Emerging military systems, driven by new mission requirements and enabled by quickly advancing technologies, demand sophisticated test equipment and methods to develop and characterize. For example, the incorporation of wider bandwidth signals in military systems, for better resolution in radar systems and higher data rates in communications and lower susceptibility to jamming for both, drives the need for signal simulation and analysis tools capable of comparable or more bandwidth and performance.
As defense budgets shrink and contracts continue to migrate from cost based to firm fixed price, walking the line between instrument performance and reducing the cost of test has become much more complicated. Military systems now in development still require the highest performance from test equipment to ensure operational readiness and success. But close scrutiny is now given to the exact level of performance required when purchase decisions are made. Additionally, consideration is given to the complete cost-of-test over the life of the system.
There are various mission requirements, technical trends and economic drivers that affect the design of new test solutions and development of advanced measurement science. Below we will review some of the most significant trends and how the test and measurement industry supports these needs.
Mission requirements for military radar and communication systems have been continuously evolving since the beginning of the Viet Nam war. While this evolution accelerated at the end of the cold war, change in mission requirements has never been as drastic as with the recent conflicts in the Middle East. Let’s take a look at some of the newest priorities.
Unmanned aerial systems (UAS) have garnered a large share of the pentagon budget. Much of this allocation is destined not for the airframe itself but for the sensor and data processing payloads that these platforms carry. First, the transmission of high resolution video (encrypted) from the UAS from anywhere in the world via satellite back to the United States requires a large block of bandwidth to attain the needed data rates. This also means the use of higher order modulation schemes. The ability to characterize performance under the stringent conditions of a satellite link is essential. A signal analyzer capable of analyzing non-standard signals can be used but a higher bandwidth digitizer or perhaps even an oscilloscope with vector signal analysis capability maybe needed. In the latter case some dynamic range will be traded away for the extra analysis bandwidth. Second, the sensors that make up UAS payloads have continuously increasing levels of performance and capability to deal with ever changing threats. Active array radars that have a small form factor in addition to high resolution require a coherent multi-channel measuring receiver platform to quickly and accurately calibrate the array (see figure 1).
Another priority mission for the military involves finding and disabling improvised explosive devices (IEDs). As you can imagine this is a complicated effort and many different technologies have been employed to try and solve the problem. The triggering of IEDs has been devised to operate in a number of ways. These include using radio signals emitted by everything from cell phones and two-way radios to garage door openers. Passive infrared (PIR) sensors or simple pressure plates are also employed.
Identifying threat signals and jamming them is a challenging task. Often, jamming all frequencies at once (barrage jamming) may be the most effective solution.
To put the challenge in perspective, figure 3 shows a squelch tone from a handheld radio operating at a carrier frequency of 300 MHz. This signal has audio modulation; a squelch tone at about 100 Hz. We can see that the rise time of this tone is around 50 ms. In the case of counter-IED systems, we must detect and jam the transmission before the tone rises. To do this we want to selectively jam emitters that are deemed a threat. The jammer must have a lot of sensitivity, dynamic range, and search speed to deal with the emitters, and distinguish enemy transmissions from civilian transmissions while preserving system resources, namely power.
In order to evaluate jamming systems, a robust test capability for simulating multiple emitters is essential. A wide bandwidth arbitrary waveform generator (AWG) with sufficient dynamic range of about 12 to 14 bits can be used to simulate width swaths of spectrum and multiple emitters. Because of these stringent requirements and new types of threats, AWGs of this type have been developed and are currently available with 8 to 12 Msamples/s providing an RF modulation bandwidth of 3.2 to 5 GHz and a spur free dynamic range (SFDR) of over 75 dBc.
The technical trends that facilitate moving the defense industry toward higher performance and more capable systems also affect the requirements of test and measurement equipment. Recent advances in semiconductor materials and processing has aided the development of solid state power amplifiers with high power capability, efficiency and linearity in a much smaller form factor. Gallium Nitride (GaN) amplifiers are replacing traveling wave tube amplifiers (TWTA) in radar applications and have led to the development of active electronically scanned array (AESA) radars. The elements of the array are made up of transmit/receive modules (TRM) as shown in figure 4. Beams are formed and steered through the application of phase offsets between the various elements of the array.
The key test challenge is characterizing element to element phase and amplitude errors or misalignment at the point of radiation. These errors can be contributed from various components in the antenna array, but must be calibrated out so that the system in use operates as intended. Figure 5 suggests a possible block diagram for the test and calibration of the array. Conventionally a network analyzer has been used as the measuring receiver for antenna testing. With the number of array elements increasing and the waveforms employed having wider bandwidths, new methods are needed to reduce test times and characterize the system under operational conditions.
Digital array radars (DAR) currently in the research phase will change the paradigm of radar antenna test even further. In today’s active arrays the TRM is a classical analog in and analog out device. With DAR, the analog to digital conversion (ADC) takes place within the TRM making it an analog-in and digital-out type of device, making conventional network analysis difficult to implement. New test methodologies will need to be developed to keep pace with these new systems.
Signal processing and computing capabilities are another technological trend that has had a profound effect on both the commercial and defense markets where performance, power consumption and size are concerned. Advances in devices such as digital signal processors (DSP), graphics processors (GPU) and field programmable gate arrays (FPGA) have enabled more sophisticated algorithms and signals within radar, electronic warfare, satellites and terrestrial communication systems. These devices allow greater performance and capability in systems where power and size constraints previously required tradeoffs to be made. With these advanced tools, systems have the flexibility to perform multiple functions as their operating mode is now software defined. For example, an airborne radar can support functions such as communications, target tracking, fire control, ground mapping and then to adapt to multiple detected targets, electronic counter measures (ECM) and channel conditions.
Verifying the operation and performance of these multi-function systems requires a test capability that is equally or more flexible than the system under test itself. The newest generation of instruments includes internal signal processing capabilities to enhance data analysis. With FPGA-based data reduction techniques such as digital down conversion (DDC), the measurement noise floor is reduced and signal capture memory conserved. On the signal generation side, digital up conversion (DUC) in modern AWGs gives the user greater flexibility in creating long and unique signal scenarios that simulate numerous operational environments.
Current economic conditions in the defense industry have been disruptive to the old ways of thinking. As budgets trend downward, a creative approach to meeting mission requirements, staying on top of technological trends, and finding ways to reduce the cost of test is paramount. While this priority is being aggressively addressed by the test and measurement industry, we cannot lose sight of the important role high performance and broad capability in measuring equipment play in verifying and characterizing military systems.