because of the different frequency bands used for satcom systems, a well-designed satellite simulator should accept uplink signals at C-, X-, Ku-, and Ka-band frequencies and translate them to the appropriate downlink frequencies for retransmission to the downlink/receive terminal. For maximum flexibility, a satellite simulator that can handle different frequency uplink signals simultaneously can greatly increase test throughput and, for training purposes, allow operation by multiple users.

Training exercises in commercial settings can help service personnel to install and maintain satcom system performance across a communications network. The capability of quickly installing and establishing communications with satcom gear in military environments can be critical on the battlefield. A satellite simulator can also be employed as a temporary communications solution onboard aircraft, on an unmanned aerial vehicle (UAV), or on a tethered balloon when a portable communications system is required. Even in some commercial situations, like electronic news gathering (ENG) applications, training for quick set-up and operation of satcom equipment can be beneficial.

For realistic satcom testing and training, an SES system is placed between uplink/transmit and downlink/receive terminals (Figure 2). The SES provides the required frequency translation between the uplink and downlink frequency bands along with signal impairments, such as propagation delays, Doppler effects, path fading, and noise. The SES can emulate a single satcom propagation path (the uplink or downlink path), round-trip signal paths from uplink to downlink, and multiple hops between uplinks and downlinks. A standard SES system is designed for radiated, free-space testing that allows for testing of a complete satcom terminal. The SES receives a satcom terminal’s transmitted uplink signal(s), processes that signal with the desired channel effects, and re-transmits the signal(s) to the terminal receiver at the proper frequency and amplitude.

An SES system includes two outdoor enclosures for testing at a satcom ground station: the RF Conversion Unit (RFCU) with input and output antennas is mounted on top of a mast; and the L-Band Processing Unit (LBPU) is mounted at the base of the mast or in a co-located enclosure. The RFCU and LBPU are connected via coaxial cables, while control signals for the LBPU are provided by an Ethernet connection. The RFCU is usually 20 to 300 feet from the satcom ground terminal. Due to US Federal Communications Commission (FCC) regulations, the satellite terminal antennas must be pointed at least 5º above the horizon to minimize interference with terrestrial communications systems. As a consequence, distances greater than a few hundred feet between an SES and a satellite terminal under test require a higher elevation.

The RFCU downconverts input satcom frequency bands to an L-band intermediate frequency (IF). The LBPU adds signal impairments to these signals. The processed signals are then returned to the RFCU for upconversion to satcom frequency bands and retransmission. The RFCU incorporates two antennas, a frequency downconverter, and a frequency upconverter for each operating frequency band.The frequency downcoverter receives signals from an uplink/transmit terminal and translates them to L-band. The frequency upconverter translates L-band frequencies back to the satcom band. For each band, the L-band output of the downconverter and the L-band input of the upconverter are connected to switches that select the desired operating frequency band. The LBPU (Figure 3) provides an L-Band IF output with amplitude optimized for analog-to-digital conversion.

Variable attenuators in the RFCU are used to set signal attenuation over a 30 dB range (with less than +/- 1.5 dB gain ripple) for establishing optimum amplitude levels to the LBPU. The RFCU employs phase-locked local oscillators (LOs) for good stability in its superheterodyne frequency translation process, maintaining frequency translation accuracy of 0.01 ppm and nominal phase linearity of +/-10 deg. for any 40 MHz band segment. SES systems are designed for use with vertically or circularly-polarized transmitters and horizontal or circularly-polarized receivers.

The SES LBPU (Figure 4) accepts L-band signals over coaxial cables from the RFCU and performs frequency translation to downconvert these signals to a 70 MHz IF signal suitable for sampling by the LBPU’s 12-b analog-to-digital converter (ADC). The DSP board contains a field-programmable gate array (FPGA) with custom firmware to process digital signals from the ADC with variable time delays, Doppler frequency shift, signal attenuation, and pseudo-random white Gaussian noise. To ensure good signal fidelity on uplink signals, the LBPU incorporates a 16-b interpolating digital-to-analog converter (DAC) to return the DSP’s processed outputs to the analog realm. A frequency-agile up converter is then used to translate the 70 MHz IF signals to L-band outputs that are returned via coaxial cables to the RFCU for retransmission.

