nRF Cloud P-GPS
The nRF Cloud P-GPS library enables applications to request and process Predicted GPS (P-GPS) data from nRF Cloud to be used with the nRF9160 SiP. This library is an enhancement to the nRF Cloud library. It can be used with or without Assisted GPS (A-GPS) data from nRF Cloud.
Overview
To get a position fix, a Global Navigation Satellite System (GNSS) receiver needs information such as the satellite orbital data, exact date and time of the day, and accurate hardware clock frequency data. GPS satellites broadcast this information in a pattern, which repeats every 12.5 minutes.
Predicted GPS (P-GPS) is a form of assistance that reduces the Time to First Fix (TTFF), the time needed by a GNSS module to estimate its position. It is provided through nRF Cloud services. In P-GPS, nRF Cloud provides data containing information about the estimated orbits (Ephemerides) of the 32 GPS satellites for up to two weeks. Each set of ephemerides predictions is valid for a specific four-hour period within the set of all provided predictions. A device using P-GPS downloads the ephemeris predictions from the cloud, it stores them in its flash memory, and later it injects them into the GNSS module when needed.
P-GPS is designed for devices that are frequently disconnected from the cloud but need periodic GNSS fixes as quickly as possible to save power. This is possible because a device can download the broadcasted information and predictions of satellite data provided through P-GPS (or also A-GPS) at a faster rate from nRF Cloud than from the data links of the satellites. However, P-GPS should not be used for general use cases that already work with Assisted GPS (A-GPS) only.
Note
When using two-week ephemeris prediction sets, the TTFF towards the end of the second week will increase due to the accumulated errors in the predictions and the decrease in the number of satellite ephemerides in the later prediction periods.
P-GPS requires a cloud connection approximately once a week, depending on the configuration settings. A-GPS requires a cloud connection each time.
A device can use P-GPS together with A-GPS. This provides the following advantages:
It shortens TTFF compared to using only P-GPS.
It requires less cloud data during each fix compared to using only A-GPS.
With proper configuration, A-GPS can be used with P-GPS when a cloud connection is available, and it can acquire fast fixes even without a cloud connection. This is possible as long as the stored P-GPS data is still valid, and the current date and time (accurate to a few seconds) and the most recent location (accurate to a few dozen kilometers) are known.
Note
To use the nRF Cloud P-GPS service, an nRF Cloud account is needed, and the device needs to be associated with a user’s account.
Configuration
Configure the following options to enable or disable the use of this library:
Configure these additional options to refine the behavior of P-GPS:
Configure the following option if you need your application to also use A-GPS, for coarse time and position data and to get the fastest TTFF:
-
Note
Disable this option if you do not want to use A-GPS (due to data costs, low power requirements, or expected frequent loss of cloud connectivity).
You must also configure the following options for storing settings, for having accurate clock time, and for having a location to store predictions:
The P-GPS library requires a storage location in the flash memory where to store the P-GPS prediction data. There are three ways to define this storage location:
To use a dedicated partition, enable the
CONFIG_NRF_CLOUD_PGPS_STORAGE_PARTITION
option.To use the MCUboot secondary partition as storage, enable the
CONFIG_NRF_CLOUD_PGPS_STORAGE_MCUBOOT_SECONDARY
option.Use this option if the flash memory for your application is too full to use a dedicated partition, but the application uses MCUboot for FOTA updates but not for MCUboot itself. Do not use this option if you are using MCUboot as a second-stage upgradable bootloader and also have FOTA updates enabled for MCUboot itself, and not just the application (using
CONFIG_SECURE_BOOT
andCONFIG_BUILD_S1_VARIANT
). The P-GPS library will otherwise prevent the MCUboot update from fully completing, and the first-stage immutable bootloader will revert MCUboot to its previous image.To use an application-specific storage, enable the
CONFIG_NRF_CLOUD_PGPS_STORAGE_CUSTOM
option. You must also pass the address and the size of your custom location in the flash memory to thenrf_cloud_pgps_init()
function.Note
The address must be aligned to a flash page boundary, and the size must be equal to or greater than 2048 bytes times the
CONFIG_NRF_CLOUD_PGPS_NUM_PREDICTIONS
option.Use this third option if you do not use MCUboot and if you want complete control over where to store P-GPS data in the flash memory.
See Configuring your application for information on how to change configuration options.
Initialization
Ideally, once the device has connected to the cloud, the application must call the P-GPS initialization function.
If a connection is not available, initialization must still be called.
If the CONFIG_NRF_CLOUD_PGPS_REQUEST_UPON_INIT
option is disabled, the initialization function does not automatically download missing P-GPS data.
In these cases, predictions might be unavailable until a connection is established to the cloud.
