Self Introduction

[Ceros]

Hi Guys

A little about myself……..

I have worked in the UAV business since the mid 1980's – just as it was starting to go “commercial” in a big way – by way of qualifying with, and subsequently working for BAE Systems as a electrical engineer on their UAV projects, and stayed working on/with UAV's for the rest of my career.

My particular area of specialty: data links. My very first posting a few weeks back – was tongue in cheek – I was hoping for more responses, but in hindsight having read through a few forum pages now, PC104 and I/O isn’t really much of a subject for the UAV projects discussed by most members on the forum.

So, what can I offer the forum?

Well, on UAV subject matter, truth is my mechanical knowledge is next to zero, my aerodynamic and structual knowledge is equally close to zero and my ability to fly anything is also equally poor – but my understanding of rf comm’s & data links is pretty good. So by all means, on this one part of the overall subject of UAV’s - please feel free to ask: retirement can be boring and I’m only to happy to share my experience and knowledge to help forum members as much as I practically can.

In due course, with the permission of the forum Moderator(s) I'd like to write up some notes on the subject of rf/data links which I think will go some way to helping folk get the best (or more) out of their current datalink(s).

Regards to all

Ceros

[pthompson24]

Hi Ceros,

Welcome to the forum.

So, do you have your own personal hobby UAV yet? Are you going to build one?

Personally, I'm flying and building as much as I can with on board video but I know I can't do the UAV thing myself so I'm waiting for Attopilot to go commercial to get into that part. I want to be able to send video missions on a path and have it come back to me reliably with great footage.

You don't happen to live in the Northern VA area do you? I know there are BAE buildings around this area but I'm sure they are all over the US as well.

Paul T.

[Tom Harper]

Ceros,

Welcome!

Anything you can contribute on data com is welcome. What are the licensing rules? What are the limits on 'do it yourselfers'?

Tom

[zik]

Ceros, I'd be interested to know if you've worked with any UAVs which have a gimbaled directional antenna on board. Do you think a setup like that is likely to work well?

[Ceros]

Paul - the key to good quality video downlink, other than TX power and Rx sensitivity, is basically as follows - and in mentioning all these things they are not esoteric beyond the means of hobby builders - both financialy or technicaly - all well within hobby means:

1) RF Modulation type used: Your best modulation type is going to be COFDM because it is inherently resistant to the problems that are associated with low power and dynamic platforms (read as: constantly moving) - in fact it was designed to deal with those very problems - so look for the words COFDM on the printed material that comes with whaytever type of modem/tranmitter you choose.

2) Then look for the word Digital - Digital is better than analouge - the s/n (signal to noise ratio) is far far superior - especially with video links. Where analouge picture is completly drowned out in snow, a digital picture will still be crystal clear.

3) Then look for QAM, QPSK 16 or 64 - the rule is: the lower the amount of bits the more reliable and durable the signal will be - and in many cases you can select whtever setting you ant, or the equipment will default to the lowest setting it can use for a given amount of data. Yes 64 QAM sounds mighty impresive, the truth of the matter is that it offers absolutely ZERO advantage - in fact it is counter productive on a singla video stream, because you don't need all those bits and they just take up extra power (in simple terms).

4) Again as I said in earlier comments to a member named Gary - the lower your frequency the better your link will be.

In summary your best video results at an amature level will come with using a - full resolution digital type progresive scan (as opposed to interlaced type - look for the description 480 or 525 lines with a p after this number, not an i) PAL camera, which is encoded with MPEG2 Transport Stream (this is different to MPEG2 - the encoding can be downloded free from a number of sites on the internet), using COFDM modulation. If you are current using analogue Tx, change over to digital as soon as circumstances permitt - you will notice a big improvement.

On the ground that old rule applies again - the more gain your Rx antenna has the better will be the receive signal, and as I said before, unless you want to carry around a big Parabolic dish, nothing beats a well sorted out helical antenna.

No - I have no personal UAV. I can't fly anything! But time permiting I will be working on datalink/TV link ideas using off the shelf equipment that amatures can easily afford.

Tom - there are no limits on do it yourselfers. It comes down to how much time you want spend on the project and how much you want to invest. Take the Aerosonde UAV. A very simple and very small UAV that is capable of flying its self across the Atlantic!

Zik - yes, I have worked with directional/sterrable antenna's on UAV and they are used, as I am sure you know, by a number of miltiary models - mainly in the form of microwave dishes.

The problem for do it your selfers is that they add weight (which means a larger UAV is required) and the add complexity - both mechanical and eletrical complexity which has to be intergrated into the UAV's on board positional electronics .... and if it goes wrong then you mess on your hands. In short the small benefits are outweighed by the potential con's. Steerable antenna's should never be used as a flight control Rx antenna on a UAV - we don;t even do it on military models because of the inherent risks. As a downlink for non-critical data (e.g. high resolution real time visual data) - fine, no prob's.

