UPRA GND.RF69x Modem PC ICD

UPRA-GND.RF69x Modem

The GND transceiver hardware is based on the UHF flight COM hardware (COM.RF69x) expanded with a USB-UART interface.

GND.RF69x Modem

The firmware is modified so the hardware acts as a UHF modem. The GND.RF69x forwards the received radio packets on serial port to the PC. For the most basic operation the modem can be hooked up with a PC and a terminal software to read the radio packets.

As additional features the modem can receive NMEA commands on serial port to fine tune the frequency and send Telecommand packets to the balloon.

Communication Protocol

PC-Modem Interface

The physical layer of the PC-Modem communication is serial (UART) port. The modem has an FTDI USB-UART converter which establish communication on a virtual COM port.

Default UART configuration

Parameter	Value
BaudRate	57600
Data bits	8
Stop bits	1
Parity	none
Flow control	none

Radio Packets

Incoming radio packets are forwarded directly to the PC via serial port.

Telemetry Packet

The Telemetry Packet is a fix 61 byte long UKHAS telemetry packet without checksum. Data segments in the packet are fixed length and also terminated by [,] (0x2C) character.

$$CCCCCCC,iii,hhmmss,(+/-)xxxx.xxx,(+/-)xxxxx.xxx,aaaaa,eeee,ooo,rrr,

Segment	Description
$$	START bytes
CCCCCC	callsign
iii	Message ID*
hhmmss	GPS time (UTC) (hh-hours, mm-minutes, ss-seconds)
(+/-)xxxx.xxx	latitude (NMEA format)
(+/-)xxxxx.xxx	longitude (NMEA format)
aaaaa	altitude (m)
eeee	external temperature (*10 °C -> eee.e°C)
ooo	OBC module temperature (*10 °C -> ooo°C)
rrr	COM module temperature (*10 °C -> rrr°C)

* Message ID shows the last three digits of the sent message and increments with every new message. After COM restart the Message ID set back to 0.

House-keeping Packet (TBD)

The Telemetry Packet is a fix 61 byte long custom packet. Data segments in the packet are terminated by [,] (0x2C) character.

$$CCCCCCC,iii,hhmmss,...TBD...

Segment	Description
$$	START bytes
CCCCCC	callsign
iii	Message ID*
hhmmss	GPS time (UTC) (hh-hours, mm-minutes, ss-seconds)
TBD	TBD

* Message ID shows the last three digits of the sent message and increments with every new message. After COM restart the Message ID set back to 0.

Data Packet (TBD)

TBD

Modem Control

The modem communicates with the PC via NMEA protocol.

All transmitted data are printable ASCII characters between 0x20 (space) to 0x7e (~)
Data characters are all the above characters less the reserved characters (See next line)
Reserved characters are used by NMEA0183 for the following uses:

ASCII	Hex	Dec	Use
[CR]	0x0d	13	Carriage return
[LF]	0x0a	10	Line feed, end delimiter
!	0x21	33	Start of encapsulation sentence delimiter
$	0x24	36	Start delimiter
*	0x2a	42	Checksum delimiter
,	0x2c	44	Field delimiter
\	0x5c	92	TAG block delimiter
^	0x5e	94	Code delimiter for HEX representation of ISO/IEC 8859-1 (ASCII) characters
~	0x7e	126	Reserved

Messages have a maximum length of 82 characters, including the $ or ! starting character and the ending
The start character for each message can be either a $ (For conventional field delimited messages) or ! (for messages that have special encapsulation in them)
The next five characters identify the talker (two characters) and the type of message (three characters).
All data fields that follow are comma-delimited.
Where data is unavailable, the corresponding field remains blank (it contains no character before the next delimiter ).
The first character that immediately follows the last data field character is an asterisk, but it is only included if a checksum is supplied.
The asterisk is immediately followed by a checksum represented as a two-digit hexadecimal number. The checksum is the bitwise exclusive OR of ASCII codes of all characters between the $ and *. According to the official specification, the checksum is optional for most data sentences, but is compulsory for RMA, RMB, and RMC (among others).
[CR][LF] ends the message.

As an example, a waypoint arrival alarm has the form:

$GPAAM,A,A,0.10,N,WPTNME*32

Another example for AIS messages is:

!AIVDM,1,1,,A,14eG;o@034o8sd<L9i:a;WF>062D,0*7D

Source: Wikipedia

Message Types

Message	Description
GRHKR	House-keeping packet request
GRSFQ	Set radio frequency
GRACK	ACK message sent by the modem

GRHKR - House-keeping request

If GRHKR message received on serial port the modem sends a house-keeping request package to the balloon. If the packet sent by the RF TCVR the modem respond with a GRACK message to the PC.

Request: $GRHKR,S,*17

Response: $GRACK,S,*2B

GRSFQ - Set frequency request

If GRSFQ message received on serial port the modem sets its RF frequency and transmits a dummy packet for testing. If the packet sent by the RF TCVR the modem respond with a GRACK message to the PC.

Request: $GRSFQ,ffffff,*cc

Response: $GRACK,F,*3E

Segment	Description
$	Start byte
GRFSQ	NMEA Header
ffffff	RF Frequency in [kHz]
*cc	Checksum*

*optional

GRACK - ACK message

Acknowledge message sent by the modem, if handshake is needed

$GRACK,a,*cc

Segment	Description
$	Start byte
GRACK	NMEA Header
a	Acknowledge ID*
*cc	Checksum**

**optional

*Acknowledge ID:

S - Radio Message Sent
F - GND Frequency Set

Checksum

The checksum at the end of each sentence is the XOR of all of the bytes in the sentence, excluding the initial dollar sign.

C Code:

#include <stdio.h>
#include <string.h>

int checksum(const char *s) 
{
    int c = 0;

    while(*s)
        c ^= *s++;

    return c;
}

int main()
{
    char mystring[] = "GPRMC,092751.000,A,5321.6802,N,00630.3371,W,0.06,31.66,280511,,,A";

    printf("String: %s\nChecksum: 0x%02X\n", mystring, checksum(mystring));

    return 0;
}