Low Frequency Radio Transmission for long distance.

I'm sorry, but that is plain wrong when you are not receiving signals by direct line of sight.

When you are receiving reflected signals the paths are *all* too long. The bad part is they are off by differing amounts which very much screws the calculations.

Newer units have better algorithms to help deal with the multipath, but it is still a significant problem in the high rise portion of the city.

Selective availability added a "jitter" to the signal that looked like random noise. Average your locations long enough and you still got a good fix. Bad for guiding missiles, but ok for finding your house on a map.

I think this assumes you can receive the direct path. In a dense urban environment only a small number of sats can be viewed by direct path which is often insufficient to get a fix without the indirect paths to the other sats.

Oops (again). Y'er right. As you note, the reflected paths are always longer than the direct path. However, I was thinking of the averaging the computed fixes based on the mix of reflections and direct paths. I think that would produce something that could be averaged.

It's less of a problem with HSGPS receivers, which offer a larger number of data points from which to compute a location. As I mentioned, they also produce these locations faster than conventional receivers, which make it easier to distinguish a fairly stable direct path, from a less stable reflected path. I previous found an article that compared HSGPS with conventional GPS tracking in urban canyons, but can't seem to find it again. I'll dig some more later.

This isn't the article I wanted, but it does show what might be expected from urban canyons and some of what's being done about the problem for driverless vehicles: Note the large GPS antenna (Pg 16808) and the use of GNSS in order to obtain more satellites within view. GPS has 32 and GLONASS has 28.

"Error analysis for the Global Positioning System"

GPS clock jitter or dither will work for this. SA error averaged about 50 meters horizontally, and about 100 meters altitude. I used to use VisualGPS program: for averaging. Sample output: As I vaguely recall, it took about 500 points (about 15 mins) to get a reasonable average. However, the GPS I was using at the time had no provisions for dropping insane results due to reflections. The also program didn't handle these, so I had to scribble a PERL program to clean up the data before averaging.

Correct. For a 2D fix, one needs 3 satellites, one of which acts as a phase reference for the others. It has to be direct and stable or the other delays are garbage.

The phase measurements are all time tagged and do not need to be simultaneously present from all 3 or more satellites. That's partly how DGPS correction works. It combines the receivers measured delay and the differential receivers delay, at the same time tag, to create a corrected delay. As long as the receiver in the bottom of the urban canyon is locked (i.e. clock is accurate), and is getting signals from enough satellites, it can get reassemble delayed phases into something usable. How long a delay, I don't know. I'm not familiar with how the various GPS chips work internally.

As you indicated, the big problem is seeing enough satellites in the urban canyon in order to get a location fix. Just to make things worse, satellites directly overhead are useless because there is no Doppler shift from them. See Pg 16810 of the report I previously listed: shows the number of satellites in that were used to compute a location fix. Looks like 4 to 8 satellite. Sometimes at the same time but often rather scattered.

There are a lot of things that can be done even though the paths are not di rect.

For instance when tracking missiles , the missile has a translator and rebr oadcasts the GPS signal at a different frequency. So there is no direct pa th. But with four satellites you can get a good fix. Not the same as addi ng reflective paths of different lengths. But with a little incentive , I expect that John Hopkins could get an accurate fix.

Dan

If you are talking about a system where the missile retransmits all the sat signals with the same delay in order to give the ground a fix on where the missile is, then there is effectively no delay and a fix is trivial. That is the same as using an external antenna with a cable run. The only impact of the cable run is to cause an offset in the time calculated by the unit vs. the true time, approx 1 ns/foot.

This is nothing like getting a fix with most of the sat signals being reflected from different surfaces with different delays. Those delays cause positional errors of up to 1 foot/ns.

Qkay. So in the case of a missile that cable would be maybe 500 miles and changing. Maybe 1000 miles to another ground station. As I remember you need one additional satellite to get a fix. So you need a minimum of 4 satellites.

