Can standard components do rough distance measurement?

So far so good, but you are a bit late inventing that:

and measuring the amount of

Andy Wood snipped-for-privacy@trap.ozemail.com.au

It's been done before, so is possible.

Those figures sound about right.

Many micro's have an inbuilt hardware counter that can be externally clocked.

You are up the right tree, but it's an old tree. GPS is used now.

Dave.

Phwoar, at 100MHz? ;)

Yep, I'm well aware of GPS, but I don't want to have to rely on it. This would be used for sanity checking of GPS data (recently while walking I apparently travelled east by 80m then back within 2 seconds) or as a rough backup if the GPS lock is unreliable.

Getting the timing of packet sending and processing to be consistent is going to be the fun part.

How on earth do you plan to use this while walking or doing any other activity? You need a transmitter and the receiver. Plan to walk 10km, place your receiver, walk back, and then start your walk again? The "backup" for a GPS is a map and compass, and with proper compass/ topo navigation techniques it is possible to get the accuracy you require.

Learning how to use your GPS and knowing when to believe it is important. Trail logs allow you to see if it's telling you the truth or not, and there are lots of tricks for getting better accuracy under marginal conditions.

Care to explain exactly what it is you are trying to achieve here?, because quite frankly it's sounding very silly indeed.

Dave.

The "receiver" (they're really transceivers) will be mounted on my car, so it should be able to track correctly from the moment I move away. Here's how it would work:

- car sends TX packet, noting the point at which the very last bit was completed

- mobile device receives complete packet, then resends it

- car receives TX packet noting the point at which the very last bit was received, then deducts known delay to calculate effective round trip time.

Another idea I had was for the mobile device to receive on one frequency and transmit on another, which literally repeats a stream of data that the car mounted transceiver can use to calculate distance (averaged on a per bit basis, rather than per packet). This would not require any fancy counting of MPU cycles in order to figure out the software delay between receiving a ping packet and retransmitting it.

As I said earlier, supplementary data for sanity checking of the GPS output, and a rough backup for when GPS isn't functional. I should make it clear that I want to track where I've *been* - once I get home

- not where I'm going.

Forget it, you'll be wasting your time trying to turn this into a practical working system. The devil will be in the detail, and you'll never get it working anywhere near the accuracy of your GPS (even when it "plays up"). Get a second GPS if you really are that worried about having a backup, perhaps one with a different antenna system (like Helical and a SiRF chipset). Or simply get one with an external antenna connection and attach a better antenna onto your pack or whatever.

But like I said, learn to use your GPS properly and you'll be able to ignore those occasional "80m jump" errors. And when you get back and upload your track log you can simply remove those points from your track log manually because they are really obvious. I use my (bottom of the range patch antenna) GPS for tracking inside canyons and deep gullies and I still am able to get very usable track logs by removing only a couple of bogus data points. So in practice it's a trivial problem you are trying to solve here.

Dave.

I have a Garmin Edge 305 GPS and it's accurate to within about two metres, much more accurate than my old GPS72. It very rarely loses contact with the satelites and is very stable. Cheap on eBay.

Friday