Note
Each prediction requires 2 KB of flash. For prediction periods of 240 minutes (four hours), and with 42 predictions per week, the flash requirement adds up to 84 KB.
The P-GPS subsystem’s nrf_cloud_pgps_init()
function takes a pointer to a nrf_cloud_pgps_init_param
structure.
The structure must specify, if CONFIG_NRF_CLOUD_PGPS_STORAGE_CUSTOM
is enabled, the storage base address and the storage size in the flash memory where the P-GPS subsystem stores predictions.
It can optionally pass a pointer to a pgps_event_handler_t()
callback function.
Note
The storage base address must be aligned to a flash memory page boundary.
Time
The proper operation of the P-GPS subsystem depends on an accurate sense of time. For use cases where a cloud connection can be established easily, use the Date-Time library with NTP enabled. Otherwise, a battery-backed real-time clock calendar chip must be used so that accurate time is available regardless of cloud availability after reset.
Requesting and processing P-GPS data
P-GPS data can be requested from the cloud using one of the following methods:
Directly:
If
CONFIG_NRF_CLOUD_MQTT
is enabled:
Call the function
nrf_cloud_pgps_request_all()
to request a full set of predictions.Pass a properly initialized
gps_pgps_request
structure to thenrf_cloud_pgps_request()
function.
If
CONFIG_NRF_CLOUD_REST
is enabled:
Pass a properly initialized
nrf_cloud_rest_pgps_request
structure to thenrf_cloud_rest_pgps_data_get()
function.Pass the response to the
nrf_cloud_pgps_process()
function.If either call fails, call the
nrf_cloud_pgps_request_reset()
function.
Indirectly:
If
CONFIG_NRF_CLOUD_MQTT
is enabled:
Call
nrf_cloud_pgps_init()
, with no valid predictions present in flash, or with some or all of the predictions expired.
If
CONFIG_NRF_CLOUD_REST
is enabled:
The indirect methods are used in the nRF9160: Asset Tracker v2 application. They are simpler to use than the direct methods. The direct method is used in the nRF9160: GNSS sample.
When nRF Cloud responds with the requested P-GPS data, the application’s cloud_evt_handler_t()
function must call the nrf_cloud_pgps_process()
function when it receives the CLOUD_EVT_DATA_RECEIVED
event.
The function parses the data and stores it.
Finding a prediction and injecting to modem
A P-GPS prediction for the current date and time can be retrieved using one of the following methods:
Directly, by calling the function
nrf_cloud_pgps_find_prediction()
Indirectly, by calling the function
nrf_cloud_pgps_notify_prediction()
The indirect method is used in the nRF9160: GNSS sample and in the nRF9160: Asset Tracker v2 application.
The application can inject the data contained in the prediction to the GNSS module in the modem by calling the nrf_cloud_pgps_inject()
function.
This must be done when event NRF_MODEM_GNSS_EVT_AGPS_REQ
is received from the GNSS interface.
After injecting the prediction, call the nrf_cloud_pgps_preemptive_updates()
function to update the prediction set as needed.
A prediction is also automatically injected to the modem every four hours whenever the current prediction expires and the next one begins (if the next one is available in flash).
Interaction with the GNSS interface
The P-GPS subsystem, like several other nRF Cloud subsystems, is event driven.
Following are the two GNSS events relating to P-GPS that an application receives through the GNSS interface:
NRF_MODEM_GNSS_EVT_AGPS_REQ
- Occurs when the GNSS module requires assistance data.NRF_MODEM_GNSS_EVT_FIX
- Occurs once a fix is attained.
When the application receives the NRF_MODEM_GNSS_EVT_AGPS_REQ
event, it must call nrf_cloud_pgps_notify_prediction()
.
This event results in the call back of the application’s pgps_event_handler_t()
function when a valid P-GPS prediction set is available.
It will pass the PGPS_EVT_AVAILABLE
event and a pointer to nrf_cloud_pgps_prediction
to the handler.
The application must pass this prediction to nrf_cloud_pgps_inject()
, along with either the nrf_modem_gnss_agps_data_frame
read from the GNSS interface after the NRF_MODEM_GNSS_EVT_AGPS_REQ
event or NULL.
If the use case for the application is such that the device will not move distances greater than a few dozen kilometers before it gets a new GNSS fix, it can pass the latitude and longitude read after the NRF_MODEM_GNSS_EVT_FIX
event to nrf_cloud_pgps_set_location()
.
The P-GPS subsystem will use this stored location for the next GNSS request for position assistance when A-GPS assistance is not enabled or is unavailable.
If the use case involves possible long-distance travel between fix attempts, such a mechanism can be detrimental to short TTFF, as the saved position might be too inaccurate to be a benefit.
