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:

  • CONFIG_NRF_CLOUD_AGPS

    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 and CONFIG_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 the nrf_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:

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:

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

Header file: include/net/nrf_cloud_pgps.h
Source files: 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.

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.

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.

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.

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().

Public Members

char *host

User-provided buffer to hold download host name

size_t host_sz

Size of user-provided host buffer

char *path

User-provided buffer to hold download path/file name

size_t path_sz

Size of user-provided path buffer

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.