Dan

The cable has no impact on the equations. The equations are all based on differential delays. So you are finding the location of the antenna which in this case is the missile. The only possible issue with a variable delay in the cable would impact the apparent speed of the missile which, in fact, is the cause of the delay.

Not sure why you would need an additional sat. The equations don't change, so the solution shouldn't change. Do you understand that part?

I thought I could find something on the internet that would have a clear explanation , but failed. It has been about 30 years since I was involved. So I could easily be wrong now.

Dan

I didn't know that there were missiles that did that. Getting the group delay identical over 300 to 500 MHz is not a trivial exercise.

I once setup a rooftop GPS antenna and amplifier for a company that wanted indoor GPS in their factory and burnin rooms. With only about

I originally expected the indoor reflections would be a problem and was pleasantly surprised that this was not the case. All the reflections, from all the satellites, were together and had approximately the same reflection paths with (almost) identical delays. This appeared as an odd change in altitude, but the 2D position remained fairly constant. I did have to deal with some frequency selective fading from indoor reflections, but those were dealt with by moving the indoor antennas and installing a choke rings to eliminate bounces off the steel ceiling (both indoors and outdoors):

There were only 4 indoor antennas involved. One over the factory floor and 3 in the shielded burnin rooms so I didn't have to deal with multiple internal sources (except when the screen room door was open).

It was a fun project because at the time (about 1989) nobody available had a clue how it worked or what was needed to make it work. Today, one can buy off the shelf GPS repeaters: and is commonly used to illuminate indoor and underground areas.

Test yourself: You should be able to answer this question with the info I supplied above:

I would hope you understand why this is and not think of it as an "oddity". The point is you are receiving the signals picked up by the antenna no matter where you were inside the factory.

Of course, once you get a few sats directly, the path differences get to be very off and may not even give you a solution.

Was it not practical to cable the roof top antenna signal to the other receivers rather than retransmit it? Or do I not understand the setup?

I refereed to it and other phenomenon as an oddity because mere mortals and those not well informed as to how GPS operates, would thought that moving around the factory should have produced a change in location. I will confess that it took me a few hours, accelerated by an injection of designer coffee, to determine that it was not broken and that this was the way it was suppose to work. It took overnight to determine that it was normal to locate the rooftop antenna, and not the GPS receiver. I would not expect a person of average intelligence to understand these things at first glance, which is why I labeled them as an oddity.

That actually caused a small crisis. A local fire department had purchased GPS trackers for their larger engines. The engines were inside the somewhat RF proof fire station most of the time, where GPS was not reliable. The problem was that the GPS receivers of the day (about 1990) took something like 10 mins to obtain initial lock and would never get an initial lock if the GPS was moving. Once they got an initial lock, things were fine.

Someone installed a GPS repeater and indoor antenna which was suppose to help. It provided the necessary initial lock. The GPS would get an initial lock, save its location, and then go into standby. Knowing its current location, warm lock only took a few seconds (as long as the engine wasn't moving). The problem was that with the receiver getting signals via multiple paths, with plenty of internal building and ground reflections, it would never get an initial or warm lock. The engines frequently left the station in an unlocked state. The GPS would be "searching the sky" en route, which didn't work. Only when it arrived at the scene of the fire would it lock.

I was at the fire station working on some of the firemans personal computahs, when I got involved in the problem. Since I was unlikely to get paid for this, I decided that a fast fix was the easiest. Since the problem was the reflections from multiple sources, the easiest fix was to block all the reflections and have ALL the RF coming from the repeater antenna. The patch antenna on the fire engine didn't need to have a view of the entire equipment bay or sky, just the repeater antenna. So, I built a "waveguide" of sorts out of a cardboard box covered with aluminum foil, leaving a circular hole pointing upwards at the repeater antenna. It worked and produced quick and reliable initial locks.

