Simple GPSDO - Questions and Answers

                        by James Miller G3RUH

I often receive questions about the GPSDO; this document summarises
the replies.

Why do you call it 'simple'?
It is simple as in 'non-complex'.  It represents pretty much the minimum
set of hardware elements you can use to construct a GPS controlled
oscillator.  This minimalism is possible because the Rockwell Jupiter GPS
RX has a precise 10 kHz output which greatly simplifies the PLL phase

What software is needed to use the GPSDO?
You do not need any software to use the GPSDO; but you might want to for
interest.  has a good collection of
related material, including Rockwell's original Labmon (for x86 computers).
Labmon uses the Jupiter GPS native binary protocols for control and display.

The Jupiter also reports in NMEA-0183 plaintext, and the GPSDO is shipped
in that mode.  Any software that can process industry standard NMEA data
can display the data.  Examples (for x86 computers):

A dumb terminal program will show the NMEA raw data as text lines.  Set the
program to 4800 baud, format 8N1.

Explanations of NMEA sentences can readily be found using an internet search
engine.  For example.

Please note that the GPSDO serial port is configured as a "modem".  It can be
connected directly to a computer's serial port; do not use a "null modem"
interconnection cable or device.

How can I tell when the GPS is in Lock?
Using a dumb terminal program, look at byte 15 of the NMEA  $GPRMC
message.  When this shows "A", the GPS is in lock.  Example:


Another method is to watch the 1-pulse-per-second LED jump into synchronism
with WWVB and similar HF radio time signals, which will happen a minute or
two after power-on.

PIC-based methods are described here:
These devices work best if the Jupiter's Solution Validity Criteria is set to
"the minimum number of satellites for solution = 4" (instead of the zero
default).  Then, if the antenna is disconnected, the PIC-based lock
detectors give an immediate reaction.  To change the Jupiter setting, see the
section "How do I RESET a Jupiter GPS?" which refers to a file of commands
you can download.

A GPS lock indicator requires added complexity, which is contrary to the
minimalist principle espoused here.

Why is the date is wrong via RS-232?
1. Summary of the problem

After power-on and then acquiring satellite lock, the displayed date (via the
RS-232 port) is exactly 1024 weeks earlier than the true date.

This first came to light on 2015 Feb 15 [Sun], GPS Week 1832, after some
20 years of normal behaviour, and appears to be due to the inability of the
firmware of older Jupiters to retain the last used date in its EEPROM.

However, the Jupiter GPS receiver's 'Scorpio' DSP engine is processing the
satellite signals correctly; acquisition of satellites, generation of hardware
1PPS and 10 kHz output signals are perfectly OK.  The GPSDO produces its
precise 10.0 MHz output as normal.

If your application does not use the RS-232 data stream, NO action is required.
If otherwise, you might want to reset the date.

2. How do I reset the displayed date?

If the Jupiter's date output is important to you, then you must initialise the
date yourself, as follows.

With the GPS receiver RS-232 stream in NMEA mode, send the command string:
with the appropriate date.

NOTE: It is essential that the I/O connector pin 8 is in the "1" (high) state
      or not connected (the latter as with the author's GPSDO 0091-000).
      Otherwise the date will default to that stored in ROM, 1997-Jun-28.

A file,  init_date.txt  that contains the above command can be found in the
archive linked in this FAQ's section "How do I RESET a Jupiter GPS?"

Example; for 2015 Dec 31 you would send:

Note: this is 36 ASCII bytes plus 2 terminating bytes CR LF, (hexadecimal
0x0D  0x0A), making a total of 38 bytes.  The CR+LF are essential, and must be
in that order.  There are 14 ","s in the string.  There is NO tolerance for
error in this byte sequence.

Investigation shows that you can actually choose any initial date in the range:

  Today-919weeks to Today+104weeks

Thus it follows that the initialisation date used in the above example
(2015 Dec 31) will work for you unchanged until near the end of year 2033.

If you use Hyperterm, use VT100 Emulation mode.  Note: a user has
reported difficulty with Hyperterm, but advised that Teraterm (for x86
computers) in binary mode does work OK.

Ensure that the file or string sent is 38 bytes.  Many terminal programs will
add [CR][LF] to your string, so on-screen, what you should see is 36 characters,
as from the "$" to the "5" in the above example, with the terminal adding the
two terminating bytes.  However exactly [CR][LF] is required; NOT [LF][CR] or
only [CR] or only [LF]

How long does it take to lock up?
To be usable, three things have to happen; the GPS receiver must acquire
satellites; the OCXO oven must warm up; the PLL must pull into lock with the
GPS.  For practical purposes the GPSDO is ready for use after 15 minutes.
By then it will be accurate to better than 10-10 equivalent to
1 Hz at 10 GHz, and improving.

Why do you not use a modern M12+ GPS receiver?
Because it does not have a 10 kHz output.

What GPS antenna should I use?
Almost any antenna will do.  It should be 'active' and have enough gain to
compensate for cable loss, typically 1 dB/m for RG174, with 6 dB to spare.
RG174 cable is the very thin stuff, about 3mm diameter.

