DARTS
Frequently Asked Questions (FAQ)

Updated:  April 18, 1999

You wet-behind-the-ears whippersnapper! I was designing radars while you were still sucking your thumb! You couldn't possibly do anything like this because <presumptive_babble> ! <sour_grapes_and_vitriol>!!!

To all sour-grapes-spewing know-it-alls: Leave this site immediately! It will do nothing but offend, make you mad, etc, etc. If you don't like "unqualified" people doing the things you do for a living, you can leave my site and never return again. People like you ALWAYS 'humbug' anyone doing anything new and different. Mr. Roosevelt said it best:
"It is not the critic who counts; not the man who points out where the strong man stumbled or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred with dust and sweat and blood. At best, he knows the triumph of high achievement; if he fails, at least he fails while daring greatly, so that his place shall never be with those cold and timid souls who knew neither victory nor defeat."

Theodore Roosevelt

I like this one, too:
"The greater the difficulty, the more glory in surmounting it. Skillful pilots [captains] gain their reputation from storms and tempests."
Epicurus, Greek philosopher
For the rest of you, I'm sorry I had to add this one. It seems a few curmudgeons were spoiling the enthusiasm of some.

[Top]

"What taking you so #%^&* long to get this system working?"

Watch your language, buster! Anyway, I work for a living, so I can't be playing with cool radar stuff ALL the time.
Besides--DARTS is a hobby project, remember?

[Top]

"When will DARTS be commercially available?

Most likely never.   I am not developing DARTS for commercial purposes.  I plan to use it at regional rocket launches, but as it is experimental, it will be changing all the time.  Nobody wants a system that's changing all the time--look at all the backlash of "software version" in the PC world.

Once I get a stable system I might revisit this question.

"If DARTS is not a commercial product that will generate profit, why develop it?"

A very good question.  It think the answer boils down to two points:

   1.Contributions to the experimenter community:  DARTS expands the bounds of what amateur radio experimenters can do, with frequencies and design  concepts that are at the very edge of the experimental envelope.  Also, such a capability does not exist within the mainstream of amateur (sport) rocketry, so DARTS represents the leading edge in experimental technologies in that field, too.

   2.Technical Challenge / Notoriety:  Alright, I admit to being proud of what I and my team members have been able to do here, our successes and what we've learned from our failures.  I originally took up this project back in 1989 because it represented a high technical challenge to me then.  Even after I have learned so much and come so far, it still contains significant technical challenge and opportunity for creative expression.

[Top]

"Great! Just what I need! Where can I get the schematics, PCB layouts, etc.?  How about a kit?"

No, none of these things are available. Nor will they likely ever be.

I have absolutely no interest in creating a complete technical documentation package for the base station.  Such an undertaking would consume ALL my time, with nothing to show for it.  Besides, some of the parts are custom.  For instance, the base station antenna is hand-built.  It will be a long time (if ever) before any other units than the development vehicle are built.

I had some interest there for awhile in making kits of the transponder available, but after being exploited by several in the amateur rocket
community, I have no interest in that.  In fact, it would be fair to say that my bad experiences with the amateur rocket community as a whole is the reason my interest taking this project to the commercial level has waned.  There are so many people out there who just want to "strap on" thge technology and don't want to help pay for its development.  They think every electronic thing should just work and be dirt cheap, like cell phones...

[Top]
 

"I've got an altitude attempt coming up.  Can I get a DARTS unit on loan?"

Not likely.  At any given moment, the DARTS unit is probably apart, under development, or profoundly broken from the last experiment I did.   I have to admit I'm more interested in playing with the technology than actually tracking people's rockets.

I sympathize with your plight of having absolutely no verification of vehicle performance, except for what little data an accelerometer or
barometer provides.  That's why I started DARTS in the first place

And,  one would think by now that there would be a system on the market that does this.  Where are they?  I suspect people have tried, and like me, discovered that it costs too much to develop with darned little financial return.

[Top]
 

"If you're not offering units for sale, why do you have a Web page?" or, "It's  vaporware!  Why are you 'advertising' it on the Internet?"

Good question.  Maybe if I shut down my website, I'd have fewer people threatening me, calling me names, etc.

In the beginning, the World Wide Web was a place where people could share information on interesting technical projects they were doing.  Now, people see it as just a gigantic shopping mall.  It's not my intent to "advertise" anything, but many in the consumer-oriented rocketry crowd see it that way.

Times change, I guess.  Maybe I should consider taking the site down.

[Top]
 
 

"Why don't you use a recording altimeter / accelerometer? They're so cheap, etc."

