The two way ranging protocol allows a tag to measure its distance to an
anchor. The protocol contains 4 messages, two are sent by the tag and two by
the anchor. The message sequence is as follow: 
The time of flight is calculated using the formula:
TWR packet IDs
| ID | Description |
|---|---|
| 0x01 | TWR_POLL |
| 0x02 | TWR_ANSWER |
| 0x03 | TWR_FINAL |
| 0x04 | TWR_REPORT |
TWR_POLL
Format:
| Payload byte | Name | Note |
|---|---|---|
| 0 | SEQ | 1 Byte sequence number, is repeated in subsequent packet of the same TWR transaction |
TWR_ANSWER
Format:
| Payload byte | Name | Note |
|---|---|---|
| 0 | SEQ | 1 Byte sequence number, matches the POLL sequence number |
| 1.. | LPP management message | Optional LPP management data |
The optional management data can be any LPP management packet deemed useful for the tag. Currently this is used to send the anchor position from the anchor to the tag using the LPP Short anchor position packet
TWR_FINAL
Format:
| Payload byte | Name | Note |
|---|---|---|
| 0 | SEQ | 1 Byte sequence number, matches the POLL sequence number |
TWR_REPORT
Format:
| Payload byte | Name | Note |
|---|---|---|
| 0 | SEQ | 1 Byte sequence number, matches the POLL sequence number |
| 1.. | Report | Report structure, see format bellow |
Report structure format:
typedef struct {
uint8_t pollRx[5]; // R1 timestamp
uint8_t answerTx[5]; // T2 timestamp
uint8_t finalRx[5]; // R3 timestamp
float pressure; // Pressure measurement at the anchor
float temperature; // Temperature of the pressure sensor
float asl; // Above Sea Level altitude
uint8_t pressure_ok; // Not 0 if the pressure information is correct
} __attribute__((packed)) lpsTwrTagReportPayload_t;