rooftop antenna range

Given two vanilla wifi radios (say of the wrt54g class in tx power and
rx sensetivity) what sort of range can I expect for rooftop, line of
sight use with a pair of high-gain 15dbi omni's (eg. Hyperlink
HG2415U) ? They won't be obstructed, but the antennas won't really be
much above the roof-lines either.

Does anyone have any words of advice about rooftop wifi antenna
placement, anchoring, grounding etc? I'm lead to believe that 15dbi
8-degree omni's are a bit touchy on how they get mounted. The MIT
roofnet folks had pretty mixed reviews about using the narrower angle,
higher gain omni's saying that in practice they tended to need
straightening after strong winds. Before I found that info I was
planning to simply bolt the antenna to an existing steel antenna pole
that was mounted with two W-shaped gable mounts. I'm now wondering if
I need something more rigid.

-wolfgang
--
Wolfgang S. Rupprecht http://www.wsrcc.com/wolfgang/

On Tue, 30 May 2006 13:07:03 -0700, "Wolfgang S. Rupprecht"
<wolfgang+(E-Mail Removed) .wsrcc.com> wrote:

>Given two vanilla wifi radios

I prefer my radios chocolate flavored. If you insist on vanilla, at
least try chocolate chip flavored.

>(say of the wrt54g class in tx power and
>rx sensetivity) what sort of range can I expect for rooftop, line of
>sight use with a pair of high-gain 15dbi omni's (eg. Hyperlink
>HG2415U) ?

That's easy but depends on what speed you're expecting. The receive
sensitivity varies with connection speed. See the FAQ at:
http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations
The trick is to get 20dB of fade margin. Grinding the numbers for a
pair of vanilla flavored wireless bridge radios.

TX power = +15dBm
TX coax loss = 2dB (1ft LMR-240 plus a mess of connectors)
TX ant gain = +15dBi
Distance = unknown
RX ant gain = +15dBi
RX coax loss = 2dB (same at other end)
RX sens = -84dBm (at 12Mbits/sec)
Fade margin = 20dB

Plugging into:
http://www.terabeam.com/support/calculations/som.php
I conveniently get 1 mile range. You can go somewhat farther with
bigger antennas or slower speed. For example, if you go to 24dBi dish
antennas as both ends, the calcs show that you can go 8 miles with
everything else being the same. As a rule of thumb, 6dB gain is worth
double the range. 12dB gain is 4 times the range. Etc.

>They won't be obstructed, but the antennas won't really be
>much above the roof-lines either.

That's obstructed. You need *MORE* than line of sight at RF
frequencies. You need at least 0.8 times the Fresnel Zone diameter at
midpoint. See:
http://www.terabeam.com/support/calc...esnel-zone.php
For 1 mile, you need a radius of 19ft around the line of sight at
midpoint. For 8 miles, you need 53ft radius. Skimming the rooftops
might work if the roofs are wood or shingle. However, if there are
trees, masonry construction, or steel buildings, forget it.

>Does anyone have any words of advice about rooftop wifi antenna
>placement, anchoring, grounding etc?

Yes. However, it's difficult to give general advice. The basic
question is whether you can mount it on the building, on a bracket of
sorts, on a tubular pipe, or on a tower. Give me some idea of the
roof constuction and height involved and I can offer some hints.

>I'm lead to believe that 15dbi
>8-degree omni's are a bit touchy on how they get mounted.

Omni's suck for point to point for exactly that reason. The vertical
radiation angle is far too narrow. Use a dish, sector antenna, or
panel.

>The MIT
>roofnet folks had pretty mixed reviews about using the narrower angle,
>higher gain omni's saying that in practice they tended to need
>straightening after strong winds.

I think they were using the typical Antenna Specialists vertical that
has a right angle aluminum bracket to mount it. That's fine for VHF
and UHF antennas, but the requirements of 2.4GHz antennas to be
exactly vertical makes this bracket useless. Also, the typical
rooftop pole used for home mesh networks is not known for its
stability or precision.

