The KA7OEI GPS page


Figure 1:
Click here for another page showing more-detailed GPS receiver statistics.
Left:  A plot showing the paths of the recently-tracked and currently-tracked satellites.  The receiver's location is the "bull's eye" in the center while the top of the plot represents the North pole.
Right:  Graphs showing receiver statistics.
The red line shows the "EFC" value - the setting of the electronic tuning adjustment which is an indication of how far the Z3801's internal oscillator departs from GPS time and the tuning required to correct it.
The green line - "Predicted Uncertainty " or "PU" shows the expected accuracy of the receiver over the next 24 hours should the receiver lose its GPS lock.
The blue line ("T1") shows the short-term timing error between the GPS signals and the receiver's internal clock. 
The pink line is a "smoothed" version the T1 value.
Finally, the brown-ish line across the bottom shows the number of GPS satellites that have been tracked.
The above graphs were created by the GPSCon program and are updated about once per hour.
Look at the bottom of this page for recent news/info on the receiver's operation.
 Map showing recently-tracked GPS satellites Graph showing some of the timing/satellite stats from the receiver.

Note:

This page is intended to document only how I put my system together and a few other miscellaneous items.  It should go without saying that there are probably better ways to do these things, that your mileage may vary, blah blah blah...

Back in 2003 I bought a used HP Z3801A GPS receiver (often called a "Z-Box") with the intent of using it as a time/frequency reference.  As with most projects, this one was ongoing, with incremental progress dependent on available time and current whim.  Nevertheless, it didn't take too long to get it up and running, following the "make it work, then make it look good..." mantra.


Use as a frequency reference: 

Why would one need a frequency reference?

A known-good frequency reference is handy for making sure that everything else is on frequency.  At this location (Utah) reception of WWV on 10 MHz (the most convenient frequency for calibrating against) is very sporadic and it is often difficult to get a good zero beat owing to the constantly changing path and the usual presence of WWVH.

Over the years the Z3801 has been invaluable for use as a frequency reference.  With it, I have been able to assess the stability of OCXOs (Oven-Control Crystal Oscillators) and Rubidium references for use with microwave (10 and 24 GHz transverters) as well as use its 10 MHz output as an external reference for a frequency synthesizer to generate reference markers for use in the ARRL Frequency Measurement Tests (FMT's) - among many other things - and with accuracy that is greater than that which might be obtained by almost any other means.


Uses as a time reference:

The Z3801 is now monitored by an Aopen M-945D computer - a small, form-factor desktop that draws only a few 10's of watts with a clock speed of about 1.7 GHz..  Having a computer with a modest power consumption allows it to be left on continuously without impacting the electric bill much or too-quickly killing the UPS - and it has enough horsepower to do about anything that would be required!

Via a USB-to-serial adapter, this computers interrogates the Z3801 and provides logging of the unit using the GPSCon program noted in Figure 1.  It also provides a GPS-referenced SNTP Time server which is used by the local Ham Shack computer which is helpful when running programs that require accurate timing such as those in the WSJT suite.  The version of GPSCon that I use does not use the 1pps signal from the receiver, but rather by determining the time by querying the serial port.  By this method the timing is accurate only to within a few 10's of milliseconds - but that is good enough for my purposes.  Having a "local" SNTP time server is nice as it means that I can query it several times an hour:  Most publicly-available internet servers do not allow more than a few queries a day a given IP address!

Another use of this computer is to decode and process weather satellite pictures:  Those seen on my Weather Satellite page are the result of this work and having a continually-updated clock helps keep the maps accurate, too!

Important note concerning the use of serial ports under Windows:

"What does this 'Signal Strength' (SS) reading really mean?"

On the status screen of both the HP SatStat program as well as GPSCon (see below) a signal strength indication is given.  All the Z3801 manual really says is that this value should be above 20-25 for reliable tracking, and it should be higher than this for best accuracy. 

In digging around the Motorola Oncore GPS documentation (the receiver used in the Z3801) I determined that this is an indication of "Signal to Noise Density ratio" (S/No) and is shown in db. 

