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Appendix A. TeleGPS System Operation
A.1. 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.
A.2. 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
using TeleGPS; that process is described in Section 3.13, “Configure Device”.
AltOS supports both compressed and uncompressed APRS position report data formats. The
compressed format provides for higher position precision and shorter packets than the uncompressed
APRS format. We’ve found some older APRS receivers that do not handle the compressed format. The
Kenwood TH-72A requires the use of uncompressed format to display altitude information correctly.
The Yaesu FT1D requires the use of compressed format to display altitude information.
APRS packets include an SSID (Secondary Station Identifier) field that allows one operator to have
multiple transmitters. AltOS allows you to set this to a single digit from 0 to 9, allowing you to fly multiple
transmitters at the same time while keeping the identify of each one separate in the receiver. By default,
the SSID is set to the last digit of the device serial number.
The APRS packet format includes a comment field that can have arbitrary text in it. AltOS uses this to
send status information as shown in the following table.
Table A.1. Altus Metrum APRS Comments
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 primary battery
at 4.0V from device 1876.
L6 B4.0 1876
Make sure your primary battery is above 3.8V and 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.