>Before I found that info I was
>planning to simply bolt the antenna to an existing steel antenna pole
>that was mounted with two W-shaped gable mounts. I'm now wondering if
>I need something more rigid.

Those area actually fairly rigid for an omni. The problem is getting
the mounting pipe exactly vertical. There's no easy adjustments once
the bolts go into the side of the house. That leaves shims and pipe
benders.

The side mounts are also a real problem for big dish antennas. They're
just too heavy for such a mount as there's no vertical supports.

Gotta work... good luck.

>-wolfgang
--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 (E-Mail Removed)
# http://802.11junk.com (E-Mail Removed)
# http://www.LearnByDestroying.com AE6KS

On Tue, 30 May 2006 22:08:52 GMT, Jeff Liebermann
<(E-Mail Removed)> wrote:

>On Tue, 30 May 2006 13:07:03 -0700, "Wolfgang S. Rupprecht"
><wolfgang+(E-Mail Removed) m.wsrcc.com> wrote:

Duh. I thought the name sounded familiar. I've been using your web
page on DGPS over the Internet as reference material for some of my
DGPS experiments and projects. Thanks much.

You're going to have an additional problem with an omni antenna if
you're in Fremont CA. You'll get LOTS of interference from other
2.4GHz users. See:
http://wireless.wikia.com/wiki/Wi-Fi#Interference
for a list of potential interference sources. The problem with an
omni is that it picks up this junk from all directions. The more gain
the omni has, the more junk it picks up.

However, a directional (dish) antenna only picks up junk along the
line of sight. This can still be a problem, but is much less a
problem than the junk picked up by an omni.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 (E-Mail Removed)
# http://802.11junk.com (E-Mail Removed)
# http://www.LearnByDestroying.com AE6KS

Jeff Liebermann <(E-Mail Removed)> writes:
> On Tue, 30 May 2006 13:07:03 -0700, "Wolfgang S. Rupprecht"
> <wolfgang+(E-Mail Removed) .wsrcc.com> wrote:
>>Given two vanilla wifi radios
>
> I prefer my radios chocolate flavored. If you insist on vanilla, at
> least try chocolate chip flavored.

Me too. I was just going to talk about the vanilla though and
secretly add the chocolate when nobody was looking. ;-)

> That's easy but depends on what speed you're expecting.

Ah, I should have mentioned. I'm not going for any speed record.
1Mbs is fine. I'm just curious if I can expect to hit a rooftop omni
from Mission Peak 4.3 miles away or get at it from a Fremont's Central
Park 1.5 miles away.

> http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations

Great link. Thanks for all the information!

Ok, it looks like 6Mbps OFDM is roughly the same as 1Mbps BPSK.
Cranking it down to 1Mbps seems to buy me very little with that radio.

> The trick is to get 20dB of fade margin. Grinding the numbers for a
> pair of vanilla flavored wireless bridge radios.
>
> TX power = +15dBm
> TX coax loss = 2dB (1ft LMR-240 plus a mess of connectors)
> TX ant gain = +15dBi
> Distance = unknown
> RX ant gain = +15dBi
> RX coax loss = 2dB (same at other end)
> RX sens = -84dBm (at 12Mbits/sec)
> Fade margin = 20dB
>
> Plugging into:
> http://www.terabeam.com/support/calculations/som.php
> I conveniently get 1 mile range. You can go somewhat farther with
> bigger antennas or slower speed. For example, if you go to 24dBi dish
> antennas as both ends, the calcs show that you can go 8 miles with
> everything else being the same. As a rule of thumb, 6dB gain is worth
> double the range. 12dB gain is 4 times the range. Etc.

I wouldn't have guessed that one wants to shoot for a 20db margin.