It's mostly the amateur rocket people who ask this one, mainly because digital recording altimeters are the state-of-the-art. Let me count the problems:

1. You crash the rocket, you lose your data. It's that simple.

2. No velocity or acceleration data.

3. Only vertical coordinates, so you have no idea what the altitude "could have been" if the rocket hadn't tilted at launch.

Accelerometers are even more problematic, because integration of noisy acceleration data gives erroneous velocity estimates, and integrating those gives a very bad estimate of position. And what's more, you only get these (erroneous) data back IF you recover the rocket!

Why don't _you_ use an accelerometer?  DARTS isn't commercially available, anyway.

[Top]

"But is it legal ?"

I've had a LOT of people ask me this, mostly non-amateur-radio-operators. I suppose it strikes me as strange because I thought amateurs radio operators were always supposed to be on the leading edge. By definition, to make a law against something, you've got to be able to define it. And if something is new... Oh, well. We'll let the lawyers figure it all out...

The DARTS system is a combination of things:

Digi-peater: It echoes the 'backbone' bitstream so the range counter can measure range.

Telecommand system: Coded on the uplink 'backbone' are commands to ignite motors, blow 'chutes, etc.

Telemetry system: Coded on the downlink 'backbone' can be 'chute status, motor temperature, airspeed, etc.

All these things are legal by the Commission's current Part 97 rules. Anyone with a Technician or higher license class can operate DARTS.

The only issue at present is space operation (higher than 50 km = 165,000 ft = 31 miles).

[Top]
 
 

"Does the flyer have to have an amateur radio license, and any other for that matter?"

Let me be totally clear about this: only the BASE STATION OPERATOR has to be licensed.  The transponder is TOTALLY under the control of the base station, so it qualifies as an "automatic/remote control transmitter" under FCC Part 97 rules.  I doesn't matter who OWNS the transponder--so long as when it's brought to the flying field, its code is entered into the base station and is therefore controlled by a licensed base station operator. By default, the transponders only receive -- they don't transmit until specifically commanded to do so.

[Top]
 
 

"Why don't you use GPS? It's so so simple, easy, great, etc."

Civilian GPS units are intentionally crippled for high-altitude, high-speed operation--they simply stop working.  See the GPS info on APRS.NET.
I had to re-think the answer to this question since the little "GPS chips" have come out.  So here is my answer:  Go ahead.  Use a GPS receiver and a telemetry transmitter in your rocket.  DARTS isn't commercially available, anyway.

[Top]

"Why are you wasting your time on flight tracking when all people want is a ground recovery system?"

Well, I'm convinced that sport rocketeers, by and large, simply don't know what the technology is capable of, and think that ground recovery is all they can hope for.  Others think flight tracking equipment would be so expensive that they don't dare ask for it, setting instead for ground recovery only.

A point that needs to be made is this:  DARTS is a ground recovery system.  If it simply keeps tracking the rocket until touchdown, that is, until the time deriviative of elevation angle is near zero for several seconds, the last position measurement yields the exact bearing and range to the rocket on the ground. The uplink/command capability of the transponder could even be used to turn on a sonic beacon.

However, there are practical difficulties with this.  Rockets sometimes get into weeds, no-tresspass zones, and other nasties.  It would be nice to know the exact bearing and distance to the rocket with respect to a close-by point so that, for example, you could approach the landowner and tell him/her exactly where the rocket is.  For this case, you need a miniature interrogator ("hand scanner").  Go ahead, build one.
 

"Why are you using such a high frequency ? 'Nobody' uses those frequencies. Why not just use VHF, etc."

The microwave frequencies was chosen for two reasons, which are inter-related. The higher the freq, for a given size antenna, the higher the gain and *the tighter the beam. A tighter beam (within reason) means more gain in a small ground antenna to pull in the transponder's weak signal.

Higher gain gives you a better SNR on the downlink, which is critical because it's desirable to keep the transponder package as small, light, and cheap as possible. The more receive antenna gain you have on the ground, the less power you have to output in the rocket, up to a point.

Many of my early DARTS experiments used 220 and 440 MHz, because I had the equipment on hand and it was cheap and easy to do tests with. See the next question...

[Top]

"But you're just a Tech-class ham, with 'limited' privileges... how can you possibly do all this?"

Our 'limited' priviledges encompass more than 99.99% of all amateur radio bandwidth. There are many, many of us who feel that the higher frequencies (especially above 1 GHz) represent the future of ham radio. I'm a Tech Plus operator, and occasionally use HF. But it's worthless for DARTS.

[Top]
 

"What does that Latin phrase on your logo mean?"

"Ad astra per aspera", loosely translated, means "to the stars the hard way." And, boy does it seem to be the hard way sometimes ! By the way, this is also the state logo of Kansas. Which has no real bearing on why I chose the phrase. Oh well.

[Top]

"What is the measured or expected range of the transponder? The system?"

