# A History of Channel Coding in Aeronautical Mobile Telemetry and Deep-Space Telemetry
## 4 Aeronautical Mobile Telemetry
### Channel Coding in the 21 st Century
#### 4.2.1 Enhanced Flight Termination System (Efts)  
To prevent unmanned test articles from leaving the controlled airspace of a test range, unmanned test articles are equipped with a flight termination system (FTS). An FTS comprises two elements: a ground-based transmitter that uplinks commands to the unmanned test article, and a flight termination receiver installed on the unmanned test article that receives the uplink commands. When an unmanned test article veers off the range, a range safety officer issues a \"terminate\" command that \"halt[s] the forward motion of an errant vehicle by placing it in a condition of zero lift (nominally) and zero thrust to establish a known impact point or debris footprint\". [190]. The mechanism for termination varies and includes zero thrust with a parachute, a flat spin, or destruction by an explosive charge (usually for a missile or rocket). FTS dates from the early 1950s [190] and is based on a series of combinations of three or four tones modulating an FM carrier in the 420 to 450 MHz band. A block diagram of the transmitter is shown in Figure 23a. The three tones, used to define four commands, are drawn from a set of 20 RCC tones in the range 7.5 kHz to 73.95 kHz. The optional fourth tone provides a check channel. The tones are either on or off, depending on the command. The sum of the tones is used to modulate the FM carrier. A block diagram of the flight termination receiver is shown in Figure 23b. The received signal is applied to a limiter/discriminator to produce a noisy version of the three/four-tone combination. The power at each of the tone frequencies is monitored by the command decoder that identifies the uplink command. While primitive by today's standards, the FTS worked well for almost 50 years. FTS was an example of the well-worn adage, \"if it ain't broke, don't fix it\". In March 1999, test flights with the then-new Global Hawk [191] unmanned air vehicle broke FTS [192; 193]. The Global Hawk event occurred on the last day of its tests at Edwards AFB, CA, USA. Because the Global Hawk was scheduled for tests at Nellis AFB (just north of Las Vegas, NV, USA) the next day, the Nellis AFB range safety office, following standard procedure, tested the Nellis AFB flight termination system by transmitting a flight termination sequence and monitoring the transmitted signal. The Global Hawk flying high above the Edwards AFB range received the command, successfully decoded it, and terminated by crashing into the desert on the Edwards AFB range. The post-event analysis identified two issues: (1) the Edwards AFB FTS and the Nellis AFB FTS operated on the same carrier frequency and (2) there is no provision in FTS for source and destination addresses. The first issue was caused by the fact that in normal circumstances Edwards AFB and Nellis AFB are too far away from each other for simultaneous line-of-sight RF propagation to/from a test article on the Edwards AFB range. Global Hawk's high-altitude operation violated this assumption. For the second issue, if FTS had a provision for even simple source and destination addressing, the accident would not have occurred even without the resolution of the first issue. The second observation prompted a program called the _enhanced_ flight termination system (EFFS) in April 2000 [190; 194; 195]. The result was a system that provided source/destination addressing, enhanced reliability through channel coding, and increased security through encryption. The last two items nudged the design toward a \"digital system\" and away from a modified tone-based system, such as the one described in [196]. The result was the system summarized by the transmitter block diagram in Figure 24 [197]. The EFFS message includes test range, transmit, and vehicle identification codes with the 6-bit command code as shown. After encryption (and the insertion of a spare bit to match the code dimension), the encrypted message was encoded by a (25,13) Reed-Solomon code with elements from GF(2\({}^{5}\)).