Using the propeller for a gps signal generator/simulator?
turbosupra
Posts: 1,088
I've seen lots of gps projects with the propeller that are for receivers, are there any that act as a signal simulator/emulator? I believe it is in the 1575.42 mhz range. I'd be interested in building one for a test project.
Comments
-Phil
at 1227.60 or 1575.42 may get you noticed by
monitoring stations that process data from several
geosynchronous sats. I advise caution!
GPS devices that transmit data are probably frowned on
by the mil.
I have heard that GPS spoofing devices that can trick those GPS tracking units
that are placed on peoples cars to log where they go are meant to be placed
into a small metal container along with the tracker to feed it false data. You might do
this to avoid radiating rf. Other than this I would advise against generating rf at
those frequencies.
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"Where am I? Where am I going? Why am I in a handbasket?"
If they can retransmit at low power, they can transmit I would think?
http://www.mobilegpsonline.com/mgpsstore/index.php?main_page=product_info&cPath=6_97&products_id=347
with perhaps a teeny bit of amplification of the original signal...
But with a max range of 1.5mtrs it seems totally passive to me.
Such a thing is nothing like generating your own GPS signals.
The repeater would not ever compete with the GPS signals since
any rf re-radiated by it's antenna would be in nearly perfect
phase with the original signals.
You can create passive repeaters on any freq very easily.
They are basically one antenna connected by feedline to
a second antenna at another location. If you have super poor
cell coverage in your house I would think you could make up
a quick repeater to allow coverage close to an interior ant.
Just cut a folded dipole made of 300ohm line and locate it
on the roof in vertical polarity in a good signal spot. Then run
300ohm cable inside to a 2nd identical antenna and you should
get coverage inside if you are close to the ant. For a real signal boost
an external yagi would outperform a vertical dipole by many db.
I suggest 300ohm line because it has extremely low signal loss
even at cell frequencies.
p.s A broken old microwave oven is a great place to test small
rf generators at GPS freqs as the signal would be almost entirely
contained within. They use one at work when testing boards that
have data burst ICs on them...so the signal does not escape and
cause interference.
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"Where am I? Where am I going? Why am I in a handbasket?"
There should be no legality issues with that
Maybe I'd be better off though with software simulation and a hardwired connection?
Just so you know - the GNSS received signal levels are below the noise level. High sensitivity receivers have to deal with < -160 dBm signals indoors. (Here's a good reference book -"A-GPS: Assisted GPS, GNSS, and SBAS." http://www.frankvandiggelen.com/)
We use plenty of GNSS simulators at work they are indispensable and legal - I didn't see anything in the original post about intentionally radiating in-band GPS signals. Search for Spirent if you want to see what the high-end simulators look like. And search for GPS reradiator or pseudolite if you want to see what kind of trouble you could get into radiating in-band. We've had low-end simulators in the past the leaked, and it was very disruptive.
Search this for a fun story - "Jamming in Moss Landing Harbor, CA 15 Apr 01 – 22 May 01, VHF/UHF television antennawith pre-amplifier caused GPS failures to all of MossLanding Harbor"
I'm not sure why you want to build a GPS simulator though. There are three levels of complexity that I can think of (this just for L1 C/A codes too) (1) just generating a single PRN code with selectable doppler and data bits (although the data bits wouldn't mean anything) (2) Actually simulating a constellation of satellites in orbit, and generating the appropriate ephemeris (3) doing #2 but with the addition of delivering assistance data - look for UMTS at the Spirent site
For a different take on simulation see this GPS recorder/player - you could generate the signals off-line and just play them back - www.labsat.co.uk/ I think that there to be another similar based on an RFMD chip - maybe at Sparkfun?
For any of these you'll need a very accurate clock for starters - I would expect < 1 ppm error and I think that most people would lock their simulators to a more accurate clock. Here's a good clock - www.trimble.com/timing/thunderbolt-e.aspx?dtID=overview You really need to understand phase noise for this type of project too.
Depending upon what you would want to do the prop doesn't seem to make much sense. Maybe it could do some PRN generation for #1 above, but this is quite simple stuff. See the GPS ICD for the G1 and G2 LFSRs etcetera www.navcen.uscg.gov/pubs/gps/icd200/icd200cw1234.pdf
If you actually modulated this up to L1 then you would want to be very careful about leakage.
Old article:
www.gpsworld.com/lbs/indoor-positioning/indoor-gps-the-no-chip-challenge-1823
Search this for a fun story - "Jamming in Moss Landing Harbor, CA 15 Apr 01 – 22 May 01, VHF/UHF television antennawith pre-amplifier caused GPS failures to all of MossLanding Harbor"
In 2001, Jamming caused Missed Landings... All successful attempts are reported as "the works of Bin Laden".
I have a deep dark shadow of doubt about TV antennas jamming GPS. The preamp can't even jam channel 3
(or send a clear picture to your neighbor) if you hook it up backwards to your VCR.
GPS signals are so jam-proof that they all transmit on the same two frequencies at the same time and the receivers
need to decode 3 or 4 of them at the same time to work. The signals are weaker than static and yet the static and
interferences and the individual satellite signals are clearly unmixed in the receiver.
I'm shocked!
If they really got GPS reception inside a microwave oven that
is just amazing.
The signal would be incredibly attenuated.
Just consider that a microwave oven attenuates
a 1kw 2450mhz signal inside to such a low level that
it is safe to stand in front while it makes your popcorn
The GPS signal inside a closed microwave must be at least
25-30 db down.... to get more attenuation I guess you would have to
go deep underground.
I wish I could find a cheap USB wifi adapter that had that sort
of sensitivity!
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"Where am I? Where am I going? Why am I in a handbasket?"
www.gpsworld.com/gps/system-challenge/the-hunt-rfi-776
A autonomous receiver would need a minimum of 4 satellites to compute time and position.
I'll bet the the receivers in question were L1 only.
The GPS C/A PRN code is 1023 bits long and repeats every ms, and you integrate in order to get it out of the noise.
They have it directly connected to a receiver.
James
www.sparkfun.com/commerce/product_info.php?products_id=8238
I believe that book also has a software simulator in Appendix B.
Maybe this is a good business to be in?
I really doubt the attenuation is exactly the same in all directions
but I'm sure there is some upper limit of leakage in any direction
that cannot be exceeded if the oven is to be certified. 5mw per cm2
at a few inches if I remember correctly.
I would think that the attenuation is similar or even greater at the
lower freq than the higher one. Now if you went significantly higher
in freq then it seems like more signal would start to pass through
the small holes in the front screen and small gaps like the door seal.
I guess you could place a tiny oven inside a larger one to get more
attenuation.
Or make your own metal box and solder the door shut before a test.
I wonder if any signal would get in? I wonder if a powerful EMP pulse
could get in? Perhaps it would since it would seem that the metal surfaces
would re-radiate significant rf inside if they were hit by powerful enough
rf.... the space inside the box would sort of become the interior of a sealed
antenna element??
Maybe it would be important to ground the case also? Seems like the oven I
saw used to attenuate signals was left plugged in...perhaps just to ground the case.
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"Where am I? Where am I going? Why am I in a handbasket?"
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Timothy D. Swieter, E.I.
www.brilldea.com - Prop Blade, LED Painter, RGB LEDs, 3.0" LCD Composite video display, eProto for SunSPOT
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