I've devised my own scheme, documented below, which is heavily based on the one used by VE3KCL, which he documented heavily in his S-4 flight blog. My wisp1 telemetry makes several modifications. I'll document them below.
My pico balloon data will be encoded into two WSPR transmissions, which is fairly standard for picos. As a refresher, a typical WSPR transmission might look something like this:
KD2EAT FN12 27
The fields include Callsign, Grid locator, and a power level. There are 19 discrete values permitted in in the power level field.
Primary packet
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:
| DBM | Meters |
|---|---|
| 0 | 0 |
| 3 | 1,000 |
| 7 | 2,000 |
| 10 | 3,000 |
| 13 | 4,000 |
| 17 | 5,000 |
| 20 | 6,000 |
| 23 | 7,000 |
| 27 | 8,000 |
| 30 | 9,000 |
| 33 | 10,000 |
| 37 | 11,000 |
| 40 | 12,000 |
| 43 | 13,000 |
| 47 | 14,000 |
| 50 | 15,000 |
| 53 | 16,000 |
| 57 | 17,000 |
| 60 | 18,000 |
So, the packet above indicates that the balloon is flying at 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).
Secondary 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, in this wisp1 version of telemetry, the grid square 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.
Before going into the telemetry available, let's look at the amount of data we can encode.
The callsign consists of 6 positions with potential values as follows:
| Position | Possible Values | Use in Wisp1 | Number of Values |
|---|---|---|---|
| 1 | 0,Q | Telemetry Flag | 2 |
| 2 | A-Z,0-9 | Data | 36 |
| 3 | 0-9 | Telemetry Flag | 10 |
| 4 | A-Z | Data | 26 |
| 5 | A-Z | Data | 26 |
| 6 | A-Z, space | Data | 27 |
Positions 1 & 3 allow for 2*10 = 20 possible values. This allows for up to 20 pico flights to be in operation simultaneously without colliding, provided everyone cooperates and uses a unique pair of characters in those two positions.
Positions 2,4,5,6 allow for 36*26*26*27 = 657,072 possible values.
The "Power" field accounts for another 19 possible values, 19 * 657,072 = 12,484,368 possible telemetry values.
Within those possible values, I encode the following parameters:
Positions 2,4,5,6 allow for 36*26*26*27 = 657,072 possible values.
The "Power" field accounts for another 19 possible values, 19 * 657,072 = 12,484,368 possible telemetry values.
Within those possible values, I encode the following parameters:
| Telemetry | Possible Values | Granularity | Number of Values |
|---|---|---|---|
| Grid Char 5 | A-X | One Character | 24 |
| Grid Char 6 | A-X | One Character | 24 |
| Altitude Granular | 0-666 | 333 meters | 3 |
| Temperature | -45c - 5c | 5c | 11 |
| Lipo Battery | 3.2v - 4.8v | 0.2v | 9 |
| Solar Cell | 0 - 1.2v | 0.2v | 7 |
| Satellites | 0-9 | 1 | 10 |
This scheme allows for 24*24*3*11*9*7*10 = 11,975,040 needed values, which comfortably fits into the 12,484,368 available.
Calculating the value (example)
- Actual telemetry:
- Callsign: KD2EAT
- Grid: FN12MX
- Altitude: 8,500m
- Temperature: -21c
- Lipo: 4.35v
- Solar: 0.8v
- Satellites: 6
- Pico tag letters "0" and "9"
- The two WSPR packets for this data would be:
- KD2EAT FN12 27
- 0S9SBU FN12 17
How we got Packet 1
- Packet 1 is standard WSPR except the DBM field. 27 DBM encodes the value "8", for "8,000 meters" (See the table above). So we know the balloon is between 8000-9000 meters. We'll get more granularity from packet 2.
