Emlid Reach RS2 GNSS Receiver – Too Good to Be True? Interview with a Professional Surveyor & GNSS Expert

GNSS receivers are one of the most vital tools at a surveyor’s disposal. They speed up surveys and provide much-needed accuracy to the data. So as a firm that conducts many surveys, we can never really have enough GNSS receivers, but this posed a challenge— most industry-leading GNSS receivers are pretty expensive. The price of these receivers can be rationalized, they do provide accuracy up to the mm, but when you’re doing land parcel surveys, earthwork etc., you don’t really need this high accuracy. To address this, the minds at Emlid came out with the Emlid Reach RS2, a multi-band RTK GNSS receiver.

To give a little intro to Emlid Reach RS2– they are known for their affordable GNSS modules and receivers that support RTK and PPK. The Reach receivers work right out of the box and are a valuable addition to both experienced and beginner surveyors.

The Reach RS 2 from Emlid takes GNSS receivers to their core, making them functionally simpler, easier to use, and integration painless. So let’s get down to the basic fact— the RS2 is placed at an extremely competitive price point; the question is— does that price come at a cost? We enlisted the help of one of our in-house experts Muhammad W. Pramujati (aka Jati), to find out.

Right Jati, let’s slowly dig into it— what do you think are the advantages of the Emlid Reach RS2?

It’s super simple to use, almost 1/10th the price of some competing GNSS receivers, and it is a lot simpler to use. But I’d say my personal favorite is the interface with the receiver with a phone application. With other receivers, you need a controller to set up, change settings and configure, and oftentimes, the controller may end up giving you trouble. So it’s just one less thing to carry and one less issue to deal with.

Okay, and how do these advantages translate as benefits?

We need a lot of receivers, but our capabilities may be limited by the number available to us, and that’s an issue when the receiver is expensive. When you have a more accessible solution like the RS2, you have the budget to support your capabilities and even expand them fully. The RS2 has an endurance of 16 hours with its built-in batteries, which boosts your capacity. The RTK function from Emlid unlocks the potential and efficiency of surveying applications and improves surveys’ time efficiency.

Are there any limitations to the Emlid Reach RS2?

Of course, as with any practical tool or software, the Emlid Reach RS 2 does have it’s limitations. The radio range is pretty limited; in my testing, I found that when using the LoRa radio, I couldn’t effectively go further than 1.8km— However, you can receive GNSS data over the internet, so this didn’t really impede anything. You can achieve a centimeter-level accuracy, when you compare it to the more expensive options, which can go down to a millimeter-level accuracy; this is a limitation. Though it’s pretty rare that you’d need accuracy going down to the millimeter, the Emlid Reach RS2’s output is more than sufficient for most jobs.

Below we can find Jati’s report on his tests with Emlid Reach RS2. He gets pretty technical and in-depth so make sure to read it to get a comprehensive understanding of how the RS2 held up in our field test.

Emlid Reach RS2 Performance Assessment-

1. Rapid Static VS RTK Measurement

Reach RS2 is a dual-band GNSS receiver released by EMLID that is capable of capturing numerous GNSS constellations; GPS, GLONASS, Beidou, GALILEO, QZSS. It is a reliable receiver to conduct surveying, mapping or navigation task. Furthermore, Real-Time Kinematic (RTK) mode is made available in this device through UHF radio communication and 3.5G modem for internet connectivity. Thus, static and RTK positioning modes could be performed using Reach RS2. In RTK mode, differential correction is sent through the RCTM message from the reference station and allows the single receiver user to obtain centimetre accuracy in a short amount of time [1-2]. There are two ways of doing RTK with Reach RS2, through LoRa (radio) Communication or Networked Transport of RTCM via Internet Protocol (NTRIP) caster.

In this document, we compared the positioning performance of the Reach RS2 using two different modes, rapid-static and RTK.

2. METHODS

The reason why rapid-static and RTK were employed is that these two modes are the most time-efficient and accurate ways to measure ground control points specifically for aerial survey. Rapid-static gives flexibility to the user, they could measure any point without any geographic constraint while the RTK enables the user to achieve the same level of accuracy as rapid-static in a shorter amount of time. At a glance, RTK is more appealing to the user. However, there are also certain limitations applied to this mode. Due to distance dependant bias (satellite orbit, atmosphere and ionosphere bias), RTK is more efficient when the reference station and rover are close to each other [3]. When the rover is too far apart, the atmosphere, ionosphere, and multipath distortion between the reference station and the rover are no longer the same.

To assess the performance, repeated measurements were taken on the benchmark points with known coordinates. Thus we could compare the accuracy of the measured coordinate against the known coordinate. On the single test point, 20 observations were made. Each rapid static observation took seven minutes long while the RTK took 30 seconds.

There are two RTK communication options available in Reach RS2 to send the correction from the reference station, through LoRa radio and NTRIP Caster. LoRa transmits carrier-phase and pseudorange data over the radio to the rover [4]. In this scenario, both the reference station and the rover has to be operated within radio range. Emlid provides a service that acts as a pipeline to relay correction data over the internet. Both the reference station and the rover has to be connected to the internet in order to exchange the correction data. We tested these two communication ways.

