GPS upended?

[a64pilot]

Originally Posted by Dockhead

Yeah, tell that to the U.S. military, and NATO. Smart bombs guided by BeiDou?

People want to argue this apparently but the only thing common in Military and Civilian GPS is they MAY be located on the same Satellite.
It would be difficult to jam or even more so to spoof a JDAM
I’m sure there are others but the JDAM is the only GPS guided munition that I’m aware of, but I’ve been Retired for almost 20 years too.
Many munitions use GPS for positional data, for example the Longbow RF missile is given an extremely precise transfer alignment from the launch platform just prior to launch, then its given the North, East and Down to the target, it’s autopilot flies the trajectory and in the terminal phase of the flight the Radar goes active for terminal guidance, it uses GPS, but not for guidance.

The JDAM is just a “kit” that’s attached to a “dumb” bomb, just like the LGB is, but it’s a laser guided system this been around since Vietnam.

I think that GPS isn’t considered reliable enough for munitions guidance, by reliable I mean it could be jammed, often times it’s not hard to jam guidance systems

[rondelais]

The problem is that GPS (or other L-Band GNSS) signals are very weak, coming from 12,500 miles (20,000 km) up. An interfering signal from say 10 miles away on the ground has an advantage of 1.5 million to one, or in engineering speak 62 dB (square law). These signals must be at fairly high power, so there might be another 10 dB required. It’s hard to make a filter selective enough to reject a signal close to the desired frequency, so frequency allocation authorities require “guard bands” to keep frequencies part far enough to allow reasonable filters. The guard bands being proposed in this latest frequency grab are, in at least my opinion (MIT engineer, 40 years in military satellite communications design), just too small, requiring heroic filters. Although they can be built, it is hard to get both good (70 dB) stop-band attenuation and low in-band loss in anything with modest size. An additional complication is that there are millions (maybe billions - I count 12 just at my house) of GPS receivers in use, and almost all of them (such as in cell phones) have no way to add a filter. For new-design receivers, a combination of modest (say 30 dB or so) filtering plus high dynamic range digital signal processing (another 40 dB) could get there. The GPS signal uses spread-spectrum modulation, so there is some signal processing built in. However, new chipsets would have to be designed to provide this - the earlier generations were not designed with this threat in mind. These would be bigger, and take more power than exsisting chip designs. The US government has made a practice over the last 20 years or so of SELLING (generally at auction) radio frequency spectrum, which to me is a limited natural resource. Until this idea came along, spectrum was allocated based on the merits of a proposed use. The auction idea resulted in some spectrum being awarded to speculators, which then “flipped” it to others. The auctions did not provide much in the way of limits on how the spectrum was to be used. For various technical reasons, L-band is a highly desirable frequency, and other raids on it have been proposed.
There are only two ways to design receivers to reject interference from signals close to the operating frequency of the system. One is to use frequency selective filters,

[Auspicious]

Originally Posted by a64pilot

SA can be defeated by a non moving GPS by averaging, my Apollo Precedus for example had a survey mode where it averaged hundreds of position fixes and that took out the SA errors, but wasn’t effective on a moving platform of course.

That is the entire foundation for differential GPS (DGPS). That takes care of supporting moving platforms.

Originally Posted by a64pilot

People want to argue this apparently but the only thing common in Military and Civilian GPS is they MAY be located on the same Satellite.

As far as I know they are on the same satellites. M-code is on GPS Block III birds.

Originally Posted by a64pilot

I’m sure there are others but the JDAM is the only GPS guided munition that I’m aware of, but I’ve been Retired for almost 20 years too.

Not relevant to the application I was involved in. Various –INTs identified a location. Position went to forward targeting teams who called for appropriate strike teams. That could be anything from special operations to cruise missiles including everything in between. Very specific position was important at every step. I suspect you are of an age to remember the t-shirts that floated around the precision munitions community of a cruise missile flying in a specific window.

Do you remember the missile systems that did automated celestial?

Originally Posted by rondelais

An interfering signal from say 10 miles away on the ground has an advantage of 1.5 million to one, or in engineering speak 62 dB (square law).

