Satellite Testing Demands RF Link Emulation
With the global emphasis on counter-terrorism, the need for surveillance in geographic areas that are inaccessible or too dangerous to put military personnel on the ground and the push for electronic information gathering, satellite-based communications have seen a dramatic resurgence. As the United States continues its prosecution of the War on Terrorism, more and more systems are being developed and deployed to aid in tactical battlefield communications. Rapid technology improvements also are driving new developments in commercial applications.
Traditional satellite communications roughly segment into two categories: units that may or may not be transportable but are used in a fixed location and mobile satellite services (MSS). Fixed-location satellite communications typically use very small aperture terminals (VSAT). This technology most commonly transmits narrowband data such as credit card point-of-sale transactions, polling or RFID data, and supervisory control and data acquisition. It also can carry broadband data such as VoIP or video for the provision of satellite Internet access to remote locations.
VSATs also are used for transportable or mobile maritime communications such as Vizada or Eutelsat services and by two-way satellite Internet providers such as HughesNet, StarBand, and WildBlue in the United States and Bluestream, SatLynx, and Technologie Satelitarne in Europe. Across the world, these commercial services deliver broadband Internet access to often remote or rural locations that cannot get less expensive broadband connections such as ADSL or cable Internet access.
The newest push is MSS, which uses portable terrestrial terminals. MSS terminals may be mounted on a ship, an airplane, or an automobile and even carried by an individual. Some terminals also may augment their communications capabilities with terrestrial cellular wireless communications.
A typical example is the new Mobile User Objective System (MUOS), the U.S. military's next-generation narrowband global mobile satellite communications system. This will give troops in the battlefield the ability to communicate directly with their commanders.
However, in the military world, there also exists the need for wider bandwidth mobile communications than is possible with MUOS. A typical example of such a system is the Warfighter Information Network-Tactical (WIN-T), a telecommunications system consisting of communications infrastructure and network components from the maneuver battalion to the theater rear boundary. The WIN-T network will allow 21st century Army commanders and other communications network users at all echelons to exchange information internal and external to the theater from wired or wireless telephones, computers that will provide Internet-like capability, and video terminals.
Regardless of which type of SATCOM system is being developed, all commercial and military communications satellite systems must be thoroughly tested before deployment. This testing normally starts at the subassembly level and progresses to full system test. Subassembly testing is highly specialized and very hardware specific.
Once all major components are assembled, system-level testing can commence. A major part of this involves verifying operation of the complete system using realistic ephemeris test data or satellite orbit models.
Evaluating a satellite system presents challenges found in no other communications application. In addition to the usual RF performance parameters, SATCOM systems experience transmission-path conditions that vary with the spacecraft's position in the sky and whether it has a low or medium Earth orbit or a polar or geosynchronous one. This creates a unique set of operational parameters for every SATCOM system and a corresponding need for ATE that can accurately emulate a link.
Dynamic emulation is required from uplink through the satellite's transponder to the downlink and back and perhaps from transponder to transponder if hand-offs to multiple satellites are required to complete the link. To achieve this, system integrators build large closed-loop ATE systems that perform hundreds of tests. These systems must be capable of establishing the communications link regardless of the modulation format and adding dynamically varying impairments such as flat fading, phase shift, Doppler shift, and delay to precisely emulate the impairments that would occur in actual use.
Varying time delay is one impairment parameter that often is the hardest to accurately simulate and can cause havoc with complex SATCOM systems using full duplex data communications. As a satellite moves across the sky, typically in low or medium Earth orbit, the distance between the satellite and Earth station or from satellite to satellite is continuously changing. This results in varying time delay and corresponding Doppler shifts.
Traditionally, some form of analog-to-digital conversion with a memory storage system was used in test systems to generate the necessary delays. Changing delays were simulated by dropping or adding data samples, and the resultant data stream was converted back to analog.
While this technique was adequate for fixed delays or TDMA systems in which the delay could be varied during the dead time between time-division slots, it proved disastrous for the newer form of CMDA systems being deployed. Dropping or adding samples resulted in very significant discontinuous phase shifts. New SATCOM testing demanded the capability to vary delay in a phase-continuous manner.
