Sunday, May 15, 2016

Wisp1 Telemetry Revisited

Wisp1 Telemetry Revisited

So, in my previous blog post about telemetry, I outlined a WSPR telemetry scheme to maximize the amount of data packed into the few bits available.  Alan Adamson (W7QO) and I were both thinking of using it.  However, after some initial implementation, Alan realized it required a hefty bit of coding.  He counter-proposed a more "positional" concept.  We went back and forth for a day or two, and came up with the scheme below.  It's agreeable to both of us, so we each intend to use it on our trackers.  Since he did the heavy lifting with most of the design, I'm writing it up for the interwebz to enjoy.

First Packet

     KD2EAT FN12 37

The fields include Callsign, Grid locator, and a power level.  There are 19 discrete values permitted in in the power level field.

The first modification is to re-purpose the power level field in the WSPR packet as an "Altitude" indicator. Each of the 19 discrete values will represent 1000 meters of altitude.   The altitude mapping ends up looking like this:

DBMEncoded Altitude Meters
00
31,000
72,000
103,000
134,000
175,000
206,000
237,000
278,000
309,000
3310,000
3711,000
4012,000
4313,000
4714,000
5015,000
5316,000
5717,000
6018,000

So, the packet above indicates that the balloon is flying at, at least, 11,000 meters.  This provides altitude data in one packet with no additional modifications to the protocol, if desired.  It provides for altitudes from 0-18,000 meters (59,055 feet).

Second Packet

This is where the rubber meets the road.  We are encoding a lot of data in the callsign field, as well as the power field.  However, the grid square field is NOT used to carry telemetry data.  This means that, though the callsign will be bogus, the telemetry data will appear in the same grid square as the primary packet.  It is assumed that the same data will be used to generate BOTH the primary and secondary packet, since the altitude encoding in the primary packet is refined by the telemetry packet.

We'll illustrate the encoding scheme by decoding an example.

     QK1SKN FN12 33

The callsign consists of 6 positions with potential values as follows:

Position Possible Values Use in Scheme (number of values) Number of Used Values /
Number of Possible Values
Callsign 1 Q,0 Telemetry Channel (2) 2 / 2
Callsign 2 0-9,A-Z Battery volts (12), Altitude_fine(3) 36 / 36
Callsign 3 0-9Telemetry Channel (10) 10 / 10
Callsign 4 A-Z Grid Square 5th char (A-X) (24) 24 / 26
Callsign 5 A-Z Grid Square 6th char (A-X) (24) 24 / 26
Callsign 6 A-Z, space Temp (9), Altitude_super_fine(3) 27 / 27
Gridsquare 1 A-RSame as Packet 1 n/a
Gridsquare 2 A-R Same as Packet 1 n/a
Gridsquare 30-9 Same as Packet 1 n/a
Gridsquare 4 0-9 Same as Packet 1 n/a
DBM 0-18 Solar_volts(6), Sats(3) 18 / 19

Callsign Positions 1 & 3: Telemetry "Flight" or "Channel" number

Positions 1 & 3 allow for 2*10 = 20 possible values.  This allows for up to 20 pico flights to be in operation simultaneously without telemetry confusion, provided everyone cooperates and uses a unique pair of characters in those two positions.  In the example above, the two telemetry channel bytes are "Q" and "1". We can interpret a "Q" in position 1 as the 10's value, so this is "Flight Number 11" or "Channel 11".

Altitude encoding

Altitude is encoded across both the first and second WSPR packet.  The first WSPR packet gives us the altitude with 1 km of granularity, from 0-18,000 meters, as described above in the first packet.

The second packet, gives us two more levels of granularity:

Altitude_fine:  0, 333, 666.
Altitude_super_fine: 0, 111, 222

Balloon altitude is thus calculated by adding the three values together.  As we expand the telemetry below, we'll see in this example that:

Altitude_fine = 2, so we add 666 to the altitude.
Altitude_super_fine = 1, so we add another 111 to the altitude.