What does this mean, then?  This explanation requires a bit of communications theory - so I'll keep the explanation as simple as practical: 

Take a relatively weak CW carrier as an example.  If your receiver has a bandpass filter 1 kHz wide, let's say that the strength of the that CW signal is the same as total energy in the noise everywhere else in that 1 kHz bandpass.  In this case, one has a 0 db Signal to Noise ratio, for the noise and signal are of the same total strength. 

It would make sense, then, if you were to cut your bandpass filter down to 100 Hz bandwidth, you'd still be intercepting all of your original CW signal, but you'd be intercepting only 1/10th as much noise.  In this case, the CW signal is now 10 db stronger than the noise. 

If you take it the other way, let's say that you widen the bandwidth to 10000 Hz (10 kHz) - the signal/noise ratio is now -10 db. 

The GPS signal consists of several different signals, all modulated atop the same carrier.  If all you want to do is recover the GPS receiver's carrier (but no information) you could (theoretically) use an extremely narrow bandwidth to detect the carrier.  For receiving the data, you need wider bandwidth - a bandwidth proportional to the data rate. 

So, to keep lock on the GPS signal, a C/No ratio of 20-25 db  is the minimum required.  This represents a pretty "ratty" signal, however, and the amount of noise present makes it difficult to recover the data stream and timing with great accuracy.  As you might expect, increasing the the C/No will reduce the amount of noise - and improve the accuracy of the recovered timing. 

How much signal is "enough?"  As it turns out, by the time one gets up to a C/No of 45-50 db, enough of the uncertainty (noise) is gone and relatively little improvement may be had (comparatively speaking) by a further increase - at least for the "typical" L1-only (non-differential) GPS receiver. 

As it turns out, most receivers don't go through a lot of pains to obtain really good C/No values, so it is more reasonable to take a "sliding average" to get a picture of how well a particular satellite is being received. 

(I'm a bit skeptical about the receiver's reporting, though:  I have  difficulty believing that this parameter is, in fact,  accurate when it reports a C/No of >200 db...)

An interesting article that appeared in GPSWorld that describes C/No as well as effects of interference on GPS systems may be found here

The Z3801 receiver:

The receiver itself needs several things to operate:

Taking these things one-at-a-time:

The "PBJ" (Peanut Butter Jar) antenna:

Generally, an amplified (powered) antenna is employed with a GPS receiver such as this with the receiver (usually) located at some convenient distance - say, in the ham shack.  Owing to the very high frequency of the L1 GPS frequency (centered on 1575.42 MHz) typical coaxial cabling has high losses.  Because these losses directly contribute to receiver noise figure (and thus, overall sensitivity) it doesn't take very much coax loss before the GPS signal from an unamplified antenna disappears into the thermal noise.  Having an antenna with a built-in preamplifier allows the receive system to tolerate significant cable losses without degradation of the actual signal - a factor that also translates to being able to use inexpensive, smaller-diameter coaxial cable such as type RG-6 which is often used for TV and satellite reception. 

While a wide variety of amplified antennas are commercially available, I decided to "roll my own" GPS antenna starting from the article "An Inexpensive External GPS Antenna" by Mark Kesauer, N7KKQ (QST, October 2002, Pp. 36-39) - ARRL Members only link. 

Note:


I constructed the turnstile antenna more-or-less described in the article, departing from the article on several points:

Figure 2 shows how the two turnstile conductors (comprising 4 elements, actually...) are mounted atop the short length of UT-141 coax.  While not obvious from Figure 2, the bottom end of the UT-141 coax goes through the circuit-board disk with its shield soldered on both sides of the board.

Figure 2:
View of the top side of the "PBJ" antenna.
Click on the image for a larger version.

On the bottom side of the board is a simple preamplifier (see Figure 3) constructed using the inexpensive Mini-Circuits MAR-6 MMIC (see the schematic, Figure 5, below.)  This amplifier has a moderate noise figure (3-4 db) and reasonable gain (13-15 db or so) at this frequency - enough to overcome reasonable coax losses.  This amplifier is powered from the 5 volts superimposed on the coaxial cable:  The DC is first decoupled with a coil with the RF being blocked by a chip capacitor, then the DC is bypassed with an electrolytic capacitor.  Finally, DC is re-injected into the circuit via another choke.  Note that there is no blocking capacitor in the input of the MMIC, so a small amount of DC (about 1.5 volts) is actually present on the antenna - but this isn't any sort of problem.

