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How to Make a GPS System
The GPS has three segments:
1. Space segment: The original requirement was for 24 satellites in circular orbits in three symmetrical planes (120 degrees apart) at 12-hour orbits. Today, the Air Force uses 31 satellites in six symmetrical planes at 55 degrees of inclination (the satellites go as far as 55 degrees north and south). The satellites contain synchronized atomic clocks (mainly rubidiums).
2. Control or ground segment: These stations control and monitor the satellites. Originally, the stations were in the U.S. or secure U.S. territories which led to the Indian Ocean gap. Here are the stations from the 1971 Timation Development Plan (pre-GPS) (note the eight-hour orbits).
Here are the stations today:
3. User segment: These are your receivers (smartphones, dedicated GPS receivers, etc.). Note that GPS does not track anyone. The signals are sent from the satellites; no information is sent back. Tracking is based on information retained by the receivers or external systems.
There’s more information on my website.
Published in Group Writing
Well done, Richard.
My first experience with GPS was somewhere in the 1989-1990 period. We had to look up the positions of the satellites to make sure we had enough of a constellation to conduct our flight tests.
Yes, and every time Richard posts something about GPS I recall the Sat Nav system I used in 1972. Took a series of fixes over many hours to get a good position, and used a program that we ran on a DEC PDP 8-I minicomputer that was definitely bigger than a breadbox.
It’s like a Chinese telephone book to me, but I like having it.
Thanks, Richard.
Your post was so well done, I almost understood it.
Mongo like magic GPS box…
This week, GPS interference testing is being conducted by a Carrier Strike Group off the coast of Georgia. Note the distances and altitude bands at which GPS may be unavailable while this is going on:
https://www.faasafety.gov/files/notices/2019/Feb/CSG4_19-01_(Revision_1)_GPS_Flight_Advisory.pdf
This kind of interference testing happens occasionally, but the time period involved seems longer for the current one.
Meanwhile, the FAA is progressing with the reduction in the ground-based navigational beacon network in favor of more emphasis on GPS.
It is better to have more than one way of answering the question “where the hell am I?”
Really fine post, worth bookmarking. The text is succinct, the images are clear and free of distractions, and it all could fit on a folded quick reference card. @richardeaston are you a fan of Edward Tufte?
Thanks for your kind words. I’m familiar with his book but had forgotten his name.
Yes I maintain the VHF VOR’s in my airplanes, and play with them regularly. As much as I love having GPS in the cockpit, along with all of the fabulous applications that make aviation navigation child’s play, I have seen the occasional glitch in my GPS location which I can verify that I am not where it show I am by ground references.
To be fair I think it was the DoD testing methods to spoof users located in and around the DC area since it was occurring shortly after 9/11, and GPS was not as prominent a device in most pilots flight bags at that time.
I have not seen it much since then, however I can envision rogue actors doing the same spoofing as we enter a “Drone vs Airspace” security battles in the near future.
The Navy reintroduced celestial navigation a few years ago. Better to have it and not need it than to need it and not have it.
Making the obscure clear is a gift.
This conversation is part of our Group Writing Series under the February 2019 Theme Writing: How Do You Make That? There are plenty of dates still available. Tell us about anything from knitting a sweater to building a mega-structure. Share your proudest success or most memorable failure (how not to make that). Do you agree with Arahants’ General Theory of Creativity? “Mostly it was knowing a few techniques, having the right tools, and having a love for building and creating whatever it was.” Our schedule and sign-up sheet awaits.
I will post March’s theme mid month.