A Work In Progress

a comprehensive history of one man's dream

The DARTS Project is a quest that I have undertaken (along with many others) to develop rocket flight performance measurement equipment . Here is a brief documentation of the trials, failures, and successes we have had in this quest.

Remember that the pictures are clickable to bring up larger, bandwidth-eating versions.

To view the AVI clips on this page, you'll need to install one of the Intel CODECs below. Sorry they're not Mac compatible.

Original Intel CODEC (OK for Windows 95, but not WinNT)
New Intel CODEC (Works under NT for me)

Generation II: FM/FSK (1993 - 1996)

I wrote a short paper on the problems with the Doppler radar, and proposed pulse-time-of-arrival ranging. Associated Aerospace of Bethany, OK was very receptive to funding my new ideas, since the hardware was flexible enough to support the new mode with few changes. The basic parameters of the proposed system were:

220 MHz NBFM uplink: 2 Watts + 5-element YAGI on ground
1298 MHz NBFM downlink: 50 mW + (22-turn helical + 0.8 dB NF preamp) on ground
Transponder: 220 MHz RECEIVER-> 144 MHz EXCITER & 1154 MHz PLL SYNTH-> MIXER & AMP-> 1298 MHz DOWNLINK
Range measurement by pulse-time-arrival measurement, using 10 MHz-clocked counter (precision: +/- 15 meters)

A DSP (of course) was used to process the time-of-arrival data from a high-speed counter. The thought was that a VHF uplink was easy to implement, and the 1296 MHz stuff was mostly left over from the Doppler fiasco. I did build a nice Hamtronics 220 MHz transmitter kit to use with this unit. Below is a picture of the transmitter:

In April of 1994, I changed from a pattern-correlating range measurement system to a pulse-time-of-arrival type system, where a single pulse is sent and the ground receiver waits until it is detected to send another one. This increased the ranging speed of DARTS and reduced the complexity of the system.

I found that a range-code-correlation system (also used in the Apollo spacecraft) is useful only if the entire uplink and downlink path can be filled with the code. This requires long distances, a large bandwidth, or both. Even in the case of the Apollo system, the range code was only used during acquisition; a single-pulse system was used during tracking.

I also discovered that the time/range difference between using 300E6 m/s as the velocity of light and using the vacuum value of 299,792,458 m/s is less than 0.07%, well below the DARTS systems' smallest range resolution even when the error is largest.

But--the long rise and fall times of the VHF radios, combined with range/velocity crosstalk from using FSK instead of ASK, doomed this scheme to failure.

Here's the only known picture of the earliest FM/FSK DARTS system, from about May 1995. My son Daniel (2 years old here) and I are testing the panel lights of the DARTS system, in preparation for a launch test that summer.

"What's this one do, Dad?"

At the Sooner-Boomer launch that summer, I flew some DARTS hardware on board Terry Birchett's RC airplane. Terry and the other two Birchett brothers, Sam (pictured below with the ever-popular "DC-3 from Hell") and Dale, were early sponsors of DARTS.

During this time, I also experimented with conical-scan systems.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 wish I had some pictures of that beast...

Even back then, I knew that at 1296 MHz, antennas were large. So around December 1995, I built a 3.5 GHz transmitter beacon, hoping to eventually migrate to that band, using surplus TVRO equipment. Below is a picture of a dish I put together from TVRO stuff and had painted:

One of the pivotal papers which was inspiring me at this time was "Gemini Rendezvous Radar" by William W. Quigley at Westinghouse Electric's Air Arm Division. (AIAA Guidance and Control Conference August 12-14, 1963, MIT, Cambridge, Mass. Article No. 63-350) I was really impressed by how much this author rejected the assumptions of the radar people of his day. Also, many of the premises of this radar still apply to DARTS. Someday, I'll ask for permission to scan this fine document and present it here.

Also, I got tired of nothing being available in the way of single-chip FM receivers that covered the 220 MHz ham band and switched to 144 MHz. Here's the notes from my logs:

March 13, 1996

The 220 MHz / 3.5 GHz version of the transponder is scheduled to undergo tests onboard a sailplane the weekend of March 31st.

For reasons that have escaped me now (maybe hoping for partial Doppler cancellation?), I changed the downlink frequency to 440 MHz. This also allowed me to use commercial radios for test transponders, a tactic I would later regret. Crashing radios is expensive...

April 2, 1996

I'm currently integrating a new 144 MHz uplink transmitter with the DARTS ground unit. The 220 MHz transmitter I have been using isn't compatible (sigh) with the new transponder's 2-meter receiver.

