With Help from NXP Engineers, Young Students Chat with ISS Astronauts

I began my undergraduate studies with no knowledge of amateur radio. I grew up in the digital age when dissecting electronic devices produced little more than uninspiring green boards populated by anonymous black rectangles, and when circuit tinkering involved microcontrollers and CAD software (EAGLE, of course). Today, prospective EEs are even more likely to begin their journey with software and processor platforms instead of a radio kit.

However, after a recent conversation with two engineers from NXP’s facility in Chandler, Arizona, I have a renewed appreciation for the things that you cannot do with an MCU evaluation board. For example: establishing a real-time voice link with someone who lives 250 miles above the Earth and is careening through space at 17,500 miles per hour.

 

NXP Helps Elementary Students Reach ISS Astronauts

NXP’s collaboration with Tarwater Elementary was supported by ARISS (Amateur Radio on the International Space Station). This multinational organization facilitates educational projects that introduce students (and adults) to wireless technology in general and ham radio in particular. It is difficult to imagine a more effective way to convince young folks that “old-fashioned” RF techniques are still useful and interesting. A thousand-dollar iPhone can do many things, but it can’t call an astronaut on the ISS.

 

NXP engineer Lionel Mongin with the ground station equipment. Image used courtesy of NXP

 

Though delightfully simple compared to cutting-edge wireless devices, a ham-radio system capable of reliable, clear, bidirectional communication with the ISS is not something that can be designed and operated by fourth graders. NXP engineers supplied the required technical expertise, but the school didn’t miss an opportunity to combine some STEM lessons with an impressive demo of STEM in action: before the scheduled radio call, NXP’s Amateur Radio Club taught students about wave speed, refraction, modulation, and other RF fundamentals.

 

The BOM to Contact the ISS

So what exactly does a school need if it wants to communicate with the ISS? The ground station consists of a VHF radio, a two-meter amplifier, an antenna, and some software.

The VHF (very high frequency) band extends from 30 MHz to 300 MHz. The ISS downlink frequency, at 145.800 MHz, is near the middle of that range. A two-meter amplifier is so called because of the wavelengths corresponding to the module’s operating frequency range (λ = 2.06 m for f = 145.8 MHz).

The radio and amplifier can be bought off the shelf. An optimal antenna system, however, may present some procurement difficulties if you don’t have a company like NXP to help you out:

 

The team at NXP affixed a rotating beam antenna to the roof of a converted news van to maintain contact. Image used courtesy of NXP

 

The antenna itself was a cross-polarized Yagi. The minor complication here is the importance of rotating the antenna as the ISS passes overhead—hence the news van, which not only elevates the antenna but also allows the ground station’s software to control the antenna’s orientation according to the changing position of the orbiting space station.

During my conversation with Lionel Mongin and Jim Davies, the two NXP engineers who operated the equipment, I asked about the feasibility of using a fixed antenna. Though some communication would be possible in this scenario, the duration of high-SNR reception would be seriously reduced. My impression is that the results aren’t worth the effort if you don’t have an ISS-tracking antenna system.

Another thing to keep in mind is the importance of reliable equipment, proven software, and skilled operators. The line-of-sight communication window is short, and as Jim said, “Everything [has to be] working properly at the moment because you don’t get a second chance. You get about nine minutes.”  

   

Factoring in Doppler Shift

When Lionel and Jim explained the variations in carrier frequency required to compensate for Doppler shift, I was initially surprised. It hadn’t occurred to me that radio communication with the ISS would require that type of adjustment.

First of all, I haven’t spent much time with RF systems that involve high relative velocity between transmitter and receiver, so the Doppler effect is not prominent in my mind when I’m pondering radio communication. Years ago I worked on a military data link that was subject to instances of high relative velocity, but if I recall correctly, we didn’t attempt to correct for it, because even with data loss due to Doppler shift we still expected to achieve acceptable packet error rate.

Second, I wasn’t thinking carefully about the spatial relationship between an object in orbit and an observer on the ground. Even though the ISS seems to be moving “up there” rather than directly toward or away from a ground station, the relative velocity between the two is still sufficient to noticeably degrade SNR if the carrier frequency is not adjusted.

The importance of Doppler compensation depends on the space station’s angle of elevation above the horizon as it passes through the ground station’s line-of-sight region. For example, at an elevation of 26 degrees, a system without Doppler compensation will lose about 65% of the communication interval; at 83 degrees, you lose about 85%.

 

The amount of RF Doppler shift depends on a satellite’s elevation angle. Image used courtesy of Ron Hashiro

 

Engendering Interest in STEM

Jim and Lionel are both long-time ham enthusiasts. Jim acknowledges that young people’s interest in amateur radio is waning; it seems less relevant in the smartphone era of easy, instant global communication. But calling the ISS via radio—as with other types of engaging, collaborative engineering projects—has effects that extend beyond ham technology.

Jim said, “If we spark some interest in any STEM-related fields through this, I’m sure someday these kids are going to look back and remember this and then maybe find it cool and interesting and want to get into engineering.”

Math and science are difficult subjects for many kids, and the work won’t get any easier when they later enter advanced high school courses and embark on university engineering programs. Motivation is a critical factor in academic success at all educational levels, and NXP certainly brought math, physics, and engineering to life for students at Tarwater Elementary.