Really, these questions are interrelated, since the transponder range depends so heavily on the ground station receiver noise figure and uplink power.  However, in my initial implementation, I am designing the system to have adequate SNR at the range counter's maximum extent.

See the Interrogator and Transponder specifications for more.

[Top]

"What is the resolution cell dimensions you anticipate or have achieved?"

I'm using a 30 MHz range counter clock, which works out to a range cell of +/-5m.  My angular resolution on the pedestal is 0.01 degree in elevation and 0.04 degree in azimuth, but I doubt the mechanics can hold an equivalent accuracy.  Further, the non-coherent nature of my angle-tracking circuitry will probably limit overall angular accuracy to within about +/- 0.5 degree.   That's much better than the accuracy of any optical tracking means now emplyed in amateur rocketry.

This is a good place to remind you about the Interrogator and Transponder specifications...

[Top]

"Is a three station doppler tracking system is more expensive than a single monopulse base station?"

Most definitely.  I've been down that road, taking my cues from Bjork's work for in the NASA Trailblazer project in the early sixties (see my History page).  First, you must have THREE nearly-duplicate stations which still have to keep their antennas pointed to some degree of accuracy, if any reasonable antenna gain is to be realized.  Further, the intercommunication and station coordination issues are very difficult for a three-station real-time trilateration setup.  And, Bjork's work showed that you really need FOUR stations to subtract out the apparent doppler shift due to error in transmitter frequency.

I won't rule out multilateration completely; for extremely high-altitude flights, a multi-station chain is going to be very important.  But most of my customers are staying under 30,000 ft, so I'll leave station coordination for a later excercise. If you have need for it, please contact me by E-mail.

[Top]

"Must the interog antenna be pointed slightly above the rocket at take off to minimize ground return?"

Ground return is the signal that bounces from the ground; it's causd by sidelobes of the antenna which radate energy in undesirable directions, like toward the ground.

Ground return isn't a problem for DARTS because the transponder return is shifted both in time and freqeuency.  This is very different than the situation in a conventional radar (i.e., one not using a transponder).  The transponder downlink is lower in frequency than the uplink.  In addition, the transponder implements a delay from the time it receives the uplink pulse.  The delay is about twice as long as the uplink pulse is wide. This gives the ground receiver time to come out of "blocking" (overload due to the proximity of the transmitter), and switch to the "sum" lobe before the return signal arrives.  In addition, the delay allows tracking the rocket into essentially zero range.

There is a reason to "lead" the rocket slightly at takeoff, though, which isn't related to ground return. The highest elevation angular rates that the antenna sees are the ones at takeoff. Pointing the antenna slightly above that, and acquiring the rocket sometime after liftoff, lowers the angular rates and allows the system to work with slower motors, bringing overall system cost down.

The only real problem with this scheme is the possibility of acquiring on an antenna "sidelobe", which would bring error into every measurement DARTS makes. See the next question...

[Top]

"It has always been difficult to acquire a high angular acceleration target. How does DARTS do it?"

Indeed, it has been the subject of much interest and it is very difficult to do.  There is a region of high angular acceleration from launch until a few seconds into the flight.  In radar literature, this is called the "pass-course" problem, but in their application is usually associated with the horizontal movement of an aircraft past the radar rather than the vertical.

However, for DARTS, we already have significant information about the flight dynamics the target: rockets mostly go straight up after launch. We can use this information in the tracking algorithms.

One scheme for doing this is to "lead" the rocket.  At launch, the antenna beams are pointed well up into the sky, and ranging/command/telemetry are performed using the parasitic sidelobes of the ground antenna, since the transponder is close enough for these signals to be strong.  At launch , the pedestal is commanded to accelerate vertically.  At some point (hopefully far above the region of rapid angular acceleration), acqusition will occur.  I realize this means that for the first few seconds of flight, 3-D flight parameters will not be available. However, they can be extrapolated from the range data and the assumption of a straight-line flight from launch to acquisition.

[Top]

"It's a bit primitive, but have you considered a conical-scan tracking scheme?"

You've hit a sensitive nerve :)  I toyed with the idea of using some sort of conical-scan back in 1993, and I am kind of ashamed of those experiments.  Would you believe a car-window motor (sans gears) spinning a subreflector with a Casegrain 1296-MHz backfire-helical feed?  The whole structure vibrated, and the idea of that 8" metal subreflector becoming an 1800 RPM Frisbee was a little scary.  I'm just not mechanically adept enough to dynamically balance something like that.  I figured I'd better do scanning electronically.

