1955, Fred Whipple had just been appointed to head the Smithsonian Astrophysical Observatory in Cambridge. The International Geophysical Year was less than two years away and he wanted to involve citizen scientists as much as possible. Whipple was one of the few who realized a satellite was likely and set about creating a web of radio operators to monitor and relay transmissions from satellites along with groups of amateur astronomers who could optically track their orbits. Operation Moonwatch was formed and by Sputnik's launch over 200 teams were in place.
Tracking a satellite was done with a clock and a low power telescopes that could read azimuth and declination. The National Bureau of Standards time signal broadcast on WWV in Fort Collins, Colorado used as the reference clock and most people into amateur astronomy had a shortwave radio. Readings from stations separated by hundreds of miles were sent by amateur radio operators to a computer at MIT that gave the project a bit of its precious time. More clusters of readings would come in from around the world and a model of the satellite's orbit generated an ephemerides - predictions of where it would be. Later specialized cameras were developed by Bell Labs to automatically generate satellite ephemerides as well as tracking ICBM tests, but Moonwatch was a success and lit the imagination of a few people.
In physics it's common to turn a problem on its head to gain a bit of insight. Operation Moonwatch used known positions on earth and an accurate clock to measure a position in orbit. What if you put synchronized clocks in space and measured how long it took radio signals to reach a position on earth? With enough satellites with synchronized clocks you could triangulate a position anywhere on the Earth's surface. The technical details were difficult details, but the basic idea was simple.
It turns out accurate navigation is very difficult and expensive and a limiting factor in fighting the cold war. It was so difficult that as technology progressed ARPA decided to push ahead with a satellite based global positioning system - the inverse of Operation Moonwatch. It was a bargain for what it gave to the military. Finally a good enough signal was made available to civilians and navigation changed.1
GPS doesn't work everywhere. It would be nice to track things indoors and at a lower cost. Bluetooth and WiFi are currently used - usually measuring signal strength from several transmitting beacons and triangulate a position. Sometimes it's just proximity to a single beacon - a lighthouse model. The approaches aren't very accurate - three to ten meters is common. Good enough to track your phone as you move around, say, in a mall or subway, but doing better would be nice.
This is where the Titanic comes in..
On April 15, 1912 the Titanic struck an iceberg. Frantic radio signals went out, but there was confusion among rescue vessels and people on land trying to coordinate the rescue effort. Sometimes it takes a disaster to create order and an effort to regulate radio frequencies began soon after. Transmitters were assigned narrow swaths of the electromagnetic spectrum that were large enough to effectively communicate with receivers that could tune to those frequencies. Regulations changed as larger chunks of this resource were needed - AM radio required more than Morse Code, FM radio even more and television much more. Government would parcel out these increasingly scarce resources, but power was also regulated and it was possible to give much larger chunks to low power short range communications - WiFi and Bluetooth are examples. Most of radio engineering tried to pack as much information into as little spectrum as possible and to build transmitters and receivers that could be accurately tuned to specific frequency ranges If you waited for some technology to catch up there was another way (actually several, but I'll only mention one) way to do radio.
You may have heard about ultrawideband (UWB). In the mid 2000s it was supposed to be a good way to send large amounts of information over short distances. Cheaper and higher bandwidth than the WiFi of the day. Often existing technologies are improved with scale and an embedded base making it difficult for new players. Sophisticated coding technologies and enormous penetration (WiFi has greater penetration than smartphones) cemented WiFi as the prime high bandwidth low range wireless connection. In a few years UWB went from redundant to lagging. Many thought it was dead, but it had a few advantages in some use cases.
Here's how it works. Rather than sending out a precisely tuned narrow band frequency, UWB sends out narrow (in time) digital pulses that "splash" over a very wide frequency range. The signals are usually only one or two billionths of a second long and aren't sent that often - a hundred thousand to ten million a second are common rates. The frequency range is so wide that usually at least some of it can pass through objects that block signals like GPS and WiFi. It's also so weak that narrowband communication can't detect it.2
Imagine scattering a few UWB transmitters around where you know their location with some accuracy. Now start transmitting synchronized pulses. A receiver notices the time of arrival of each signal and, using the fact that light travels about 30 centimeters in a billionth of a second, can triangulate it's own position. Accuracy is about ten centimeters - about four inches.
A use case I'm familiar with is finding the locations of players in a beach volleyball game. In international FIVB games a very small (less than a half ounce) UWB device goes in the back of each player's shirt or sports bra. Their positions are tracked within ten centimeters about a hundred times a second and accelerometers and a compass send out rotation and orientation information on the UWB signal that the announcers, coaches and sports science people use. Four transmitters are used and all four players are easily tracked.
It would also be useful for augmented reality and Apple has included an UWB chip in every iPhone 11 - the greatest deployment of the technology so far. For now it's being used for data transfer, but wait a bit. Someone could also make very cheap ($10 or less) postage stamp devices that might be powered by available light that could be location tags. Put them on your keys, glasses, remote control .. anything you tend to lose track of. We went through trying to manage dementia with my mother. I would have paid a lot for this capability. And thousands of other uses from amateur sports to tracking boxes accurately in warehouses.
There are probably some bad things that can be done with it. We need to be thinking these things through, but our track record isn't exactly good.
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1 It also gives a very good frequency standard and a timing signal. Without this our mobile phone system, high frequency trading and a host of other core technologies we depend on would break. This is an issue as GPS is vulnerable to attack and we've done almost nothing on backup technologies.
2 The FCC defines it as taking up at least 500 MHz of spectrum somewhere from 3.1 to 10.6 GHz. Power is under -41.3 dBm/MHz .. or 75 nanowatts. Since it only deposits a tiny bit of power in any narrowband frequency, it's just below the noise floor of a narrowband receiver. Fortunately it's straightforward to build a receiver that can detect the signal.
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