The output of the amplifier is fed to another length of UT-141 coax which is soldered directly to an "N" connector (refer again to Figure 2.)  This piece of coax is used to support the antenna and its circuit board, and the entire assembly is "captured" by the glass jar.  If preferred, one could use a BNC or even an "F" type connector (if one used RG-6 coax, for example) but in any case, one must assure that the connector is adequately waterproofed upon installation.

Figure 4 shows the PBJ antenna installed on the roof.  In this installation, the coax (about 60' - or approximately 20 meters of RG-11) is fed up through the center of the pipe to which another antenna is mounted.  The antenna is secured partly by the weight of the coax pulling down, and partly by the black nylon wire ties which tie to a mast clamp below the GPS antenna.  It is worth mentioning that while the antenna mast to the right of the PBJ antenna (due south) somewhat blocks the view to the south, it doesn't appear to have any significant effect on the received signals.

Figure 3:
A close-up view of the bottom of the "PBJ" GPS antenna.  (The antenna was turned upside-down for this picture.)
Click on the image for a larger view.
As may be seen from Figure 2, some holes were drilled in the "lid" (on the bottom) to allow ventilation.  After the picture was taken, small pieces of nylon window screening were glued into place with RTV (to help keep out insects) and all of the pieces were sprayed with clear lacquer to protect against the possible effects of moisture and condensation.

In this case, ventilation is important:  When placing a piece of equipment outside, one must either hermetically seal it (usually not practical) or one must provide ventilation to avoid accumulation of moisture due to repeated condensation.

A layer of oil or grease was smeared over the metal lid's threads to reduce the accumulation of rust and its "seizing" the lid onto the jar making it difficult or impossible to remove.  Finally, electrical tape was applied at the joint between the lid and the jar itself to minimize accumulation of moisture in the lid's threads.

How well does it all work?  Surprisingly well.  The impedance mismatch of the 75-ohm RG-11 and the 50 ohm GPS input is, in this application, inconsequential.  Even though the loss of the coax is around 6dB for this run, the signal strength peaks at about 180-190 for satellites that are between 60 and 90 degrees elevation and at 50-60 for those satellites that have gained enough elevation to clear ground effects and clutter.  In retrospect I should have used ordinary RG-6 TV-type coaxial cable - with the proper adapters, of course!

Figure 4:
The "PBJ" antenna as installed.
Click on the image for a larger view.

Some eyebrows may be raised concerning the use of the MAR-6 as the amplifier.  True, there are more modern, lower-noise MMICs around - and if you have one, by all means, use it!  The MAR-6 (also known as the MSA-0685) is relatively noisy at this frequency - a noise figure of 3-4 db or so - but because the current generation of GPS satellites have such strong signals, a full-sized antenna that is in the clear gets plenty of margin.  Also, owing partly to aperture, a full-sized turnstile antenna such as this is likely to intercept a little more signal than a dielectrically-loaded (i.e. smaller) patch antenna - something that can help make up a bit of lost signal.

In looking at the track map and statistics, the tracking ability of the receiver/antenna system appears to be limited only by horizon visibility.  To the southeast and west/northwest, visibility is somewhat limited by some tall trees - but in other directions (due south, due east, southwest) only mountains (elevations of <3 degrees, typically) limit the view.  During initial testing I'd seen the GPS receiver track satellites as low as 4 degrees - but this was rare because there are usually enough satellites at higher elevations and the receiver rarely has to try to track those...

Comparison of the homebrew antenna with a commercially-made antenna:

For several weeks (ending 3/10/04) I used a commercially-made GPS antenna instead of the homebrew.  The antenna used was a Magellan OEM-type of antenna (the round, white one.)  The results were observed to be as follows:

Figure 5:
The schematic of the "PBJ" antenna.
Click on the image for a larger view.


How well has this antenna held up?  As noted below, I had repeated problems with one solder-tack joint breaking loose (until I encapsulated it in clear epoxy) but it has been on the roof since at least 2003 and shows no obvious signs of degradation.