The cards I was really lusting after were these:

http://www.ubnt.com/super_range_cardbus.php4

Receive Sensitivity

Rate Level
1 Mbps -96 dBm, +/- 2dB
2 Mbps -95 dBm, +/- 2dB
5.5 Mbps -91 dBm, +/- 2dB
11 Mbps -91 dBm, +/- 2dB
6 Mbps -93 dBm, +/- 2dB
9 Mbps -92 dBm, +/- 2dB
11 Mbps -91 dBm, +/- 2dB
12 Mbps -90 dBm, +/- 2dB
18 Mbps -89 dBm, +/- 2dB
24 Mbps -85 dBm, +/- 2dB
36 Mbps -82 dBm, +/- 2dB
48 Mbps -76 dBm, +/- 2dB
54 Mbps -73 dBm, +/- 2dB

Transmit Power 802.11b/g

Rate Level
1-24 Mbps 24 dBm, +/- 1dB
36 Mbps 23 dBm, +/- 1dB
48 Mbps 22 dBm, +/- 1dB
54 Mbps 20 dBm, +/- 1dB

It looks like I could buy myself another 12db on the RX side (-96dbm
vs -84dbm) and another 9db on the tx side (24db vs 15db). That 21 db
should be good for 3.5 doublings in distance - call it

(* 2.0 2.0 2.0 1.414) 11.312 ; ~11x further

> That's obstructed. You need *MORE* than line of sight at RF
> frequencies. You need at least 0.8 times the Fresnel Zone diameter at
> midpoint. See:
> http://www.terabeam.com/support/calc...esnel-zone.php
> For 1 mile, you need a radius of 19ft around the line of sight at
> midpoint. For 8 miles, you need 53ft radius. Skimming the rooftops
> might work if the roofs are wood or shingle. However, if there are
> trees, masonry construction, or steel buildings, forget it.

The houses are mostly stucco over chicken-wire and concrete roof tiles
painted to look like clay.

So it looks like the lower half-cylinder of the fresnel zone will get
clobbered. Is this equivalent to half the signal getting lost or
would it be worse? If it is only half the signal getting lost, what's
another 3 db among friends?

>>Does anyone have any words of advice about rooftop wifi antenna
>>placement, anchoring, grounding etc?
>
> Yes. However, it's difficult to give general advice. The basic
> question is whether you can mount it on the building, on a bracket of
> sorts, on a tubular pipe, or on a tower. Give me some idea of the
> roof constuction and height involved and I can offer some hints.

Concrete roof tiles, stucco building, wide eves with a 2x12 beam
running along the end of the roof.

I've already got a weather station mounted on an steel antenna pole
thats lag-bolted to the 2x12 near the apex using 2 of those bent steel
legs. I think it was the same RCA badged tv-antenna mounting brackets
as in the following kit.

http://www.sjgreatdeals.com/rcavh120x.html

Because of the way the legs are bent, there is considerable flex. I
have no doubt that I could mount it within a few degrees of correct
and then just gently bend it until it was perfect. Of course that is
what worries me a bit too.

> Omni's suck for point to point for exactly that reason. The vertical
> radiation angle is far too narrow. Use a dish, sector antenna, or
> panel.

I was actually going to use a lightweight 15dbi yagi for the laptop
antenna. Its only 18" long and weighs 3oz. I figured the weight and
size were ideal for laptop use.

http://www.mfjenterprises.com/produc...rodid=MFJ-1800

> Those area actually fairly rigid for an omni. The problem is getting
> the mounting pipe exactly vertical. There's no easy adjustments once
> the bolts go into the side of the house. That leaves shims and pipe
> benders.

Sounds like I should probably just use the 4ft pipe attached to my
brackets.

Thanks again for the great info.

-wolfgang
--
Wolfgang S. Rupprecht http://www.wsrcc.com/wolfgang/

On Tue, 30 May 2006 16:55:55 -0700, "Wolfgang S. Rupprecht"
<wolfgang+(E-Mail Removed) .wsrcc.com> wrote:

>Ah, I should have mentioned. I'm not going for any speed record.
>1Mbs is fine.