Inverse square law? *grin* Welcome aboard.

Originally Posted by rondelais

requiring heroic filters.

Agreed. The problem with heroism in this context is that it gets physically big. DSP is great but that takes bigger, hotter processors and more energy from batteries. GPS is already a big hit on battery life for portable devices. Even then, DSP doesn’t/can’t make up for things like front-end desense.

Did I miss any of the big stuff? *grin*

Originally Posted by rondelais

The GPS signal uses spread-spectrum modulation, so there is some signal processing built in.

Are you sure? I think GPS C/A and P(Y) are BPSK. M-code is something orthogonal – I forget but it’s in one of the links I posted above. Not spread spectrum, even NB. Am I missing something? Footnotes?

Your point about the relative of spectrum auctions vice allocation based on societal good is worthy of discussion. Even during allocation days license fees were staggering. Auctions just shifted the pricing model to market-based.

With respect to using L band for phones and broadband it really does come down to the terms of the licensing agreement with the FCC. This topic does lend some perspective (NOT merit) to the COVID-19/5G cellular conspiracy theories doesn’t it? *grin*

[Midnight Son]

Sorry late to the party.

A64 and some others know most/all of this.


For a little background on precision uses of GPS. Speaking as a retired Surveyor (amongst other careers), before they turned off SA.


1) High precision GPS survey systems (0.0001m in N,E,Z) utilize two stations with the full L1, L2 receivers, (special license required to own/operate, at least in Canada).
2) Master set on a known point, local or National Geodetic Mark, or a point referenced to one.
3) Radio signal, from the master, with correction data (known point location vs GPS calculated location) is sent to the rover.
4) All data points recorded at both ends. Data reduced on computer to further refine positional accuracy.


That is how you get to 0.1 mm in three axis.


WAAS does not need that accuracy, so it just sends out the correction for it's location, via radio (aircraft). Onboard system calculates true position.

For agricultural uses the correction (from WAAS? or system suppliers?) is sent via cell services data feed (farm tractors, combines, sprayers).


If the interference, from the Legato system, gets to be too much, they might have to revive all or part of the eLoran idea, ~50 kW LW shore transmitters sending timing/correction signal, for either eLoran or GPS. This also would mean one more piece of electronics on the boat.


Best wishes, hopefully I haven't bored anyone to tears. That was definitely not my intent.

[Auspicious]

Originally Posted by Midnight Son

3) Radio signal, from the master, with correction data (known point location vs GPS calculated location) is sent to the rover.

Not necessarily. We built a non-real-time DGPS system in 1991 for subsea survey purposes. Civilian GPS under SA was fine for navigation. We used the master station data and the at-sea data with post processing to correct the data. No radio link. That really reduced licensing burdens.

I know I'm being pedantic but real-time (including time-of-flight propagation delay) is important for some applications but not all. I suspect land survey is the same.

[Midnight Son]

Originally Posted by Auspicious

Not necessarily. We built a non-real-time DGPS system in 1991 for subsea survey purposes. Civilian GPS under SA was fine for navigation. We used the master station data and the at-sea data with post processing to correct the data. No radio link. That really reduced licensing burdens.

I know I'm being pedantic but real-time (including time-of-flight propagation delay) is important for some applications but not all. I suspect land survey is the same.

No problem, just filled in for information.

High accuracy for surveys, not so much for aircraft/agriculture.
Best wishes

[StuM]

Originally Posted by Midnight Son

WAAS does not need that accuracy, so it just sends out the correction for it's location, via radio (aircraft). Onboard system calculates true position.

WAAS is the US implementation of a SBAS (Satellite Based Augmentation System)

It sends out corrections through geostationary satellites which are serving as relays for fixed ground stations.

For agricultural uses the correction (from WAAS? or system suppliers?) is sent via cell services data feed (farm tractors, combines, sprayers).