Increase your Tx power has to be a simply and more relaible way to go.

When the standards were being set for ISM type modems/transmitters, it was established that 10watts could safely be used in 5.8Ghz commercial ISM type equipment (the sort of modems you buy in computer shops), without risk of inteferance to neighbours. I'll get no thanks from the DTI for publicising this, but there you have it - you can safely use a 10watt bi-directional PA with each modem without worring about intefering with your neighbours. I mention this because so many folk use ISM/home computer type modems running at a few hundred milliwatts as data links.

.... and this goes back to Tom's question about licesning rules: 10watt's is way way above what the DTI permitt in the UK for ISM transmitters - but good quality PA's can be purchased from any one of many sources.

I am not to clued up on UAV reg's, but as far as the rules go for radiofrequency datalinks in the UK, they are long and complicated.

The few amatures I do know mostly use ISM type 5.8GHz computer modems - which although he regs are not yet finalised, are easily avalible of the shelf now. 2.4GHz spectrum has just become to saturated - so if anyone is still using 2.4GHz - go purchase yourslef a 5.8GHz modem.

.... it;s such a big subject Tom - I think the first thing I can do is write up some notes on the comparisions of different types data link equipment that are easily/cheaply used by do-it-yourselfer UAV builders - the pros & cons of each type.

[zik]

Thanks Ceros, very interesting. I'll be keeping an eye on what you come up with for your data link / video link ideas.

[Connexxion]

Hi Ceros

First thing first: Welcome to RCG! :D

We really appreciate people like you who are willing to share their knowledge with us instead of watching us "drowning".

I'm sure there will be lots of members who will be interested in your knowledge and will be very appreciated by your affords taken to explain and answer our questions,especially the FPV guys.

Connexxion

[nick.sargeant]

Welcome to RC Groups.
I'm sure your knowledge of rf data links/comm's and the commercial UAV industry in general will prove to be invaluable to some of the members developing projects both here and in the FPV section.
Nick

[pthompson24]

Ceros,

Thanks for all the info you provided but I'm afraid some of it is over my head.

I've been getting great video recorded from a HD camcorder of many flights. I've only flown FPV a few times and I loved it and I want to do it more but a few things have prevented me so far.

I was up on an FPV flight far far away and enjoying the few when my specktrum 2.4 radio glitched and I almost lost the plane. That was nerve racking and the fear of that happening is a powerful disincentive.

I long to fly further away with confidence while still recording in HD and as I see it there are two ways I could go. 1) With a good visual link that does not disturb the range of the specktrum or 2) with an autopilot which will do the flying for me or at least act as a backup if the RX link fails.

I'm wary of adding too much complexity and expense as those are disincentives too. Flying with video running is easy and fun but I'll soon be ready for the next step as soon as it becomes simpler and not too expensive. I have my hopes on attopilot right now but there are other projects out there as well that may bear fruit someday. If you can create a reliable and clean video link that works well with 2.4 specktrum systems then I would be interested.

Thanks,
Paul T.

[Ceros]

Paul - getting a basic understanding of the points that are describe as beyond you will go a long way to understanding the limitations and how you can improve it all. The points I raised are big plus points that will considerably improve the performance of your system, but all that aside changing the links to 900MHz units will give you an immediate and very noticable real world improvement.

900MHz systems to replace 2.4GHz are avalible off the shelf at much the same cost.

[Jack Crossfire]

There are lot of ex aerospace/cold war engineers on rcgroups. islewis is another one. They tend to live in Florida. The 80's were definitely the golden age of aerospace investment.

[eddymoore]

Ceros,

I'd be *very* interested to talk to you about a downlink (and indeed uplink) for small high altitude Balloons that we're running out of Cambridge, UK. My understanding on the regs is that we're limited to 10mW on unlicensed bands (Radio license or not - ofcom seem upset by unmanned airborne radio's). If there are things that actually are legal that are better but that we don't know about, I'd love to know :)

We're using radiometrix 434Mhz 10mW modules at the moment, sending RTTY at 300 baud, we do raised cosine shaping on the input pulses (input goes into a built in VCO on the Tx). We've got about 300km off it in real world conditions - the Tx signal still sounded absolutely fine by the balloon burst and dropped the payload into the sea at that point.

Receiving is done with a high gain yagi, yaesu transceiver and PC soundcard rtty decoding software. We currently have no uplink - about to start working on it but advice would be useful!

Am pretty happy messing with micros, am looking for an excuse to get into SDR, but on the balloon the main concerns are small, light, and low power. A modulation scheme that be generated by a 40MIPS micro/dsp chip (like a dspic, arm7 (arm9, maybe) etc) would be absolutely ideal.

Any advice you can give would be immensely helpful and appreciated!

[Ceros]

Eddymore

Actually I am suprized that you are getting a reliable and constant 300km with 10dBm off a 434MHz ISM type modem - that is excellent by any standard and reflects a good Rx front end setup.

