AVR Freaks

Get a GPS made for walkers and rangers. These typically are made to register much finer displacements and slower speeds.

GPS by itself can be precise to a couple inches, and millitary implementations are. The limitations the civilian apparatus has are artificial. Governments don't want you to be able to strap a GPS on a rocket and send it in someone's chimney, like they do themselves... ;)

I am much more interested in the equipment you will be using to probe the terrain itself!

In this scenario, with a good quality GPS:

Autonomous GPS = 5m, $50 for bog-standard receiver
Autonomous GPS = 2m, $500 for good quality receiver
WAAS/EGNOS = 0.6m, $500
Differential GPS = 0.4-0.5m, 2x $500 receivers
Omnistar XP = 0.2m (requires subscription), $3K-$5K receiver plus signal subscription fees
L1 Carrier phase Differential DPS = 0.2m (Note 1), 2x $500 receivers, plus whatever the software upgrades are.
Omnistar HP = 0.1m (requires subscription), $3K-$5K receiver plus signal subscription fees
Real-time Kinematic (RTK) = 2-5cm, $30K total (base + rover + sw upgrades)

It goes up rapidly as the volume sold rapidly falls and the complexity of the software increases. And people who want accuracy are usually willing to pay for it.

Numbers are ballpark and will vary between manufactures and receivers. A good antenna is also a must, like a Pinwheel antenna instead of a patch antenna.

Once L2 and L5 signals become civilian and "open", I would expect the prices for hardware to fall. Current L2 requires funky cross-correlation of signals to recover the carrier phase of the otherwise encrypted code.

-- Damien

Behind the scenes, WAAS **is** differential GPS, but by differential GPS, I'm referring to pseudorage corrections (i.e. measurement errors) from a stationary base being broadcast to a rover so that the rover can apply the corrections to its position solution. The stationary base (as long as it's close by) will supersede the corrections from WAAS.

WAAS is built for integrity ('can I trust the solution it gives me') rather than accuracy per se. The accuracy part is a nice by by-product.

Rule 1. NEVER believe the marketing figures! They'll be stated for a particular set of conditions that may or may not apply in the area that you live in. Furthermore, some will optimise to use anything it can find in the sky and others will reject anything that looks fishy - it's a trade-off of accuracy vs availability.

Use one that has an external antenna. Try hunting around for a good antenna that rejects multipath, which I'd say is most of the garbage on the west-side of the building in that tutorial. At very least, get a patch antenna and put it on a large (say, 20-30cm) ground plane. On-board or integrated antennas are not that flash.

I'm loathe to make a recommendation without testing any of them, but Trimble has been doing survey-grade GPS for years and is likely to use at least some of that technology in its RF front-end and algorithms (and hence, the most stable result during that test). However, it appears not to support WAAS or differential corrections (look for "RTCM", "RTCA", or "CMR", which are common correction formats), so you may be out of luck. Maybe looks to see which ones support what features like that?

The alternative is to get the raw data (pseudorange, carrier phase, satellite navigation data) out of the receiver - the Trimble does it - and use GPSTK to generate what you want, but this solution does involve a lot of effort and you'll need to learn a few GPS fundamentals to do so.

-- Damien

That is NOT what Differential GPS is! It's one receiver (GPS) that also listen to a local station (within about 1000Km)in the 200KHz band, and do the calc from that, and remember GPS is about 3 times more precise at nigth that in the day.

If and only if precisely the same set of satellites are received on both the rover and the base, and only if they are processed in the same way.

It only takes a single extra satellite to throw this out, though it should be better than an autonomous solution alone.

-- Damien

How big is the field overall?

The IR idea is cool, but you'll have issues resolving height accurately, Maybe, just maybe, the horizontal resolution might be good enough, depending on how accurate the angle measurements are. At 100m, you'll need an angle resolution of 0.14deg or better to get 25cm over 100m. I think :)

-- Damien

I'm wondering about a laser rangefinder... Maybe something like this, but I don't know how well it works outdoors or at long ranges.

-- Damien

I think I originally misunderstood... I thought you wanted to scan what's UNDER the field and just correlate data with coordinates to map it.

Hey who knows maybe you got a spaceship under there...

On a more serious note usually surveying large area like this requires some segmentation, you don't usually do it one shot. Say you have an area 1KM by 1KM, you might wanna split it down in areas 10M x 10M and just do them square by square. Each area thus has absolute coordinates. This also makes it easier to track what has been done, and what's left to do.