For example "26dB gain" would be suitable for a 20m cable run.  The GPS
receiver supplies +5V DC along the cable to the antenna, and the load should
not exceed 100 mA.  Typical antenna consumption is 5-20 mA.  A male SMA is
required on the cable at the GPSDO end.

There are hundreds of GPS antennas available on eBay.  In a portable
situation, you'll probably want something small that can rest on top of the
equipment, and the 'puck' antenna is entirely adequate.  You can of course
use a puck antenna in a fixed station/laboratory situation.

If you require a laboratory grade antenna, but do not wish to spend
hundreds of $$, then look out for the HP/Symmetricom 58532A or the
Panasonic VIC100.  The HP58504A is good too.  Often on eBay.

Why doesn't my system receive GPS satellites?
Every problem of this kind reported to me has been due to a poorly positioned
antenna, usually indoors in a window.  This is far from ideal.  Some glass
does not pass L-band signals too well.  An indoor antenna picks up the warm
room's noise. A window location sees half of the sky at best.  North facing
views see fewer satellites than South facing (in the N. hemisphere).

Give it a chance!  The performance of a radio-based equipment, such as this
GPSDO, is strongly influenced by the received signal quality.  GPS reception
is line-of-sight.  Be sure to mount your antenna where it has an unobstructed
view of the sky.

How accurate is this GPSDO?
Oscillators have several defining properties, in particular accuracy and
stability.  These are interrelated, and summarised nicely at:

Refer to the image in the above link; the OCXO and the GPSDO are measured
for these factors.

The accuracy, taken over a very long period, is the same as that of the GPS
system itself, parts in 1014, so accurate as to be neglected for most

In the short term, over a 10 second period, the accuracy expressed as a
fractional deviation from 10 MHz is within +/-3x10-11  99% of the time.
This equates to around 0.1 Hz RMS at 10 GHz.

Stability, or "jitter" is conventionally described by the statistic called
Allan Deviation (see the NIST link above).  Typically, at T=1 it's
2x10-12, T=10 is 5x10-12, T=100 is 4x10-12 and then it falls steadily to
under 1x10-13 at 1 day.  There is some spread from sample to sample, but
it is unlikely to be more than 2x greater than the above.

The phase noise (i.e. very short term jitter) is excellent; multiplied by
1000, a pure tone at 10 GHz has barely a hint of noise.  Mainly it reveals
the instabilities of one's downconverter ...

An interesting comparison of several GPSDOs, including this one, is at:

What is the delivery time?
Shipping is normally ex-stock, on receipt of payment.  Insured postal
service is always used, and air-mail for non-UK.  Fedex is also available.

How can I order this GPSDO?
Just email me.  The address (a graphic) is at the bottom of this page.

How can I pay for this GPSDO?
Ways to pay are described at: and include PayPal.

Can you send me the schematic and parts list?
Documentation is supplied with the GPSDO.

Where can I get a 10 MHz Distribution Amplifier?
Look at the (sometimes available) TAPR TADD-1.  It takes a 1V RMS sinewave
input and has six outputs.  The input and all outputs are DC isolated.
A smart enclosure is available: (USA).

A similar 4-way distribution amplifier kit is at (UK)

A 4-way unit, available made and tested or at PCB and kit level is available
from Down East Microwave (USA).

For a 2-way splitter, try a simple passive Wilkinson divider.

How can I generate 10 MHz harmonics?
If you are comfortable with self-build take a look at:
This unit generates harmonics at 1 MHz intervals to beyond 10 GHz.

I have a Jupiter GPS and a 10 MHz oscillator I found on eBay.  Can I make
myself a simple GPSDO?

Yes; see

Where can I find Jupiter GPS documentation?  has a good collection of Rockwell
documentation in one place.  Navman, who acquired the interests in
Rockwell/Conexant's GPS brand 'Jupiter', no longer list legacy documents,
which is reprehensible.

How do I RESET a Jupiter GPS?
Instructions for restoring a Jupiter GPS receiver to the factory default
state and/or GPSDO-friendly state are here.

Other useful commands are included.

How do I identify a Jupiter GPS?
A typical Jupiter product number is TU30-D140-221

You can tell explicitly by looking at the underside.  There's a small dark
panel with some fat white numbers in it.  They look something like this:

 221 9932 GPXO
In this example: 221 is the model suffix (as in TU30-D140-221) 9932 is the date code (1999 week 32) GPXO is a production ID The TU30-D140 product identification is in white legend just above the fat white numbers. Underneath the fat white numbers is the part number of the bare PCB itself, in copper trace under green, typically TU30-D145-027 You can also tell something from the eprom label on the topside. -141 models have "JUP7" and "V118" (and straight up MCX connector) or "JUP8" and "V180" -221 models have "JUP8" and "V180" (and straight up MCX connector) -151 and -231 are as above, but with a right angle MCX connector. "Vxxx" is the firmware version number, e.g. V1.80 Abbreviations GPS Global Positioning System GPSDO GPS Disciplined Oscillator NIST National Institute of Standards. Formerly NBS. NMEA National Marine Electronics Association MCX A sub-miniature coaxial connector SMA A sub-miniature coaxial connector OCXO Oven Controlled Crystal Oscillator PLL Phase-locked loop RX Receiver, as in radio RX

Last updated: 2017 Jan 18