No, don't go to 802.11b speeds. The slowest 802.11g speed will give
you better sensitivity, range, and reliability. Stay with OFDM.

>I'm just curious if I can expect to hit a rooftop omni
>from Mission Peak 4.3 miles away or get at it from a Fremont's Central
>Park 1.5 miles away.

Both should be possible with a proper antenna. Note that my calcs
assume that the radio goes on the roof along with the antenna to
minimize coax cable loses.

Reading between the lines, it appears that both these locations have
existing wireless installations. However you didn't disclose the
hardware and antenna types and gains. I can't do a calculation
without these. Got photos? Got inside info?

If these locations (park or peak) have the usual 2ft long omni, it has
a gain of about 7dBi. No clue on the radios. Many of these public
access points are 802.11b only and are limited to 5.5Mbits/sec
connection speed. That will limit the range somewhat due to lousy
sensitivity.

>> http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations
>Great link. Thanks for all the information!

If you find any errors, please either let me know or just fix them
yourself. The FAQ is a user supported effort.

>Ok, it looks like 6Mbps OFDM is roughly the same as 1Mbps BPSK.
>Cranking it down to 1Mbps seems to buy me very little with that radio.

Yep. That's it. 1Mbit/sec is also a problem in that the packets are
flying through the air for a longer time to send the same amount of
data. That creates a higher probability that a noise hit or
interference blast is going eat a given packet. 1Mbit/sec is not
terribly reliable because of this. The problem is that with 802.11b,
*ALL* management packets are sent at 1Mbits/sec. That guarantees lots
of loss even if you manage to get an 11Mbit/sec connection. With
802.11g, I think (not sure) that the management packets are sent at
the connection speed.

>I wouldn't have guessed that one wants to shoot for a 20db margin.

Don't guess. That's my job.

Receiver sensitivity is measured at at a BER (bit error rate) of
either 1 bit in 10^5 or 10^6 (depending on standard). One error every
million bits isn't all that bad but you wouldn't want to operate
there. Below the BER reference of 10^5, and drop in signal results in
a rather drastic decrease in error rate. Looking at the chart on my
wall, a 3dB drop in signal level will reduce the error rate to about
10^3. (It varies with different modulation types and speeds). At 1
error every 1000 bits, you're guaranteed to have an error in just
about every packet. In other words, this is a threshold effect with
10^5 or 10^6 being at the knee of the threshold.

A 3dB drop in signal is really easy to produce. It can come from
water incursion, rain, blockage, fresnel zone diffraction,
reflections, and possibly the position of the moon. Please note that
my calculations are seriously simplified and offer the *BEST* case
model. It can only get worse as errors and losses are introduced. For
example, I have yet to see a 24dBi dish that I actually can test with
a resultant gain of 24dBi. Same with the +15 to +17dBm the typical
router is suppose to belch. It's more like +13dBm on my junk test
equipment. Everyone lies, but that's ok because nobody calculates.

>The cards I was really lusting after were these:
> http://www.ubnt.com/super_range_cardbus.php4

I don't have time to tear these apart. I've only played with one for
a few minutes. However, the trend for increased power output seems to
be epidemic. I don't like it because it creates an alligator (big
mouth, small ears) which transmits far further than it can hear.

>It looks like I could buy myself another 12db on the RX side (-96dbm
>vs -84dbm) and another 9db on the tx side (24db vs 15db). That 21 db
>should be good for 3.5 doublings in distance - call it
>
>(* 2.0 2.0 2.0 1.414) 11.312 ; ~11x further

That's only true if you have a symmetrical system, where you increase
the transmit power and sensitivity on BOTH ends of the link. If only
one side of the link has a big transmitter, then there's no guarantee
that it can hear the other side.