The application can also call nrf_cloud_pgps_preemptive_updates()
to discard expired predictions and replace them with newer ones, prior to the expiration of the entire set of predictions.
This can be useful for customer use cases where cloud connections are available infrequently.
The CONFIG_NRF_CLOUD_PGPS_REPLACEMENT_THRESHOLD
sets the minimum number of valid predictions remaining before such an update occurs.
For best performance, applications can call the P-GPS functions mentioned in this section from workqueue handlers rather than directly from various callback functions.
The P-GPS subsystem itself generates events that can be passed to a registered callback function.
See nrf_cloud_pgps_event_type
.
API documentation
include/net/nrf_cloud_pgps.h
subsys/net/lib/nrf_cloud/src/
- group nrf_cloud_pgps
Defines
-
NRF_CLOUD_PGPS_NUM_SV
-
ETIMEUNKNOWN
P-GPS error code: current time unknown.
-
ELOADING
P-GPS error code: not found but loading in progress.
-
NRF_CLOUD_PGPS_EMPTY_EPHEM_HEALTH
Value to mark the ephemeris as unavailable for satellites for which no predictions are available from the cloud.
Typedefs
-
typedef void (*pgps_event_handler_t)(struct nrf_cloud_pgps_event *event)
Event handler registered with the module to handle asynchronous events from the module.
- Param event
[in] The event that just occurred.
Enums
-
enum nrf_cloud_pgps_event_type
P-GPS event passed to the registered pgps_event_handler.
Values:
-
enumerator PGPS_EVT_INIT
P-GPS initialization beginning.
-
enumerator PGPS_EVT_UNAVAILABLE
There are currently no P-GPS predictions available.
-
enumerator PGPS_EVT_LOADING
P-GPS predictions are being loaded from the cloud.
-
enumerator PGPS_EVT_AVAILABLE
A P-GPS prediction is available now for the current date and time.
-
enumerator PGPS_EVT_READY
All P-GPS predictions are available.
-
enumerator PGPS_EVT_REQUEST
A P-GPS request has been created for missing predictions. The event has payload in the form of gps_pgps_request. The event is intended to be used when CONFIG_NRF_CLOUD_PGPS_TRANSPORT_MQTT is disabled to let the application decide when and how to use the request information. This event type is not received if CONFIG_NRF_CLOUD_PGPS_TRANSPORT_MQTT is enabled.
-
enumerator PGPS_EVT_INIT
Functions
-
void nrf_cloud_pgps_set_location_normalized(int32_t latitude, int32_t longitude)
Update storage of the most recent known location, in modem-specific normalized format (int32_t). Current time is also stored. Normalization: (latitude / 90.0) * (1 << 23) (longitude / 360.0) * (1 << 24)
- Parameters
latitude – Current latitude normalized.
longitude – Current longitude in normalized.
-
void nrf_cloud_pgps_set_location(double latitude, double longitude)
Update the storage of the most recent known location in degrees. This will be injected along with the current time and relevant predicted ephemerides to the GPS unit in order to get the fastest possible fix, when the P-GPS subsystem is built with A-GPS disabled, or when A-GPS data is unavailable due to lack of a cloud connection. Current time is also stored.
- Parameters
latitude – Current latitude in degrees.
longitude – Current longitude in degrees.
-
void nrf_cloud_pgps_clear_location(void)
If location has previously been set, clear it. The application should do this if significant distances might have been travelled since the previous location was saved.
-
void nrf_cloud_pgps_set_leap_seconds(int leap_seconds)
Update the storage of the leap second offset between GPS time and UTC. This called automatically by the A-GPS subsystem (if enabled) when it receives a UTC assistance element, setting leap_seconds to the delta_tls field.
- Parameters
leap_seconds – Offset in seconds.
-
int nrf_cloud_pgps_notify_prediction(void)
Schedule a callback when prediction for current time is available. Callback could be immediate, if data already stored in Flash, or later, after loading from the cloud.
- Returns
0 if scheduled successfully, or negative error code if could not send request to cloud.
-
int nrf_cloud_pgps_find_prediction(struct nrf_cloud_pgps_prediction **prediction)
Tries to find an appropriate GPS prediction for the current time.
- Parameters
prediction – Pointer to a pointer to a prediction; the pointer at this pointer will be modified to point to the prediction if the return value is 0. Will be set to NULL on failure.
- Returns
0..NumPredictions-1 if successful; -ETIMEUNKNOWN if current date and time not known; -ETIMEDOUT if all predictions stored are expired; -EINVAL if prediction for the current time is invalid.
-
int nrf_cloud_pgps_process(const char *buf, size_t buf_len)
Processes binary P-GPS data received from nRF Cloud over MQTT or REST.
