Loco Positioning Protocol

The Loco Positioning Protocol (LPP) defines the messages send on the Loco Positioning System UWB radio to configure and use the system. There is two main class of messages: the messages used for ranging and positioning and the messages used for management.

The ranging messages are sent and received directly by the ranging algorithms in the Crazyflie and in the Nodes. The management message are sent by management tools like the Crazyflie client.

Ranging protocols

All ranging packet share the same format:

byte	value	note
0	ID	Packet ID
1..	Payload	Format is defined per ID

Two Way Ranging

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 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;

Management protocols

LPP Short packets

The LPP short packets are management packet short enough to be sent using a single Crazyflie CRTP packet. This means that the payload part of an LPP packet should be smaller than 28 Bytes. This makes these packet also well suited to be transferred as extra data appended to a ranging packet (this is for example used for the anchor position packet).

LPP Short packet format:

byte	value	note
0	0xF0	LPP short
1	LPPS_ID	LPP Short Packet ID
2..17	Payload	Format is defined per ID

LPP Short packet IDs:

LPPS_ID	Description
0x01	Anchor position

0x01: Anchor position

Format:

struct lppShortAnchorPosition_t {
  float x;
  float y;
  float z;
} __attribute__((packed)); 

When sent using unicast to an anchor, the anchor will save the (x,y,z) position as its new position.

When appended to a ranging message, the tag uses the (x,y,z) position as the anchor position.