Of the top of my head that leaves you with an Rx strength of something like
-130dBm.

In general the rule has to be KISS - keep it simple stupid, as far as is possible - especially when it comes to running electronics at high altitudes - but, some questions if I can beofre I comment:

- what is the Yaesu receiver sensitivity (dBm)?
- what sort of range are you wanting to achieve?
- what sort of data rate are you wanting to achieve at max range?
- why are you using a Yagi and is it Horizontaly or Verticaly polarised?
- what type of modulation are you using (FM, QAM, QPSK etc etc ......)?
- what mass an the balloon lift?

And lastly, just out of interest, just what sort of altitude are you hoping to achieve?

[eddymoore]

Hi Ceros,

Agreed on KISS - simplicity, reliability and low current consumption are key for these balloons - the performance of the batteries is reduced at such cold temperatures.

To answer your questions in order:

This is the receiver: http://www.qsl.net/sm7vhs/radio/yaesu/ft790/specs.htm and there is is a 13dB yagi hanging off that (I think, anyway. All a bit hazy :) )

The maximum range possible is 600km as at the standard operating altitudes (~30km) we loose LoS at that point because of curvature of the earth. However, realistically we want to keep stuff in the UK, so lets say we'd like everything to be quite happy up to about 200km. The 300km was as a result of a failing pyrotechnic which releases the payload from the balloon if it strays too far.

At the moment we work on 300baud rtty with no error correction - it's all just eyeballed to see if it makes sense. Truthfully, the next useful stage of telemetry would be an order of magnitude faster (to send pictures and the like) but that's not going to happen, so we'll just stick to text. Let's say 1.2kbps would be lovely but we can live with less.

The Yagi is used for the high gain. Its polarisation depends on how I hold it in my hand :)

FSK. But that's just at the moment.

We typically send stuff up to 2kg up to about 30km altitude. In such a setup, we'd expect to have about 1kg free lift after payload. But it varies - we can get balloon envelopes from 500g to 3kg (where the weight specifies the weight of the balloon) so we tend to tailor stuff on a flight to flight basis.

Right now we're working for fun on breaking the amateur altitude record (about 120,000ft) by getting to 140,000ft (40km). That's using a 150g payload and a big envelope with very little free lift - the balloons burst at a specified diameter so the less free lift, the less helium, so the higher you can get before the helium expands to such an extent that it bursts the balloon. Hoping to launch around May when the atmopheric conditions are a little more sympathetic.

The uplink is the big thing we'd like to include. So far we've used the first every thing we did - a simple rtty transmitter with pulse shaping and decoding with our eyeballs. It worked, and we had other stuff to develop and make reliable. Now we've got to the stage where the radio is holding us back so we'd like to sit down and make a good system that'll last us a while. We'd like to keep the Yaesu transceiver as we've invested in the hardware (student project - money is tight) but don't mind ditching the radiometrix Tx modules - with the VCO on the input they rather limit us to FSK (as opposed to, say, audio-modulated PSK like the PSK31 standard). We're not afraid of getting hands dirty with embedded stuff and signal processing - it's all about the journey on this project!

I forgot to mention - the antenna on the balloon (tx) is a 1/4 wave monopole - I think it has a decent radiation pattern for ballooning (more or less conically downwards but radially symmetric - the balloon tends to spin around and generally does it's own thing as it gets through the turbulent rubbish into the upper Stratosphere. But the caveat to all this is that I'm a 2nd year undergrad on a general engineering course, so I have no idea what I'm talking about!

Ed

[Ceros]

I want to chew this over for a couple days, but in the meantime here are some brief comments in point form:

- 1.2kbps is no problem - in fact a couple Mbps should be realistic. The issue will be bandwidth and modulation. FSK is going to have to be dumped and replaced with QAM or similar - just which constelation is going to be best is something I want to chew over. 64 would be great especially if you want to shift pictures (would that be still frames or streaming video?), but in all reality you are probably going to have to settle for 16 (QAM) at most if not straight forward QPSK. It really will come down to what size the picture frames are going to be, and how many over what period of time - so some feedback on that please.

The next consideration - and this is going to have ultimate control of what you can and cannot achieve - is temperature: are you aware of the temperature and its impact on electronics at 40 000m? You will have temp of between -40 and -60 C.

Some thought to insulation and/or correct packaging is going to be in order.

Out will go your yagi and out will go your airbourne dipole - will discuss with you later. A wide angle patch (mounted facing down) is a possibility, and if not, then at least a tuned larger element - maybe a 5/8, a homemade conical, or even a pcb mounted spiral or patch of sorts

Out wil go your Radiometrix modules, and more than likely a rise in Tx frequency will be beneficial.

The uplink is no problem - just what purpose wil it serve? - payload control, and if so some detail on the data type it will be carrying as this will determine the airbourne Rx type (and weight).

