lotus

previous page: 44  Can I actually get traffic conditions over the Internet?
  
page up: California Driving (and Surviving) FAQ
  
next page: 46 But aren't most citizens prohibited from using mobile radio scanners?

45 What is a "CHiPs detector"? What's the complete story on CHP radios?




Description

This article is from the California Driving (and Surviving) FAQ, by "George J Wu" georgewu@netcom.com with numerous contributions by others.

45 What is a "CHiPs detector"? What's the complete story on CHP radios?

from Chucko@charon.arc.nasa.gov (Chuck Fry):

That's right. Many CHP cars are equipped with repeaters so that when the
officers get out of their cars, their walkie-talkies need only reach the car
instead of the base station. The CHiPs Detector (tm) takes advantage of the
fact that the CHiPpies rarely turn off the repeater when they're IN the car.
Note that the CHP may change this frequency at any time, although they're
not likely to.

The disadvantages are that you just know at least one CHiPpie is in the
area, not how close, what they're doing, or whether they're after you; and
there's no signal transmitted from the repeater when the base station is
silent. So it's hardly foolproof.

morris@grian.cps.altadena.ca.us (Mike Morris) posted on 12 oct 1991:

The following info was compiled from several sources, none of which have
1st-hand knowledge of the new CHP radios, but what I have been able to put
together seems to agree. So with that caveat, ...

The older Motorola Micor mobile radios had "mobile extenders" by GE. These
extenders were 1/4 watt transmitting units that repeated the audio from the
42mhz CHP mobile radio to 154.905mhz. The mobile extender time-sliced the
channel to transmit for roughly 9/10 second and receive for 1/10 of a second
to see if the officer was replying. Hence the "yakyakyak-chuff-yakyakyak-
chuff-yakyakyak-chuff-..." sound of the repeated traffic on the 154mhz
channel. There was a writeup of the single-channel time-slicing technique
in a ham radio magazine back in the early 70s, and the technique has taken
off tremendously. It has mostly been used to allow single-channel
radio-to-telephone interconnects called simplex autopatches (because they
use one channel - a "simplex" channel.)

Anyway the mobile extender technique works very well, and allows the officer
to use a relatively low power high-band hand-held to communicate with the
dispatcher via the > 100w low-band mobile radio in his/her patrol car with
very little trouble.

A low-band hand-held would have to use a 6' antenna to be resonant, or a
"rubber duckie" over a foot long. And the hand-held couldn't have enough RF
power to reach the dispatch center in 99% of the state. Hence a 150mhz
handheld (where a 18" antenna is the norm, and a "rubber duck" is < 9") and
a mobile extender.

A bit of history:
The bid for mobile radios was won by Motorola around 10 years ago. The
mobile extenders were an afterthought, and that bid/contract was won by GE.
The user interface was a simple on/off switch, and the state radio shop
people mounted it in the Motorola control head. It was a toggle switch
labeled with a Dymo tape "repeat enable/disable" (or "extender on/off" or
"portable on/off").

The average officer soon discovered that leaving the extender switch in the
"on" position worked just fine. They turned off the hand-held to shut off
the "repeater", not realizing that the mobile side of the extender was still
on. Probably 99% of the CHP officers left it on for the entire shift. With
the almost constant activity on the CHP dispatch channels, this 154.905mhz
vehicle transmitter behaved like a 1/4w beacon, providing between 1/4 to 1
mile notification of the location of a patrol car.

Now the spoiler: The CHP is replacing (has replaced here in my area) _all_
of their Motorola Micor/GE extender radio packages. The new radios are all
GE, with CHP-designed control groups. (The state Red Cross got 90% of the
radios for the 47.42 - 47.62 freqs. A few of them went to other state
agencies, like the Office of Emergency Services). The 1991 Southern
California edition of the "Police Call" frequency listing has a nice writeup
on the CHP-designed control groups, as I remember. They even got 90% of it
right.

The new design forces the officers to disable the extender when they are in
the patrol car. Listening to 154.905 while mobile now just tells you where
a CHP car is _stopped_, with the officer out of the car, as opposed to
before when it would tell you where a stopped or a moving one was...

Here is the frequency map of the CHP hand-helds as I have it.

F1: 154.905 with the primary tone. (NOTE 1)
F2: same 1st alternate tone
F3: same 2nd alternate tone
F4: 154.920 (CLEMARS 1) - Base side of CLEMARS
F5: 154.935 (CLEMARS 2) - Mobile/Portable CLEMARS
F6: 156.075 (CALCORD) (NOTE 2)
F7: 155.475 (CLEMARS 3 / NALEMARS) (NOTE 3)

Abbreviations: CLEMARS: California Law Enforcement Mutual Radio System.
CALCORD: California Coordination - a statewide "on-scene"
channel
NALEMARS: National Law Enforcement.... A federal version
of CLEMARS.

Note 1: With the old hand-helds (2 freqs - 154.905 and 154.920) there was no
way that two units from different areas (i.e. different dispatch
frequencies) could have their extenders operational at an out-of-vehicle
scene -- when an officer transmitted, both mobiles would be brought up.

the remainder of Note 1 explanation is from the post of scotto@ipars.cts.com
(Scott O'Connell) on 14 oct 1991:

The receiver of the extender has an attenuator making a low wattage HT
usable for only a short distance (typically less than 50yds). To make sure
there is only one extender being used within close proximity each vehicle
extender sends a short burst tone to see if others are active. If it is
within range of another active extender it doesn't turn on at all. The HT
is then using the other vehicle radio (the one that was already turned on).

Now for the PL explanation. There are three channels on CHP HT's that
relate directly to the extender. Channel 1 (also called PP or Person to
Person) does not transmit any tone nor does it decode. It is meant for HT
to HT use. Channel 2 has a subaudible tone on transmit allowing the officer
to talk to dispatch. (ie, transmits on the input freq of the lowband radio)
Channel 3 has a different subaudible tone on transmit allowing the officer
to talk to other officers. (ie, transmits on the output of the lowband
radio) All channels are carrier squelch on the receive so that PP can be
heard regardless of other traffic.

I hope this clears up why there are three 154.905 channels on the HT's.

Note 2: 156.075 is also the Ship TX side of Marine channel 61 (paired with
160.675 Ship RX). I understand some re-thinking of the use of this
frequency is going on. It seems to be pretty useless in coastal areas.

Note 3: 155.475 I have been told that this channel has multiple PL tones. I
have also been told that the CHP handheld is 10 freq - capable. Maybe this
channel has multiple appearances like F1-F2-F3. More info is needed.

Another rehash of the low band channels is in the works since LA County
Sheriffs is moving to 800 or 900 mhz. The CHP has acquired all of the 39mhz
LASO channels and is slowly moving to change all of the low band dispatch
operation to full repeat. My sources do not know if the mobiles will be
transmitting on 39mhz and listening on 42mhz or vice versa. It does not
make much difference to the GE mobiles since they cover the full 30-50 mhz
just fine (as opposed to the old Micors that covered 42mhz to 50mhz only.
Does anybody have any info?


 

Continue to:













TOP
previous page: 44  Can I actually get traffic conditions over the Internet?
  
page up: California Driving (and Surviving) FAQ
  
next page: 46 But aren't most citizens prohibited from using mobile radio scanners?