- Parameters
buf – Pointer to data received from nRF Cloud.
buf_len – Buffer size of data to be processed.
- Return values
0 – A-GPS data successfully processed.
-EFAULT – An nRF Cloud P-GPS error code was processed.
- Returns
A negative value indicates an error.
-
int nrf_cloud_pgps_inject(struct nrf_cloud_pgps_prediction *p, const struct nrf_modem_gnss_agps_data_frame *request)
Injects binary P-GPS data to the modem. If request is NULL, it is assumed that only ephemerides assistance should be injected.
- Parameters
p – Pointer to a prediction.
request – Which assistance elements the modem needs. May be NULL.
- Returns
0 if successful, otherwise a (negative) error code.
-
bool nrf_cloud_pgps_loading(void)
Find out if P-GPS update is in progress.
- Returns
True if request sent but loading not yet completed.
-
int nrf_cloud_pgps_preemptive_updates(void)
Download more predictions if less than CONFIG_NRF_CLOUD_PGPS_REPLACEMENT_THRESHOLD predictions remain which are still valid.
- Returns
0 if successful, otherwise a (negative) error code.
-
int nrf_cloud_pgps_init(struct nrf_cloud_pgps_init_param *param)
Initialize P-GPS subsystem. Validates what is stored, then requests any missing predictions, or full set if expired or missing. When successful, it is ready to provide valid ephemeris predictions.
Note
It must return successfully before using P-GPS services.
- Parameters
param – [in] Initialization parameters.
- Returns
0 if valid or request begun; nonzero on error.
-
struct nrf_cloud_pgps_system_time
- #include <nrf_cloud_pgps.h>
nrf_cloud_pgps_system_time is a special version of nrf_cloud_agps_system_time that does not include the full array of sv_tow values; this is transferred from the cloud as part of each prediction, to indicate the date_day and time_full_s for the center of each prediction’s validity period.
Public Members
-
uint16_t date_day
Number of days since GPS time began on 6 Jan 1980 for the center of a prediction’s validity period.
-
uint32_t time_full_s
Seconds into GPS day for center of validity period.
-
uint16_t time_frac_ms
Will be 0.
-
uint32_t sv_mask
Will be 0.
-
uint32_t pad
Placeholder where sv_tow[32] is for A-GPS.
-
uint16_t date_day
-
struct nrf_cloud_pgps_prediction
- #include <nrf_cloud_pgps.h>
P-GPS prediction Flash storage format.
Public Members
-
uint8_t time_type
Set to NRF_CLOUD_AGPS_GPS_SYSTEM_CLOCK.
-
uint16_t time_count
Will be 1.
-
struct nrf_cloud_pgps_system_time time
Information about when this prediction is applicable.
-
uint8_t schema_version
Not from cloud; inserted during storage to ease reusing A-GPS code.
-
uint8_t ephemeris_type
Set to NRF_CLOUD_AGPS_EPHEMERIDES.
-
uint16_t ephemeris_count
Usually will be NRF_CLOUD_PGPS_NUM_SV.
-
struct nrf_cloud_agps_ephemeris ephemerii[(32U)]
Array of satellite orbital equation coefficients.
-
uint32_t sentinel
Not from cloud; appended during storage to verify integrity on retrieval.
-
uint8_t time_type
-
struct gps_pgps_request
- #include <nrf_cloud_pgps.h>
P-GPS request type.
Public Members
-
uint16_t prediction_count
Number of predictions desired.
-
uint16_t prediction_period_min
Validity time per prediction, in minutes. Valid range 120 to 480.
-
uint16_t gps_day
Number of days since GPS time began on 6 Jan 1980 for the center of the first prediction desired.
-
uint32_t gps_time_of_day
Number of seconds since the start of this GPS day for the center of the first prediction desired. Valid range 0 to 86399.
-
uint16_t prediction_count
-
struct nrf_cloud_pgps_result
- #include <nrf_cloud_pgps.h>
nRF Cloud Predicted GPS (P-GPS) result; the location of the P-GPS data file which is to be downloaded and provided to nrf_cloud_pgps_process().
-
struct nrf_cloud_pgps_event
- #include <nrf_cloud_pgps.h>
-
struct nrf_cloud_pgps_init_param
- #include <nrf_cloud_pgps.h>
Initialization parameters for the module.
Public Members
-
pgps_event_handler_t event_handler
Event handler that is registered with the module.
-
uint32_t storage_base
Flash storage address. Must be on a Flash page boundary.
-
uint32_t storage_size
Flash storage size. Must be a multiple of a Flash page in size, in bytes.
-
pgps_event_handler_t event_handler
-
NRF_CLOUD_PGPS_NUM_SV