A drawback of that decision is that the 2m antennas were physically large, and since I used two seperate antennas (one for the 2m uplink, and one for the 440 MHz downlink), that made for lots of space to operate the thing.

In April and May, I conducted some experiments using our local amateur repeater and with mockup DARTS transponders in cars and vans, driving around town. I was able to measure distance, but had some strange readings occasionally. I would later learn that this was a result of Doppler and noise crosstalk.

Also, I repackaged the DARTS interrogator into a smaller package:

Also I added transmit amplification on the ground (later deemed unnecessary) to compensate for the unknown attitude of the transponder antenna.

Here's a look at the DARTS FM/FSK system undergoing range testing in Argonia, KS in August of the same year.

Here's the system all spread out so we can see the individual components, which are labelled.

Below is a look at the antennas I used.

At Sooner-Boomer 13 in Medford, Oklahoma, the first flight test of DARTS hardware took place...

July 5, 1996

The first flight test of DARTS happened at "Sooner Boomer 13", sponsored by rocket outfit Tripli Oklahoma on June 28-29, 1996. The 3.5 GHz downlink transmitter was still "on the bench", so I substituted a 440 MHz transmitter I had on had. Tom Skahan of Bella Vista, AR, provided the DARTS transponder a ride to 3500 ft (approx) on a J-powered 5-ft tall 4-in diameter rocket. Below is (I think) a picture of that liftoff.

I had good signal contact all the way up and down, including coverage when the rocket was on the ground, before it was recovered by CAP (Civil Air Patrol) volunteers. The only problem was: the range counter refused to measure distance ! After some analysis, I determined that reversed connections between the 2m uplink receiver and the 70-cm downlink transmitter caused the ranging waveform to be inverted, causing the range counter to overflow and read zero. Sigh.

I had hopes to participate in the JP Aerospace Project Spaceflight launch in July, 1996. However, they could not ever get their rockoon project off the ground, and still have not (as of mid-1998).

JP was provided a mock-up of the DARTS transponder, to integrate into their high-altitude vehicle. I have a picture of that unit somewhere...

Realize that none of this hardware had any direction-finding or angle-of-arrival capability at all, and required the operator to manually track the rocket. That is really tough, and luckily the antenna beamwidths were wide enough that just pointing in the general direction was good enough.

I spent a lot of time on the FM/FSK system, because I was convinced it would work. It didn't. The short story is: too much timing jitter, too many multipath reflections, and range / velocity crosstalk. What really conviced me was when I crashed $250 worth of radio hardware at supersonic velocity into the Medford soil...

August 29, 1996
The 2nd DARTS test flight at Medford on Aug 24 provided valuable data. This data helped me make two very important design decisions:

FSK introduces crosstalk between range and velocity (see the [FAQ]); I'm going to pulse modulation.

The VHF uplink must be replaced with a narrow-beam microwave link, due to spurious reflections from nearby metal objects, causing range errors.

That aside, I can report a small triumph (hey, we take what we can get): DARTS measured the rocket's peak altitude at 0.9 km. This jibes with the estimate of fellow Tripoli OK 'rocket scientists' Warren Ballard and Dick Embry of about 3500 ft.
There was an altimeter on board, but it and its precious data were destroyed when the rocket's chute didn't open (see what I told you?). My transponder also took a beating, but the altimeter wasn't secured as well, and the metal motor impaled it.
This second test flight provided two valuable insights:

The VHF uplink 'jitters' in phase due to spurious reflections from nearby metal objects. For example, there was a clean ranging signal being echoed when the transponder was on the ground, but when the rocket got up to about 100 ft, the echoes from a nearby metal building turned the clean square wave into a jittering, noisy mess. The jittering causes major error in range measurement.

FSK introduces crosstalk between range and velocity. This means that small changes in signal frequency due to Doppler shift are perceived by the interrogator as a range difference. It's small but still measurable at VHF frequencies, and downright disabling at microwave frequencies.

The decision to replace the VHF uplink with a narrow-beam pulsed microwave link was dictated by the above data, but it was a hard decision to live with. Many months of preparation were ahead to bring the all-microwave DARTS radar to fruition. I had to draw on all aspects of my electronics engineering knowledge to make it happen: digital, analog, RF, and microwave circuitry, and software engineering besides. I guess that's one of the things I like best about this project, its all-inclusiveness. Never a dull moment.

Generation III: Pulse Microwave (1996 - )
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This document copyright Steve Bragg, KA9MVA. Updated: 08/6/98