Visualize a large tilt up building. One big room with maybe a 30ft high ceiling and a few modular offices around the periphery. Along one wall was a series of home made burnin, screen, and QA rooms. The big floor area was a mix of desks, benches, tables, partitions, etc. Hanging from the ceiling was the usual HVAC plumbing and electrical wiring. Lots of hung fluorescent lights, but no suspended acoustic ceiling. Typical silicon valley company for the time.

The problem was that the floor layout would change approximately quarterly as the company reorganized and grew. They were particularly thrilled with moving wires and cables around every time a move was needed. I had the same problem at a previous employer about 10 years previously, where I named our group "the portable radio division". The radio was not portable, but the division was. Also, there was some mention of being able to demonstrate GPS products without going outside, but that was never a real issue. Anyway, management was convinced that the GPS signal could be distributed cheaper and easier by wireless, but had no clue how to do it properly. Someone else had designed the system, purchased the parts, and disappeared. I took over and made it work.

Incidentally, indoor DAS (distributed antenna system) cellular installations often include a GPS repeater. This can be a problem for emergency services, where the victims cell phone GPS shows that they're on the roof, or wherever the DAS base station antenna is located, not where the victim is actually located.

That may have been the case in the past, when the GPS signal processing was expensive in both weight and power consumption, so just doing a frequency translator in flight and the actual signal processing on ground.

A similar system has been used with weather balloons. Earlier Loran-C was used and the signals from all base stations were retransmitted on a single downlink for signal processing on the ground. This would then calculate the position of the balloon (or actually receiver antenna).

It worked just as expected.

For antennas intended for geodetic GPS receiver, the center of radiation is specified to a precision of less than 1 mm. With the antenna sitting on tripod, the height over ground in millimeters can be used to compensate. Otherwise, the GPS receiver would display the location and height of the antenna center of radiation, not the elevation of the ground beneath the antenna tripod.

It seems to be PWM which is similar to Morse code. Apparently, the signal is repeated several times so the receiver can compare the results and detrmine which bits are correct and which are just noise. You can get a pretty good idea of what the message is with several repeated transmissions, but I don't see how you know for sure. Maybe the clock rejects time information that doesn't agree within some tollerance?

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.... Phil

If you are referring to the WWVB signal, it is repeated once a minute, but with updated information. There is no defined protocol for rejecting a noisy signal or even knowing it is noisy. Of course you can easily develop your own. Typically there is a clock running while receiving the signal. So the received data is compared to the running clock time. The received data can be stored and the consistency of the received time checked. If no errors were received the time should increment by a minute each time. Once the time was received correctly the running clock can be updated.

The WWVB signal also sends data by two methods, AM and PM. lol These can be compared to one another as well.

I believe some of the other time codes send parity bits so errors can be detected more readily.

There's the 5 bit ECC (error correcting code): "An additional 5 error correcting bits produce a 31-bit Hamming code that can correct single-bit errors or detect double-bit errors (but not both)." There are also parity check bits for DST and leap year.

The usual method is that the clock looks for 2 or 3 consecutive successful decodes before updating the clock.

Sure, if you can find a PM (BPSK phase modulated) WWVB receiver. Most clocks still use only AM modulation. There are also some home brew kits and designs for the PM version, but no commercial products. The PM method is patented by Xtendwave (Everset Technologies), who is dragging their feet on producing chips: Patents and applications: and a whole bunch more patents and applications: Patent ownership is now in the hands of Grindstone Capital. See "Legal Events" at the bottom of the page on most of the patents. We should see PM chips and products when hell freezes over.

Yep. Methinks they all do some kind of error detection and correction.

.... Phil

On Sun, 24 Apr 2016 21:41:54 -0700, Jeff Liebermann Gave us:

You already said it. Three successful decodes and it hard updates.

This is why radio clocks (not clock radios) take a minimum of three minutes to set themselves. Most take around five.

The WWVB time code only has parity on the PM signal, not the AM. If you don't decode the PM signal, you aren't getting any parity bits.