Putting this antenna on the roof:

The best location for any GPS antenna is one with a clear view of the eastern, western and southern horizons (or northern horizon, if you are down 'unda, or both if you are nearing the equator.)  In North America and Europe, for example, the inclination of the GPS constellation causes the minimum elevation to be fairly high.  In most cases, therefore, the preferred location for mounting the antenna is on the roof.

What are the mounting options?  A few come to mind:

Unfortunately, none of these are viable options on my roof::

The other option is a non-penetrating or "ballast" mount.  As the name implies, this sort of mount simply sits on the roof, being weighed down for stability.  These mounts are often used to hold down the satellite dishes that are seen atop gas stations and are used because they are relatively easy to deploy and don't involve putting holes in the roof.  These ballast mounts are also available for the smaller direct-to-home satellite dishes - although one may have to mail order it as they aren't always found at local sources.  For the GPS antenna, one of these types of mounts - along with the requisite number of cement blocks - is overkill and you are likely to lose your shingles before the mount blows away!

You could also make your own mount from a metal plate sized to accommodate two side-by-side cement blocks.  Using angle brackets and a piece of pipe, one can make a suitable mount for small antennas.  One would also use one or two pieces of metal as a "strut" not only to stabilize the pipe but to make the pipe sit vertical to accommodate the pitch of your roof.  Under this plate, one would put the protective mat, and atop the plate, the two cement blocks:  Two are recommended as they form a "square" footprint, aiding in mechanical stability.

A few comments:

Figure 6 shows my particular installation.  The "PBJ" antenna may be seen (especially if the larger-sized image is displayed) most of the way down at the top edge of the black ballast mount bracket (follow the white cable down from the top...)  The antenna on top is actually a DF (Direction Finding) array, with its top at about 8 feet above the roof.  You may notice that there are cement blocks under the mount as well as on top of it:  This was done because the metal roof has ridges (that is, it is not flat) and these blocks are used to elevate the ballast mount above these ridges.

Needless to say, these are suggestions:  It is up to you to do the appropriate research and take appropriate safety measures!

Figure 6:
The GPS antenna on its ballast mount.  See text for more details.
Click on the image for a larger view.

The power supply:

After originally using a voltage-doubled 24 volt AC "wall wart" I replaced it with a a cast-off "48 volt" 2 amp switching-type supply from a telephone switch.  A simple modification was made to raise its voltage from 50 to 56 volts (the addition of a parallel resistor which was done to keep the Z3801 happy.)  The supply was mounted in a metal box and "brute-force" L/C EMI/RFI filtering (their circuits similar to that depicted in Figure 7) was added on both the input and output to keep its switching supply noises out of my amateur receivers.  In the years since it was put into service, I have had no troubles with it at all - but that should be expected from a high-reliability device intended for telephone use.

There is one "gotcha" about using switching supplies with the Z3801A:  The Z3801A itself has a switching-type supply - and like typical switching supplies, it has its own start-up issues:

The upshot of all of this?  If you are powering your Z3801 with a switching supply, this outboard supply may, itself, have a "soft start" circuit built-in:  Upon application of line power, the two switching supplies (the line-powered one and the one in the '3801) may get "stuck" and not fire up.  Also, unless it has a rating significantly higher than the peak current consumption of the receiver, it too may balk at the initial start-up current requirements of the receiver - especially when trying to charge the input capacitors as well..

The solution to this problem?  Actually, the fix is a very old one:  Delayed start.  If you power up the supply - then connect it to the GPS receiver - it may do just fine.  This "delay" may be done with an old-fashioned relay and delay circuit, or one could take a solid-state approach and use a transistor (bipolar or MOSFET) switch.

Power supply RFI:

As mentioned before, the Z3801 uses a switching supply to generate its internal voltages.  Actually, it uses several...

One of my "sub-hobbies" is LF/VLF listening.  Several years ago, with winter arriving and the summer static subsiding, I decided to test my LF receive setup once again and was greeted with a noise floor about 30 db higher than I was expecting.  Presuming that the antenna itself was having some problems, I checked further and no, there wasn't a problem with the antenna at all - but that it was an external noise source.  It was now time to start shutting things down and/or disconnecting things.

