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[appendix]
== TeleGPS System Operation
=== GFSK Telemetry
TeleGPS's native telemetry system doesn't use a
'normal packet radio' mode like APRS because it's not
very efficient. The GFSK modulation we use is FSK
with the base-band pulses passed through a Gaussian
filter before they go into the modulator to limit the
transmitted bandwidth. When combined with forward
error correction and interleaving, this allows us to
have a very robust 19.2 kilobit data link with only
10-40 milliwatts of transmit power, a whip antenna in
the rocket, and a hand-held Yagi on the ground. We've
had flights to above 21k feet AGL with great
reception, and calculations suggest we should be good
to well over 40k feet AGL with a 5-element yagi on the
ground with our 10mW units and over 100k feet AGL with
the 40mW devices.
=== APRS
TeleGPS can send APRS if desired, and the interval
between APRS packets can be configured. As each APRS
packet takes a full second to transmit, we recommend
an interval of at least 5 seconds to avoid consuming
too much battery power or radio channel bandwidth. You
can configure the APRS interval; that
process is described in the Configure TeleGPS
section of the TeleGPS Application chapter.
AltOS uses the APRS compressed position report data
format, which provides for higher position precision
and shorter packets than the original APRS format. It
also includes altitude data, which is invaluable when
tracking rockets. We haven't found a receiver which
doesn't handle compressed positions, but it's just
possible that you have one, so if you have an older
device that can receive the raw packets but isn't
displaying position information, it's possible that
this is the cause.
The APRS packet format includes a comment field that
can have arbitrary text in it. AltOS uses this to send
status information about the flight computer. It sends
four fields as shown in the following table.
.TeleGPS APRS Comments
[options="header",cols="1,1,3"]
|====
|Field |Example |Description
|1
|L
|GPS Status U for unlocked, L for locked
|2
|6
|Number of Satellites in View
|3
|B4.0
|Altimeter Battery Voltage
|4
|1286
|Device Serial Number
|====
Here's an example of an APRS comment showing GPS lock with 6
satellites in view and a battery at 4.0V from device 1286.
....
L6 B4.0 1286
....
Make sure your battery is above 3.8V GPS is locked
with at least 5 or 6 satellites in view before
flying. If GPS is switching between L and U regularly,
then it doesn't have a good lock and you should wait
until it becomes stable.
If the GPS receiver loses lock, the APRS data
transmitted will contain the last position for which
GPS lock was available. You can tell that this has
happened by noticing that the GPS status character
switches from 'L' to 'U'. Before GPS has locked, APRS
will transmit zero for latitude, longitude and
altitude.
=== Configurable Parameters
Configuring TeleGPS is very
simple; the few configurable parameters can all be set
using the TeleGPS application over USB. Read
the Configure TeleGPS section in the TeleGPS Software chapter below
for more information.
==== Radio Frequency
Altus Metrum boards support radio frequencies in the 70cm
band. By default, the configuration interface provides a
list of 10 “standard” frequencies in 100kHz channels starting at
434.550MHz. However, the firmware supports use of
any 50kHz multiple within the 70cm band. At any given
launch, we highly recommend coordinating when and by whom each
frequency will be used to avoid interference. And of course, both
TeleGPS and the receiver must be configured to the same
frequency to successfully communicate with each other.
==== Callsign
This sets the callsign used for telemetry and APRS to
identify the device.
==== Telemetry/RDF/APRS Enable
You can completely disable the radio, if necessary, leaving
TeleGPS only logging data to internal memory.
==== APRS Interval
This selects how often APRS packets are transmitted. Set
this to zero to disable APRS without also disabling the
regular telemetry and RDF transmissions. As APRS takes a
full second to transmit a single position report, we
recommend sending packets no more than once every 5 seconds.
==== Maximum Flight Log
Changing this value will set the maximum amount of flight
log storage that an individual flight will use. The
available storage is divided into as many flights of the
specified size as can fit in the available space. You can
download and erase individual flight logs. If you fill up
the available storage, future flights will not get logged
until you erase some of the stored ones.
==== Logging Trigger Motion
If TeleGPS moves less than this distance over a long period
of time, it will not log that location, saving storage space.
==== Position Reporting Interval
This sets how often TeleGPS reports position information via
telemetry and to the on-board log. Reducing this value will
save power and logging memory consumption.
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