Something we've asked the AltusMetrum guys to implement is a base station
software update that allows you to ignore call signs other than your own.
On the ground with rockets being ground tested, rockets on the pads ready for
launch all squaking and trying to share just 10 available channels, it can be
tough to get a signal from your rocket even with a directional yagi antenna.
At Airfest, Balls etc you end up with multiple telemetry files written to your
computer and the screen swapping between different call signs. It doesn't keep
the altimeter from doing its functions, but isn't very helpful for tracking
lock on the ground. Once in the air the directional Yagi should improve your
chances of getting a steady fix on your rocket.
Ken and Bdale should be able to implement a software update to the ground
station software to ignore APRS call signs other than your own. That would
extend the usability of the Telemetrum/mega/GPS method to up to 10 birds in the
air instead of 10 birds at a launch. That hasn't been a priority, but if you
want it let them know.
I use the Telemetrum and Telemega, but also sometimes use a big red bee for the
reason Ken explained.
Eric Smedstad L3
Tripoli Houston
PS trying to stay dry down here..
Sent from my Phone
On Aug 25, 2017, at 10:42 PM, Stan Hemphill <stanley.hemphill@xxxxxxxxx>
wrote:
😲
why did I understand all that?
From: aeropac-bounce@xxxxxxxxxxxxx [mailto:aeropac-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Ken
Sent: Friday, August 25, 2017 9:41 PM
To: aeropac@xxxxxxxxxxxxx
Subject: [AeroPAC Chat] A Bit About Telemetry Radios - APRS and Altus Metrum
A bit of a tutorial then - please tune out if not interested. Hope this
helps.
APRS and the Altus Metrum radios are radio technologies for telemetry. They
transmit the location of the object being tracked to one or more ground
stations. They do not interoperate - that is one station does not directly
communicate to the other (with one exception - wait for it below). Both
operate in the 70 cm HAM band (~430 MHz) and require a HAM license to use.
APRS (Automatic Position Reporting System) is the elder technology - designed
by an engineer at the Naval Academy some 30 years ago. It uses a very
simple and slow modulation system (FSK at 1200 baud - kinda like AOL access
in the late 1980s) to broadcast a data packet containing the location (and
some other stuff) from the object being tracked. This can then be received
by any compatible ground station. It originally operated in the 144 MHz HAM
band but now also operates in the 70 cm band. It is widely used for high
altitude balloons, LEO satellites as well as rockets. A ground station is
an amateur radio, a modem to decode the packets, and a computer to display
the position. Position packets can be relayed to an an internet server for
display on a common web page to anyone in the Internet.
APRS uses a very old (50 years?) packet radio technology (ALOHA) that allows
different tracked objects to share the same frequency. Each object has a
specified radio frequency for transmission. At 1200 baud, each transmission
takes a bit less than 1 second. If different objects transmit on the same
frequencies at different times, multiple objects can share the same
frequency. APRS works best when the time between transmissions is (much
longer) than the transmission time (say position reports > 5 seconds). To
help keep multiple transmissions on the same frequency from overlapping (and
thus colliding and destroying each other), we can assign each transmitter to
a time slot. Different stations will (and do) sync their time (on about a 1
second boundary) via GPS. Let’s say we want 5 stations to share the same
frequency, each station transmitting once/5 seconds. We assign five
transmission time slots, synched via GPS. Slot one begins exactly at a
minute mark (for every transmitting station), slot two begins at a minute + 1
second, slot three at a minute + 2 seconds, etc. We assign each station to
transmit in its own time slot thus ensuring they do not interfere with each
other.
The Altus Metrum system is a proprietary radio telemetry system for
transmitting position (and other data) from the tracked object to a ground
station. It operates in the 70 cm radio band.
It (mostly) uses a proprietary protocol for its telemetry packets, encoded in
a more modern way than APRS, with a bit more efficiency. However, it does
like to transmit more data so it transmits it faster (e.g. default 38,200
baud or 32x APRS). Going faster means less radio energy per bit transmitted
and hence less range (in principle). While an APRS system at 100 mW could
likely be heard in LEO, an Altus Metrum system will go substantially less
(but still a lot farther than most rockets). Just to make it more complex,
the AM radio system does redundant coding of bits which in principle should
recover some energy/bit -
Because the default protocol is proprietary, it needs a proprietary ground
station (radio + modem) to receive and decode packets.