How we got Packet 2
- Each telemetry value is converted to an integer. Ex:
- Grid Char 5: "M" = 12
- Grid Char 6: "X" = 23
- Altitude Granular: 8,500 (8000 was in packet 1. 500 remains). Rounds to 666. = 2
- values: (0=0, 333=1, 666=2)
- Temperature: -21c. Rounds to -20c. = 5
- (-45=0, -40=1, -35=2, -30=3, -25=4, -20=5, etc)
- Lipo: 4.35v. Rounds to 4.4v = 6
- (3.2=0, 3.4=1, 3.6=2, 3.8=3, 4.0=4, 4.2=5, 4.4=6, 4.6=7, 4.8=8)
- Solar: 0.89v. Rounds to 0.8v. = 4
- (0.0=0, 0.2=1, 0.4=2, 0.6=3, 0.8=4, etc)
- Satellites: 6 = 6
- Making the big number
- Big number =
- num = grid5
- num = num * 24 + grid6
- num = num * 3 + altitude
- num = num * 11 + temperature
- num = num * 9 + lipo
- num = num * 7 + solar
- num = num * 10 + satellites
- For this example, the telemetry would be:
- 12
- 12 * 24 + 23= 311
- 311 * 3 + 2 = 935
- 935 * 11 + 5 = 10,290
- 10,290 * 9 + 6 = 92,616
- 92,616 * 7 + 4 = 648,316
- 648,316 * 10 + 6 = 6,483,166
- 6,483,166
Making WSPR from the telemetry value
- WSPR char 1: "0" or "Q". Hard coded.
- WSPR char 3: "0" - "9". Hard Coded.
- Remaining WSPR Characters, as follows:
- Num = telemetry number as above.
- DBM = num % 19; num = num / 19 // Values 0,3,7,10,13,17,20,23,27,30,33,37
- WSPR6 = num % 27; num = num / 27 // Values a-z,space
- WSPR5 = num % 26; num = num / 26 // Values a-z
- WSPR4 = num % 26; num = num / 26 // Values a-z
- WSPR2 = num (should be 0-35) // Values a-z,0-9
WSPR encoding our telemetry above
- Telemetry value: 6,483,166
- DBM = 6,483,166 % 19 = 5 = "17 DBM"
- WSPR6 = 341,219 % 27 = 20 = "U"
- WSPR5 = 12,637 % 26 = 1 = "B"
- WSPR4 = 486 % 26 = 18 = ""S"
- WSPR2 = 18 = "S"
- Assuming we chose our hard=coded ID characters as "0" and "9"
- Assuming Grid is FN12
- WSPR string:
- 0S9SBU FN12 17
Checking our work
- Decoding the WSPR
- Starting value: 0S9SBU FN12 17
- Pico ID: 09
- Grid: FN12
- WSPR2 = "S" = 18; num = 18
- WSPR4 = "S" = 18; num = 18 * 26 + 18 = 486
- WSPR5 = "B" = 1; num = 486 * 26 + 1 = 12,637
- WSPR6 = "U" = 20; num = 12,637 * 27 + 20 = 341,219
- DBM = "17" = 5; num = 341,219 * 19 + 5 = 6,483,166
- Converting to telemetry
- 6,483,166 % 10 = 6; num = 6,483,166 /10; Sats = 6 (6 sats)
- 648,316 % 7 = 4; num = 648,316 / 7; Solar = 4 (0.8v)
- 92,616 % 9 = 6; num = 92,616 / 9; Lipo = 6 (4.4v)
- 10,290 % 11 = 5; num = num / 11; Temp = 5 (-20c)
- 935 % 3 = 2; num = num / 3; Alt Granular = 2 (666m)
- 311% 24 = 23; num = num / 24; Grid6 = 23 = "X"
- 12; Grid 5 = 12 = "M"
- Any temperature warmer than 10c, will be sent as 10c.
- Any temperature colder than -45c will be sent as -45c.
- Battery and Solar voltages are assuming LiPo battery, and solar cells wired in parallel, not series, to keep the voltage low.
- Satellites of "0" will mean NO GPS lock. Satellites of 9 will represent 9 or more.
Differences from VE3KCL's scheme
- The Grid locator is not overlayed, so that the map view on wsprnet.org will show telemetry beside the balloon.
- I use the option of a space in the 6th character of the callsign to squeeze a little more data in.
- I encode altitude data in the first WSPR packet.
- I encode data differently in the second packet.
Next steps
I'd like a few fellow pico/WSPR folks to review this document and provide feedback.
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