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Looking for technical details of RMT target: What is the IR carrier signal? (Trimble/Geodimeter)
chris-mills replied 4 years, 10 months ago 6 Members · 31 Replies
While looking online for some other Geodimeter related stuff, and found this site:
https://confluence.qps.nl/dm/geodimeter-600-active-radio-link-10-26280243.html#0002–1433161306
Not sure what’s going on here, and I’ve got to go out to do some survey so don’t have time to read further, but it looks like something related to tracking.
Excellent.
When I was looking at this some 10 years ago the intent was to see if a miniature target could be produced rather than the larger ones using a full size prism. Problem at that time was cutting down the size of the board. Modern technology should enable a very small circuit board to be produced, so I await developments with interest.
For conventional use (2 person) we made some small targets with a LED (for sighting purposes in the dark) using a 20mm. acrylic prism. Overall size, including button cell battery, is around 100x20mm. With the prism on the back plate this means that there is no offset for distance and in a corner a 10mm. offset laterally. Typical range is over 100 metres in daylight. We use them in large buildings (store re-fits, etc.), m/s car parks (where sighting between columns often leaves only very small gaps) and on small topos with lots of buildings.
Cool, can you post a photo of that tiny 100x20mm target?
Who has worked with old (Geodimeter) und new (Trimble S-series) instruments and can rate the following assumptions?
- Old Geodimeters/Trimble instruments have the quad photodiode for tracking the IR transmitter mounted below the optics, aiming parallel.
- With old instruments the retroreflector is actually not used for acquisition and tracking. If a target has the wrong offset, it is possible that no EDM can be performed.
- New (S-series) and future instruments measure co-axial with no offset between retroreflector and IR transmitter.
- New instruments can use the retroreflector at the target to look for the strongest signal. Is this only the case for tracking passive targets? Or also with active targets?
- New instruments can’t use the old active targets as they work on different carrier frequencies and possibly transmit data with a different encoding.
If I want to design a target that is agnostic and suitable for many instruments, what would be a good approach? How about a ring shaped PCB with the LEDs transmitting radially on the bottom most part, on top of that a cylinder wrapped in reflective tape with a height of at least the tracker<->edm offset of the old instruments.
Shu, what you are doing is awesome!
- Old Geodimeters/Trimble instruments have the quad photodiode for tracking the IR transmitter mounted below the optics, aiming parallel. Yes this is my understanding as well.
- With old instruments the retroreflector is actually not used for acquisition and tracking. If a target has the wrong offset, it is possible that no EDM can be performed. Yes this is my understanding as well. It is possible to move the scope off the glass center, but, not a huge way
- New (S-series) and future instruments measure co-axial with no offset between retroreflector and IR transmitter. I don’t think this is true as there sure appears to be a cell below the scope
- New instruments can use the retroreflector at the target to look for the strongest signal. Is this only the case for tracking passive targets? Or also with active targets? I don’t know the answer, but, my guess is one or the other
- New instruments can’t use the old active targets as they work on different carrier frequencies and possibly transmit data with a different encoding. Designed obsolescence
This is the S6 active target and I have always wondered why the ring of LED’s on top when there is clearly one sensor on the instrument
As requested, two views of the mini-target. Designed to be used in normal two-person use. It should be possible with today’s electronics to fit a cut down version of the RMT circuit board into it so that it can be used in solo mode. For our usage it is mainly for inside work, often in derelict buildings when two persons are a safety requirement.
Body is heavy brass angle, brazed to make a channel of the required width. (Brass so as to stand up to being dropped). Top and bottom plates also of brass.
Swivel base tapped to standard 5/8″ thread and held by bolt inside target.
Front panel covers a standard coin cell battery (2032) which is operated by the front waterproof switch. At the centre of the prism is a 2mm. yellow LED which enables the target to be seen in darkness or poor light. Cover hooks into the bottom plate and is held in place by the two small screws.