[eddymoore]

Thanks for your answer :)

We'd like to keep this all absolutely legit - it's a requirement for getting some assistance/workshop time from the engineering department. You're probably much more clued in on what is and isn't. We probably can't afford the £ks it costs to get a completely new radio licensed...

... but to answer the questions :)

Pictures would be fun but not entirely necessary - video would be absolutely great - we could do a lot more with it than we could with vids (text overlay for telem and the like). 640 x 480? As I say, it's all very much more than what we're used to which is text typing itself like in a soviet era film.

We've had a fair bit of experience now with high altitude electronics. Most of the stuff we sue is rated to -40 anyway. Crystals do tend to drift. We insulate well. active temperature control is do-able but a bit of a power hog. Most of the stuff we use now (8 bit and 32bit micros, general hobby electronics) seems to work no probs in the ambient temperature.

The uplink at the moment would only need to do basics like command a cut-down, command a chute deployment and so on. It's all done by onboard logic and would continue to be, but an over-ride would be nice for unforseen circumstances. I'm currently working on flying the payloads back under guided parafoil. The ultimate solution would be a video downlink to a joystick which is then uplinked, again if we need to take manual control of anything. But all fairly low bandwidth. Won't be sending up any new programs or anything :)

Ed

[kfong]

Hi Ceros,

Glad to have some rf expertise on board. You brought on some interesting ideas. If you were to design such a system from off the shelf parts. Is there a setup that can be afforded by the DIY hobbyist. Even the equipment I've collected are not as high tech as what you have described, your stuff sounds like high end commercial stuff. The info is good to know since I browse the surplus stores and ham shows. I've just recently become aware of 5.8ghz band, some recommendation or links would be great to have and best choices for antennas.


Kin

http://www.embeddedtronics.com/

[Ceros]

Kfong - in asking me what COTS equipment I would use - are you asking me in respect of brandnames, or in respect of general technical configuration?

[kfong]

Ceros,

Both, but brand names so I can become familar with what's out there. That way if I come across such stuff I might be more alert to it. It will also give a reference to compare other products with.

Kfong - in asking me what COTS equipment I would use - are you asking me in respect of brandnames, or in respect of general technical configuration?

[Ceros]

Oh hell - could write all day on that.

In terms of rf the rule has to be go for as low a frequency as you can (I don't think I need explain the reasons for that - the lower the frequency, the better it will propergate and the less power it will need to propergate for a given distance), and in that regard I like the:

Ubiquiti Xtreme Range7 (700MHz) and the
Zcomax mPCI (also 700MHz)

I have worked with both professionaly, so I know they do "what it says on the tin" so to speak, but are many similar products out there to choose from.

If money is not an issue, take a look at VideoPropulsion intergrated professional PCI cards - the C1LP is superb (but I'd hardly call it a budget product - it's around $3000). I've done a lot of work with this card - it works fantastically. It will carry flight control and telemtry data multiplexed with the video signal in way few other cards can.

In terms of modulation type, well, all things considered it's my humble opinion that in the absense of being able to configure PWM (Pulse Width Modulation - which not many hobbyists have sufficient background or knowledge to do), the way to go has to be OFDM/COFDM - but it needs to be set up bearing in mind power/gain figures avalible at both the TX and Rx ends of the link, and one needs to configure the the data rate properly and in line with the amount of data they are going to move backwards and forwards.

Try to source out rf cards that allow user setting of all the parameters - like data rates and modulation settings.

Digital as opposed to Analogue - no argument about that. Digital every time will beat an analogue rf signal - both in terms of amount of information carried, distance carried and Rx quality over any given distance for a given data amount at a given Tx/Rx output/gain. Go for digital.

Camera - CCD type as opposed to CMOS types, and Progressive scan as opposed to Interlaced - and added to this, always look at the size of the image sensor (i.e. 1/4"" or 1/3" or 1/2" or 1" or whatever) - the larger the better.

Then look at the number of horizontal and vertical lines. The horizontal figure is the important one and you are looking for a number as close to (or bove) 480 - 525, with a small "p" after it, as opposed to a "i" (Progressive as opposed to Interlaced).

There is ongoing dispute regards which is more important: larger image sensor or more horizontal lines?
It is my opinion that larger image sensor coupled with pixel size should always be considered before the number of lines.

Encoding - Used to be MPEG2 Transport Stream (Transport Stream is slightly different to normal MPEG but is need to move stills and streaming video over rf links if it is digital), but it has now being superceded by H.264. MPEG2 can compress up to around 10 x before you start loosing siginificant amounts of picture quailty/detail, whereas with H.264, you can compress up to around 50 x before loosing significant amounts of picture detail/quality. In effect it means that you can move around 5 x the amount of imagery data with H.264, than what you could move with MPEG2 for agiven bandwidth or data rate/data avalibility.