WAAS is picked up by the receiver directly from the WAAS satellite. DGPS uses ground based communications

[Auspicious]

Originally Posted by StuM

WAAS is picked up by the receiver directly from the WAAS satellite. DGPS uses ground based communications

Agreed. I'm not aware of any application that uses cellular for corrective data. Asset tracking does sometimes use cellular for outbound data. The latter is how Amazon Prime can show your package (actually the delivery vehicle) during the last few miles of delivery. I wish UPS, FedEx, USPS could do as well.

Do agricultural applications use dedicated DGPS for combines and other harvesters? It couldn't be more than $500US for a dedicated master station and radios at each end. Some frequency coordination might be needed between neighbors which could be interesting.

Does anyone have any insight? Footnotes? Large agricultural equipment is beyond my experience.

[StuM]

Originally Posted by Auspicious

Agreed. I'm not aware of any application that uses cellular for corrective data. Asset tracking does sometimes use cellular for outbound data. The latter is how Amazon Prime can show your package (actually the delivery vehicle) during the last few miles of delivery. I wish UPS, FedEx, USPS could do as well.

Mobile phones frequently use cellular networks (not data) as well as GPS for location services.

My Samsung phone has three "Location Methods" which can be selected:
1. High accuracy - uses GPS,Wi-Fi and mobile networks to estimate your location
2. Battery saving - uses Wi-Fi and mobile networks to estimate your position
3. Phone only - us3s GPS to estimate your location.

The majority of vehicle tracking systems that I am familiar with us cellular data to send their location.

Do agricultural applications use dedicated DGPS for combines and other harvesters? It couldn't be more than $500US for a dedicated master station and radios at each end. Some frequency coordination might be needed between neighbors which could be interesting.

Does anyone have any insight? Footnotes? Large agricultural equipment is beyond my experience.

RTK is the buzzword (Real Time Kinematic).


Here's one widely used base station



https://agriculture.trimble.com/prod...-base-station/


Around $10,000 + for the base station. The in-vehicle system can cost as much again.

[Auspicious]

Originally Posted by StuM

Mobile phones frequently use cellular networks (not data) as well as GPS for location services.

My Samsung phone has three "Location Methods" which can be selected:
1. High accuracy - uses GPS,Wi-Fi and mobile networks to estimate your location
2. Battery saving - uses Wi-Fi and mobile networks to estimate your position
3. Phone only - us3s GPS to estimate your location.

Sure. Those choices have nothing to do with operational location. This is called assisted GPS (AGPS) in which other information is used as a 'seed' for initial solution. That greatly speeds up acquisition. Once GPS is running each GPS position is used as the seed for the next calculation.

You'll find 'battery saving' location to be very crude. It's just averaging the locations of the cell sites and WiFi APs it can hear and recognize.

Originally Posted by StuM

Around $10,000 + for the base station. The in-vehicle system can cost as much again.

Jeepers. Someone is making a lot of money.

[StuM]

Originally Posted by Auspicious

Sure. Those choices have nothing to do with operational location. This is called assisted GPS (AGPS) in which other information is used as a 'seed' for initial solution. That greatly speeds up acquisition. Once GPS is running each GPS position is used as the seed for the next calculation.

You'll find 'battery saving' location to be very crude. It's just averaging the locations of the cell sites and WiFi APs it can hear and recognize.

I'd never tried it, just seen the option.
You are correct. In Battery saver mode, my navigation apps say GPS not connected and Google map has my location jumping around up to several hundred meters away with a constantly varyiing "circle of error". Guess I won't be using that setting

[Auspicious]

Originally Posted by StuM

I'd never tried it, just seen the option.
You are correct. In Battery saver mode, my navigation apps say GPS not connected and Google map has my location jumping around up to several hundred meters away with a constantly varyiing "circle of error". Guess I won't be using that setting

The other piece of the puzzle is that way offshore you have to be patient. Without the 'A' in AGPS it takes a while for the GPS to get started. It may take five minutes depending on how long and how far the last fix was. Once your phone GPS is connected it does great. In fact, you can improve battery life by switching on airplane mode so your phone doesn't use energy hunting for a cell site that may be hundreds or thousands of miles away.

From a practical point of view, once located updates once or twice a day are plenty for fast location without cell signals.