The "Ah Ha!" moment came when I disconnected the antenna from the GPS receiver and the noise dropped 15-20 db.  Shutting the receiver off (something that I hate to do...) returned the LF receive antenna system to its expected performance levels...

The Fix:  

At this point, I knew what I had to do:  Add a power supply filter in the Z3801.  Note that this racket was not coming from the outboard AC supply mentioned above which I'd already filtered, but from the Z3801 itself, via its DC power connector/cable.

Removing the top cover, the obvious place to mount a filter was in a clear spot against the back panel - next to the power connector, so  I carefully cut a piece of circuit board material to the maximum size that would fit in the space and mounted it with two screws.

Then, the power connector was removed from the chassis and disconnected from the input on the power supply board.  I then carefully "un-crimped" the "board-end" of the internal power cable and soldered longer wires to it - although I could have simply cut and spliced.  Doing this gave me a long enough run of wire to go from the power supply input to the (to be added) filter.

Effective filtering at VLF/LF/MF frequencies isn't as straightforward as it might seem - especially when higher currents are involved.  Too often, I have seen hams simply slap ferrite beads on QRM sources - only to be disappointed when doing so wasn't particularly effective.  Furthermore, simple ferrite beads and snap-on chokes only have effects above several MHz anyway and will do absolutely nothing at VLF/LF and MF frequencies and are only mildly effective at HF in most cases.

The Filter:

Referring to the schematic in Figure 7, L1 was salvaged from a computer power supply.  Typically, these chokes come in three flavors:  A bifilar-wound toroidal choke, a toroidal choke that has a winding on each half of the core, or one that looks like a small, square power transformer with two distinctly separate windings side-by-side.  In either case, both windings are tightly coupled together and any common-mode energy passing through them is suppressed significantly.  If you have a means to do so, make certain that this choke has quite high inductance:  The choke that I used measured about 1.5 mH (that's Millihenries) per winding and thus has enough inductance to offer significant reactance at LF frequencies (nearly 100 ohms at 10 kHz.)  If you don't care about frequencies that low, then a choke with lower inductance may be used.  Remember:  We are trying to keep LF energy from the switching supply inside the GPS receiver from being conducted onto the power cable!
Figure 7:
Schematic of the power supply input filter.
Click on the image for a larger version

As mentioned before, capacitors C3-C7 re-assert common-mode signals and ground them out and by this time, LF energy is mostly quashed by L1 and is then routed to "ground."  Both electrolytic and ceramic capacitors are used to help guarantee that both low and high frequencies are bypassed.

L2 can be either a choke similar to that of L1, or it may be two smaller inductors.  Their primary purpose is to keep HF out of the receiver and thus, the inductance of each winding (or inductor) may be in the 10-100 uH range (or higher, if you wish.)  Finally, C8 is used to make any energy entering (or exiting, for that matter) the GPS receiver's power supply common-mode.  Resist the temptation to connect a capacitor to ground at this point because any RF energy entering on the power supply lead will find it as a low-impedance path to the chassis:  Let L2 do its job and choke it out, first!

Finally, make certain that all capacitors that you use are rated for the voltage:  Ratings of at least 100 VDC are recommended on all capacitors.  If you don't see a voltage rating plainly printed on the component, don't use it!

Results:

The results of adding the filter were good:  No detectable noise on LF, MF, or HF emanates from the GPS receiver anymore!
 
Other modifications - Improved heat-sinking of the power converter modules:

While I had the GPS receiver apart I decided to make another modification:  The addition of more heat-sinking to the DC-to-DC converters inside the GPS receiver.

These are flat, square-ish modules with obvious printing on them indicating their input and output voltage ratings.  During testing, I couldn't help but notice that they were "hotter than hell" - too warm to touch comfortably.  Knowing that the reliability of any electronic device is inversely proportional to its operating temperature I decided to do something about it.

Rummaging around the junkbox I found a few small CPU-type heat sinks.  Noting that the DC-DC converter modules already had tapped holes in them for mounting, I found some mating machine screws in my hardware collection and, along with white heat-sink grease, I attached heat sinks to these supplies.