If it took 1 minute to transfer x amount of video with MPEG2, with H.264 it will take 12seconds whil maintaing the same detail and quality. Conversely, if you were encoding MPEG2 5x, you can encode the same digital still 25x with H.264, and get the same quality picture at the other end - and heres the really important part: it is achieved with 20% the amount of data bits you would need to move over the same amount of time using MPEG2 Transport Stream. Anything that allows less bandwidth or less data to be used, makes the link more durable and easier to maintain.

Taking things up a level, for the technicaly minded, it is easier to multiplex telemetry and control data into video data with H.264 than it is with MPEG2. But in both cases, a bit more than a basic knowledge and understand of how to multiplex and mix compressed and uncompressed signals together in the same data stream is required, because of course you cannot compress control data - it has to be sent at "full resolution".

In short it is now possible for hobbyists to transmitt full frame 25 or 30fps, D1 broadcast resolution video (PAL or NTSC - makes no difference) with affordable COTS equipment.

Another point to keep in mind as far as imagery is concerned is what is known as frame capture. Today, the way most hobbyists (and budget level professionals) implement digital still picture caputre is to load a digital still camera into the UAV and use it to take pictures.

While this is not bad, and indeed, as consumer level (and prosumer level) digital still camera's are increasing every year in resolution, this method accounts for the large majoirty of situations, it none the less has it's limits.

An alternative to this - which will provide better resolution pictures is to implement a framegrabber card, coupled to a dedicated digital camera of the type used in machine vision systems. This method enables one to capture single megapixel sized frames from streaming video - the limit on the frame size being a function of how much you can afford to spend on a framegrabber card and camera unit. anything from 1 or 2Megapixel sized single frames up to 12 - 16Megapixel single frames can be "captured" from streaming video using a dedicated framegrabber.

Using the industry standard Camerlink standard to interface your digital camera to your framegrabber will allow the following single digital picture sizes to be caught from streaming video:

1) 100pfs @ around 1Mbpf
2) 60fps @ around 2Mbpf
3) 30fps @ around 3Mbpf
4) 25fps @ around 4Mbpf
5) 15fps @ around 5Mbpf
6) 10fps @ around 6 - 7Mbpf
7) 5fps @ around 8 - 16Mbpf

So it possible to extract very very high resolution digital stills from streaming video using a CamerLink and a dedicated framegrabber. I have no particular choice here, but have used Silicon Graphics and Imperx's 16Mbpf camera's - through a CameraLink and Matrox framegrabber, stroing the stills at full resolution on the UAV on a removeable solid state PCMCIA type 8 - 32Gbt card, and using a 2nd camera to stream to ground PAL video at 25 frames per second with single frame sizes of 3Megs - thats enough to set a 24" computer screen to 1920 x 1200 resolution (complete overkill - but shows what is possible).

You can get a 3Meg machine vision camera from Imperx and any other industrial vision camera manufactuer for around Euro 500 - 800, but a thermocooled 16meg camera will cost from Euro 10K - 20K.

A framegrabber to handle single image capture at 3Megs per frame and stream video to ground at PAL or NTSC 25 or 30fps will set you back Euro 800 and upwards - that will give you a 24" digital TV screen quality video!

On the issue of MEMS type attitude and heading sensors. Like motorcycle or motorcar horsepower figures, they are given for cirucmstances seldom repeatable in real life, and they are seldom defined against background definetions that are realwolrd applicable.

So what is important in reading and understanding MEMS performance figures?

It is NOT the drift figures manufacturers give for these components ( - especialy when used in and with prebuilt autopilot modules), but whether or not those figures are pre or post Kalman filtering, and secondly, what log's are used to filter the MEMs outputs(?).

From a navigation/flight control perspective this is, in my opinion the most important consideration. However, it is also the aspect about autopilots/navigation and flight control least understood by UAV enthusiasts. Before you spend your hard earned money purchasing any autopilot, find out the following from the manufacturer:

1)what are the MEMS drift figures pre and post Kalman filter implementation?
2) how are the figures given/described?
3) if a single figure - is it pre or post Kalman Filter correction or not?
4) is there any access to the Kalman filter source code?

...... and if Kalman filtering is not implemented for MEMS output correction, then ignore completly the printed drift figures - for they will be given for conditions that you are extremely unlikley to encounter in real world conditions and will therefore not experiance.

Kalman filtering is an absolute must must must for MEMS used in UAV's - and how well it works is very much a function of the code writers' skill and experiance. To implement K/filtering properly requires a thorough understanding of the subject, lots of experiance with UAV's (as opposed to ground based robots) and last but not least in my humble opinion, an intuition and skill not many have - it is as much a science as it is a skill that is refined over time with experiance.