Prism is a 25mm. acrylic with an ABS surround – made to full prism diameter and then filed down to fit the channel exactly.
The main section of the body is angled relative to the base, so that the measuring point when mounted on a target rod is directly above the centreline of the rod. Leaning back onto a vertical face, the prism constant for the acrylic is fortunately the same as the backplate thickness, so again the distance is correct.
Height of unit matches the standard GDM 600 target, so it is interchangeable. For small sites with quite a few buildings or tall walls, etc. we tend to use it for all the survey, as there isn’t a longitudinal offset when it is up against a face and the range exceeds 100 metres.
With the LED switched on to assist identification it means you can shoot through visual gaps of 10mm (why do columns almost always line up in front of things you want to measure?). I also have a couple of 20mm. wide steel strips with screw mounts on the end, so if you can reach into a gap you can also place the target on a corner beyond the access.
Here’s a link to the S series tech manual. It explains the optical scope and the tracker coaxial bit. I read as while they are coaxial, they aren’t, and still require collimation. The window below the scope looks likely to be for tracklights, although I think further into the manual it says for HP instruments it is used for additional tracker? There’s a bit in there that I can’t work out, where it seems to indicate the instrument sends out an IR signal for tracking, and that this has 5m spread at 100m – I thought (and still do) that it is the multi prism (like the 5600/GDM RMT’s) that send the signal, however the S manual does I think indicate that the instrument has a IR receiver.
https://fccid.io/ANATEL/03503-11-07503/Manual/1C0C0E35-8B24-43C0-9A75-3A5EFAE5F244/PDF
Here’s a guide on multi prisms, including a tip on vertical accuracy:
The main section of the body is angled relative to the base, so that the measuring point when mounted on a target rod is directly above the centreline of the rod.
Great idea to tilt the retroreflector to have the same offset while upright and while on the wall. But remember that the EDM triggers on the first pulse and with the usual beam divergence you will essentially measure the shortest distance to the target, ie the top most part of the retroreflector. The error is insignificant in most cases. With one (1) prism there is no such error, only with multi-prism targets, and the larger their planar surface, the more significant the error.
Thanks again Richard, very valuable information. Yes, if I understand the maintenance manual right, there is a tracklight below the EDM lens with a beam divergence of 38.5 mrad, roughly 2 degrees. But that is just used for the person who is wandering around not knowing where they should stand 🙂 ie. while staking out.
The Tracklight® is a visible guide light. It emits two flashing coloured lights, red and green. The colours represents each side of the centre. The red and green lights enables the rod person to align himself with the line of sight to the instrument
The tracking is done coaxially through the same lens with the EDM and sighting, with the help of filter mirrors. This works as the tracklight has a different wavelength compared to the EDM. While they share the same lens, EDM and tracking have offsets (errors) introduced versus the straight sight, due to their additional mirror. With the 785nm tracklight emitted from the instrument, it can track passive targets.
Difference between the long range tracker below the EDM (available in some models in place of the tracklight) and a standard coaxial tracker seems to be the beam divergence. Standard tracker: 2gon, Long range tracker: 0.2gon.
The tracklight from the instrument can be used to search and lock onto passive targets.
Yes, I realised that, but this target is basically designed for internal building survey, so one ot two mm. variation is not critical.To be absolutely factual it measures 1mm. short when leant against a wall and 1mm. long when plumbed vertically over a point – a case of splitting the difference when designed. It is only used on a detail pole; never as part of a traverse. When I took the photo I forgot to switch on the LED, but you can see where it is in the centre of the prism. The battery lasts for many weeks.
I did quite a bit of experimentation before making the final one. With those acrylic prisms it appears that the instrument doesn’t fully differentiate what it is hitting, so several adjacent ones appear to merge into the final reading.
Just as a point of interest, in those experiments I found that there is a slight change of distsnce depending upon whether the acrylic is orientated with the “mini lines” of prisms set horizontally or vertically – strange.
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