The maximum height of cellphone reception varies quite a bit depending on where you are – we’ve seen 30,000 ft in New Mexico and 4,000 ft in North Carolina. In this post, I’ll give a few examples of cellphone reception from altitude from our own experience. I’ll also explain what factors probably led to the surprising range of maximum altitudes we observed.
Cell phones have been used to track high altitude balloons for a while now: at least since these guys from MIT launched a camera into near space for $150. Here at Bovine Aerospace, we used cell phones for three launches: a near-space flight in New Mexico, a solar balloon near Norwood, Massachusetts, and a solar balloon near Chapel Hill, North Carolina. We used the instamapper app, which went offline in 2012 but apparently is back (just realized this about 2 minutes ago, it’s like hearing back from an old friend).
The Jake 2 high altitude balloon launch carried a SPOT tracker and a Boost Mobile phone. The phone continued to transmit latitude, longitude and altitude until the balloon reached 20,000 ft, after which the altitude ceiling was reached (some GPS chips stop sending altitude above a certain height). It continued to report latitude and longitude to approximately 9,000 meters (30,000 ft), when the connection was lost. The cell phone did not transmit data on the way down (good thing we had the SPOT!). This is not too surprising because the balloon came down over a remote desert area and ended up in the mountains.
Unfortunately we do not have the cell phone data, because we never copied it down and the online repository expired. Rookie mistake!
The Jake 3 solar balloon carried my coworker’s old Android phone to about 3,000 m (9,000 ft):
Elevation versus time plot for the Jake 3 solar balloon launch in Norwood, Massachusetts.
We had cellphone tracking from the surface (35 meters above sea level) to 350 m (1161 ft) above sea level. We lost tracking for about twenty minutes, then regained it at 1867 m (6123 ft) above sea level. Tracking was lost for good at 2755 m (9038 ft) above sea level, after about 43 minutes in the air. The balloon was visible for 15 or 30 minutes after we lost tracking. We’re not sure how high the balloon went, but we do know it came down in the Atlantic…7 of the 9 messages in bottles in the payload were discovered up and down the Massachusetts coastline.
The gap in coverage is particularly interesting since the balloon was never more than a mile from the launch site during tracking (despite being well over a mile in the air when contact was finally lost). So, clearly there is not a simple relationship between cell reception and altitude. I have a pretty good idea what is causing this – see the discussion at the end of the post. You can download the position data for Jake 3 here.
We repeated this experiment with a different solar balloon design (and a different Android, obviously) in Chapel Hill, North Carolina a few months later. Once we lost reception, we never expected to see the balloon again. However, something went wrong during the flight and the balloon came back down a few miles away. The resulting altitude plot looks like this:
Elevation versus time for the Jake 4 solar balloon flight in Chapel Hill, North Carolina.
Cellphone tracking was present from the surface (100 m, 328 ft) above sea level to 800 m (2624 ft) above sea level. The balloon was too high for cellphone reception for about 21 minutes. It reappeared, descending rapidly, at an elevation of 1164 m (3818 ft) above sea level. We recovered the balloon, cell phone, and the messages in bottles we’d hoped to distribute at sea in a forest about 6 miles away. Over a thousand foot difference in maximum height of cellphone reception over 6 miles! Evidently, the maximum cellphone reception height is variable spatially as well. You can download the position data for Jake 4 here.
While we still have no idea what made it come down suddenly, it is certainly possible to make a rough estimate of the maximum height of the balloon using linear regression. Turns out it made it almost exactly a mile above sea level before something went wrong!
Maximum elevation reached during the Jake 4 flight estimated using linear regression.
Evidently cellphone reception with altitude varies with location (highest in New Mexico, lowest in North Carolina), varies with horizontal distance in the same region (i. e. North Carolina), and can periodically disappear and reappear with altitude (Massachusetts). All three of these variations make sense. In New Mexico, cellphone networks must cover large areas of mountainous land with very low population density. My guess is that cellphone transmitters in New Mexico send a very powerful signal in order to cover the largest land area with the fewest possible towers. So it makes sense that we would still have cellphone reception above 25,000 ft in New Mexico.
The gap in cellphone transmission in Massachusetts was likely not due to the phone malfunctioning but actually a consequence of how radio antennas operate. It turns out that directional antennas (such as those used to transmit cell phone signals along the ground) produce side lobes in addition to the intended radiation direction. So this balloon remained in the main signal zone until about 2600 ft, where it entered a null region. It then crossed into a side lobe at about 6100 ft, finally losing the signal again just over 9,000 ft above sea level.
Illustration of main and side lobes. Figure from the Wikipedia article on side lobes in radio antenna design.
As an aside, this same radiation pattern shows up in some unexpected places – such as the sound pattern produced by speakers.
The North Carolina launch showed something a bit more mundane – cell phone reception varies with location!