measuring 10 km with 1m accuracy

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I suspect that the cheap handheld units don't -- the cost of the logic would go up as something like N^1 or N^2 as the size of the time window increased, and to catch a reflection you need a time window big enough to see it and the 'correct' signal, or you need a multiplicity of receive channels.

One hopes. But a $150 GPS receiver is a pretty amazing exercise in cost reduction already -- I wouldn't count on it.

Yes. The receiver has to solve for three spatial dimensions and time. Solving those four equations demands four unknowns. Height is generally known to a much lesser accuracy than horizontal position, because the slant angle to the satellites is often quite shallow, which makes it harder to resolve height.

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Low resolution GPS chip sets are available for $10 to $20. Logic cost is not the issue. High precision GPS requires high precision clock, but such product would likely be classified as amunitions and sale restricted.

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Take a look what happens to the elevation display, when you walk below a bridge, that obscures at least some of the satellites.

I also wonder why any tripod mounted GPS antennas intended for geodetic survey clearly try to avoid any ground reflections or signals from low elevation angles (and hence reflected from a building) with a special constructions (e.g. using a ground plane sheet and a few concentric rings).

Of course, a receiver capable of receiving more than the minimum of four satellites might be capable of determining, if some of the satellite signals are received through a reflection and hence ignore it.

No firearms necessary. With the explosion of the number of GPS units in use in non-permissive environments, lots of the units -- especially handheld and vehicle mounted -- use "black" (unencrypted) keys. For example, see

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PPS data are encrypted and still subject to strict guide lines. Civilian codes are unencrypted, but subject to availability (can be turned off) in war threaters.

"After manufacturing the SAASM unit, the GPS receiver manufacturer ships the SAASM hardware to the KLIF for the loading of the Key Data Processor (KDP) crypto software"

Meanwhile, back in a real world, questions to the OP:

Are both measurement points static? Or one mobile? Both mobile?

How often are you doing the measurement? Every second/minute/hour/day/week/etc.?

Is elevation difference important?

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Absolutely, and I mis-typed up above. The keys are *encrypted* and thus they can be distributed as black or *unclassified* material, which is what I meant to say. The earlier generation of keys were themselves classified, so if your battery backup failed during a power outage, it was time to go humbly to the classified material custodian and 'fess up to needing somebody to come around with the magic juujuu. Do that too often and you'd better have fresh donuts waiting...

Even the "unclassified keys" are distributed on "need to know" or "need to have" basis. However, there are ways to build precision devices without the keys altogether. I am sure such projects exists in friendly and unfriendly countries. In a way, PDW (Precision Destructive Weapons) are more dangerous than MDW.

Rich,

Em 15/2/2010 14:45, Rich Webb escreveu:

When I started in the profession, the portable [and affordable] computing device was a slide rule, and it had a scale for sine and tangent which was essentially homologous to the arc itself (in radians) for angles lesser than six degrees (for a 360° circumference), so it should not be a surprise. . .

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Many modern civilian GPS units have 16 to 32 recieve channels=20 (correlators). I don't know if they are used to detect multipath or not. I'm sure that the extra channels are useful for locking in new satellites as they appear without having to drop old ones.

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Mark Borgerson

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$150? I have $50 bluetooth GPS and I have seen $25 GPS modules that use nothing more than an ARM7 to do the actual calculations for a fix. I can't imagine that it would take a significantly more expensive CPU to do the culling of outliers. In fact, one of the vendors we considered supported the sharing of the CPU to run application code potentially saving an MCU in an application. That shows even in a little ol' ARM7 there are CPU cycles to spare.

But then I don't know diddly about the actual calculations being done. So I can's say for sure.

Rick

Op Mon, 15 Feb 2010 19:22:25 +0100 schreef Paul Keinanen :

Unless you would compensate for this predictable effect.

Unless you would compensate for this predictable effect.

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Just convolutions and trigonometry. Nothing heavy. 10 to 20MIPS should work.

.> GPS could be good for greater distance but accuracy seems coarse.

Surveying grade GPS code based instruments can provide sufficient accuracy at the cost of more time (and money) than the shirt pocket GPS receivers take. If you are only interested in distance and not position you can use real time kinematic (RTK) GPS instruments that process relative positions based on GPS carrier phases instead of codes. See:

Dale B. Dalrymple

Years and years ago SGS published a booklet that explained in detail how to write the firmware to drive their GPS frontend chips and decode the data. It was very interesting reading but alas, I lost my copy. Anyone seen a pdf version around?

.> and skewed. I am not sure if it would be possible to improve the .> accuracy by averaging and how much of averaging it would take.

To improve accuracy by averaging you need to average over samples with independent errors. With GPS that can require collecting samples over a long enough period to collect from different sets of satellites in view. How many years did you plot?

If you have the time you can measure continental drift:

For an example plot see:

Dale B. Dalrymple

The initial solution of the GPS system of equations starting from zero knowledge requires a lot of number crunching. It could take several minutes on the 10 MIPS machine without FPU.

VLV

Perfectly true, if you do temperature and humidity measurements along the path to determine the refractive coefficient.

Unfortunately, in practice, this is the hard part.

While I understand that in the old days, in the worst case, it takes quite a long time (several minutes) for a single channel GPS receiver to try out all the 32 PRN sequences in order to lock into at least _one_ GPS satellite and then wait until the whole GPS satellite constellation _almanac_ has been downloaded and after that, lock into at least 4 satellites to get the fix.

However, these days, with a multichannel receivers, each "receiver" can search for a specific satellite PRN sequence in parallel and perform the required navigational calculations, without waiting for the satellite almanac.