It is possible to extend the range of the 40 mW transmission power of the
Altus Metrum trackers (TeleGPS, TeleMetrum and TeleMega) by decreasing the
transmission speed (a 4x decrease in transmission speed should yield about a
2x increase in range), or using gain antennas on the ground.
To my belief, the AltusMetrum radio protocol does not allow multiple
transmitting stations to share the same frequency. Each must have its own
frequency distinct from other stations within range. There are no slots to
coordinate multiple radios on the same frequency.
The interoperability exception. There is an option in (some/all?) Altus
Metrum trackers to transmit APRS packets interleaved with proprietary Altus
Metrum packets. For example, every 5 seconds, the tracker will stop
transmitting Altus Metrum packets and send an APRS encoded packet and then
resume sending Altus Metrum packets. The APRS packet must be received by an
APRS compatible ground station. The Altus Metrum packets by an AM ground
station. This requires two ground stations to receive both the Altus Metrum
proprietary packets AND the APRS packets.
Now … I have not measured this (and in fact there is a lively friendly
disagreement between the Altus Metrum folks and me on this topic) but … the
fact that the lowest speed of Altus Metrum is 4x the speed (9600 baud) as the
1200 baud APRS packet should mean that the range of the APRS packet might be
2x (or more or less) the Altus Metrum packet. The AM folks asset this
compensated by the redundant error coding in the propriety packet format. I
remain a tad skeptical but happy if true.
We use both radios on the ARLISS Extreme and we configure our Altus Metrum
TeleMegas to transmit both APRS and proprietary packets. Just to make sure
something gets through. We’ve received Beeline 100 mW APRS packets from as
far as ~50 km line of sight.
As my rule of thumb … Beelines (and other APRS systems) use an older, less
efficient technology but likely go further, more reliably than more modern,
elegant systems like Altus Metrum which send more information.
Enjoy.
K
On Aug 25, 2017, at 3:18 PM, Jonathan DuBose (Redacted sender "jmdubose2000"
for DMARC) <dmarc-noreply@xxxxxxxxxxxxx> wrote:
Cliff,
You'd be surprised at what I don't know. Sounds like something I should
know.
So Ken, how does this work with the Altus Metrum technology?
I had lock on the BRB and Telemega at XPRS last year - all the way to 90k'
without knowing anything about "Channel / Slot / ID". Stupid lucky or
blissful ignorance?
J
From: Ken Biba <kenbiba@xxxxxx>
To: "aeropac@xxxxxxxxxxxxx" <aeropac@xxxxxxxxxxxxx>
Sent: Friday, August 25, 2017, 3:00:58 PM PDT
Subject: [AeroPAC Chat] Re: aeropac Digest V1 #8
Actually, slot comes from old ALOHA packet radio - and old TDMA terminology.
Permission to transmit is coordinated on 1 second boundaries - slots. APRS
packets < 1 second in duration. Slotting distributes packet transmission
over time in a coordinated manner and dramatically improves transmission
efficiency.
Good to do.
K
Ken Biba
Novarum, Inc.
415-577-5496
On Aug 25, 2017, at 2:52 PM, Cliff Sojourner <cls@xxxxxxxxxxxxx> wrote:
Jonathan, surprised you don't know this
Channel is the transmitter frequency
Slot is a big red bee thing, an option in transmitter software to share the
frequency by only transmitting on specific 6 second boundaries
ID is amateur call sign
On August 25, 2017 2:07:25 PM PDT, Jonathan DuBose
<dmarc-noreply@xxxxxxxxxxxxx> wrote:
Now that I look at the list again, I guess we should sign up for the days we
need the freq, n'est pas?
Also, can anyone explain what the "Channel / Slot / ID" is?
Jonathan
PS Greg Ruhf, did you get your TeleGPS talking to your radio?
From: Lowell Hart <lowellh@xxxxxxx>
To: "aeropac@xxxxxxxxxxxxx" <aeropac@xxxxxxxxxxxxx>
Sent: Friday, August 25, 2017, 1:59:03 PM PDT
Subject: [AeroPAC Chat] Re: aeropac Digest V1 #8
I added one today.
Lowell
Date: Thu, 24 Aug 2017 03:19:38 +0000 (UTC)
From: "Jonathan DuBose" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender
Subject: [AeroPAC Chat] Radio Tracking for XPRS
I have added a tab for 2017 and reserved a couple of frequencies for XPRS.
Jonathan