The result was that the temperature of these modules went from "too hot to touch" to just "very warm" - a definite improvement!

Knowing that doing so would make the receiver potentially more-sensitive to variations of ambient (room) temperature, I resisted the temptation to put a small DC-powered fan inside the case.

The RS-422 interface:

One popular scheme is to utilize the Z3801's built-in RS-232 circuitry.  Although installed, it is not connected by default - a process that takes some minor surgery to complete.  Because this procedure may be found on the K8CU page (see below) it is not detailed here.

Jamming myself...

Several years ago, I was hiking with a group of hams, and we happened to be using a simplex frequency of 146.52 MHz.  I also happened to have my GPS receiver with me (back then, a Magellan GPS-4000 - non-XL version) turned on.  I couldn't help but notice that it didn't seem to be working very well at all:  It was constantly losing lock on all satellites. 

When we stopped for a break, I happened to let my HT (a Yeasu FT-530) do it's timed auto power-off:  Since our group was all together in one place, there was no need to leave the radio on.  I then noticed that my GPS receiver was working normally. 

Putting two-and-two together, I surmised (and later verified) that the '530 was, in fact, jamming the GPS receiver.  Here's how:

At 146.52, the FT-530 uses low-side injection for its local oscillator.  With an IF of 15.25 MHz, this places the LO at 131.27 MHz - the 12th harmonic of which is 1575.24 MHz - almost exactly on the L1 GPS frequency of 1575.42 MHz.  Apparently, when the GPS receiver and the HT are within a few feet of each other, there's enough LO energy present to "jam" the GPS receiver! 

The cure for this problem?  We now use a 147 MHz simplex frequency - which isn't as busy, anyway...
There are also numerous schemes to convert RS-422 levels to RS-232 levels - also found on the K8CU page.  If you are in a real hurry, however, you can do a faux conversion using just a single (optional) resistor:

There are several caveats with this method, however:

A question that may come to mind is, "Why use RS-422 in the first place?"  The question is one of robustness.  RS-422 is a differential signaling scheme - that is, each signal is transmitted on a twisted pair - with the two conductors carrying equal and opposite signals.  This has the advantage of being able to handle "common mode" interference without corrupting data.  What is common mode interference?  Simply put, a common mode signal is one that travels down the pair of wires in unison - that is, not differentially.  The common mode signal, affecting both wires equally (in ideal cases) is ignored by the receiver - which is looking only at the difference between the two wires.

Because of the differential nature of this scheme, it is possible to run RS-422 signals for miles - the distance being limited mostly by the high frequency response of the wire pair limiting the data rate.  A note here:  Even though RS-422 doesn't use the "ground" wire for signaling, per se, one is strongly recommended to limit the swing of the common mode signals on the wire pair to a level that the receivers can handle.

Another important reason for using RS-422 over RS-232 is that '232 has in inherently slow slew rate - that is, it can't swing very fast.  While RS-422 is capable of carrying signaling rates of several megabaud (for short distances) this simply isn't the case with RS-232.  What this also means is that if you want to use the GPS receiver for absolute timing accuracy with the 1PPS output, you don't want to use the RS-232 output unless you take into account the "slowness" of the RS-232 hardware - and the likelihood that this reference is likely to drift around a bit as thresholds in the RS-232 hardware change with time and temperature.  This isn't likely to be too much of a problem for the vast majority of users, as the difference is only likely to be a fraction of a microsecond, anyway...

All of that being said, I've been too lazy to build an RS-422<>RS-232 and have stayed with the "kludge" from the beginning.  The only "problem" that I had was when I recently interfaced the receiver with a computer that had no built-in RS-232 port and I had to use a USB<>RS-232 adapter.  When doing this I noticed that the more-expensive Belkin adapter didn't work at all, but the cheap, no-name adapter that used the "Prolific" chipset has worked perfectly and with no errors at all.

Use as a time reference:

It would be nice to be able to, say, drive a clock and supply GPS information to a host computer.  Being that there is only one serial port on the Z3801, one would have to multiplex both systems in a manner that the Z3801 can support.  I'm currently working on a PIC-based controller that will do the following:

As noted above, I use the GPSCon program to both gather statistics from the Z3801 as well as provide a local NTP service.