Kalman filtering written and implemented for a UAV weighing 200lbs, will not be suitable for a UAV weighing 20 or 30lbs - so when manufacturers say their autopilot is suitable for a UAV of around a given size/mass, it is often because the Kalman filtering used has being written up and implemented for a UAV whose flight/mass characteristics are suited for the Kalman filtering they have chosen to implement. As, said, access to the Kalman source code as implemented in any autopilot you have purchased is always a nice thing to have - though its often something manufacturers are reluctant to share. So add to the list of questions: what is the ideal UAV size for the Kalman filtering as implemented in the autopilot you wish to purchase - and if it is ideal for a 100 - 200lbs UAV, and yours is around 20 - 30lbs, i would look elsewhere for my autopilot if there was no onboard function to adjust the Kalman filtering for something more suited to your UAV mass.

Trust this helps Kfong ...... if there is anything else you would like me to comment on, please ask - only to happy to help, and if any member feels I have stated something inacurrately or incorrectly, please do feel free to question/challenge me - the last thing I want is someone to setup their pride and joy on the basis of advise or comments I have made, only to later find out my information was wrong - so please anyone, if you do not concurr with me, please do feel free to challenge me.

Note - I have no affiliation with any company whose name is mentioned in these comments. I only mention a company name because I am familiare with the equipment I refer to and have personal experiance using it in one or other UAV project.

[kfong]

Wow, such a wealth of info. Definitely taking notes, all very well researched for those using high end UAV's. I like how you covered a broad range of topics. Thanks for writing such a detailed explanation. I have a ham licence, but never got involved in that hobby, only wanted it for video transmission, so some of the rf you covered makes good sense. I also have a degree in electronics and work in that field, so I have been exposed to many of the things you have talked about. Just wanted to know I appreciate the time you took to write up all of this. My UAV stuff is just getting started, but I spend a lot of time doing the research and learning as much as I can. My first attempt will be my own Mirokopter design. Using 4 props to lift a camera up for aerial photography.

What are good antenna choices for the 2.4ghz band. That is what I have currently, the ones I see used most often by the hobbiest is a patch antenna for the ground base and for the transmission a simple dipole. The better setups use a diversity antenna system having as much as 4 patch antennas to recieve. What are your thoughts.

Kin

[Ceros]

Good - after 20years plus in the business it is nice to find a forum in which I can share that experience for the benefit of others. Need to take care I keep to a level or context that is comprehendible by all - mind you, most UAV folk have a technical grasp of computers and electronic subjects way above the average person.

The problem with 2.4GHz is that it is in principal good for nothing - other than it has being allocated by the powers to be for all to use freely and as such their is a large range of equipment to choose from that is within the budgets of most. Other than for that, 2.4GHz is a lousy frequency for UAV comm.’s - except in one respect: the higher the frequency the smaller the antenna. Other than that it’s useless.

So - what antenna. Well, by and large off the shelf 2.4GHz antenna's are a waste, and as a HAM you should have no problem at all putting together an antenna that beats hand's down anything out of a shop.

A number of points to keep in mind: horizontal Tx polarization from the air to ground works better than vertical polarization. But the problem is that in implementing h/polarization from a UAV with a single element antenna will mean trailing it at the rear of the fuselage (or sticking it out the front of the nose - in the way of any camera!) and that means your H and E lobes are not going to give you good coverage at all.

So, a single element antenna has to be facing up or down (either on top of the fuselage or underneath it) - and if you want to use a 3/4 or 5/8's then on top is usually easier than underneath - at least if the UAV is smallish. And the problem with that is, when the UAV banks into a circle facing the Rx antenna on the ground, you stand a chance of loosing the link for as long as the fuselage is between the UAV Tx antenna and the ground Rx antenna.

So back we go to placing it underneath.

However, if you have the tenacity to design a patch antenna - which can be placed underneath the fuselage - then that is a viable option. My choice would be to design a circular polarized helix - left or right hand twist makes no difference - which could be etched out of copper foil and stuck flush to the UAV underside. Applying so that it follows the fuselage curves is not ideal, but the advantages would outweigh the disadvantages.

Fortunately, for 2.4GHz the ground plane for such and antenna is placed literally a few mm behind the actual etched element - which means that you can build up the inside of the fuselage with a few mm's of lightweight ridged foam - cut and sanded to the correct profile and then resined inside the fuselage, over which you could then lay some thin copper foil or common kitchen tin foil.

This would extremely lightweight and would provide you with an omni directional Tx and Rx ability - which would be unaffected by UAV attitude or position at any given time.

On the ground side (and I have mentioned this in a few postings to date), there is very little that beats an axial helical (and for 2.4GHz there are hundreds of confirmed designs and measurements on the net).

Just one thing - make sure that you give the UAV antenna helix a right hand twist, then your ground based axial helix should be a right hand twist. If one is rh and the other is LH you will loose at least 3dB - if not more. For its size, weight and cost, there is nothing that beats an axial helical - and the Rx/Tx gain it provides for its size/weight/simplicity (not to mention its almost equal performance with both circular as well as vertical as well horizontally polarized signals - which means it doesn’t matter what angle the UAV is flying at), the only thing that beats it is a parabolic dish.

