Before FireFox, customers had to purchase a separate GPS disciplined Cesium or Rubidium frequency reference, and a synthesizer unit and connect these together to get both accuracy, and resolution. The cost of these units easily exceeds $5000, and leaves the user with two power-hungry, clunky units that have to be connected to each other.
FireFox uses an internal very high-end Motorola M12 timing receiver, which
is specialized for reference timing applications. The GPS timing receiver
provides the best available timing reference signal on the market by compensating
for the receivers' location, thus providing a timing signal that is synchronized
to within 20ns of UTC anywhere in the world. This timing signal is used
to synchronize a high-end, low-noise 10MHz OCXO to UTC which itself provides
the reference signal to all FireFox PLL's and the DDS. Combining both the
GPS disciplined Frequency reference and the Synthesizer functionality into
one unit allows exceptional accuracy (the frequency accuracy is much better
than 10-12 long term for the 10MHz internal OCXO reference) while
at the same time reducing the size, cost, power consumption, and failure
rate of the units.
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Other commercially available reference-timing receivers boast of their long hold-over circuitry performance when not being able to receive any satellites. The FireFox synthesizer uses a slightly different approach.
Instead of hoping for the best and designing circuitry for hold-over mode
when the receiver cannot receive GPS satellites due to jamming or due to
the low sensitivity of competitive receivers, the FireFox uses an optimized
design and the best receiver available on the market to avoid any GPS receiver
drop-outs in the first place. Once the receiver has established its position
and is in position-hold mode, only one satellite needs to be seen by the
receiver for the FireFox synthesizer to work properly. The sensitivity and
jamming immunity of the FireFox GPS receiver is absolutely exceptional.
Thus long hold-over periods will likely never occur even if the antenna
is just placed in a window sill.
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The FireFox internal microprocessor controls and configures the GPS receiver with default settings during power-on. A menu option allows access to the GPS serial signals via an RS-232 connector on the back panel to override these default settings. This connector may be used to optionally control the GPS receiver setup via Motorola's WinOncore12 PC software. The software can be downloaded from:
Download WinOncore12
All features of the internal Motorola M12+ receiver can be checked and
controlled via a simple null-modem serial cable. Antenna cable offset, satellite
mask angles, timing information, satellite health information as well as
GPS vs. UTC time-base settings are some of the parameters that may be controlled
and checked via the PC software, overriding the FireFox default settings.
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The FireFox GPS receiver is controlled by its internal microprocessor by
default. You can monitor all GPS parameters via WinOncore12 (see
FAQ) but you cannot change its settings in the default mode. To change
the receiver settings, go to "MENU/SETTINGS/COMMUNICATION/GPS CONTROL"
and change the item to "REMOTE". You must stay in this menu item
for as long as you want to change the receiver settings via WinOncore12.
Exiting this menu item will return the receiver to "LOCAL" control,
and you will again only be able to monitor the receiver parameters without
being able to change them.
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The FireFox system uses a specialized GPS timing receiver. Instead of calculating
the geographic position, this receiver uses a fixed geographic position
setting to achieve a very high timing accuracy. When the present position
is unknown (such as when the receiver was moved) the satellite receiver
will measure and average ten thousand position measurements to achieve a
position average with very high accuracy. This measurement mode is called
Auto Survey Mode. The satellite receiver locks into its final position-hold
mode within 2.5-5 hours, after which it will receive the position-corrected
timing information from the GPS receivers. The Auto Survey mode settings
are saved in NV memory, and the mode will not have to be invoked again unless
the unit is turned off for longer than 2 days (typically) or the units'
antenna is moved more than 30 feet.
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The FireFox GPS receiver includes an optional Motorola Time RAIM (Time-Receiver
Autonomous Integrity Monitoring) algorithm for checking the timing solution
integrity. This mode is invoked via the front-panel menu, and allows the
GPS receiver to check the quality of a satellite receivers' signal. It will
mask off signals from Satellites that indicate any fault conditions, or
if the particular GPS signal is outside certain limits compared to three
or more other satellite signals. This Algorithm prevents faulty satellites,
or adverse atmospheric conditions from affecting the precision of the FireFox
synthesizer. T-RAIM is unique to the FireFox system and its' Motorola GPS
timing receiver.
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Competitive products use standard GPS receivers to save cost, or because
they were designed prior to GPS timing receivers becoming available. These
receivers are optimized for navigational purposes, and do not include the
benefits of position-hold mode and T-RAIM (see above). Their tracking filters
need to be very wide to allow for speed and directional changes, translating
into much larger inaccuracies in the timing reference output. The timing
receiver is typically stationary, and thus allows for much smaller bandwidth
tracking filters, increasing the timing solution accuracy significantly.
Comparable navigational receivers have a typical accuracy of 100ns rms,
whereas the receiver used in the FireFox units has a down to 10ns or better
1-Sigma timing resolution due to its optimized small bandwidth tracking
filters (2ns solutions are possible when using optional sawtooth correction).
The FireFox units further increase the accuracy of the timing solution and
the receiver sensitivity by enclosing the receiver into a shielded metal
can to mitigate the effects of temperature changes and EMI, and by rigorously
filtering all power supply voltages prior to entering the timing receiver.
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The FireFox system supports both 3.3V and 5V GPS antennae via a standard
BNC connector. Jumper JP4 on the PCB can be set for either 3.3V or 5V output.
Many 5V rated antennae will also work with a 3.3V setting. Jackson Labs
recommends mounting any outdoor antennae with proper lightning precautions.
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The SAT number indicates the number of satellites that are presently being received and used for the timing solution. The STAT item is a status indicator, having the following meaning:
BG - Bad Geometry. This indicates that the satellite positions are not differentiated enough to be usable in Auto-Survey mode.
AQ - Aquiring. This indicates that the unit is looking for satellites to track.
UL - Unlocked. The unit is receiving satellite timing information, but has not finished locking the OCXO to UTC yet.
AS, 3D, 2D - Position fixes. The unit is receiving 3D or 2D position fixes when in Auto Survey mode. Ten thousand readings are required to finish Auto Survey mode.
OK - Position hold mode with locked OCXO. The unit has locked the OCXO to UTC, and sufficient satellites are being received to maintain lock.