Status of the GPS receiver at KA7OEI:

- March 2011:  Two problems:

1.  It looks as though the "PBJ" antenna has gone intermittent again.  So far, it only seems to do it for a short time, when it is cold.  As noted before, this could have been prevented at the beginning by better-connecting the MMIC's input lead to the UT-141 coax center connector.  When the weather is nice - and I have more time - I'll pull it off the roof to see if it's that same problem, or maybe a problem elsewhere.  Comment:  After a few weeks, it stopped dropping out, so it may have been snow-related.  If it happens again in the summer I'll pull it down and take a closer look.

2.  I (think that I) finally solved a problem that showed up with the new computer - and had showed up on the old one, too.  Since computers no longer routinely come with RS-232 serial ports, I had to use some USB<>Serial adapters.  I'd tried this on the old 366 MHz laptop with its single built-in serial port, but when I tried to use a USB adapter, it would crash after a while.  I just chalked that up to problems with the old computer and Windows 98SE.

When I switched to the "newer" computer (1.7 GHz) which had no RS-232 ports at all, I had to use a serial port for the WX Satellite receive and another for the GPS receiver.  As noted above, the Belkin didn't talk to the "Pseudo RS-232" level from the RS-422 port on the GPS receiver - but the no-name "Prolific" chipset-based adapter did!

All was well for a while - and then I started to get more and more "BSOD" crashes with a cryptic "USB_BUGCODE_DRIVER" error.  By the process of elimination, I determined that it was the Belkin USB<>Serial driver.  Putting another generic "Prolific" chipset driver on there solved the problem - and it was likely the same problem on the W98SE computer.

It seems as though, if you were to put the same, identical USB device on the same physical USB port (this may not apply to an outboard USB hub, though) then it will uniquely identify it for that USB port.  In other words, if you put a particular USB<>Serial port on, say, the 1st USB port and assign it COM1, it will ALWAYS be COM1.  If you move it to the 2nd USB port and assign it to COM2, it will always be COM2 when plugged in that USB port.  (Note:  To pick your COM port you must force it to the desired COM port in the hardware configuration screen or else it may just pick the next-available COM port - usually a high number like "COM5" or above.  Note that the "Com port already in use" warning you may get can refer to a USB<>Serial port that had been previously installed - but not necessarily one that is plugged in at that moment!  In other words, if you EVER had a COM1 in the past, it will mark that is "in use".)

The advantage of having two different-brand USB<>Serial adapters is that they will never be confused with each other as the computer cannot mistake them for each other.  In this case, however, since I ended up using two "Prolific" USB<>Serial adapters, I just had to make sure that I always knew that "COM1" was associated with one USB port and that "COM2" was associated with the other.  (Unlike other reports, I've had no problems with the "Prolific" chipsets on XP...  'dunno...)

- January 2011:  After being online for about 8 years everything continues to work fine - including the "PBJ" antenna, which has been in the weather, on the roof for just as long!  The receiver is now being monitored full-time as I now have a low-power computer dedicated to the task - one which also provides a local SNTP server for digital modes - see above.

I also noted the GPSCon wouldn't always start up in the "running" mode when the computer came up after a power bump:  While I'd scheduled GPSCon to be started just after auto-login, it just came up in "idle" mode which meant that it wasn't pulling the GPS receiver, providing SNTP service or collecting log data.

Looking at the GPSCon web page I noticed that the very next version after mine included a check-box to auto-start the program - but I'm a bit reluctant to pay for yet another upgrade for a program that hasn't been updated recently and wouldn't offer any other feature that I don't currently need.

What seems to have worked as a "fix" is to go into the registry and set a key.  Using Regedit, I went to:

HKEY_CURRENT_USER, Sofware, GPSCon, Initial, Was Running

and changed that value from 0 (zero) to 1 (one.)

So far, so good... mostly...  After a power interruption it failed to come up automatically on one occasion - and the above registry key had changed back to zero:  I suppose that I could have the registry entry automatically be written when the computer boots, but I now have my UPS working properly so there shouldn't be as much need to restart the computer.