Incidentally, the RX specs look a bit too good. I can usually get 2dB
more sensitivity by simply tweaking the BER reference level. The
numbers are very close to the noise floor of the receiver. I gotta
grind some numbers to be sure. Let's just say I'm suspicious.

>The houses are mostly stucco over chicken-wire and concrete roof tiles
>painted to look like clay.

Concrete roof tiles are like a brick wall. Hell, they *ARE* a brick
wall. Nothing goes through them.

>So it looks like the lower half-cylinder of the fresnel zone will get
>clobbered. Is this equivalent to half the signal getting lost or
>would it be worse? If it is only half the signal getting lost, what's
>another 3 db among friends?

I wish it were that easy. Fresnel Zone is a knife edge diffraction
zone. The signal hits the edge of the obstruction and is diffracted
away from the line of sight target. In extreme cases, it will setup
reinforcement and cancellation interference patterns as in Newton's
rings.
http://en.wikipedia.org/wiki/Newton's_rings
The important point is that it's not stable. As things move around,
the destination signal strength varies radically. You might be lucky
and find a location with a reinforcement peak, only to find it a null
when someone moves their vehicle in a midpoint parking lot.

>Concrete roof tiles, stucco building, wide eves with a 2x12 beam
>running along the end of the roof.

Concrete and chicken wire supported stucco are like a wall. Nothing
goes through. You'll need clearance and altitude. Also, don't forget
about trees. I'm sure there are some in the area. If they're in the
way, they'll block the signal (about 10dB per "average" tree).

>I've already got a weather station mounted on an steel antenna pole
>thats lag-bolted to the 2x12 near the apex using 2 of those bent steel
>legs. I think it was the same RCA badged tv-antenna mounting brackets
>as in the following kit.
> http://www.sjgreatdeals.com/rcavh120x.html

Retch. That's even more flimsy than the Radio Shack flavor. The
problem is lack of vertical support. Also no adjustments.

>Because of the way the legs are bent, there is considerable flex.

That's an understatement. I used a cousin of this abomination to do a
temporary 24dBi dish mount to a large pipe. Of course, during the
week the wind decided to blow about 50knts. The mount literally
twisted loose from the wind load on the antenna. Dish mounts have to
be very rigid. You would do better with a satellite DBS dish mount.
http://www.fab-corp.com/home.php?cat=277
or maybe a replacement for your roof peak mount:
http://www.glenmartin.com/catalog/page14.html

>I
>have no doubt that I could mount it within a few degrees of correct
>and then just gently bend it until it was perfect. Of course that is
>what worries me a bit too.

If you can bend the mount, then it's not going to stay put.

>I was actually going to use a lightweight 15dbi yagi for the laptop
>antenna. Its only 18" long and weighs 3oz. I figured the weight and
>size were ideal for laptop use.
> http://www.mfjenterprises.com/produc...rodid=MFJ-1800

I don't like yagi's for 2.4GHz. Too expensive per dB of gain. Also
too narrow a radiation angle and they get huge for higher gains.
Double the length for only 3dB more gain. However, 15dBi for $30 is
rather cheap. Still, methinks a similar gain panel would be better.
How about 14dBi for $30?
http://www.fab-corp.com/product.php?...cat=255&page=1

>Sounds like I should probably just use the 4ft pipe attached to my
>brackets.

Something like that. It really depends on the size, weight, and wind
load of the antenna. Anything over about 10ft will require guy wires
(yet another complication).

Search Google for "wireless antenna mast".

>Thanks again for the great info.

Drivel: Of course you could do something interesting like:
http://www.boschaero.com/tower.htm
The movie clip at:
http://www.boschaero.com/movies/towers.wmv (5.3MB)
is worth watching. Just an idea...