The LBPU’s capability includes broad control over range delays, which are free-space propagation time delays between a satcom transmitter and receiver. Onboard memory is used to implement the delays typically found in satcom links (Figure 5). The unit’s time delays are phase continuous and can be programmed from 35 microseconds to over 500 milliseconds. Dynamic time delay is appropriately filtered to ensure a smooth and phase-continuous variation representative of an actual link. Performing tests with dynamic propagation delays while maintaining phase continuity can be critical to evaluating the performance of large systems capable of QAM modulation, and spread spectrum formats as used in satcom On-the-Move terminals.

The LBPU adds the effects of Doppler shifts to both signal carriers and their data. The embedded DSP implements time delays or expansion due to Doppler shifts that correlate with the Doppler frequency variations (Figure 6). Doppler shifts are also phase continuous and can be programmed over a total range of +/-500 kHz with frequency resolution of better than 1 Hz. Combining Doppler effects with time-varying delays, for example, can help reproduce the signal impairments that take place from Doppler shifts during a satellite overpass.

An SES system stores numerous predefined channel profiles representing typical satellite orbits, to accommodate testing and training with a variety of satcom systems. In addition, operators can load custom profiles to create a time-sequenced table of dynamic channel effects. The SES system includes a variety of channel models, including Rayleigh and Rician statistical fading models.

As an option, an SES can interface with orbit propagation tools, such as STK or OASYS, for added programmability. The SES can be operated manually, relying on user inputs to create required signal impairments for testing, or under the automated control of a personal computer running a custom or standard test application. An SES system under PC control is also ideal for training applications.

In addition to supporting static and dynamic satellite channel simulation, an SES system with multiple DSP cards can also perform signal generation and spectrum/modulation analysis. Part of that signal generation is control of Additive White Gaussian Noise (AWGN) that is combined with simulated signals for proper simulation of satcom system signal-to-noise ratios (SNRs) under dynamic conditions, such as interference and rainfall. An SES system allows simulation of noise levels from various sources within a satcom system, including low-noise amplifiers (LNAs), cables, passive components, and noise received by and re-radiated from the satellite transponder. The SES system can utilize both digital Gaussian random number generators and analog noise sources to produce controlled levels of noise. An SES is equipped with enough memory and processing power to simulate at least 10 minutes worth of a satellite’s orbit. Because the signals appear to originate from an orbiting satellite, a wide range of system and subsystem tests can be performed with the SES system under dynamic conditions.

The LBPU can also be loaded with firmware to function as a digitally controlled signal generator capable of as many as four carriers with independent control of frequency, modulation, amplitude, and data rate. A wide range of modulation types can be generated, including amplitude modulation (AM), frequency modulation (FM), and varieties of frequency-shift keying (FSK), minimum shift keying (MSK), and phase-shift keying (PSK) modulation. The digital signal source can also be used to simulate jammers, adjacent-satellite or adjacent-channel interference.

Conclusions
An SES system can emulate a satellite uplink, downlink, or complete earth station-to-satellite-to-earth station link. It can emulate a full satellite transponder, with as much as 40 MHz bandwidth and as many different modulation types as desired. The SES can also emulate a wide range of different satcom systems, so that signals appear to be originating from LEOS, MEOS, or geosynchronous orbiting satellites. An SES equipped with additional DSP cards can also create jamming and interference signals for military communications and electronic-warfare (EW) applications, training, and for tactic validation. The system can fully exercise and test L-band satcom modems beyond traditional measurements, with transmission signals and modulation types only limited by the number of installed DSP cards.

TAMPA MICROWAVE
www.tampamicrowave.com
TXTLINX.COM127
Email this article to a friend!