3. RESULT

3.1 Rapid Static

In rapid static mode, Continuously Operating Reference Stations (CORS) and nearby government benchmarks were used to perform network adjustment. The measured point is tied to the CORS and government benchmarks. We obtained the precise ephemerises from NASA’s The Crustal Dynamics Data Information System (CDDIS) to replace the navigation file from the field. The deviation of the measured point against the reference station’s coordinates is depicted in Figure 3.

The accuracy (RMSE) of rapid-static mode in this measurement mode is as follow:

Easting : 0.014 m, 
Northing : 0.018 m, 
Ellipsoid Height : 0.027 m.

Figure 3. Deviation of the measured point against the benchmark’s coordinates in Easting-Northing (a) and Easting-Ellipsoid Height (b) in Rapid Static Mode.

3.2 RTK – LoRa Radio Modem
The landscape of the test site is predominantly farm and residential areas. The closest distance between the reference station and the test point is about 3.4 km, meanwhile, the rover lost its radio reception on 1.8 km. Thus, the measurement could not be made due to the limited range of the radio.

3.3. RTK – NTRIP Emlid Caster
Emlid provides each user with five NTRIP mounts points ID available to be connected to the reference station and each mount point could serve up to ten rovers at once. Mount points come with a distinct password to connect the reference station to the caster and connecting the rover is possible by specifying the NTRIP address, port, mount point ID and password (Figure 4).

One advantage of NTRIP Caster is it is free of charge while most NTRIP services cost some money for their subscription and the service region also varies depending upon the availability. Some countries might not have NTRIP Services available in their area. NTRIP Caster enables users to use their service anywhere in the world as long they have access to the internet.

Three rover points were measured in 20 repetitions with keeping certain pauses time in between repetitions. With the baseline between the reference point and the rovers are 6.157 – 9.168, the accuracy (RMSE) in this RTK measurement mode are as follow:

Easting : 0.015 m
Northing : 0.011 m
Ellipsoid Height : 0.022 m

Figure 5. Deviation of the measured point against the benchmark’s coordinates in Easting-Northing (a) and Easting-Ellipsoid Height (b) in RTK mode using Emlid Caster.

At the maximum distance of 9.168 km, the RMSE behaves relatively stable on all the axes (Easting, Northing and Ellipsoid Height) regardless of the distance between the reference point and rover (Table 1). At this stage, we suspect the obstruction (multipath) surrounding the rover gives a stronger influence on the accuracy rather than the distance between the reference point and the rovers. However, a further test is required to justify this assumption.

Baseline (m)	Easting	Northing	Ellipsoid Height
6157.132	0.011	0.008	0.026
9167.551	0.023	0.014	0.016
8099.122	0.003	0.007	0.021

4. REMARK
The ability to conduct a survey on RTK mode increases the time efficiency of the surveys. Emlid Reach RS2 has the ability to perform RTK mode surveys in two ways either using radio communication or using NTRIP Caster. According to our 30 seconds observation time test using RTK Emlid Caster, the result has relatively the same accuracy as seven minutes rapid static mode (1.5cm horizontal and 2.2cm vertical). However, due to the limited range of radio communication, RTK mode using LoRa radio could not be operated at a distance of more than 1.8 km.

NTRIP Emlid Caster enables any device with RTK module to use the NTRIP service and that include Drones as well. For instance, P4P RTK from DJI could receive the correction from the NTRIP by connecting to the Emlid Caster. In this manner, the accuracy of the images taken by the drone will be improved as well. We see Emlid RS2 not only as a device but a comprehensive service. The RTK function from Emlid unlocks the potential and efficiency of any other application.

Reference
[1] Y. Feng and J. Wang, "GPS RTK Performance Characteristics and Analysis," Positioning, Vol. 1 No. 13, 2008, pp. -.
[2] Teunissen, P. J. G.; Khodabandeh, A. (2015). Review and principles of PPP-RTK methods. Journal of Geodesy, 89(3), 217–240. doi:10.1007/s00190-014-0771-3 
[3] Rizos, C., Han, S. Reference station network based RTK systems-concepts and progress. Wuhan Univ. J. Nat. Sci. 8, 566–574 (2003)
[4] Langley, Richard B. "Rtk gps." Gps World 9.9 (1998): 70-76.

Coming back to our main question, does the RS2’s competitive price point come at a cost— What is your response?

Yes, the Emlid Reach RS2 is phenomenally priced— almost too good to be true to a surveyor’s ear. But upon testing it, I can say that the Emlid RS2 is a capable addition to the surveyor’s tool belt. We exclusively conduct drone surveys and we very rarely need a sub-centimeter level (mm) accuracy so the accuracy provided by the Emlid is satisfactory. So yes, I’d say the RS2 holds up much better than expected.