--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 (E-Mail Removed)
# http://802.11junk.com (E-Mail Removed)
# http://www.LearnByDestroying.com AE6KS

Jeff Liebermann <(E-Mail Removed)> writes:
> Duh. I thought the name sounded familiar. I've been using your web
> page on DGPS over the Internet as reference material for some of my
> DGPS experiments and projects. Thanks much.

You're welcome! I'm glad my spare dgps radio went to good use. After
dragging it up many of the local mountains along with a 4.5 AH 12v
battery, I decided I needed to find a good home for it that didn't
involve me carrying large hunks of lead and sulfuric acid up and down
45-degree inclines.

> You're going to have an additional problem with an omni antenna if
> you're in Fremont CA. You'll get LOTS of interference from other
> 2.4GHz users. See:
> http://wireless.wikia.com/wiki/Wi-Fi#Interference
> for a list of potential interference sources. The problem with an
> omni is that it picks up this junk from all directions. The more gain
> the omni has, the more junk it picks up.

Hmm. I hadn't considered that.

After years without a laptop (after my last one walked out my back
door) I did finally get another one. One of the first field trips was
to drive around to two of the close by free Fremont hotspots to see
how usable they were. Simply running "iwlist wlan0 scan" in a loop as
I drove to the sites showed that there were tons of wifi transmitters
on every residential block. There were even a few hits on what I
could have sworn came from a rent-a-cop car with a ssid of
'something-or-other-security'.

My thought at the time was: this is great, look at all the folks that
might be interested in helping to form a mesh network. I see now I
was looking at it from the wrong side. All this chatter is going to
be a real problem.

> However, a directional (dish) antenna only picks up junk along the
> line of sight. This can still be a problem, but is much less a
> problem than the junk picked up by an omni.

I might need to do that. I don't mind the cost as much as the
neighborly karma points I burn. I figured a small omni is going to
raise a lot less ire than even a wire dish pointed at what looks like
some poor person's roof.

-wolfgang
--
Wolfgang S. Rupprecht http://www.wsrcc.com/wolfgang/

On Tue, 30 May 2006 23:09:12 -0700, "Wolfgang S. Rupprecht"
<wolfgang+(E-Mail Removed) .wsrcc.com> wrote:

>You're welcome! I'm glad my spare dgps radio went to good use. After
>dragging it up many of the local mountains along with a 4.5 AH 12v
>battery, I decided I needed to find a good home for it that didn't
>involve me carrying large hunks of lead and sulfuric acid up and down
>45-degree inclines.

Ah... dedication. I found some GPS boards that will belch raw data
and are suitable for DGPS. I'll be installing one at the local ham
radio repeater site shortly. I've been using the one at MBARI on Mt
Toro, but the baseline is too far from Santa Cruz and their antenna is
aimed over the bay, not the land. Yet another project.

>Hmm. I hadn't considered that.

Interference isn't fatal. However, substantial interference from
multiple systems is a big problem. Your best defense is to narrow the
antenna pattern to only pickup junk in one direction. That means no
omnis.

>There were even a few hits on what I
>could have sworn came from a rent-a-cop car with a ssid of
>'something-or-other-security'.

Say hello to Jason if you run into him. I forgot which security
company he works for in Fremont. I can provide contact info in you
wanna meet.

>My thought at the time was: this is great, look at all the folks that
>might be interested in helping to form a mesh network. I see now I
>was looking at it from the wrong side. All this chatter is going to
>be a real problem.

Mesh networks only make it worse. The problem with these is that
there are duplicate packets floating around. If a client is perhaps 4
hops away from the wired access point (where it hits the internet via
conventional backhauls), there will be 4 packets flying through the
air to deliver just one packet. That leaves 1/4th the airtime
available for other users. I can rant on about the evils of mesh
networks but not now. It's 11:30PM and I'm still in my palatial
office.

>I might need to do that. I don't mind the cost as much as the
>neighborly karma points I burn. I figured a small omni is going to
>raise a lot less ire than even a wire dish pointed at what looks like
>some poor person's roof.