Nothing wrong with diversity - however, with COFDM set at low modulation rates of QPSK or 16QAM, diversity offers little to no real world gain or benefit. I am not against it, it has its role to play, but it needs to be understood and used correctly for it to realize real world benefits - and like claims by modems manufacturers that there modems are capable of x amount of data per second, they are performance specs given for circumstances that are seldom realizable in real world circumstances. And the same applies to most diversity antenna claims!

Diversity addresses multipath reflection - but only so long as the echo arrives at the Rx antenna within certain software programmed time period (measured in nano and micro secs). Fine for built up areas, where you want to maintain a steady link. No, I am not going to get into a software/source code level discussion on diversity programming, suffice to say that at a consumer/budget level, it seldom realizes the benefits it is in theory capable of.

It is my opinion a well thought out and setup COFDM system will out perform a Diversity in most cases (but not all). It has its role to play and as far as UAV's are concerned the conditions are seldom suited more so towards diversity than they are towards COFDM. Diversity is brilliant in GP/motorace type scenarios, where both the path and the obstructions are well defined and do not change from time to time or lap to lap - in this case, yes Diversity works very well indeed.


As a ground antenna, yes patches are good choice – so long as they are pointed in the correct general direction and they are of a type with a wide beam width they can be very good options indeed – so I would not discard a patch for ground based Tx/Rx, but I should add that there are monopoles for 2.4GHz that can provide very similar gain performance to patches and their benfit is that they are ominidrectional - so you'll never have to worry about the occassion that the UAV is going to fly out of coverage as it could quite easily with a patch antenna.

Are you familiar with the old Bow tie type patch antennas?

Indeed, I have not done this, but in theory, 3 of them placed on a UAV (one on each side of the fuselage and one underneath – all coupled and balanced through a good 3-way splitter) would be a fantastic and easily implemented UAV Tx and Rx solution. At 2.4GHz they are small, easily made, have very little coefficient of air resistance, can be attached to the fuselage via a small screw on type co-axial terminal, or indeed embed into the fuselage during construction (and again the backplane could constructed just as I described earlier), and they weigh next to nothing. They would provide between 30% - 70% more gain than a single flat helical – and would outstrip the performance of a monopole of any length and design.

[kfong]

Wow, a lot to digest. Let me stew over this for a while. Gotta do my taxes as well, so I'll be a bit tied up. Will get back to this interesting thread soon.

Kin

[zik]

Very interesting stuff Ceros, thanks. You've definitely got me thinking about some of these setups.

[solarsystems]

...PC104 and I/O isn’t really much of a subject for the UAV projects discussed
by most members on the forum...

Ceros

I'll hang with you on that one Ceros!!! My highest embedded system runs 500MHz at 800km altitude doing ~17,000mph! :eek:
But my RF skills pale in comparison to your voodoo... :cool:

Thom

[Ceros]

Horses for courses - I know nothing about solar modules.

I have worked on various satellite data link projects in my career - and one thing I have never forgotten about the cost of the "psu" solar cells used was the number of zero's after the first figure in their price - for no more than a few percent extra eficiency over consumer cell costs. Frightening!!

[Kisssys]

Well I hate to see this thread die, very interesting. After reading the last few Posts of Ceros I spent some time poking around for PC104 and modems etc. This has brought up a lot of questions that maybe everyone has that's interested in using a digital link.

There are lots of PC104 boards and it seems a couple of different versions. Knowing a good board, a suitable RF modem for both the US and elsewhere.

The receiver side also, preferable a modem that could talk to a laptop for portable use.

What language would the code be written in, and are drivers available for the modems. Free is the answer, and running a pc in Linux limits it usefullness.

I think you could get a number of guys to take on the venture if they didn't think they would get so bogged down with a very long learning curve. I've spent many hours finding out I had one bit in one register wrong. I'm in to immediate gratification now with hard work to perfect it.

Tell us more

Kisssys

[Prof100]

Ceros,

Here is my vision for system to fly my RC plane. I am simply looking for a decent system that allows a cockpit view of the aircraft for me to view from on the ground on a bright sunny day. I picture an eyepiece or a set of goggles to avoid shading a LED screen. Airplane remote control would be via the standard RC TX and Receiver.

I picture (excus the pun) a wide angle camera on the nose of the plane transmitting to a receiver on the ground that displays the video real time into an eypiece or set of viewing goggles.

I would expect minimial transmission delays so the control inputs can be matched to the plane's performance and roll, pitch, yaw and throttle needs can be provided in a timely fashion.

Can you picture a simple, hobby system that could do what I described?

Bill

[NG_Mike]

Good morning Gents!

