The main purpose of a telemetry system is to collect data at a place that is remote or inconvenient and relay that data to a location where it can be stored and flight test analysis can occur. Real-time telemetry is important to the flight test community for a number of reasons including speeding up the test program through real-time data analysis and to facilitate repeating test points in a single flight. However, the one essential reason real-time telemetry must be employed is for safety.
Aircraft Flight Testing - fixed or rotary wing – has an elevated degree of risk, not only because of the newness of structure, control or avionics but also due to the maneuvers that are inherent in evaluating the performance, certification and mission needs of a new program. For example all military and civilian aircraft manufacturers must ensure the aircraft can cope with high G maneuvers, high crosswind landings and pass the highly dangerous spinning and flutter test scenarios - the most critical of all tests since any airframe failure can be catastrophic. Military aircraft can also encounter additional tests such as inverted flight, carrier landing and high risk weapon release tests. All these high risk maneuvers require real-time flight test telemetry to ensure the test pilots have the greatest chance of avoiding danger and completing the mission successfully.
There are challenges in telemetering data that cannot be overcome and in all practical cases data will be lost. How much depends on a number of factors including the aircraft’s flight profile, local environment effects and the design of the transmission and reception hardware. As the flight profile of the aircraft is largely set by the requirements of the test program, accounting for environmental effects and designing the telemetry hardware are the crucial elements to recovering as much data as possible.
Local environmental effects like fading, multi-path, blind-spots, interference from other RF equipment, thermal noise, base-line drift and Doppler effects can make recovering the data from the PCM/RF link difficult (e.g. an IRIG-106 link). One method of mitigating many of these effects is to use multiple receivers and bit-synchronizers. Multiple receiving sites can ‘cover’ blind spots as well as provide an additional data source with independent noise. Some of today’s bit-syncs are very close to performing as well as theoretically possible (Smart Source Selectors, can yield a possible 1000 times fewer errors (at least 3dB). Smart Source Selectors, use best data metrics and/or best source metrics to decide which streams are “good enough to combine” – they then time align the selected streams and use the soft-bit powers for each bit to decide if the bit is a “1” or a “0”. Such setups help decrease the number of lost PCM frames resulting in benefits such as major time savings due to increased coverage, higher viable data rates and fewer data-gaps.
In practice, cost is often the major limiting factor in designing a telemetry system. How much data can be sent and/or what range the aircraft can fly to is related to the number, size and gain of the receiving antennas. For example, a 2.4m dish with a gain of 30dB can in theory cope with 10 times the data rate at the same range as a 19dB flat panel antenna – however, the required ground station equipment cost could be upwards of $300K more. Ultimately, the quantity of real-time data essential for safety must be telemetered. A balance can then be found between the desired data rate and the resources available, using onboard recorders to fill in data gaps and capture additional data.