- March 2008: Not much to report, really:  The "PBJ" antenna and the Z3801 continue to work.  Note that the computer connected to the Z3801 is not used very much, so the graphs on this web page are not updated vary often.  I had one more instance of the solder tack joint breaking loose as described below, but I repaired it and then encapsulated the joint with clear epoxy.

- July 2004: I recently had some trouble with the Oncore GPS module:  After a power cycle, it would never lock and making it load defaults resulted in it believing that it was in an impossible location (e.g. at "596 degrees west and east at an altitude of 13k miles or so) and due to "bad geometry" the survey would never complete.  I remedied this by building an interface to power the Oncore module and connect it to the computer and, using the Motorola "WinOncore" program was able to reset the receiver and successfully test it.  Upon re-installation, it again exhibited some strange behavior - but I was able to work around it.  (It may be heat-related:  Further investigation is warranted.)  Update:  I was able to find an 8-channel Oncore unit and installed it - along with a "super-cap" to hold the RAM during power failures - in the Z3801, solving the receiver problems.  I also noticed previously-noted susceptibility to about any VHF or UHF transmission from my shack causing the receiver to go into holdover was now gone - probably related to the fact that the new receiver had a bandpass filter and the old one did not! 

- March 2004:  For several weeks (until 3/10/04) I was using an OEM Magellan GPS antenna to compare it with the homebrew unit.  The results of this testing are in the text, above.  

- September 2003:  On about 9/27/03, I fixed the GPS antenna.  For some weeks prior to this, I'd intermittently lose all GPS signals.  The problem got worse - to the point of total loss, but I didn't have time to look at it until the 27th.  The problem turned out to be a solder tack joint had failed where the UT-141 coax from the turnstile connected to the input of the MAR-6. 

This change was done as part of the "permanentizing" of the receiver installation.  Much work remains to be done...

- May 2003:  Initial installation of the Z3801.

Additional comments:

One problem that I need to address is receiver jamming.  When I operate 2 meter SSB, for example, the receiver loses lock on all satellites.  Why is this?  There are several possible explanations:

What to do about this?  In the first case, I could build a simple GaAsFET amplifier to replace the MAR-6.  By necessity, the GaAsFET would have a matching network on its input - and it would surely have enough out-of-band rejection to prevent significant VHF/UHF energy from entering the preamplifier itself.  This "fix" would also take care of the second situation (overload of the GPS receiver itself) at the same time.

As it turns out, I answered the question (at least partially) by substituting the homebrew antenna with a commercial one.  Because the commercial antenna was unaffected by the presence of a 2 meter transmitter, the "theory" that weak harmonics are the culprit was discounted.  It could still be that the amplifier and/or the receiver itself are being overloaded by the presence of the local 2 meter signal.

What if it is the second situation?  A simple bandpass filter stuck inline (a relatively tightly-coupled 1/4 wave cavity filter) would take care of the problem.  The "awkardness" here is that I'd have to feed the DC (for the amplifier) around the filter.  (I'll probably just build a GaAsFET preamp and avoid the problem...)

What about the last one - weak harmonics of the 2 meter signal?  Because I don't think that this is likely to be too serious a problem, I haven't given it much thought.  In reality, the occasional nature of the SSB operation - and the fact that the receiver seems to recover nicely from "holdover" (resulting from loss of GPS lock) may make it so that I'll just live with the problem...

(Again, since I replaced the original GPS module in my Z3801 with one that used a bandpass filter, the loss-of-lock that occurred when a local transmitter was used is now very rare.)

Now, I wonder how things will work when I fire up my 23cm transverter???

This page will be updated as time goes on - check back later...

For more info on the Z3801A, the GPSCon program and other GPS receivers used as time/frequency references, go to K8CU's page at: 

http://www.realhamradio.com/GPS_Frequency_Standard.htm
 

Any comments or questions?  Send an email!

This page and its content copyright 2004-2011 and maintained by Clint Turner, KA7OEI and yes, I know the background is for the MedFER beacon...  Any trademarks are the property of their respective owners.  This page last updated on 20111214

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