I came upon this website looking for information and you all seem to be the ones who can help me. I am looking to see if it is feasible to find a UAV/RPV that can do the following:

Airspeed: 40 - 120 KTAS
Altitude: S/L - 5,000 feet
Perform pre-programmed maneuvers including a 90 degree, nose down, vertical dive
Fly from land to a predetermined point at sea (say 50 NM) over the horizon using GPS, Irdium, etc
Fly in an external mode or via a laptop display while over water
Maintain an on station time of 4 hours
Then return to base

It sounds ominous but I assure you I am looking for an asset that we can use to perform testing for the US Navy. There are places out there that tell me they can do it but then the cost grows to a point where I could buy a Cessna 152, load it with an awesome autopilot and fly it as needed. I am looking to prove that we can do this and keep it cheap (under 10K for A/C and nav system).

You all sound like the folks to talk to in order to get smart on the subject. I am looking forward to hearing your thoughts on the subject.

Sincerely,

Mike Lowry
Lowryme@msn.com

[air]

Mike, I dont think you've got a snowballs chance in hell of getting that done with that budget to be honest, however I think that Steve Morris (Mlblco here on rcgroups) of spyplanes.com probably has a VTOL platform that would meet most if not all of your requirements.

[treehog]

Good morning Gents!

I came upon this website looking for information and you all seem to be the ones who can help me. I am looking to see if it is feasible to find a UAV/RPV that can do the following:

Airspeed: 40 - 120 KTAS
Altitude: S/L - 5,000 feet
Perform pre-programmed maneuvers including a 90 degree, nose down, vertical dive
Fly from land to a predetermined point at sea (say 50 NM) over the horizon using GPS, Irdium, etc
Fly in an external mode or via a laptop display while over water
Maintain an on station time of 4 hours
Then return to base

It sounds ominous but I assure you I am looking for an asset that we can use to perform testing for the US Navy. There are places out there that tell me they can do it but then the cost grows to a point where I could buy a Cessna 152, load it with an awesome autopilot and fly it as needed. I am looking to prove that we can do this and keep it cheap (under 10K for A/C and nav system).

You all sound like the folks to talk to in order to get smart on the subject. I am looking forward to hearing your thoughts on the subject.

Sincerely,

Mike Lowry
Lowryme@msn.com


Elements of this are possible for the $10,000 budjet and would require you to make nearly everything from circuit boards to planes and buy a lot of second hand gear and not pay yourself or any team members or outside helpers

But even if you managed all the targets at that price the idea to try to sell to a government department is probably a non runner

The typical budget for a several day Junket for 4 star plus Generals with 5 star hotels first class travel suitable retirement slush funds such as cushy job after leaving military as advisor at $500,000 PA and other kick backs to make them choose your military solution is in the millions of dollars
Even then they might another solution or Congress might block it whatever

Elements possible using gasoline engine ~50cc or similar big stable platform similar to scale cub and could manage to fly at 5000 feet 50 miles out to sea at ~50MPH max 80MPH on pre program flight mode and return

The Vertical dive will cause a lots of problems and would require a symetrical wing such as acro bat plane which would reduce payload
A short deep dive of 1000 feet would cause less issues but a dive from 5000 feet to 100 feet or less would pose serious problems to do as dive speeds could be 200MPH in terminal velocity and impose lots of problems for the systems

It is a strange requirement to dive verticaly and steep 45 degree dives flat or spiral dives are more easy to accomplish
If the dive is nessary so as to be a kamakasi bomb device or to drop a bomb load or a devise or similar onto the deck of a ship the ability to carry a small JDAM to do that job might solve that issue and if its to drop a payload the JDAM would not be armed and could pop a suitable chute close to the deck to slow its decent

If the craft was a wheapon system then configing it to drop its wings and take a JDAM mode could solve these issues

If anywhere in the testing you suffered a failure the budjet10K would probably be exceeded

You need to be more precise as what size you require
Speeds exceeding 80MPH make thing expensive
Unless you can get a loan of the Irdium for testing the cost for Irdium or similar would be very high so would be something to do late in the program as pre programmed flights are more easy to do

staying in station for four hours makes it a total flight time of 6 hours and would require a big tank and less holding station time which could be done at 40mph
the problems are this would be a project that would on that budget only operate in good conditions of light and rain cloud etc
requiring to work in cold would require heating for engine carberator and wings to stop freezing ice or similar and costs would be orbital to be all weather

strong winds suchas head winds on return leg could drop flying speeds down
to 10mph over the sea and therefore a resesrve tank of 5 extra hours would be required if the craft was to fly in strong winds

YOU tube
http://www.youtube.com/watch?v=TPYCAGNA9Jo&feature=related

http://www.youtube.com/watch?v=tgWIdpegfuc&feature=related

http://www.youtube.com/watch?v=sLC1sCdl5Kk

http://www.youtube.com/watch?v=FO0WZc8wzYk&feature=related

show a few brazilialin modelers with small simple models such as slow stick things in good sunny conditions going several miles to sea looking at ships and coming back home and entering the high rise appartment building window for a landing
But in brazil they nuts and just do things without getting sucked into the American desire to try to sell junk ideas to to the army who don't want it from back garage operators anyway who cant supply the retirment plans :(

Ralf