That is a problem. The tin foil hat crowd seems to be everywhere. If
you can live with less gain, a panel antenna will yield 19dBi of gain
and not attract the lunatic fringe.


--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 (E-Mail Removed)
# http://802.11junk.com (E-Mail Removed)
# http://www.LearnByDestroying.com AE6KS

Jeff Liebermann <(E-Mail Removed)> wrote in
news:(E-Mail Removed):

>
> Ah... dedication. I found some GPS boards that will belch raw data
> and are suitable for DGPS. I'll be installing one at the local ham
> radio repeater site shortly. I've been using the one at MBARI on Mt
> Toro, but the baseline is too far from Santa Cruz and their antenna is
> aimed over the bay, not the land. Yet another project.

Hey Jeff,

GPS boards that belch raw data ? Is that individual satellite info vs.
ascii GPS strings ? Interesting. I didn't really think DGPS was really
necessary any more.

I used to work for GLB Electronics, since your a ham, you may have heard
of them. (The owner designed the first synthesized radio gear for hams
back in the 70's, the GLB Channelizer.)

Anyway, before the GPS 'error' was turned off, people doing DGPS was
GLB's bread and butter providing narrow-band data link's for the
differential signal.

Regards,

DanS

DanS <(E-Mail Removed)> hath wroth:

>Hey Jeff,

Huh? I guess that's me. 14 hour work day today. Dinner at midnight.
Brain is gone. I guess I'm ready to answer questions.

>GPS boards that belch raw data ? Is that individual satellite info vs.
>ascii GPS strings ? Interesting.

Disclaimer: I are not an expert in GPS.

To do DGPS, you need a GPS that will belch individual satellite
doppler delays (also known as raw data), and not NEMA-183 ASCII text.
DGPS corrects on a per satellite basis and needs the raw data to do
the corrections.

>I didn't really think DGPS was really
>necessary any more.

GPS accuracy is a subject of considerable contention. There are a
bunch of web sites that offer test results for various technologies.
One of many:
http://users.erols.com/dlwilson/gps.htm
My guess, opinion, and experience is:
Uncorrected GPS with clear view: 5 to 15 meters (1 sigma)
Uncorrected GPS with lousy view: 10 to 30 meters
GPS with WAAS: 2 to 7 meters
DGPS with 50 mile baseline: 2 to 5 meters
DGPS with 1 mile baseline: 0.1 to 2 meters
If you want cm accuracy, you gotta have a local DGPS transmitter. One
big problem is that the final accuracy is totally dependent on the
accuray of the DGPS antenna location. cm accuracy is worthless if the
location of the correcting GPS is not located with the same precision.
So, I get to do some precision surveying.

>I used to work for GLB Electronics, since your a ham, you may have heard
>of them. (The owner designed the first synthesized radio gear for hams
>back in the 70's, the GLB Channelizer.)

I plead ignorance. I was doing lots of commercial radio in the
1970's. I was also out of the country much of the early 1970's.
To keep my sanity, I completely ignored ham radio until about 1993.
I even intentionally let my ham license expire.

>Anyway, before the GPS 'error' was turned off, people doing DGPS was
>GLB's bread and butter providing narrow-band data link's for the
>differential signal.

It's a common service. However, there are now several free and open
DGPS systems on the air in the Monterey Bay area. There's also free
post mortem corrections available on the internet. I have some
applications in mind which I don't wanna disclose.


--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Wed, 31 May 2006 16:27:23 -0500, DanS
<(E-Mail Removed)> wrote:

>GPS boards that belch raw data ?

"Overview of low-cost GPS receivers which output raw data."
http://home-2.worldonline.nl/~samsvl/oemtable.htm
I have a pile of Allstar 12 receivers and a few Superstar 12
receivers.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 (E-Mail Removed)
# http://802.11junk.com (E-Mail Removed)
# http://www.LearnByDestroying.com AE6KS