Actual Altitude = 11,000 (first packet) + 666 (second packet) + 111 (second packet) = 11,777 meters.

Callsign Position 2: Battery volts, and "fine" altitude

We encode volts and altitude fine as:

Volts Encoded Volts
3.0 or lower0
3.21
3.42
3.63
3.84
4.05
4.26
4.47
4.68
4.89
5.010
5.2 or higher11

Altitude Fine Encoded Altitude Fine
00
3331
6662

Having the two encoded values, we calculate the value for this "character" as follows:

value = (EncodedVolts * 3) + EncodedAltitudeFine.

Conversely, to get the Encoded values, you do the following:

EncodedVolts = value / 3
EncodedAltitudeFine = value % 3

The value of the second column is a letter 'A' - 'Z', or number '0' - '9'.

Encoded Value Letter
00
11
......
99
A10
B11
......
K20
......
Z36


In our example, we have the letter "K" in position 2, which represents "20".

EncodedVolts = 20 / 3 = 6.  So, our Volts = 4.2v.

EncodedAltitudeFine = 20 % 3 = 2.  So, our AltitudeFine is 666.  Added to our 11,000 meters from the first packet, our altitude is at least 11,666 feet.

Callsign Position 4&5: Grid Square 5&6.

This is straightforward.  We simply take these two characters, and append them to the grid square. 

     QK1SKN FN12 33

"SK" is added to our grid square "FN12".  By convention, the last two characters of the grid square are in lower case, so our 6-character grid square is "FN12sk".  

Callsign Position 6: Temperature and Altitude "super fine"

This position is permitted the characters 'A' - 'Z', or a space, making 27 total values.  We reserve 9 values for Temperature, and 3 values for "super fine" altitude, as follows.

Temperature (c) Encoded Temperature
-35 or less 0
-301
-252
-203
-154
-105
-56
07
5 or more8

Altitude Super Fine Encoded Altitude Super Fine
00
1111
2222

Having the two encoded values, we calculate the value for this "character" as follows:

value = (EncodedTemperature * 3) + EncodedAltitudeSuperFine.

Conversely, to get the Encoded values, you do the following:

EncodedTemperature = value / 3
EncodedAltitudeSuperFine = value % 3

The value of the second column is a letter 'A' - 'Z', or a space.
Encoded Value Letter
A0
B1
......
N13
......
Z25
(space)26


In our example, we have the letter "N" in position 6, which represents "13".

EncodedTemperature = 13 / 3 = 4.  So, our Temperature = -15c.

EncodedAltitudeSuperFine = 13 % 3 = 1.  So, our AltitudeSuperFine is 111.  Added to our 11,666 meters from above, our altitude is at least 11,777 feet.


DBM value: Solar Volts, Number of Satellites

The DBM field has 19 potential values, which we consider 0..18 (as in the Altitude table in the first packet above).  We use 18 of these 19 values.

We encode 6 values for Solar Voltage, and 3 values for Number of Satellites as follows:
Solar Volts Encoded Solar Volts
0.2 or lower0
0.41
0.62
0.83
1.04
1.2 or greater5

Number of Satellites Encoded Number of Satellites
0 or no fix0
4 - 71
8 or more2

Having the two encoded values, we calculate the value for this field as follows:

value = (EncodedSolarVolts * 3) + EncodedSatellites.

Conversely, to get the Encoded values, you do the following:

EncodedSolarVolts= value / 3
EncodedSatellites = value % 3

We follow the same DBM to Encoded Value scheme as the Altitude does in packet 1.

In our example, we have a DBM of 33, which represents a "value" of 10.

EncodedSolarVolts = 10 / 3 = 3.  Solar Volts = 0.8v.

Encoded Satellites = 10 % 3 = 1.  Satellites = 1.  4-7 satellites.


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