Pinpoint by Greg Milner

One of my favorite parts of my job is working with geospatial data. I’ve been playing around with my Swarm data for years, but it’s a far cry from analyzing millions of rides with hundreds of columns of metadata. GPS is foundational to everything I look at, and while it’s sometimes frustratingly inaccurate, it’s still mind-boggling how prevalent location data has become. We now complain when Google Maps traffic estimate is off by a few minutes. Ten years ago, I was using a giant map book and had to flip pages whenever we crossed town boundaries. So how did GPS take over our lives?

1) 4 GPS satellites are needed to accurately pinpoint a location.

3 satellites create 3 spheres that intersect at 2 points. One of the two points is either miles above or below sea level. The 4th satellite is there to resolve any timing ambiguities.

2) GLONASS is the Russian version of GPS and is – for now – the only other satellite navigation system with global coverage.

The most shocking thing I learned from the book is that GPS is American.

3) Etak is a form of dead reckoning used by the Polynesians that relied on using star bearings and reference islands.

I honestly still can’t conceptualize how this works. I think you have to know where you started and how fast you’re going – all in your head.

4) Back in the day, determining longitude was much harder than determining latitude.

The equator and the poles meant that latitude was actually a fixed concept, while longitude depended on using a prime meridian as a reference point. Another gotcha about lat/long is that 1 degree translates into different distances depending on where on Earth you are.

5) The second is defined as the time that elapses during a transition between 2 states of the caesium atom.

It’s not defined by the Earth’s rotation, which fluctuates too much.

6) The Air Force was responsible for space projects like satellites, but the early users of GPS were in other parts of the military like the Army, Marines, and Navy.

The history is convoluted so I don’t remember it all from the book, but the gist is that the Air Force did not really want to run the GPS program.

7) The GPS signal has two forms: the P (precise) code and the C/A (coarse acquisition) code. Only military receivers could decipher the P code.

Interestingly, there were two broadcast channels, one with P + C/A combined and one with only P. If one channel failed, the military could still use the other channel to receive the P code.

8) In addition to the P vs C/A split, there was also selective availability, where noise was added to the C/A code to prevent enemies from using GPS against Americans.

Selective availability was briefly turned off during wars in the 1990s. Two other factors also contributed to its removal in 2000. 1) Differential GPS (DGPS) was able to correct for the noise anyway 2) Selective availability was severely hurting the commercial growth of civilian GPS. $ speaks.

9) Studies determined that voice-only instructions were the most effective compared to maps or even a combination of voice and maps.

Coincidentally, I went on my first ever solo road trip while reading this book. I’d say I still looked at my Google Maps app every time.

10) A datum is a coordinate system that turns GPS signals into latitudes and longitudes tied to real locations.

The current world standard is the Earth-centered WGS 84.

This book was a bit dry at the beginning as it walked through the history of GPS within the U.S. military, but it picked up in the second half. It was also very informational, and I learned a lot about how we came to live with something so taken for granted nowadays. Here’s to hoping I’ll find some time and energy to start a side project.


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