Little bit OT: SpaceX starlink constellation used as GPS replacement

Navigating at sea used to be really hard. I used to work with ONI, Offshore Navigation Inc, who provided location services to oil rigs and service vessels. One of their systems, Raydist, was a high precision RF thing, similar to Loran but incremental; each unit came with an operator. I designed a digital readout system for them, essentially an encoder up-down counter with displays.

They did early experiments with satnav, before GPS. I hacked a custom version of FOCAL to help them with that.

Of course GPS killed their business, but one of the group's companies, OHI, now PHI, is still going strong, transporting workers and supplies to oil rigs..

Gosh, I've done a lot of weird stuff.

On a sunny day (Sun, 26 Sep 2021 07:55:16 -0700) it happened snipped-for-privacy@highlandsniptechnology.com wrote in snipped-for-privacy@4ax.com:

Nice, I should learn to fly one one day :-)

If it really works it has a great impact. Right now China for example has the means to shoot a GPS satellite down. And as there are only a few, it an hamper US forces in a big way. When however thousands of satellites are used for navigation, then for an adversary shooting many of those becomes too expensive. So I would expect US forces to go for the SpaceX method at least as backup. At sea I have learned to use a sextant, I only have a simple plastic one:

But so much more depends on GPS, some of the GPS modules I have also support the Russian and the Chinese system, go for about 35 $ or so last time I looked on ebay, so that gives you some redundancy. GPS is easily jammed though: did some work on an artificial GPS satellite... gives you the wrong location, as in that Bond 007 film..

"Using Starlink, they identified the antenna’s location within about 7.7 meters. GPS, by comparison, generally identifies a device’s location within 0.3 and 5 meters. "

Not really a major break-thru.

I believe the Starlink stuff, like GPS, has many satellites spaced around a smaller number of individual orbits. I'd be willing to bet there is a degenerative case of orbit where a single satellite can produce a bunch of shrapnel in an elliptical orbit that can take out every sat in that circular orbit over a short time. Then you just need one sat per circular orbit.

There is a lot more to military uses than just having the sats in orbit. Military GPS has anti-spoof and anti-jam capabilities which are vital to warfare.

I assume your GPS receiver did not use any of the military features of GPS that make jamming and spoofing efforts useless.

Military GPS is much more precise. While selective availability is turned off, the commercial GPS channels are not fully functional and do not get the upgrades they have added over the years.

With stronger signals and more units, you would expect it to be better. But probably limited by restricted frequency space reserved by the military.

Good book, Tragedy At Honda by Lockwood. It's about the 1923 crash of seven navy destroyers into a rugged section of the California coast, through a navigation blunder. Part of the problem was caused by a reciprocal RDF error.

I wanted to visit the site, but it's part of the Vandenburg AFB facility and visitors aren't allowed. Maybe my cousin, the fighter pilot, can get me in.

Most accounts that I've read say that RDF was a new technology. Watson didn't trust it and chose to use the dead reckoning position.

I would guess(tm) that the tests at UC Irvine did not use any error correction or differential GPS for the Starlink location experiment. Without additional ground based receivers, at accurately surveyed locations listening on Starlink frequencies, and using the data for error correction, accuracy will be mediocre.

GPS accuracy and performance standards can be found at:

"For example, GPS-enabled smartphones are typically accurate to within a 4.9 m (16 ft.) radius under open sky. However, their accuracy worsens near buildings, bridges, and trees."

Anyway, my smartphone, using GPS, GLONASS, GALILEO, and WAAS are showing about 10 meter initial accuracy, and 1.55 meter accuracy after about 10 minutes of averaging. It can do better if I wasn't doing this indoors and surrounded by reflections, trees and hills.

Until some someone adds error correction, more visible satellites, and atmospheric corrections to the Starlink constellation data, accuracy is going to be mediocre. Still, 7.7 meter accuracy is very impressive considering that the best my GPS can do without averaging was about 10 meters. I would guess(tm) that 7.7 meters is sufficiently accurate to locate a Starlink users dish.

Don't ever assume measurement error can be accurately estimated. When you talk about the number the GPS is giving you it is only offering an estimate based on the constellation and other known factors. It does not know about additional error caused by reflections. At least I've not heard of a unit that does anything about reflections. It's a tough problem, especially while moving.

I've plotted GPS locations of a stationary receiver that roamed around on a drunkard's walk reaching as far as 50 feet (~17 meters) over the course of just a few hours even using WAAS.

How do surveyors locate stuff with their GPS-ish devices? They do not mark out crooked positions that wobble by 10 feet.

They use features of the signal others don't use because of the length of time it takes to resolve the accuracy. They are detecting the phase of the carrier which at 1.5 GHz is what, 20 cm ~ 8 inches? I'm not even sure how they do that since there are no features of the waveform or pattern that can tell you which cycle of the carrier you are on. I suppose they are detecting the timing of the modulated pulse edges to select the timing of the carrier cycles. I suppose that is not just frequency locked, but phase locked. Then you can measure to fractions of a cycle. They must have pretty good timing electronics to drive it all. GPS is actually based on timing. The process of getting the time gives you the location as a byproduct.

The don't use GPS to directly produce an absolute position. They use GPS to locate the nearest benchmarks (ornamental disks with a nail pounded through the disk into the pavement or "pipe monuments"). They then obtain a GPS position at the property corners. If the land has moved due to landslide, earthquake, continental drift, subsidence, etc, and the two measurements are fairly close together, both will drift together by the same amount. The benchmark in the pavement was previously accurately located and can be assumed to be a good reference. However, surveyors do not rely on a single benchmark, and will generally search the area for other survey markers with known locations.

Note that this is sometimes done by post processing using recorded satellite delays at various known locations to correct a GPS position. It can be done in the field via a WAAS receiver, or later by downloading the corrections from NOAA CORES:

For my area (Santa Cruz County, California), these are the maps of the major benchmarks:

This is the map of the subdivision. Look for IPM markers, which are "Iron Pipe Monuments" also known as survey markers.

These are survey reports where a neighbor paid to have done in order to settle a lot line dispute:

I'm on Lots 20 and 21.

There are various ways a surveyor can improve their accuracy. One is to simply average the measurements over some period of time. I do this with the GPS survey app I mentioned in another message. You can see the effect using the software at:

I suggest Visual GPS View, which shows the "drunkards walk" pattern and the geometric center (centroid).

Another way is to use an antenna that has a well defined phase center and a way to block ground reflections and multipath. Most common are choke ring antennas:

These are necessary if you want millimeter accuracy.

Oops. Wrong URL. Should be:

"User Friendly CORS (UFCORS)"

CORS SITE ID Map:

Nearest site to me is P231 in Pacific Grove, near Monterey, California. Photo:

Under the dome is a choke ring antenna.

DGPS and post processing.