A Real-World Example
The Globalstar satellite has a typical medium Earth orbit. Figure 1 shows the Globalstar satellite path as it passes over the Earth's surface. As the satellite moves across the sky, the distance between the Earth station and the satellite will continuously change, resulting in varying delays, Doppler shift, path loss, and signal fading. These changing values are referred to as the RF link parameters.
Figure 1. Globalstar Satellite Orbit
Figure 2 shows a snapshot of a portion of the changing link parameters for an Earth station located in Los Angeles communicating with the Globalstar satellite. At the start of the link, the delay is 37.8 ms, and over the next 10 minutes, it changes to roughly 35 ms.
Figure 2. RF Link Data for Globalstar Satellite
To test either the Earth station or satellite communications system or both, it is absolutely necessary to precisely simulate this RF link in a consistent and repeatable fashion. To address this need, dBm has manufactured a specialized test instrument called a Satellite Link Emulator SLE700, which can model the RF link between the satellite and Earth station for any type of satellite orbit and Earth station location. The higher modulation bandwidths in recent SATCOM systems such as MUOS and WIN-T are addressed by an upgraded SLE700 with 45-MHz bandwidth.
With the link emulator, the user can choose a dynamic mode in which data files defining changing impairments are used to imitate actual conditions as a satellite moves across the sky. These data files are downloaded into the instrument and executed, allowing any combination of satellite orbit and Earth station location to be tested. Alternatively, in the static mode, a fixed set of conditions such as delay and frequency is entered.
To change the delay without introducing phase discontinuities, samples must not be added or dropped. This can be achieved by varying the output sample rate independent of the input sample rate in the digital memory. The rate and magnitude of the delay change should be easily programmable, with the resulting output signal smoothly changing from the initial to the final delay value with less than 0.1-ns deviation from ideal at any point in time.
The delay varies in time and creates a true Doppler shift along with chip-period variations that simulate the frequency shift that occurs as the spacecraft's position changes in relation to the Earth terminals. The SLE700 provides a delay slew rate range of 0.003 ps/s up to 20 ms/s. The range of the absolute delay is 100 ??s to 697 ms.
In an automated test system, typically a PC-based test manager controls a multitude of instruments. The satellite link emulator should reside on a LAN along with other test devices that are driven by the test manager.
Control can be embedded in the system using custom test scripts written by the user that make calls to the emulator function library. Alternatively, a PC-based application program with a graphical user interface can support remote operation without the need to write any software.
Multiple independent channels can emulate more than one link and frequencies through K band can be achieved through the use of external frequency up- or down-converters controlled by the LAN or a GPIB interface. Converters are available that utilize double or triple conversion and can be specified with internal or external local oscillators.
An example of an application that uses converters is a test set built for interfacing a satellite payload directly to an Earth station modem. The converter translates more than 2 GHz of bandwidth at K band to the fixed IF of the dBm link emulator, and dual synthesizers provide separation of uplink and downlink frequencies.
Noise impairments can be provided by carrier-to-noise generators, which allow additive white Gaussian noise to be applied to the signals. The ratio of the carrier-to-noise power can be set and automatically programmed with an extremely high level of accuracy. A complete satellite channel simulator that includes the SLE700, a CNG carrier-to-noise generator, and a multipath fading emulator is available.
In addition, a data-generation program called SATGEN can be configured for any orbit and ground-station coordinates for use in the link emulator. The program also incorporates preset values for many major satellite systems as well as multiple ground-station coordinates and path-loss models. The models are used to generate the ephemeris data files that allow simulation of complex communications paths between an orbiting satellite and a ground station.
About the Author
Mike Cagney is vice president of sales and a founder of dBm in 1999. His career began in 1985 at Intepro Systems, later purchased by Schaffner where he directed the company's U.S. Instrument Division. In 1997, he moved to Noisecom as director of sales. Mr. Cagney received an undergraduate degree in electrical engineering at University of Limerick, a master's of engineering at Northeastern University, and an MBA at Rutgers University. dBm, 32A Spruce St., Oakland, NJ 07436, 201-677-0008, e-mail: email@example.com