Any civilian-usable GPS which does not use L1 (1575MHz)?

I don't think there is anything else.

I have a light aircraft with a combined L1/L2/IMMARSAT/THURAYA/IRIDIUM
rooftop antenna, but have not found anybody making an L2 GPS receiver
for the civilian market.

I think L2 can be used in a manner which supplements L1, for accurate
survey stuff, but I just want a 'box' which connects to an L2 antenna
and outputs straight NMEA.

I have never seen reference to anything like that. There would be a
major issue selling such a receiver to the civilian sector. All 31
healthy satellites send the L2 legacy military P code but it's
encrypted. The eight newest healthy satellites also send a modernized
military code plus the modernized civil code (L2C). L2C messages
aren't currently formatted with positioning data and eight satellites
cannot provide coverage.

--- CHAS

On 2011-04-24 08:05 , Peter wrote:

L2C - it is smack dab in the middle of L2 - should be available on one
satellite beginning in 2014 with more, of course, to follow. From there
your antenna will be usable with the appropriate receiver.

Novatel (and presumably others) make civil receivers that use L2.
http://www.novatel.com/products/gnss-receivers/oem-receiver-boards/oemv-receivers/##

Current L2 is encrypted (Y) so its use would be restricted to processes
akin to survey. (Codeless processing (phase)).

Presumably with a base station on the ground you could post process to
very accurate relative positions; or have a datalink to the aircraft and
a lot of processing to do same in real time. You could get relative
position accuracies of better than 1 - 10mm per km between the base
station and the airborne receiver. eg: at 100 km distance you could
have an error of 10 cm - 1 meter. Not shabby at all.

Frankly, pointless unless you have some engineering or scientific goal.

You would need to STC the aircraft installation, or at least the
temporary mounting racks and wiring (there's a special category of STC's
for "one of a kind", but I don't recall the number).

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That was my understanding...

What suprises me is that military receivers have not "escaped into the
wild". They must be widely used by troops etc. I recall reading a book
about the 1st war in Iraq and they had them in 1991, made by Magellan,
so this stuff is really very old. Maybe they quickly become useless.
How often are the keys changed?

Alternatively, is there some way to run off L1 and if L1 disappears
then one could run off L2, at least for say some minutes or tens of
minutes?

I ask this, as a private pilot, seeing various notams warning about
intentional GPS jamming. This is quite serious stuff, because GPS is
the only means of area nav. Here in the UK, we have poor VOR coverage
at low levels and we have to fly low to remain below various bits of
Class A. INS is obviously not feasible (FOGs are another military
technology, though big jets use them, and way too expensive), solid
state gyros are a couple of orders of magnitude not good enough (lots
and lots and lots of people are looking at this) and a backup would be
very handy because if you bust some controlled airspace you won't get
any credit for being unable to use GPS because they will just say you
should have been reading a map like our grandfathers did in WW1 and
WW2 and we beat the Germans on both occassions so it must be good....

On 2011-04-24 12:00 , Peter wrote:
http://www.novatel.com/products/gnss-receivers/oem-receiver-boards/oemv-receivers/##

Military receivers are not all that common and usually quite expensive.
Their distribution is quite controlled even though an unkeyed receiver
can't be used beyond C/A - and a keyed receiver falls back to CA after
less than 1 week (maybe up to 2 if the succeeding week's key was also
loaded - not sure if that's feasible).


I would bet that in the 1st Iraq war soldiers were equipped with
civilian receivers over military receivers at a ratio approaching
1000:1. Possibly higher. Mothers were mailing GPS receivers to their
sons. I don't recall Magellan making P/Y code receivers. At the time
of the 1st Iraq war, I'm not sure if the majority of satellites were
broadcasting Y code in any case.

Keys are distributed in 1 week chunks. I don't recall if the receivers
could hold more than 1 week of code (or current + next).


No. The C/A code contains a handover word with an index to the time
position in the P/Y code. This is because correlation search would be
very long over a 7 day long code. So the HOW tells processor what
position in the sequence the receiver should use to load the correlator.
Without the current key, of course, the receiver can't compute the
proper correlation code to load into the correlator. No tracking.

Survey receivers, on the other hand, track the carrier phase. With an
encrypted signal there is a trivial hardware solution (I don't recall
the details but something simple in hardware can 'rectify' (wrong term)
the signal so that its phase can be accurately tracked. With that, and
a reference station, the phase difference between the two can be very
accurately computed, and thence very accurate position differences
computed. Note the term: _position differences_. This is highly usable
for survey but impractical and expensive for navigation.

(I probably have muffed a lot of the detail above wrt survey receivers)


NDB's are usually better at low alt and you can use commercial AM
stations for reference as well. Not as easy to navigate as VOR, esp. in
cross track winds, but still a good orientation device.

The NOTAM's are over generous in the seriousness of the amount of test
jamming strength, coverage area and duration. eg: abundance of caution.
Don't give up your VOR/NDB skills and procedures just yet. (Is it me
or procedure turns are disappearing from most approaches?).


ILS was invented at the beginning of WW II and is still the basis for
almost all CAT II and all CAT III approaches. Of course you also need a
very good inertial integration for CAT III and beyond, not to mention
crew training.

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