I did manage to buy a surplus X-band conical-scan passive tracker from an older  Navy anti-radiation missile, just to study it.  It's an extremely well-balanced, precision piece of equipment.    After spinning the rotor up to 1200 RPM with a Dremel tool (I don't have a source for the 400 Hz motor), the thing continues to spin for more than three minutes!  That's precision, and I don't think something like that can be produced cheaply.

[Top]

How will the rocket's airframe affect the transponder signals?

Some people who ask this question have had (sad) experience with using 5.7 GHz wireless LANs (local-area computer networks) indoors. One reason they don't propogate well in indoor settings is all the metal and water-containing materials in a typical building!

5 GHz signals do pass well through thin fiberglass, paper, and phenolic.  If a flyer has a metallic airframe, I can build an antenna on a flexible circuit board that will "wrap around' the outside of the rocket.  I already have one high-altitude flyer who has asked for one of these, and I plan to have a version of the transponder that has no on-board antennas, just for the high-altitude folks.

Loss depends not only on the properties of the materials, but also on the thickness.  A thin material is desirable for airframes, and also desirable from a transmission standpoint. I don't have actual numbers for the amount of loss, but I have put thin phenolic and paper (cardboard) in the microwave oven (2 GHz), and neither absorbs enough energy to get hot.  I don't have any 5.7 GHz data on fiberglass, but the radar guys tell me that you can receive 3.5 GHz signals through a fiberglass radome.

<update>

From system testing, I can find no significant attenuation from either plastic nose cones or fiberglass body tubes.

[Top]
 

I wanna build my own tracking radar.  How do I do it?

Where to begin?  I asked the same question nine years ago when I started working on my DARTS radar.  And, there are no real answers to give.  As far as I can tell, there is no one else in the free world working on an amateur tracking radar.  This is mainly, I believe, because it is such a hard thing to do, and encompasses so many disciplines.  It sounds so conceptually _easy_, but believe me that is a self-deception of sinister proportions.

You _could_ build a 10 GHz "police" Doppler radar kit (Ramsey Electronics, www.ramsey.com I think), but I take it that isn't the direction you're going. You _could_ buy a surplus military radar unit, but in many ways that would be more confusing than developing your own, since much of that technology is so dated.  Also, they are _huge_, heavy, and mostly use 3-phase 440 Hz power (where do I get that?).

There are _no_ kits for real tracking radars anywhere in the world, as far as I can tell.  Heck, there weren't even decent microwave semiconductor components commonly available until recently.

(So, WHERE ARE ALL THE DARTS-A-LIKES, anyway?  Answer:  there aren't any, because it costs too much to develop with darned little return.)

My advice, such that it is, is to read _everything_ you can get your hands on, and to experiment. Read in all electronics disciplines, but especially in radar, antennas, signal processing, analog, digital, and RF electronics.  (Check out my Radar Resources page for some ideas) Read up on control systems, and mechanical engineering.

Learn from the people who've done similar things.  Read all the stuff on my web site.  Read N1BWT's web site, ESPECIALLY the Online Antenna Handbook.  Read _ALL_ the Microwave Updates from the ARRL, cover-to-cover.

Learn _everything_ you can about radio electronics. Get an amateur radio license, if you don't have one.  Start experimenting with microwave radio. Join North Texas Microwave Society and one of the ham VHF Societies.  Put together some microwave kits from Downeast Microwave (www.downeastmicrowave.com).  Read and understand everything in the HP RF and Mini Circuits data books.

And did I mention antennas?  Get some antenna software (like MiniNEC from www.arrl.org) and start experimenting with antennas.  You _must_ get a copy of Savatini's "Microstrip Antennas for Wireless Applications", because antenna design software comes with the book (only $90!)

Oh, yeah, DSP.  Buy a DSP demo board from Texas Instruments or ADI and write some signal processing programs.  Learn all the DSP algorithms you can, from control systems, to filters, to radio receivers in software.  You'll need 'em. Learn C if you don't know it.

Also, learn metal-working skills.  Learn to operate a drill press, a gas or electric welder and a Dremel tool.  Learn to measure twice, drill straight and cut true.  Heaven knows I've had to learn those lessons more than once.

My quest for tracking radar, nine years long (and getting darn close!), has brought me untold advances in my career, and much personal satisfaction.  Also, it has brought me sweat, tears, and handbuilt electronics driven into the ground by runaway rockets! I couldn't even _begin_ to count the money and time I have poured into my search for the tracking radar grail.

And, would I do it again?  Probably, but not for fame or fortune.  The education it's given me has been worth it.
 

Not satisfied? Who is. Ask your own question. Send to: ka9ma@qsl.net.

[Top]

[DARTS Main Page] [FAQ] [NEWS][Interrogator][Transponder]
Send comments and suggestions to: ka9ma@qsl.net.
URL: faq.htm
This document copyright Steve Bragg, KA9MVA.